Synthesis of maleimide-terminatedn-butyl acrylate oligomers by atom transfer radical polymerization: Study of their copolymerization with vinyl ethers

NOTE

Synthesis of Maleimide-Terminated n-Butyl Acrylate Oligomers by AtomTransfer Radical Polymerization: Study of Their Copolymerizationwith Vinyl Ethers

CYRILLE BOYER, BELKACEM OTAZAGHINE, BERNARD BOUTEVIN,CHRISTINE JOLY-DUHAMEL, JEAN-JACQUES ROBIN

Laboratoire de Chimie Macromoleculaire, UMR-CNRS 5076, Ecole Nationale Superieure de Chimiede Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France

Received 4 February 2005; accepted 14 April 2005DOI: 10.1002/pola.20878Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: maleimide; photochemistry; atom transfer radical polymerization

INTRODUCTION

For a few years, the photopolymerization reactionspresent a growing interest, since this technique allowsthe cross-linking of resins from a liquid state to a solidmaterial in a short time under UVradiation.1,2 Economicand ecological advantages (reactions without solvent)explain largely the interest of this method for variousindustrial applications like electronics,3–5 surface treat-ment6 (paints and varnishes) ensuring an effective pro-tection of various materials such as wood, metal, andplastics, and adhesives7 allowing to carry out assembliesquasi instantaneously. The well-known photopolymeri-zation of donor/acceptor systems requires the use of amonomer with a donor character and a monomer withan acceptor one, with or without the presence of a photo-initiator.8 Various systems have been studied, such as n-vinyl pyrolidone/maleate, vinyl ethers/maleate, andvinyl ethers/maleimide.9 The use of maleimides asacceptor monomers allows an initiation step in absenceof an added photoinitiator.10,11 The presence of a photo-initiator in the mixture is prejudicial for weathering of

coatings because its consumption during the photopoly-merization is always incomplete (accelerated aging).Furthermore, the residues remaining after the photoini-tiator decomposition are generally toxic and can give col-ored materials. Because of these various disadvantagesin the use of a photoinitiator, we developed the synthesisof maleimide-terminated oligomers to study their photo-polymerization without initiator, in presence of vinylethers.

For these last years, different techniques of con-trolled radical polymerization (CRP) have been devel-oped, as the atom transfer radical polymerization(ATRP),12 nitroxide mediated polymerization (NMP),13,14

reversible addition fragmentation transfer (RAFT),15 andthe iodine transfer polymerization (ITP).16 ATRP hasemerged as a very versatile, convenient, and powerfulstrategy for polymer synthesis. A variety of monomershave been successfully polymerized using ATRP. Thistechnique uses an alkyl halide R-X as a mediator inconjunction with a transition metal complex Mn-Xmll.The polymers obtained according to this method giverelatively narrow polydispersities with controlledmolecular weights. There are several variants in liv-ing (or controlled) radical polymerization, but theyhave a general common mechanism resulting in thealternative activation–deactivation process depictedin Scheme 1.

Correspondence to: Jean-Jacques Robin (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 4303–4322 (2005)VVC 2005 Wiley Periodicals, Inc.

4303

This method presents many advantages like easyexperimental conditions and the possibility to be appliedto a great number of monomers (methacrylates, acry-lates, acrylonitrile, styrenics, etc.) with good results.Moreover, the ATRP is an interesting technique, whichleads to functionalized oligomers17 with various func-tions (aromatic amine,18 alcohol,19 acid or anhydride18).For these reasons, this method was selected for the syn-thesis of maleimide-terminated oligomers.20,21

In our case, we were interested in the acceptor/donortype copolymerization of maleimide oligomers withvinyl ether compounds22–24 and in the comparison ofthe reactivity of aliphatic and aromatic maleimides. Wesynthesized maleimide-terminated oligomers in a two-step process: first, the amine-functionalized oligomerswere synthesized by ATRP using different initiators,and second, the amine functions were reacted withmaleic anhydride to obtain a maleimide-terminatedoligomer. Original initiators have been synthesized,since initiators bearing amine functions cannot beinvolved in ATRP process because of their high reactiv-ity either by nucleophilic substitution of the initiatorbromine atom or by addition to the monomer doublebond (Michael type reaction, Scheme 2). An initiatorcarrying a protected aliphatic amine function wasdeveloped using a t-boc protection easily cleavable inacid medium after polymerization. In a similar way, anew initiator bearing an aromatic nitro group was pre-pared, which was reduced to lead an aromatic amine.

