The use of renewable feedstock in UV-curable materials – A new age for polymers and green chemistry

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Progress in Polymer Science 38 (2013) 932– 962

Contents lists available at SciVerse ScienceDirect

Progress in Polymer Science

j ourna l ho me pag e: www.elsev ier .com/ locate /ppolysc i

he use of renewable feedstock in UV-curable materials – A new ageor polymers and green chemistry

aurent Fertier, Houria Koleilat, Mylène Stemmelen, Olivia Giani, Christine Joly-Duhamel,incent Lapinte, Jean-Jacques Robin ∗

nstitut Charles Gerhardt Montpellier UMR5253 CNRS-UM2-ENSCM-UM1 – Equipe Ingénierie et Architectures Macromoléculaires, Université Montpellier II –at 17 – cc1702, Place Eugène Bataillon 34095 Montpellier Cedex 5, France

r t i c l e i n f o

rticle history:eceived 18 July 2012eceived in revised form0 December 2012ccepted 19 December 2012vailable online 4 January 2013

a b s t r a c t

This review aims to cover the state of the art of renewable feedstock use in materials produc-tion using photopolymerization processes. This area of investigation is an emerging field ofresearch, and it combines biosourced molecules with a cheap and rapid radiative processingmethod that avoids any emission of volatile organic compounds. The main classes of natu-rally occurring molecules and macromolecules such as lipids, amino acids, carbohydrates,polyenes, etc. are detailed. The way they are used or integrated in photopolymerizable sys-tems are described in relation to their applications: coatings, biomaterials, biodegradable
eywords:
hotopolymerizationenewable resourcesegetable oil

drug delivery systems, microelectronics or optoelectronics. This critical review takes intoaccount the reactivity of the various compounds as well as their cytotoxicity, biodegrad-ability and finally their end uses.

arbohydratesmino acidsatural rubbers

© 2012 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Renewable macromolecules as raw precursors for UV-cured mate

Abbreviations: 5-CAGA, 5-ring membered cyclic acetalglycerin acrylate; 6-CAethacrylate; API, Acrylatedpolyisoprene; BMA, n-butyl methacrylate; CCCA,

MAc, Dimethylacetamide; DMAP, N,N-dimethylamino pyridine; DMF, Dimeimethylsulfoxide; DS, Substitution degree; ECLO, Epoxidized cyclohexene-dimethylaminopropyl)-carbodiimidehydrochloride; ENLO, Epoxynorbornene linoybean oil; ESO, Epoxidized sunflower oil; F, Phenylalanine; GCA, Glycerin

BA, Hyperbranched acrylate; HEMA, 2-hydroxyethyl methacrylate; HMPP, 2-hesenchymal stem cells; HPN, Hybrid polymer network; I, Isoleucine; iBM

nterpenetrating polymer network; K, Lysine; L, Leucine; LbL, Layer by layeight organogelators; LO, Linseed oil; M, Methionine; MA, Methacrylic anhydhenylisocyanate); MMA, Methyl methacrylate; NELO, Norbornenylepoxidized linMA, N-methylolacrylamide; NMP, N-methyl-2-pyrrolidone; NR, Natural rubbeolydimethylsiloxane; PEG, Poly(ethylene glycol); PEGDA, Polyethylene glycolsopropylacrylamide); PUR, Polyurethane; RAFT, Reversible addition-fragmentatiil; SIPN, Semi-interpenetrating polymer network; SMCs, Smooth muscle cells;

DI, Toluene diisocyanate; TEC, Thiol-ene coupling; TEOS, Tetraethylorthosilicate;ethyl-N-(2-hydroxyethyl)propionamide]; VAPG, Valine-alanine-proline-glycine∗ Corresponding author. Tel.: +33 4 67 14 41 57; fax: +33 4 67 14 40 28.

E-mail address: [email protected] (J.-J. Robin).

079-6700/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.progpolymsci.2012.12.002

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933rials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934

GA, 6-ring membered cyclic acetalglycerin acrylate; AEMA, Aminoethyl-Cyclic carbonate carbamateacrylate; DAAm, N,N-dimethylacrylamide;

thylformamide; DMPA, 2,2-dimethoxy-2-phenylacetophenone; DMSO,erivatized linseed oil; ECM, Extracellular matrix; EDC, 1-ethyl-3-(3-seed oil; EO, Epoxidized oil; EPI, Epoxypolyisoprene; ESBO, Epoxidizedcarbonateacrylate; GMA, Glycidyl methacrylate; HA, Hyaluronic acid;ydroxy-2-methylphenyl-1-propanone (Darocur® 1173); hMSC, HumanA, Isobutyl methacrylate; IEMA, 2-isocyanatoethylmethacrylate; IPN,er; LCST, Low critical solution temperature; LMOGs, Low molecularride; MAG, Monoacylglycerol (monoglyceride); MDI, Methylene bis(4-seed oil; NHS, N-hydroxysuccinimide; NIPAAm, N-isopropylacrylamide;

r; NVP, 1-vinyl-2-pyrrolidinone; PBS, Phosphate buffer solution; PDMS, diacrylate; PI, Polyisoprene; PLLA, Poly(L-lactide); PNIPAAm, Poly(N-on chain transfer polymerization; SA, Succinic anhydride; SBO, SoybeanSolA, Solketalacrylate; T, Threonine; TAG, Triacylglycerol (triglyceride);

TPGDA, Tripropylene glycol diacrylate; V, Valine; VA-086, 2,2′-Azobis[2-; W, Tryptophan.

dx.doi.org/10.1016/j.progpolymsci.2012.12.002

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L. Fertier et al. / Progress in Polymer Science 38 (2013) 932– 962 933

2.1. Lipids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9342.1.1. Glycerol derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9352.1.2. Unsaturated oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9362.1.3. Epoxidized oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939

2.2. Polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9392.2.1. Acrylate moiety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9402.2.2. Other photocrosslinkable moieties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 943

2.3. Natural rubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9443. Renewable molecules as functional groups for UV-cured materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946

3.1. Sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9463.1.1. Macromolecules based on (meth)acrylated monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9463.1.2. Macromolecules based on vinyl/allyl monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9473.1.3. Specific use: photoinitiator water-soluble complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 947

3.2. Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9483.2.1. Hydrogels based on (meth)acrylate precursors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9483.2.2. Photoresponsive hydrogels based on cinnamate precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 950

4. Photoreactive biosourced molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9514.1. Coumarin-derived compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9514.2. Cinnamate-derived compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9514.3. Natural acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9534.4. Furans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956. . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction

The increasing number of research studies devotedto the development of biosource-based materials revealsthe great potential of renewable raw molecules and theirability to substitute for petrochemical-based materials.The construction of biorefineries and the availability ofmolecules such as glycerin derived from biodiesel produc-tion are a great evolution of the chemical industry. Untilnow, only a few examples of biosourced polymers havebeen available, and the famous polyamide 11 is synthesizedfrom castor oil, a vegetable oil also used in the prepara-tion of polyurethanes. In recent years, there has been a realexplosion in the number of studies on the development ofmaterials derived from biomass. Typical monomers such asacrylic acid, epichlorohydrin and acrylonitrile can be nowproduced from biosourced feedstock. The industrial pro-duction of “green” polyethylene in Brazil proves that thisis not just a trend but a “mutation” in polymer chemistry.Moreover, this industrial revolution should enable agricul-tural revitalization in certain countries, thanks to the addedvalue of agricultural products.

This mutation, which started one decade ago, mustbe followed by a real strategy concerning the economicconstraints of this approach. The best example addressesthe peculiar case of lipids, which are currently used forhuman and animal food and have recently been employedin the production of biodiesel. This continuous growth oflipochemistry activities will promote competition amongthe different end uses of vegetable oils (in some countries,mainly human nutrition).

Nevertheless, some non-edible oil species may be a non-competitive alternative to this situation. Other biomass
deposits, such as algae, lignins, celluloses, polysaccharidesand vegetable proteins, are easily affordable precursorsof carbon with unlimited deposits and very easy access
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956

thanks to worldwide production. These carbon sources canbe extracted from some industrial by-products and wastes(wood, wood pulp, starch, etc.) whose actual valorizationis not secured. Developments based on these by-productswill undoubtedly be of great interest because they arenot competing with raw materials devoted to nutrition.Biosourced materials are rarely used just after harvest orextraction and often need preliminary treatments (purifi-cation, chemical or enzymatic modifications, etc.) to accessreagents usable in the elaboration of polymers and poly-meric materials. Linseed oil is a rare oil variety that canbe used in its native form, as its polymerization occursunder oxygen and UV irradiation without any preliminarymodification.

To modify biosourced raw materials to make themusable as reagents in material production will be the greatchallenge for chemical engineering and biotechnologiesduring the next decades. Some recent promising resultsopen the way in the field of vegetable oil modification(epoxidation) and enzymatic degradation of starch to pro-duce various monomers (succinic acid, glycolic acid, etc.).

The use and purification of biomass can be satisfying ifenvironmentally friendly processes limiting the productionof wastes and the emission of volatile organic com-pounds (V.O.C.) are involved. In the same way, materialsprocessing should require low temperature and energy-efficient processes. For instance, UV radiation is a simpleand convenient form of energy and does not requireexpensive devices. Thanks to its high output, this specialpolymer processing method is enjoying a new expansionand is applied at the industrial scale for inks, curableresins and also in various high-added-value products suchas liquid crystal polymers and non linear optics. Liq-
uid resins can be converted into solid resins in a fewtenths of a second, making this process very attractive tothe scientific community for the past three decades. The

934 L. Fertier et al. / Progress in Polymer Science 38 (2013) 932– 962

le raw m

pctomncm

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wtpitptwapfi

Fig. 1. Examples of renewab

hotopolymerization mechanism can be achieved either byycloaddition of photosensitive molecules (chromophoreype) or by polyaddition of double bonds under radicalr cationic initiation. It is specially adapted to biosourcedaterials and thermally sensitive molecules, as this tech-

ology works at ambient temperature [1]. It can belassified as a “green” process and its use for biosourcedaterials may be of practical interest.Thus, this review aims to record the state of knowledge

n the area of natural compounds useful for photopolymer-zation (Fig. 1).

In the first part, we describe compounds from biomassith significant molecular weights that can be brought

ogether under the designation macromolecules (lipids,olysaccharides. . .). They represent an interesting and

mportant carbon source to be used as the main architec-ural backbone for materials. Some of them are naturallyhotosensitive; others require preliminary modificationso be reactive under UV irradiation. In the second part,e report the use of biosourced molecules (sugars, amino

cids. . .) for their specific properties, such as biocom-atibility and pH sensitivity. Their incorporation intounctional materials through UV-based methods (graft-ng or polymerization) are described. In the last part,

olecules in photochemistry.

we discuss naturally occurring and abundant molecules(coumarin, natural acids. . .) that display photosensitiveactivities and are used for their photoreactive function.

2. Renewable macromolecules as raw precursorsfor UV-cured materials

2.1. Lipids

Oils and fats from vegetables and animals are his-torically and currently the most important renewablefeedstock of the chemical industry, and their productionis growing (Fig. 2) [2]. They are used for the produc-tion of surfactants, lubricants and coatings. Vegetable oilsare composed of unsaponifiable compounds such as ter-penes, steroids and fatty acids and saponifiable compoundssuch as phospholipids and glycerides. The latter, and morespecifically triglycerides (also called triacylglycerols orTAGs), are the most abundant components, and their com-positions are specific to the source plant species. Most raw
vegetable oils contain various lengths of fatty chains ran-ging from C10 to C22 and various double bond contents perchain ranging from 0 to 3. Unsaturated oils are much moreabundant than saturated ones and are classified into three


and fats
Fig. 2. World production of oils
types depending on the ratio of double bonds measured byiodine value (IV): non-drying oils (IV < 125), semi-dryingoils (125 < IV < 140) and drying oils (IV > 140).

Rare vegetable oils contain peculiar reactive functionalgroups, such as an epoxy group (vernonia oil), a hydroxylgroup (castor oil with ricinoleic acid) and a keto (oxo)group (Licaniarigida seed oil) (Fig. 3). Otherwise, variouschemical pathways to functionalize triglycerides and fattyacids, such as hydroformylation [3,4], thiol-ene reaction(TEC) [5,6] and selective oxidation [7] have already beendescribed. Epoxidation is one of the most widespread func-tionalizations, thanks to the ease of ring-opening of theepoxy group by various nucleophilic reactants (alcohol,thiol, carboxylic acid, amine, etc.).

Some raw oils, such as linseed oil, can be cured underdaylight or UV irradiation through a photooxidation mech-anism (see Section 2.1.2.1). The photooxidation remainsa long process, and metallic salts must be added toenhance the crosslinking rate. For some industrial appli-cations such as coatings, a high drying rate is required.Raw oils are therefore chemically modified with morereactive unsaturations (such as (meth)acrylates [8,9],maleic derivatives [10–12] or allyl ethers[13]) that can becured by radical reaction as illustrated in Fig. 4. Anothermethod has been proposed to enhance curing reactivityby using epoxidized oils or norbornenyl epoxidized oils.In this specific case, crosslinking occurred under cationicpolymerization.

As previously mentioned, various crosslinking mech-anisms are possible depending on the oily reactive
compounds. Fig. 5 illustrates photocured vegetable oilsobtained by photooxidation, styrenisation, thiol-ene cou-pling (TEC) or acrylate coupling of fatty chains and thering-opening of the epoxy group.
for 1999–2000 and 2009–2010.

Glycerol, the by-product of trans-esterification of veg-etable oils in presence of alcohol (methanol and ethanol),is also an interesting derivative for photochemistry.

2.1.1. Glycerol derivativesGlycerol is one of the most important feedstocks in the

modern oleochemical industry. It is obtained by the saponi-fication of fats or as a by-product in the production ofbiodiesel. Due to its competitive cost, worldwide availabil-ity, and built-in functionality, glycerin and its derivativeshave become useful for numerous commercial applica-tions. As the production of glycerin currently exceedsdemand, the valorization of glycerol has emerged as achallenging trend [14]. Thus, glycerol is currently used inindustry as an intermediate in the synthesis of various com-pounds (glycerin carbonate, Solketal®, acrylic acid, etc.)[15]. Some of these compounds, bearing a (meth)acrylategroup (e.g. glycerin carbonate acrylate (GCA), glycerin car-bonate methacrylate (GCMA), Solketal® acrylate (SolA),5-membered ring cyclic acetalglycerin acrylate (5-CAGA),6-membered ring cyclic acetalglycerin acrylate (6-CAGA)and cyclic carbonate carbamateacrylate (CCCA)) have beenstudied with respect to photopolymerization [16] (Fig. 6).Kilambi et al. [17,18] have broadly investigated the impactof the glycerol-based group on the rate of the pho-topolymerization of these (meth)acrylate monomers. Theyquantified the contribution of intramolecular interactionsand steric effects of these monomers to their reactivity.Some of these monomers show very high polymerizationrates due to some transfer reactions and the Tromms-
dorff effect. Additionally, other derivatives of glycerol,such as 1,3-glycerol dimethacrylate (GDM) and glyceroltrimethacrylate (GTM) are employed in restorative dentalmaterials to improve their mechanical properties [19,20].


Fig. 3. Various fatty chains of raw vegetable oils.

dificatio

22sueiabis

Fig. 4. Chemical mo

.1.2. Unsaturated oils

.1.2.1. Photooxidation. The high rate of unsaturation ofome vegetable oils makes them sensitive to autooxidationnder air (Fig. 7) [21–25]. This reaction is basically used tolaborate Linoleum from linseed oil [26–28]. The crosslink-ng mechanism was largely studied by Paramarta et al. [29],
nd the formation of the lipidic network was explainedy successive reactions: formation of radical species,
somerization, hydroperoxidation and crosslinking. Topeed up the oxidation process, different routes such as

ns of vegetable oils.

thermal treatment or UV irradiation [30] using metal-basedcatalysts were examined. For instance, metal oxides [31](zinc, lead, manganese, cobalt) and more recently the asso-ciation of primary and secondary driers (cobalt combinedwith zirconium or calcium and zirconium octoates) wereinvestigated.

2.1.2.2. Thiol-ene coupling (TEC) on vegetable oils. Hoyleand Bowman [32] have already presented an overview ofthiol-ene coupling (TEC) as a way to make photocurable


styrenis
Fig. 5. Vegetable oil-based networks from (a) the photooxidation, (b) theand (e) ring-opening of epoxy group.
materials using petroleum-alkenes. The polyunsaturatednature of triglycerides makes them good candidates forTEC. Basically, multifunctional thiols are added onto fattycompounds to create a polymeric network as shown inFig. 5c. The low thermal stability of lipidic compounds jus-tifies the use of UV irradiation instead of thermal activation
in the TEC reaction. Another benefit of UV-induced TECis its lower sensitivity to oxygen inhibition compared toother typical free radical polymerizations [33]. The struc-ture of both the unsaturated fatty derivatives and the thiol
Fig. 6. Structures of photocurable

ation, (c) thiol-ene coupling (TEC) or (d) acrylate coupling of fatty chains

reagents affects the reactivity [34]. Thus, as mentioned byHoyle et al. [35] and Hoyle and Bowman [32], the reactivityof unsaturated reagents with SH groups can be classifiedas conjugated diene < maleimide < (meth)acrylate < vinylether < norbornene. Samuelsson et al. [34] showed thatcoatings could be made by TEC using internal double bonds
of fatty esters (methyl oleate and methyl linoleate) andmultifunctional thiols. Bexell et al. [36,37] reported theefficiency of the TEC reaction for coating aluminum sur-faces with vegetable oil-based films. In this case, linseed oil
glycerol-based monomers.


xidatio

rm[assrp

2iitlcecoeev

olaPmadoOoih

rwidrgr[

Fig. 7. Simplified photoo

eacted with thiol groups of mercaptosilane-treated alu-inum. On the other hand, Klaasen and Van der Leeuw

38] prepared a solid alkyl paint based on natural oils by combination of TEC and oxidative processes. In the firsttep, a partial network was created by the reaction betweenunflower fatty acids and trifunctional thiols. Next, theemaining double bonds were consumed by an oxidationrocess using a vanadylbipyridyl catalyst.

.1.2.3. (Meth)acrylated oils. As stated previously, thenternal double bonds of vegetable oils have a low reactiv-ty; hence, a wise option is chemical modification to makehem more reactive under UV irradiation. Modifications ofipids have been extensively described in the literature andan be applied to various oils, such as linseed, rapeseed cal-ndula [9] or soybean [8] oils. One of the most widespreadhemical modifications of oils is the (meth)acrylation thatccurs by the reaction between (meth)acrylate acid andpoxidized oil [8,9,29]. A second pathway consists of thesterification of (meth)acryloyl acid with raw, functionalegetable oil bearing hydroxyl groups (castor oil) [39].

Another strategy to graft (meth)acrylic groups is basedn isocyanate derivatives and produces urethane acry-ated vegetable oils. Homan et al. [40] also employed ancrylate-bearing isocyanate group to acrylate castor oil.atel et al. [41] modified monoacylglycerol (also calledonoglyceride or MAG) with such diisocyanate reagents

s methylene bis(4-phenylisocyanate) (MDI) and tolueneiisocyanate (TDI); the free terminal isocyanate groupsf MAG reacted with the acrylate monomer-bearing freeH group. A final route for urethane acrylated vegetableil synthesis is the reaction of both an acrylate-bearingsocyanate group and fatty chains with a hyperbranchedydroxyl-terminated polyester [42].

These (urethane) (meth)acrylated vegetable oils can beeacted with other co-monomers or directly crosslinkedith a thiol reagent. In the first case, numerous stud-

es have investigated the influence of acrylate-reactiveiluents on the photocuring rate and have shown a
elationship between the number of acrylate functionalroups on the oil backbone and the hardness of theesulting materials [9,40,41]. Di- and trimethacrylates41], or acrylated oligomers such as acrylated-PEG or
n process of drying oils.

acrylated-poly(�-caprolactone) [43,44], were also asso-ciated with the crosslinking process. In this case, thebiodegradability properties of the resulting films wereexamined, and faster biodegradation was observed forhigh-density crosslinking as a result of low molecularweight between entanglements that might otherwise blocklipase attack sites.

The second type of acrylated oil photocuring occurred inthe presence of thiol reagents. As mentioned above, even ifTEC could occur on triglyceride double bonds, it was shownthat reactivity could be enhanced by activating the alkeneusing acrylate derivatives. Black and Rawlins [45] describedthe photocuring of vegetable oil-based thermosets using aTEC reaction between acrylated castor oil and a polythiolreagent, trimethylolpropanetris (3-mercaptopropionate).The resulting coating showed high solvent resistance, hard-ness and flexibility.

2.1.2.4. Other unsaturated oils. The dimerization of fattyderivatives by carbon-to-carbon links between unsatura-tions in separate fatty chains can occur by a thermal process[46] or by a UV process [6], thereby producing fatty diacids.These derivatives, after reduction into fatty diols, wereemployed as polyols in UV-curable polyurethane disper-sions [47]. Polyurethane dispersion technology reducesshrinkage and leads to better adhesion of films, as the lowviscosity of the fatty acid dimer UV formulations for sprayapplication gives them an advantage over competing tech-nologies.

Vegetable oils can be modified by reaction with maleicanhydride as illustrated in Fig. 4, and their crosslinksare produced by reactive diluents (e.g. 2-hydroxyethylmethacrylate [48]). Thermosets have been prepared byUV-induced copolymerization between styrene and maleicanhydride-modified oils [11,12]. In the field of unsaturatedpolyester resins, Mahmoud et al. [10] used the copolymer-ization of styrene with an unsaturated polyester based onpalm oil MAG. The polyester precursor was synthesized byesterification between the free hydroxyl groups of MAG in
the presence of maleic anhydride.
Other types of reactive unsaturations can also beinvolved in photopolymerization. For instance, allylic alco-hol can be reacted with epoxidized fatty chains (Fig. 4). In


s in cati
Fig. 8. Photoreactive group
this way, UV-curable coatings insensitive to oxygen wereprepared by a thiol-ene process through the reaction of SHgroups with allylic derivatives [13]. For instance, soybeanoil (SBO) materials were investigated by the combinationof polyallylic epoxidized soybean oil (ESBO) and trithiol.

2.1.3. Epoxidized oilsThe cationic photopolymerization of epoxidized oils

is insensitive to oxygen, making it highly attractive formany thin film applications such as inks and adhesives.Tehfe et al. [49] recently explored the cationic poly-merization process of ESO using various photoinitiatorsunder air and solar irradiation. A fundamental researchstudy on photoinitiators was described for the case ofcationic photopolymerization of triglycerides. Gu et al. [50]developed original photoinitiators based on salts contain-ing a tetrakis(pentafluorophenyl)gallate anion. Anotherapproach was examined by Ortiz et al. [51], who usedonium salt to accelerate the photopolymerization of epox-idized natural oils. Other systems were investigated [52]where the effect of the structure of both the cation andanion of diaryliodinium and triarylsulfonium photoini-tiators on the polymerization rate were described. Filmswith a high degree of flexibility and impact strengthwere obtained using this system with vernonia oil as theraw epoxidized oil. This singular oil was also studied bySamuelsson et al. [53], who synthesized UV-curable resinafter the esterification of a hydroxyl functional hyper-branched polyester with vernolic acid. The resulting resinpolymerized in the presence of vernolic acid methyl esteras the reactive diluent.

Several studies focused on nanocomposite coatings,using TiO2 filler and sol–gel precursors from metal-oxoclusters within epoxidized oil, called ceramers [54]. Organ-oclays were also incorporated into epoxidized oil togenerate coatings, in the presence of an initiator, with low
UV-radiation energy consumption during the curing pro-cess [55].
Zou and Soucek [56] studied a new generation ofepoxidized triglyceride derivatives such as norbornenyl

onic photopolymerization.

epoxidized linseed oil (NELO), also termed epoxynor-bornene linseed oil (ENLO). Using divinyl ether as a reactivediluent, the curing rate of NELO was halfway betweenthat of linseed oil and that of cycloaliphatic epoxide [57].Epoxidized cyclohexene-derivatized linseed oil (ECLO) wassynthesized at high pressure using the Diels–Alder reactionbetween cyclopentadiene and linseed oil followed by anepoxydation step (Fig. 8) [57]. ECLO was used to prepareorganic–inorganic hybrid films with tetraethylorthosili-cate (TEOS) [56]. The addition of TEOS enhanced the surfacewetting properties, solvent resistance and impact resis-tance [58].

In sum, numerous strategies for vegetable oil pho-tocuring have been described in the literature. Usingradical photopolymerization, raw unsaturated veg-etable oils can be easily crosslinked. The modificationof triglycerides by (meth)acrylation and the additionof maleic and allylic double bonds lead to higher poly-merization rates. Moreover, some natural oils bearingepoxy groups and epoxidized oils can be crosslinkedunder cationic photopolymerization. These UV-curablelipidic derivatives have mainly been applied in coatingtechnologies.

2.2. Polysaccharides

Biomass provides a very important renewable source ofmolecules as a mixture of polysaccharides (starch, cellu-lose) and oligosaccharides (sucrose, maltose and fructose).Using chemical or enzymatic processes, polysaccharidescan be converted into monosaccharides and simplermolecules (i.e. polyols) [59], unsaturated carboxylic acids[60] and furan derivatives (from hemicelluloses) [61](Fig. 9).

Polysaccharides are the most abundant natural poly-mers on Earth and are extracted from plants, micro-
organisms, fungi, marine organisms and animals. Thesemacromolecules are elements of vegetable structures (cel-lulose, alginate) and play an important role in energystorage (starch). Polysaccharides are in solid state at room


cules ob

tohos

fteaficsii[hfo

b[bsRadptrhboa

Fig. 9. Overview of refined mole

emperature, and appear as fibers, pellets or gels dependingn their chemical structure. Polysaccharides can be eitheromopolysaccharides (only one type of monosaccharide)r heteropolysaccharides (several monosaccharides), ashown in Fig. 10.

Polysaccharides are mainly used in the biomedical fieldor their biocompatibility, and they have also been usedo design biomaterials that open the way to drug delivery,ncapsulation or tissue engineering. These biocompatiblend biodegradable polymers exhibit chemically modi-able functional groups such as hydroxyls, amines orarboxylates. Moreover, all these polymers have their ownpecificities in relation to biomedical applications. Fornstance, chondroitin is a polysaccharide that is presentn cartilage and which can help its regeneration. Chitosan62] and hyaluronic acid [63] accelerate wound healing andave antimicrobial properties, while dextran is mainly used

or colonic drug delivery (the presence of dextranase in thisrgan promotes the hydrogel degradation).

Polysaccharides are also used as rheology modifiers,ut some of them can be applied to photocurable resins64,65]. Among its other properties, cellulose is also capa-le of forming a chiral nematic phase in solution ifubstituents are introduced onto the hydroxyl groups.ecently, new methodologies used polysaccharides andcrylic monomers to form liquid crystal under UV irra-iation [66–68]. Photocurable materials derived fromolysaccharides are called hydrogels, and their defini-ion is more deeply developed in Section 3.2 of thiseview. Interesting reviews about photopolymerizable
ydrogels or photoactive polysaccharides have alreadyeen published [69–71]. We therefore focus on meth-ds to insert photocrosslinkable functional groups suchs (meth)acrylates onto polysaccharide skeletons, on the
tained from biomass treatment.

different networks obtained by photocrosslinking andon the different photocrosslinkable moieties currentlyused.

2.2.1. Acrylate moietyThe acrylate moiety is commonly used to modify

polysaccharides, as illustrated in Fig. 11. (Meth)acrylatesare excellent for photopolymerization due to their highphotocuring rate and the good mechanical properties ofthe resulting materials.

2.2.1.1. Synthesis of photocrosslinkable acrylated polysac-charides. Glycidyl methacrylate (GMA) and methacrylicanhydride (MA) are the reagents most often used to mod-ify polysaccharides. The reaction of the hydroxyl groupsof polysaccharides with the epoxide ring of GMA is afavorite modification method. Polysaccharides that aremodified with GMA include hyaluronan [63,72–82], dex-tran [74,83–85], heparin [86], or chondroitin sulfate [87].The addition of GMA onto polysaccharides usually occurseither in aqueous medium [62,63,72,73,75–81] or in abuffer medium (often a phosphate buffer solution) [82,87].To a certain extent, aqueous medium permits the solubi-lization of polysaccharide, but GMA is very slightly solublein this solvent. It can even be degraded in water (via hydrol-ysis of the epoxy functional group). The esterification ofthe hydroxyl functional groups of polysaccharides (alginate[88–90], hyaluronan [88,91,92], cellulose [93], and chon-droitin [94]) with MA is also achieved in aqueous medium.Experimental conditions were deeply explored by Kim and
Chu [95] (temperature, time, and amounts of MA and tri-ethylamine). They optimized experimental conditions tocontrol the degree of the substitution, which varied from1% to 75%.


an, chito
Fig. 10. Examples of polysaccharides: heteropolysaccharides (hyaluroninulin, dextrin).
Other ways to obtain acrylated polysaccharides havebeen described, such as the use of acryloyl chloride on chi-tosan [96], 2-aminoethyl methacrylate (AEMA) on alginate[97], 2-isocyanatoethylmethacrylate (IEMA) on starch [98],
and N-methylolacrylamide (NMA) on cellulose [65]. Theuse of acryloylchloride and NMA is avoided in biomedi-cal applications. Indeed, as previously mentioned, acrylatemonomers are more toxic than methacrylate monomers
Fig. 11. (Meth)acrylatedderivatives used for

san, heparin, chondroitin) and homopolysaccharides (starch, cellulose,

despite being faster to cure; thus, methacrylate remainsthe best choice for biomedical applications. Li and Zhang[99] used acryloyl chloride to modify starch, and theyphotocrosslinked it with a zwitterionic acrylate-based
monomer. The final hydrogel had swelling properties inresponse to ionic environments as stimuli. Granat et al. [64]also used acryloyl chloride to substitute ethyl-cellulose,which was used in a further step as binder to control the
the modification of polysaccharides.


F

viaectowcnn

dTthcpsAaolomsaDbi

2pataaut

ig. 12. Modification of chitosan with PEGDA by Michael addition.

iscosity of a UV-radiation-cured ink. Ma et al. proposed annteresting method to insert acrylate functional groups on

polysaccharide: a diacrylate monomer (PEGDA [100] orthylene glycol acrylate methacrylate [101]) reacted withhitosan according to the well-known Michael-type reac-ion between the amines of chitosan and the double bondsf the diacrylate, as shown in Fig. 12. Non-toxic segmentsere introduced on the polymer backbone during the pro-

ess, and unreacted products like remaining PEGDA wereot inconvenient because they were incorporated into theetwork during the photopolymerization step.

The modification of polysaccharides by (meth)acrylateerivatives has been employed by numerous authors.he main differences among published reports concernhe experimental conditions (solvent, heterogeneous oromogenous phase reaction depending on the polysac-haride solubility). Indeed, the main drawback of someolysaccharides is their insolubility in common organicolvents. Unfortunately, the acrylated reagents (GMA, MA,EMA, etc.) used for the modification of polysaccharidesre only soluble in organic solvents. Some modificationsf polysaccharides may improve their solubility by addingipophilic groups onto the polysaccharide backbone [62,96]r by reducing their molecular weight (mostly by enzy-atic digestion) [81]. Tsai et al. [96] improved chitosan

olubility in organic solvent by modifying it with phtalicnhydride. Chitosan was then soluble in DMSO, DMAc,MF, pyridine and NMP. Jeon et al. [97] improved the solu-ility of alginate by treating it with gamma rays to decrease

ts molecular weight.

.2.1.2. UV Irradiation. The experimental conditions forhotopolymerization are often similar in the literature,nd Irgacure 2959® was most often used as the pho-oinitiator. Cytotoxicity, cell viability and cytocompatibility
re of first importance for biomedical applications. Tovoid cytotoxicity, special care should be given to residualnreacted products, such as acrylates, and to the pho-oinitiator amount and chemical structure. Baier Leach
Fig. 13. Different network structures obtained by UV irradiation.

et al. [63] studied the influence on cell viability ofthe amounts and relative ratios of Irgacure 2959® asthe photoinitiator, of N-vinyl-2-pyrrolidone (NVP) as thecatalyst and of the co-monomer. Bryant et al. [102]investigated the cytocompatibility of several photoinitia-tors (Irgacure 651®, Irgacure 184®, Irgacure 907® andDarocur 2959®) and concluded that Darocur 2959® wasthe most promising. Polacheck et al. [89,90] used VA-086as photoinitiator, as it was less cytotoxic for cells (3T3-L1) than Irgacure 2959®. Chandler et al. investigated thecytotoxicity of the photopolymerization process, takinginto account different parameters such as the photoini-tiator, UV irradiation time, and UV radical formation.They demonstrated that a longer UV exposure time coulddecrease cell viability.

2.2.1.3. Material formation. Polysaccharides can lead tohydrogels, but their properties are often tailored byblending different polysaccharides or one polysaccharidewith another polymer (such as PEG, polypeptide, etc.).Such materials can be termed hybrid polymer network(HPN), interpenetrating polymer network (IPN) or semi-interpenetrating polymer network (SIPN), as described inFig. 13.

In the most common network, HPN, the blended poly-mers are modified with the same photocrosslinkable func-tional groups. The formation of the 3D network in whichboth polymers are linked together by covalent bonds occursunder UV irradiation. Han et al. [103] photocrosslinkedacrylated chitosan with N-isopropylacrylamide (NIPAAm).A HPN was obtained that combined chitosan hydrogelproperties (pH-dependent swelling ratio) with PNI-PAAm properties (temperature-sensitive polymer, LCST of32 ◦C).

Leach and Schmidt [77] photocured a hyaluronanpolysaccharide with PEG and also a polysaccharide with
both PEG and peptide [78]. The PEG, peptides and polysac-charides had been previously modified with acrylategroups and led to a hybrid polymer network under UV irra-diation. The use of HPN enabled the tailoring of mechanical


lysacch
Fig. 14. Crosslinking of carboxylate functions of po
properties and the control of swelling ratio for the drugdelivery rate, and it improved the biocompatibility and thecytotoxicity of the final product. In the same way, Pitarresiet al. [84] elaborated a HPN of dextran and polyaspartamineto improve the degradation of the hydrogel in the colon(as polyaspartamine makes the hydrogel permeable to dex-tranase).

An IPNs is a combination of two or more net-work polymers with no link between the polymers,in contrast to an HPN [104]. An IPN allows additiveeffects of the mechanical properties of both networks.Weng et al. [73] prepared an IPN with hyaluronanand DAAm (N,N-dimethylacrylamide). Indeed, hyaluronanhydrogels alone are too brittle, but photocrosslink-ing hyaluronan with a second network of PDAAm(Poly(N,N-dimethylacrylamide)) tremendously improvedits mechanical properties.

A SIPN consists of a network in which another oligomeror polymer is entangled in as an additive. Pescosolidoet al. [105] proposed a SIPN of HEMA-grafted dextranand hyaluronan as additional elements for bioprintingapplications. Hyaluronan brought bioactive and viscoelas-tic properties to the hydrogel. Zhou et al. [106,107]described a SIPN hydrogel of poly(HEMA) as a 3D UV-curednetwork with N-carboxylethylchitosan inserted as a non-photocrosslinked ionic additional element that broughtpH-dependant behavior to the hydrogel (the swelling ratiois hence directly correlated with pH). Suri and Schmidt [75]photopolymerized methacrylated hyaluronan in a colla-
gen network obtained by fibrillogenesis; they comparedan IPN and a SIPN of hyaluronan and collagen. The IPNexhibited a higher crosslink density, so it had a higherelastic modulus and a lower swelling ratio than the SIPN,
arides with p-diazoniumdiphenyl amine polymer.

which is a flexible gel with a highly porous structure.They obtained a material suitable for regenerative medici-nal applications by biomimicry of the extracellular matrix(ECM).

2.2.2. Other photocrosslinkable moietiesThe acrylate functional group is an interesting reactive

group in photocrosslinking processes, but as mentionedabove, skin irritation can occur with acrylate derivatives.Thus, some studies found other photocrosslinkable moi-eties for in vivo applications. Liu et al. [108] developed ahydrogel of heparin and alginic acid sodium salt using adiazonium functional group as photosensitive crosslinker(p-diazoniumdiphenyl amine polymer). The diazoniumfunctional groups reacted with the carboxylate groups ofalginate and the sulfate functional groups of heparin with-out a photoinitiator under UV light, as represented inFig. 14. The presence of alginate enhanced the hydrophilic-ity and the stability of the hydrogel and decreased thesurface roughness.

The diazonium-based polymer, by allowing the con-struction of a multilayer polymer with successive heparinand alginic acid layers, was an efficient technique forthe construction of a layer-by-layer (LbL) self-assemblysystem. Diazonium functional groups are highly reactiveand are good candidates for efficient photocrosslinking,generating only nitrogen as a by-product. Nevertheless,diazonium chemistry is not easy to handle, and the syn-thesis of the diazonium polymer is complex and requires
several synthetic steps [109].
Azide groups (N3) are photocrosslinkable agents that,like diazonium groups, release nitrogen gas to yieldhighly reactive nitrene groups. They interact quickly


Fig. 15. Photocrosslinking of carboxymethylchitosan thanks to azide functions.

an as m

waceasastFbtcswc

Tyopp

mTwcmoi

fcpeaht

Fig. 16. Maleinated chitos

ith one other or with amino groups to generatezo groups ( N N ) without any photoinitiator. Thus,hitosan was substituted with lactose and azide moi-ties to make biological adhesives [110,111]. Lactabioniccid was added to chitosan to enhance its waterolubility. According to the author, replacing 2% of themino groups in chitosan with lactabionate was enough toolubilize it at neutral pH. Jameela et al. [112] improvedhe process of azide substitution in the polysaccharide.irst, 1-chloro-2-hydroxy-3-azidopropane was obtainedy reacting epichlorohydrin with sodium azide, and thenhis was reacted with the amine functional group ofhitosan. Yi et al. [113] avoided the lactabionic acid sub-titution step by using carboxymethyl chitosan, which isater soluble (Fig. 15). The application of this method con-

erned the controlled release of pesticides in agriculture.Another method was developed by Don and Chen [114].

hey used maleic anhydride to substitute chitosan andield a maleinated chitosan in which the double bondf maleic groups reacted with NIPAAm by free radicalolymerization under UV irradiation in the presence ofhotoinitiator, as shown in Fig. 16.

Zhong et al. [115] designed a similar technique, usingaleinated chitosan and PEGDA as the crosslinker (Fig. 17).

he swelling ratios, mechanical properties and pore sizeere easily controlled by varying the ratio of maleinated

hitosan to PEGDA. Cytotoxicity assays showed thataleinated chitosan and its copolymer with PEGDA were

nly slightly cytotoxic at high doses, making it safe for usen biomedical applications.

The grafting of maleic functional groups results inree carboxyl groups that are useful in biomedical appli-ations (e.g. to couple biologically active agents orromote the deposition of inorganic minerals). Monier
t al. [116] introduced an �-cyano-4-hydroxycinnamiccid moiety on chitosan to create a pH-responsiveydrogel. This cinnamic acid derivative is photosensi-ive and requires no photoinitiator. Thus, Nakamura et al.
acroinitiator of PNIPAAm.

[117] prepared cinnamate-modified heparin, producinga photocrosslinked hydrogel for biomedical applications.The use of a cinnamate group as a photocrosslink-able moiety has two major advantages in that it isbiosourced and it is naturally photosensitive. This cin-namate chemistry will be more thoroughly explained inSection 4.2.

Shen et al. [118] inserted allylic moieties onto car-boxymethyl cellulose sodium salt to form hydrogels afterUV irradiation.

Kumar et al. modified cellulose [119] or starch [120]thanks to the radical copolymerization of GMA; freeglycidyl functional groups were then used as pho-tocrosslinkers in the presence of cationic photoinitiator,thereby generating a final film that was perfectly suitablefor coating applications.

Delville et al. [121] photocrosslinked starch in bulkwith no crosslinking agent, using benzoic acid sodiumsalt as a photoadditive. They showed that addition of thissalt allowed the formation of a biodegradable and water-resistant film with properties similar to those of typicalplastic materials. A similar process was described by Zhouet al. [122], who used sodium benzoate to modify the sur-face of starch sheets.

Currently, photocrosslinkable polysaccharides aremainly used in biomedical applications. Medical appli-cations require a very low cytotoxicity, and materialsbased on polysaccharides modified with acrylate or otherphotocrosslinkable moieties can be harmless and useful forapplications such as drug delivery or tissue engineering.

2.3. Natural rubbers

Natural rubbers (NR) are renewable raw materials
extracted from the latex products of Hevea brasilieusis, therubber tree. The crosslinking of polyisoprene (PI) by UVirradiation and without degradation is difficult to achievebecause of the poor reactivity of the amylene double bonds.


ted chito
Fig. 17. Photocrosslinking of maleina
The conversion of this functional group into an epoxy groupor an acrylate double bond allows efficient crosslinkingwith short exposure time in the presence of adequate pho-toinitiator.

Epoxidized polyisoprene (EPI) was obtained by thetreatment of natural rubber with peracetic acid for 6 hat 5 ◦C [123]. The ring-opening polymerization of theepoxy group efficiently proceeds upon UV irradiation inthe presence of triarylsulfonium salt, with the forma-tion of ether links [124–126]. The cationic photoinitiatedpolymerization has the advantage of proceeding in thedark after an initiating exposure, in contrast to radical-based polymerization. Thus, all the epoxy groups reactedafter 0.5-s UV exposure and one day of storage in thedark.

Decker et al. [124–126] have extensively studied UVcuring using cationic and free radical photoinitiators ofboth epoxydized liquid natural rubber and their acrylatedderivatives. Acrylated polyisoprene (API) was transformedinto a hard and insoluble material with a photoinitiator[125]. The intra-chain amylene double bonds of poly-isoprene were shown to copolymerize with the acrylatedouble bonds, thus leading to the formation of a relativelytight polymer network [127].

Polyisoprene can also be readily photocrosslinked byintroducing pendant acrylate double bonds onto the poly-mer backbone by reacting EPI with acrylic acid. The curingof a cationic photoinitiator-induced system comprisingepoxydized natural rubber, cycloaliphatic diepoxyde andglycidyl methacrylate was studied by Kumar et al. [128].
The epoxy groups attached to the rubber particles rapidlyand efficiently reacted both under UV exposure and duringthe postcuring process. In these experimental conditions,the chain extension and crosslinking reactions occurred
san with PEGDA as photocrosslinker.

simultaneously, leading to the formation of an interpen-etrating polymer network.

NR exhibited good properties (i.e. tensile strength, flex-ibility and resistance to impacts and tears) but also hadsome drawbacks. NR had low flame resistance, limitedresistance to chemical solvents, and poor ozone aging,mainly owing to its unsaturated hydrocarbon chain struc-ture. Consequently, the chemical modification of NR haswidely been considered as a solution to compensate forthese disadvantages.

The synthesis of graft copolymers from NR has beenconducted in solution [129,130], in solid state [131] or inlatex media [132–136]. Derouet et al. [137] prepared N,N-diethyldithiocarbamate-functionalized 1,4-polyisoprenesto synthesize thermoplastic-grafted polyisoprene by thephotopolymerization of vinyl monomers initiated accord-ing to the principle of the “grafting from” route. Twodifferent methods of grafting N,N-diethyldithiocarbamatefunction in the lateral position of 1,4-polyisoprene chainshave been considered (Fig. 18). Derouet et al. [137]grafted N,N-diethyldithiocarbamate inifiter groups ontorubber chains, and these groups were used to initi-ate the photopolymerization of acrylate or methacrylatephosphorus-containing monomers [138]. Under UV irradi-ation, the rubbery macroinifiters dissociated to form stableN,N-diethyldithiocarbamate radicals capable of initiatingmonomer polymerization.

To improve NR properties (i.e. flame-retardancy and oilresistance), the grafting of polymers bearing phosphonategroups was performed. Graft copolymers [139] of NR
with poly(dimethyl(acryloyloxymethyl)phosphonate)(NR-g-PDMAMP) and of NR with poly(dimethyl-(methacryloyloxyethyl)phosphonate) (NR-g-PDMMEP)were prepared [140] in latex medium with a UV lamp to


size N,N

idsolbslomti

3U

3

3m

mree

Fig. 18. Various ways considered to synthe

nitiate the grafting reaction. Various graft copolymerserived from NR were synthesized with well-controlledtructure and size by this technique. Photopolymerizationf vinyl monomers such as alkylacrylates and methy-acrylates, styrene, acrylonitrile and acrylamide, initiatedy N,N-diethyldithiocarbamate-functionalized NR, wasuccessfully conducted in bulk in toluene solution or inatex media [139]. In summary, few studies have focusedn the photopolymerization of NR; rather, they haveainly focused on the modification of the amylene bond

o obtain functionalized PI that could be crosslinked by UVrradiation.

. Renewable molecules as functional groups forV-cured materials

.1. Sugars

.1.1. Macromolecules based on (meth)acrylatedonomers

Primary and secondary hydroxyl groups are easily
odified, mainly using carboxylic acid or acyl chloride
eagents, to form (meth)acrylate-based precursors. Consid-ring the toxicity of the reagents, most acrylate-basedxperiments described in the literature are for coatings and

-diethyldithiocarbamate-functionalized PI.

related applications. The methacrylate-based formulationsare more widely used in the biomedical field because oftheir lower cytotoxicity and biocompatibility [141–143].

Rios and Bertorello [144] showed an example of the sur-face modification of industrial poly(vinyl chloride) (PVC)substrates with photocrosslinked acrylated sucrose. Theyincorporated sucrose onto the PVC surfaces to increasethe hydrophilic character of the polymer surface. Kim andPeppas [145] highlighted the role of glucose bearing amethacrylate functional group in the construction of acopolymer network (Fig. 19). The incorporation of glucoseunits provided a water-swelling property to the copolymer,and the use of methacrylate acid as a co-monomer yieldedpH sensitivity for oral drug delivery systems.

Drug-loading systems have been engineered using poly-mers based on �-cyclodextrin molecules and hyaluronicacid hydrogels [72]. A hydrogel was obtained by thephotopolymerization of a mixture of methacrylated �-cyclodextrin and methacrylated hyaluronic acid; this newcomplex showed better interaction with hydrocortisonedrugs.

Recently, Yang et al. [146] used methacrylate-derivitized galactose to functionalize polypropylenemembranes and mimic glycol receptors on cell surfaces.This modification conferred selective bacterial recognition


Fig. 19. UV copolymerization of methacrylated-based glucose, methacrylic acid and tetraethylene glycol dimethacrylate.

hes imm
Fig. 20. Glucose-terminal brus
(Enterococcus faecalis) to the membranes. d-glucosemethacrylate was used to form polymeric brushes fromspherical polystyrene cores for the specific recogni-tion of wheat germ agglutinin lectins [147] (Fig. 20).The authors observed higher binding ability for theglycopolymer-based nanospheres than for the well-knownN-acetylglucosamine-based systems.

Further applications based on (meth)acrylatedmolecules with pendant saccharides have been inves-tigated (e.g. human cornea substitutes based on acollagen–glucose matrix [148] and an antifouling coatingbased on new glyco-membrane bioreactors [149]). Forexample, Patel et al. [150] formulated an acrylate-basedglycoside with polyurethane pendant chains. They con-verted cellulose from sugar cane to glycoside and prepareda UV-curable coating with pendant acrylate-polyurethanechains. They succeeded in the deposition of microfilmswith good adhesion to mild steel substrates with no liber-ation of volatiles. Recently, Steffier [151] found cosmeticapplications of photopolymerizable compositions forpreparing human fingernails. They used methacrylatedderivatives of sucrose and sefose (a sucrose fully modifiedwith fatty acid chains).

3.1.2. Macromolecules based on vinyl/allyl monomersFew examples of vinyl/allyl-based saccharides involved

in photopolymerization have been reported. Yang et al.[152] studied the interaction processes between carbohy-drates and proteins. They reported for the first time theUV grafting of �-d-allyl-glycoside on polypropylene mem-branes. They demonstrated that the immobilized sugarscould interact with the protein Con A when the surfacedensity did not exceed 90 �g cm−2. They highlighted that
a weak interaction (Ka = 103–104 M−1) occurred betweenglycoside structures and Con A. Pichavant et al. investi-gated the photopolymerization of unsaturated monomersderived from renewable feedstock [153,154]. The potential
obilized on polystyrene core.

of donor–acceptor type copolymerization applied to vinyl-ribose and allyl-ribose was evaluated with maleate andfumarate derivatives. Sugar-derived monomers showed aninteresting reactivity compared to commercial oil-sourcedether derivatives. The sugar-based vinyloxy monomersexhibited a higher reactivity with maleate and fumaratemonomers than oil-sourced ether. They also showed thatthe H-bonding due to the free hydroxyl groups in thesugar structure provided better access to the functionalgroups. Another method to form copolymers from unsat-urated monomers was reported by Acosta Ortiz et al.[155,156] using a thiol-ene process between polyallyl-based sucrose and thiol-based derivatives, as illustratedin Fig. 21. They noticed that photopolymerization couldproceed in the absence of photoinitiator, but addition of1% (by weight) photoinitiator permitted a complete reac-tion. The use of polyfunctional monomers gave a flexiblepolythioether network (Tg = 30 ◦C). This method has alsobeen employed with pentaerythritol-based compounds formedical research [157,158].

3.1.3. Specific use: photoinitiator water-solublecomplexes

Due to the pollution of organic volatile compoundsused in conventional photopolymerization, water-solublecomplexes represent a very promising alternative forpollutant-free processes. In 1976, Kubota et al. [159]reported the use of saccharides (e.g. glucose, cellobiose,and maltose) to initiate the photopolymerization of methylmethacrylate (MMA). They highlighted that the saccha-rides were able to form radicals under UV radiationbetween 220 and 300 nm and could stabilize the MMA radi-cals formed during irradiation. These radicals were able to
initiate the polymerization of acrylamide in aqueous solu-tion [160]. The conversion increased with the solubility ofthe saccharides in water (e.g. sucrose (2000 g/L) » d-glucose(900 g/L)). To the best of our knowledge, few studies have


based al

botfttasstetLetaot

tfmmcawa

3

a

Fig. 21. UV copolymerization of sucrose-

een published in this field. The water-soluble propertyf saccharides has been applied mainly to the modifica-ion of hydrophobic photoinitiators to make them usableor photopolymerization in aqueous media [161,162]. Forhis purpose, cyclic oligosaccharides such as cyclodex-rin have been employed for their hydrophobic cavitynd hydrophilic corona [163,164]. The �-cyclodextrin is aeven-glucose ring that has been widely used as water-oluble host/guest complex. Enmanji [165] showed thathe �-cyclodextrin/benzoin ethyl-ether complex was morefficient for the inclusion of photoinitiator in cyclodex-rin due to the electron delocalization in the structure.ougnot et al. [166] and Balta et al. [167,168] showed thefficiency of a thioxanthone-based encapsulated photoini-iator in �-cyclodextrin for the photopolymerization ofcrylamide or MMA. More recent papers describe the usef methyl-�-cyclodextrin [169–172], which is more solublehan �-cyclodextrin in water (2000 g/L).

Sugars from renewable resources are a good alterna-ive to the use of fossil feedstock. Using environmentallyriendly reactions such as those induced by UV, these

olecules have been employed as precursors to prepareaterials for coatings or biomedical applications, with

onsideration to their biocompatibility, water solubility,nd pH-responsiveness. Sugars were mainly functionalizedith UV-reactive functional groups such as (meth)acrylate

nd allyl/vinyl groups on the hydroxyl groups.

.2. Amino acids

Vegetables, legumes and grains contain the essentialmino acids in various proportions. Approximately 80

Fig. 22. Incorporation of polyamino acids w

lylic derivatives using thiol-ene process.

amino acids exist in nature, but only 20–29 are requiredfor human growth. Among the 20 amino acids includedin proteins, only eight are known as the essential aminoacids: isoleucine (I), leucine (L), lysine (K), methionine (M),threonine (T), phenylalanine (F), tryptophan (W) and valine(V). The photopolymerization of this type of compound islimited to the preparation of hydrogels for medical applica-tions. The synthesis of hydrogels by photopolymerizationdoes not use amino acids as monomers. The amino acidsare grafted or introduced along the backbone of the poly-mer to synthesize precursors. Next, the UV crosslinkingreaction takes place in water, as required for biomed-ical applications. Therefore, the synthesis of hydrogelsbased on (meth)acrylated precursors and of photorespon-sive hydrogels based on cinnamate precursors will bedescribed.

3.2.1. Hydrogels based on (meth)acrylate precursorsRadical photoinitiated polymerization is largely used

for the formation of biological hydrogels. Such productshave been extensively employed for a number of biomed-ical applications, such as scaffolds for tissue engineeringand controlled-release systems for drug delivery [173,174].Hydrogels produced by photopolymerization provide ahighly swollen three-dimensional structure similar to softtissues and allow diffusion of nutrients and cellular wastethrough the elastic network. Additionally, these materi-als, generally deposited on living tissues as a liquid, are
polymerized in situ.
Amino acid sequences can be introduced along thebackbone or can be grafted into synthetic hydrogelsduring polymerization. Among synthetic polymers that

ithout and with a PEG spacer arm.

olymer
L. Fertier et al. / Progress in P
can form hydrogels via crosslinking, poly(ethyleneglycol)(PEG) is the most-investigated polymer backbone. PEG-based hydrogels have many advantages in comparisonwith natural hydrogels: they can be photopolymerizedinto various shapes or injected for tissue repair, and theirmechanical properties can be adjusted to fit final appli-cation requirements [70]. Incorporation of amino acidshas been explored with [175] and without [176] a PEGspacer, as indicated in Fig. 22, to provide a biologicalactivity.

Using this amino acid incorporation technique, PEGhydrogels containing amino acid sequences such as IKLLI,IKVAVA, LRE, YIGSR, PDSGR, DGEA and different com-binations have been synthesized. These polymerizationconditions have been shown to be cytocompatible withvarious cell types, providing matrix–cell interactionsresponsible for the attachment of fibroblasts [175,177],aortic smooth muscle cells [178], osteoblasts [179,180], andmesenchymal stem cells [181]. However, a major limitationof this synthetic route is the heterogeneity of PEG-basedgels, appearing during the polymerization process, whichinfluences the final material properties. Synthesis by clickchemistry of hydrogels with controlled architectures wasdeveloped by Polizzotti et al. [182]. Thanks to this tech-nique, the authors improved the mechanical properties ofthe final materials. The integration of multifunctional pho-toreactive polyamino acid sequences into the network was
performed. A tetrazide-multiarm PEG and a diacetylene-functionalized allyl ester containing amino acids were usedto generate well-defined PEG-amino acid hydrogels, asshown in Fig. 23.
Fig. 23. Synthesis of hydrogels via the [3 + 2] cyclo

Science 38 (2013) 932– 962 949

A new method for attaching biological molecules tohydrogels was developed by Elbert et al. [183]. They used aMichael-type addition between a thiol-peptide and a PEGdiacrylamide so that amino acids could be attached in 2 hin aqueous solution at room temperature. The photopoly-merization step can be performed in contact with livingcells. The irradiation conditions and precursor composi-tion greatly affects the stiffness of the materials, whichsubsequently affects cell spreading. The interest in theuse of Michael-type addition between a thiol-containingpeptide and an acrylate functional group is the high selec-tivity of this reaction [174]. When proteins are modifiedexclusively via their free thiols, primary amine-dependentprotein properties and functions may therefore be pre-served. Although the primary amines of proteins can alsoreact with acrylate groups by Michael-type conjugateaddition, under physiological conditions this reaction isvery slow (hours) whereas thiols react quickly (minutes)[183,184].

Free radicals generated by the dissociation of photoini-tiator during photopolymerization can have side effectsand cause damage on cell membranes, proteins and DNA.Consequently, intensive research has been focused onthe development of cytocompatible photoinitiator; thebest-established initiator system for photo-encapsulationwas Irgacure® 2959 (I2959). However, in the case of aminoacids, the aromatic amino acids also absorb at 285 nm,
thus competing with I2959 during photoinitiation. Thus,Bahney et al. [185] developed non-toxic experimen-tal conditions for the photo-encapsulation of humanmesenchymal stem cells (hMSC) using a visible light
addition reaction. (AA)n: polyamino acids.


o acid-b

peatct

aswthtmdTaaeasr

h�o(ettwahaw

Fig. 24. Schematic representation of the photoresponse of an amin

hotoinitiator system, composed of eosin Y, tri-thanolamine and 1-vinyl-2 pyrrolidone, with a peakbsorbance close to 510 nm. This visible light photoini-iator produced hydrogel scaffolds with a more tightlyrosslinked network in one-third the UV polymerizationime of I2959.

These synthetic hydrogels are difficult to synthesizend expensive to obtain. An easier and less expensivetrategy was reported in 2010 by Miller et al. [186],ho synthesized PEG precursors. Step growth polymeriza-

ion via a Michael-type reaction was performed to createigh-molecular-weight photoactive macromonomers ofhe form of acrylate-PEG-(peptide-PEG)m-acrylate. The

acromonomers were then crosslinked into hydrogelsuring a second radical-mediated photopolymerization.his system was used to develop materials to promotengiogenesis in an ex vivo aortic arch assay. The formationnd invasion of new sprouts in the hydrogels, mediated byndothelial cells from embedded embryonic chick aorticrch, were demonstrated. Thanks to this method, the gelwelling was improved and the cost of the materials waseduced compared to previous synthesis methods.

Recently, Zhou et al. [187] developed antimicrobialydrogel films and coatings based on photopolymerizable-poly-l-Lysine-graft-methacrylamide (EPL-MA). Becausef the polymerization difficulty of the oligomeric EPL-MAMn = 3000 g mol−1), dimethylacrylamide and polyethyl-ne glycol diacrylate (PEGDA) crosslinkers were addedo copolymerize and to improve the mechanical proper-ies (Young’s modulus) of the hydrogels. These hydrogelsere able to inhibit the growth of both bacteria

nd fungi; they showed in vitro biocompatibility withuman epidermal keratinocytes. Cai et al. [176] usednother type of peptide-based precursor. Poly-l-lysineith one end-capped allyl group was synthesized by the

ased dendron functionalized with p-nitrocinnamate and its dimer.

ring-opening polymerization of carbobenzyloxy-l-Lysine-N-carboxyanhydride initiated by allylamine. Next, thepoly-l-lysine obtained (Mn = 3060 g mol−1, I = 1.23) wascovalently linked into PEGDA networks via photocrosslink-ing. This covalent immobilization of poly-l-lysine inPEGDA hydrogels has great potential for the devel-opment of injectable materials for nerve repair andregeneration.

3.2.2. Photoresponsive hydrogels based on cinnamateprecursors

Organogels from low molecular weight organogelators(LMOGs, Mw ≤ 3000 g mol−1) are considered stimulus-responsive materials. They can therefore be used assensor materials, chemical valves, catalysts, mechanicaltransducers, controlled-release systems, and artificial mus-cles for biomedical purposes. The morphology of thesematerials changes reversibly by the action of light, tem-perature [188], pH [189], coordination function [190],redox stimuli [191] and even ultrasound [192]. Iwauraand Shimizu [193] reported a thymine-containing bolaam-phiphile showing a reversible photochemical conversionof self-assembled helical nanofibers. They described forthe first time the reversible conversion by UV light ofthe helicity of the thymine moiety between helical andnon-helical nanofibers. Ji et al. [194] described an efficientphoto-reversible gelator based on azo-modified poly(Gly-Asp) dendron and demonstrated that dendrons couldrepresent a new candidate for the elaboration of smart gels.Kuang et al. [195] created a photoresponsive organogelfrom the amino acid-based dendron functionalized with
p-nitrocinnamate. This dendron could self-assemble into afibrous network in common organic solvents at low con-centrations. This remarkable gel was photoresponsive andthermoresponsive. The sol–gel transition in response to


Fig. 25. Cyclodimerization of coumarins under UV radiation.

external photostimuli was attributed to the cycloadditionand cleavage of the cinnamate groups, as shown in Fig. 24.Such materials led to the fabrication of cinnamate-baseddevices but also provided a novel path to design smartmaterials. This cinnamate chemistry will be described inmore detail in Section 4.2.

In summary, the use of amino acids in photopolymer-ization is very limited; they have mainly been used asprecursors to synthesize hydrogels for biomedical appli-cations because of their biocompatibility. UV crosslinkingreactions with amino acids require the attachment ofreactive functional groups such as (meth)acrylate, methy-lacrylamide, allyl and cinnamate groups onto peptidesequences.

4. Photoreactive biosourced molecules

4.1. Coumarin-derived compounds

Coumarins are a class of molecules comprising severalhundred derivatives, among which 7-hydroxycoumarin isthe most famous structure. These molecules are naturallypresent in many plants but have also been synthesized bydifferent routes [196,197]. They are used in various appli-cations, including fragrances, medicines, drug delivery, andliquid crystalline polymers. Their ability to photodimer-ize was discovered one century ago [198]. This couplingreaction is achieved by photoactivation but not by ther-mal processes, making this reaction very interesting for itsspecificity of activation. The reaction occurs in the solidstate for some derivatives, and can also occur in solution at� > 300 nm, leading to different isomers with the formationof a cyclobutane ring by a (2�+2�) cycloaddition. Theseisomers exhibit good thermal stability, depending on thestructure of the dimers (Fig. 25) [199–202].

Because of the peculiar behavior of this natural pseu-dolactone, the crosslinking of polymers became possibleand different applications emerged [203]. Interestingly, thereversibility of this crosslinking under short-wavelengthirradiation (<290 nm) was demonstrated in the 1960s byKrauch et al. (Fig. 26) [200].

As a consequence, a new class of thermosetting mate-rials was obtained for applications such as fluorescentcoatings, laser dyes, printing formulations, polymeric liq-uid crystals [204–208]; they could be recycled by simpleUV irradiation. Chujo et al. [209] investigated this propertyby gelifying water-soluble poly(2-methyl 2-oxazolines)
that were partially hydrolyzed and functionalized withcoumarin moieties. They observed a reversible crosslink-ing, which is typical for hydrogen-bonded systems. Thisstrategy was applied to different classes of polymers,
Fig. 26. Crosslinking of polymers through cyclodimerization ofcoumarins.

including polyacrylates [210,211] and polyamides [212].Chain extension of oligomers was also explored [213–216].As an example, hydroxytelechelic oligomers of polyetherswere modified by converting OH groups by simpleesterification before the reversible chain extension,offering chemists a versatile method to prepare high-molecular-weight polymers (Fig. 27) [214].

It must be mentioned here that the dimer of coumarincan also be used to couple diamino-terminated molecules,thereby acting as a fluorescent probe for kinetics studies,as shown in Fig. 28 [217].

The dimerization ability of coumarin derivatives hasrecently become fashionable thanks to the advent of con-trolled radical polymerization. Thus, Chen et al. and Fenget al. used RAFT polymerization to prepare polyacrylatesand polystyrenes bearing pendant coumarin moieties thatwere subsequently dimerized [218,219]. In the same way,Tian et al. used atom transfer radical polymerization to pro-duce liquid crystalline homopolymers and di- or triblockcopolymers with polystyrene segments (Fig. 29) [220].

It should be noted that there were some attemptsto modify well-known biodegradable polymers,such as polypeptides [221,222], �-caprolactone andpoly(trimethylene carbonate) [223–226], for applicationin biomaterials. The literature on the use of coumarinsand coupling mechanisms is extensively described in theexcellent review of Trenor et al. [227].

4.2. Cinnamate-derived compounds

Cinnamic acid is synthesized by numerous plantsby the action of phenylalanine ammonia-lyase on l-phenylalanine, yielding phenylpropanoid compounds. Theintra-cyclization of cinnamic acid leads to the formation ofcoumarins (described above).

The synthesis of cinnamic ester-derived polymers is awide area of investigation, and studies over many years
have focused on the polymerization of related monomersor on the modification of pre-formed polymers by cin-namate groups [228]. Cinnamate groups can be (2� + 2�)dimerized under UV light (270–310 nm) or isomerized

9 olymer

ft

h(bgi[otLmmc(ct

52 L. Fertier et al. / Progress in P

rom trans to cis form in a reversible equilibrium, as illus-rated in Fig. 30 [213,229].

The design of new monomers bearing cinnamate groupsas been widely investigated, and many syntheses ofmeth)acrylates bearing lateral cinnamoyl groups haveeen proposed. These functionalized polymers exhibitood reactivity with or without photosensitizer, mak-ng them suitable in the area of photoactivable polymers230,231] and patterned polymers [232]. The presencef substituents on the aromatic ring can slightly modifyhe reactivity of the �,�-unsaturated ketone [233,234].aschewsky and Rekai [235] synthesized a cinnamoyl-odified methacrylamide combining the cloud point of theethacrylamide moiety, the crosslinking property of the

innamoyl entity and a free OH group for further graftingof a drug, for instance). Styrenic derivatives containinginnamate groups were prepared by chemical modifica-ion of 4-chloromethylstyrene and copolymerized with

Fig. 27. Chain extension of polymers usin

Fig. 28. Coupling of polymers us

Fig. 29. Block copolymers synthesized from

Science 38 (2013) 932– 962

a styrenic monomer bearing an azobenzene moiety. Theproperties of the obtained statistic copolymer were stud-ied in the field of photoalignment materials [236]. Linear orcyclic polysiloxanes such as polydimethylsiloxane (PDMS)were designed, and the competition between dimeriza-tion and isomerization was studied in detail for liquidPDMS [237,238]. Similarly, linear and cyclic polyphosp-hazenes were synthesized either by the modification ofthe hexachlorocyclotriphosphazene monomer before itsring-opening polymerization or by the modification ofpoly(dichlorophosphazene) and its crosslinking [239,240].Polyvinyl cinnamates were mainly obtained by modifica-tion of PVA with cinnamoyl chloride [241,242] and wereused for compartmentalized reactions in microparticles
and microfibers [243]. These polyvinyl cinnamates couldalso be synthesized by the amidification of polyvinyl aminewith cinnamoyl chloride, giving hydrophilic networks withpotential applications in the biomedical field [244].
g cyclodimerization of coumarins.

ing dimers of coumarins.

controlled radical polymerization.


he cinna
Fig. 30. Reversibility of t
The recent interest in natural materials opened the wayto the design of materials bearing functionalities. As aresult, modifications of natural materials to make themphotoresponsive were investigated. Epoxidized natural oils[245], cellulose [246], chitosan [247] and starch [248]were modified using cinnamoyl chloride to give materi-als exhibiting gel properties (for example) or to modify thephysical or mechanical properties of the starting naturalproducts.

The modification of biodegradable polymers was alsostudied, and some authors crosslinked poly(l-lactide). Thispolymer was extended to produce high-molecular-weightpolymers before irradiation; the resulting polymer gelswere less sensitive to enzymatic degradation [249]. Thecopolymerization of l-lactide with a cyclic carbonate bear-ing a pendant cinnamate group was also described. Theresulting poly(ester-carbonate) copolymer with a statis-tical distribution of the two monomers was shown tobe photocrosslinkable at � > 365 nm [250]. Finally, thebio-based nanoparticles of poly(3,4-dihydroxycinnamicacid-co-4-hydroxycinnamic acid) were shown to be sen-sitive to dimensional size changes under UV irradiation at>280 nm. Thus, the diameter of the particles halved underirradiation and partially recovered their dimensions aftera second exposure at 254 nm, making them interesting assize-controlled carriers for environmental and biomedicalapplications [251].

Finally, besides the design of new monomers withcinnamate moieties, it must be mentioned that somemonomers with two cinnamate groups were devel-oped. These monomers were investigated in step growthpolymerization of PET to make it photosensitive. Thus,
PET-based copolymers were spun after melt extrusionbefore their irradiation under UV light, yielding PET withimproved mechanical and thermal properties (Fig. 31)[252].
Fig. 31. Crosslinking of PET modified with cinnamates moieties.

mate cyclodimerization.

Otherwise, many attempts focused on the area of liq-uid crystalline polymers, where cinnamoyl groups are goodcandidates for mesogenic derivatives. Thus, the nature andthe influence of the substituents of the aromatic ring, andthe length of the spacer between the polymer backboneand the cinnamoyl group, were studied in detail by manyteams [253–262]. Several authors, such as Boutevin et al.[263,264] and Hernandez et al. [265], studied various non-linear optical polymers with cinnamate as the crosslinker.

In summary, cinnamic and coumarin moieties arepromising biosourced functional groups enabling thecrosslinking of polymers under UV irradiation. Thereversibility of the reaction makes them useful in techni-cal polymers, paving the way for high technologies such asliquid crystalline polymers and optical applications such asnonlinear optics.

4.3. Natural acids

Owing to their high biocompatibility and their pH-dependent behavior, natural acids have great potentialfor biomedical applications. These molecules are naturallyavailable in the biomass (e.g. tartaric acid and caffeic acid)or obtained by chemical or enzymatic processes (e.g. cit-ric acid, succinic acid, itaconic acid, and lactic acid). Theliterature describes natural acids as pH-dependent agents,reagents for the synthesis of polymers (e.g. polyester andpolyamide) [266], photopolymerizable groups for photo-chemical processes and precursors of diols after reduction.

Some biosourced acids showing a native UV reactivityhave been used as functional groups and were grafted ontoprepolymers using their carboxylic acid group. As shown
in Fig. 32, they exhibit similar chemical structures, witha double bond close to that of carboxylic acid, which is ahighly reactive functional group under UV exposure.
Fig. 32. Chemical structures of natural acids. From the left to the right:acrylic acid, itaconic acid, cinnamic acid and caffeic acid.

9 olymer

offhbupamgspTmsddam

ptpwtapaspat

micpwu

F


Natural acids have been widely used for the productionf coatings and for biomedical applications. For instance,umaric, maleic and itaconic acids have been employedor the production of contact lenses [267] because of theirydrophilic character. From the large range of UV-sensitiveio-based acids, acrylic acid is one of the most frequentlysed molecules. Over the last three decades, thousands ofublications and patents have reported the use of acryliccid in UV-radiation processes. In many instances, thisolecule has been used as a UV-sensitive functional group

rafted onto prepolymers. Decker [268–271] described ineveral papers the potential of acrylic-based monomers asrecursors involved in the photopolymerization process.oday, acrylic acid can be obtained from biomass by severalethods. The Arkema company uses the catalytic conver-

ion of glycerol to acrolein and acrylic acid or catalyticehydration of lactic acid [272,273]. Cargill and Novozymeseveloped an enzymatic route that could produce acryliccid from 3-hydroxypropionic acid using sugars as rawaterial.Johnson and Landin [274] claimed that unsaturated

olyesters based on acrylic acid could be useful as protec-ive coating materials using UV polymerization. Recently,ressure-sensitive adhesives based on acrylic derivativesere prepared, representing a promising route for elec-

ronic coatings in applications such as touch screens. Kajtnand Krajnc [275] illustrated this application using an acrylicrepolymer resulting from a 2-ethylhexyl acrylate, acryliccid and t-butyl acrylate mixture. Bag and Rao [276]tudied the potential of difunctional silane monomers asrecursors of hybrid materials (based on methacryloxy-cryloxy-silane) and for coating applications because of thehermal stability of these UV-cured objects.

Derivatives of acrylic acid, such as cinnamic-basedolecules (also called trans-3-phenylacrylic), are mainly

nvolved in reversible dimerization processes to form a
yclobutane ring under UV light. Nagata and Inaki [249]hotocrosslinked a poly(lactide) skeleton functionalizedith cinnamic groups. Similar studies were conductedsing caffeic acid (also called trans-3,4-dihydroxycinnamic
ig. 33. Examples of resin acids: (from the left to the right) abietic acid, neoabiet

Fig. 34. Formation of hydrogel usin

Science 38 (2013) 932– 962

acid or trans-3,4-dihydroxy-3-phenylacrylic acid). Thisanalog of cinnamic acid, mainly found in lignin biomass, iscomposed of a benzenic ring disubstituted with hydroxylgroups that can easily be modified [277]. The use of cin-namic acid and its derivatives is described in Section 4.2 ofthis review.

Rosin is obtained from pines and other plants (suchas conifers) by heating fresh resins to eliminate volatileterpene components. All rosins are made up of 90% diter-penic monocarboxylic acids, also called resin acids (Fig. 33).Abietic acid is the major constituent of resin acids, andit represents the major portion of rosin. Resin acids havebeen extensively used in paper sizing, printing inks andadhesives. Kwak et al. [278] applied methacrylate-basedabietic acid to photolithographic applications. In a simi-lar way, Kim et al. [279] showed that abietic acid-basedstyrene monomers were efficient precursors in the pho-tocrosslinking of polymers with methyl methacrylate.

Tannin compounds are widely distributed in plantsand are made of polyphenolic molecules. Digallic acid(a polyphenolic acid fragment of tannic acid) has beenemployed as a precursor of photoactive films for coat-ing materials [280]. Digallic acid was also combinedwith isocyanate and methacrylate molecules to form aUV-sensitive urethane, and the reactivity of the formu-lation was compared under thermal and photochemicalcuring.

Polyacids are useful for biomedical applications such asdrug delivery agents thanks to their pH-responsive behav-ior. Itaconic acid, obtained by distillation of citric acid,is composed of one photosensitive double bond and twocarboxylic acids (pKa 3.8, 5.5) [281]. Chen et al. [282]copolymerized itaconic acid with N-vinyl-2-pyrrolidonemonomer to prepare pH-sensitive hydrogels using UV-induced methods. Other research was performed by Tomicet al. [283] and Betancourt et al. [284]. They demonstrated
the ability of itaconic acid to photocopolymerize withmethacrylate monomers and to generate pH-responsivehydrogels. Malic acid, another well-known biosourcedmonomer, was studied for its pH-responsive property and
ic acid, dehydroabietic acid, pimaric acid; digallic acid (from tannin).

g methacrylated malic acid.


Fig. 35. Synthesis of furfural and its conversion into furfuryl alcohol and furfuryl methacrylate.

ing fura

polysaccharides and oils. Moreover, some studies reportedthe toxicity of furans and their derivatives as food contam-inants [293,294].

Fig. 36. Photochemistry of copolyester bear

its biocompatibility (Fig. 34). This hydroxyl-diacid (pKa 3.4,5.2) is produced from the metabolism of carbohydrates.He et al. [285] developed the synthesis of biodegradablehydrogels using a poly(�,�-malic acid) prepolymer func-tionalized by methacryloyl-based functions, with the aimof finding biocompatible materials.

In summary, the use of natural unsaturated acidsallows the functionalization of molecules or polymersand provides photosensitive properties. Biosourced acidsrepresent a promising route for preparing materials forindustrial coating and medical applications. The carboxylicacid function gives a pH-dependant character, and thisproperty is being exploited mainly in the biomedical fieldfor drug delivery systems.

4.4. Furans

Most furans and their related derivatives come from theacid-catalyzed hydrolytic depolymerization of hemicellu-loses [61,286]. Two different aldopentoses were described:xylose and rhamnose. Their dehydration leads to furfuraland methylfurfural compounds after purification by distil-lation, as illustrated in Fig. 35. Most furfural is convertedinto furfuryl alcohol, but other molecules acting as build-ing blocks for polymers can be obtained from it, such asdialdehydes, diacids, diisocyanates, and diamines.

Furan is most often involved in Diels–Alder reactionswith maleimide derivatives. This typical reaction wasextensively described by the Gandini group [287]. It isthermally reversible and was the basis of novel smartmaterials. This group also described the photochemistryof conjugated furanic structures [288,289]. To do so, theauthors grafted this aromatic chromophore onto poly-mers before photoinitiator-free crosslinking, involving a(2�+2�) cycloaddition, under UV irradiation. This reactivechromophore was also engaged in linear polyesters usingchain end functionalizations, as shown in Fig. 36.

Fang et al. [290] investigated the grafting of conjugatedfuran chromophores on polyvinyl alcohol (PVA) (Fig. 37).
According to the range of substitutions, the glass transi-tion temperatures of the modified PVAs were lower thanthat of the starting PVA due to the decrease of intermolec-ular hydrogen bonding. By dimerization, these modified
n chromophore under near-UV irradiation.

materials were converted into insoluble thin films by irra-diation. Potential applications of this strategy have beendevoted to photoresist technology, particularly in offsetprinting plates.

Another furanic derivative, furan-2-carboxylic acid, wasemployed in the elaboration of nanoparticles by pho-tocrosslinking in the presence of dextran after a dialysisprocess [291].

Lange et al. [292] exploited the high reactivity of acrylateor methacrylate functional groups in the area of furfuryliccompounds, modeling the kinetic parameters at differenttemperatures. Using this method, they showed that thedominant step of the successive reactions (primary initi-ation, propagation, cross-termination, etc.) changed withrespect to temperature. Moreover, the crosslink density ofacrylated furanic polymers was predicted by this method.

In summary, furans and theirs derivatives are usuallyused as building blocks in the preparation of initiator-freephotocrosslinkable prepolymers. We have mentioned herethat these compounds come from renewable resources butthat they require numerous chemical transformation steps,in contrast to some other natural raw materials such as

Fig. 37. Photocyclodimerization of PVA grafted.

9 olymer

5

mfttppmmbcbpabhun

l–wtfUtwow

oaaaibspcituapadgftbfitaharfra


. Conclusion

The aim of this review was to describe the macro-olecules and the variety of reactive species derived

rom biomass that can lead to materials through a pho-opolymerization process. Biodiversity allows chemistso screen a large number of backbone structures androperties. For instance, oils are hydrophobic whileolysaccharides are hydrophilic. These biosourced macro-olecules can be easily modified to obtain UV-reactiveacromolecules. Otherwise, bio-based compounds can

e introduced into polymeric materials to obtain spe-ific properties or, when they are photosensitive, cane used as reactive functional groups to induce the UVolymerization process. Consequently, bio-based materi-ls have a large variety of applications such as coatings,iomaterials and electronic devices, which generally areigh-added-value products. Some natural elements can besed for the construction of linear or ramified polymers andetworks.

The use of UV irradiation – a clean process without pol-utant emission such as volatile organic compounds (VOC)

allows the conversion of the liquid formulation to net-orked material in a few seconds. It is more adapted to

hin-layer applications such as coatings (with a thicknessrom 1 to 100 �m), as it has penetration limitations. TheV process exhibits many advantages in comparison to

hermal curing. It can be conducted at room temperatureithout any degradation of sensitive molecules and with-

ut any modification of thermosensitive substrates (e.g.ood and plastics).

It is classified as a “green process”, and the emergencef new applications for UV-responsive biosourced materi-ls will take place in the next decade. It is true that we aret the beginning of a new age for the chemical industry,nd it is obvious that the industry is now slowly convert-ng its processes by replacing fossil-based chemicals withiosourced ones. For instance, new bio-based surfactants,olvents or monomers are used in the chemical industry,rogressively changing old habits. However, a number ofhallenges still remain in terms of the purity and chem-cal reactivity of some compounds. Moreover, one mustake into account economic aspects to make these nat-ral products attractive for users. Finally, the extractionnd purification steps of materials must involve ecologicalrocesses with low energy consumption and no pollut-nt emissions to make them acceptable for a sustainableevelopment strategy. Recent regulations have favored therowth of radiation-curing technologies, as they are VOC-ree and lead to materials with good properties. Obviously,hese technologies have some limits. First, the oxygen inhi-ition of radical polymerization has given birth to a greateld of research. Moreover, the question of the photoini-iators present in cured materials and their migration is

key area of further research. The development of non-azardous photoinitiators usable in medicinal applicationsnd in food contact coatings is a new opportunity for
esearchers. For industrial applications, the new trends inuture papers will concern the development of ultra-fasteactive monomers with low toxicity as opposed to irritantnd allergenic acrylate-derived compounds. Finally, more
Science 38 (2013) 932– 962

eco-friendly formulations will result in the emergence ofUV water-borne systems.

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