EXPERIMENTAL

Materials

n-Butyl acrylate (99%, Aldrich, USA) and anisole(99%, Aldrich) were dried and distilled prior to use.

CuBr (� 98%, Aldrich) was purified by the methodreported by Keller and Wycoff.25 2-BromoisobutyroylBromide (98%, Aldrich), 4-nitrophenol alcohol (99%,Aldrich), 3-amino-1-propanol (99%, Aldrich), di-tert-butyl dicarbonate (97%þ, Avocado), triethylamine(99.5%, SDS Society), CHCl3 (99%, SDS Society),silica (60 ACC, SDS), triethyleneglycol divinyl ether(DVE-3) (ISP), hexamethyldisilazane (HMDS) (99%,Aldrich), zinc (3 lm, 99%, Aldrich), 1,1,4,7,10,10-hex-amethyltriethylenetetramine (HMTETA) (97%, Aldrich)were used without further purification.

Characterization

1H and 13C NMRs were performed at room temperatureon a Bruker AC200 apparatus using CDCl3 as the sol-vent (chemical shifts refer to tetramethylsilane). SECanalyses were performed with a Spectra–Physics appa-ratus equipped with two PLgel 5 lmMIXED-C columnsfrom Polymer Laboratories and a Spectra–Physics

Scheme 1. Mechanism of atom transfer radical polymerization (ATRP).

Scheme 2. Side reactions occurring with an ali-phatic primary amine during the ATRP process.

4304 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 43 (2005)

SP8430 RI detector. The eluent was THF (T ¼ 30 8C,flow rate 0.8 mL/min). PS standards were purchasedfrom Polymer Laboratories. Gas Chromatography (GC)was performed with a Delsi Instruments 330 apparatusequipped with a Shimadzu C-R6A integrator and a 2-mlong Carbowax 20M (poly(ethylene glycol)) column.Nitrogen was used as the gas vector at a pressure of1 bar. The analysis was performed at 150 8C. GC wasused to determine the monomer conversion throughoutthe polymerization. Anisole was used as the internalstandard.

The FT-IR analyses were performed with a NicoletNexus apparatus, with 2 cm�1 accuracy. Sampleswere analyzed with the reflexion mode.

UV radiation was provided by an Ultracure 100SSPlus/Novacure apparatus equipped with a 250–450 nmfilter.

Synthesis of the Initiator: p-Nitrophenyl2-Bromoisobutyrate

In a double-necked round-bottom flask (500 mL)equipped with a condenser, 10 g (6.5 � 10�2 mol) of p-nitrophenol, 6.50 g (6.5�10�2 mol) of triethylamine,and 200 mL of anhydrous chloroform were introduced.Then, 16.42 g (7.2 � 10�2 mol) of 2-bromoisobutyroylbromide were added dropwise under stirring, for 1 hat 0 8C. A white-yellow precipitate appeared duringthe reaction. After 4 h of reaction under stirring atroom temperature, the reaction medium was filteredto eliminate the insoluble quaternary ammonium salt.The filtrate was washed with an aqueous solution ofhydrochloric acid (0.1 N) and then with water untilpH 7 of the aqueous phase was achieved. The organicphase was dried over anhydrous Na2SO4, concen-trated by evaporation of the solvent to recover thepure initiator in high yield (solid brown, yield ¼ 90%).

1H NMR (200 MHz, CDCl3), d (ppm): 8.25 ppm (d,2H, aromatic H), 7.25 ppm (d, 2H, aromatic H) and1.95 ppm (s, 6H, CH3).

FT-IR: 3100 cm�1 aromatic, 1730 cm�1 C¼¼O (ester).In a second step, the reduction of the nitro function

was carried out using SnCl2 � cH2O. In a one-neckedround-bottom flask (500 mL) equipped with a con-denser, 1.152 g (4 � 10�3 mol) of p-nitrophenyl 2-bro-moisobutyrate, 4.5 g (2 � 10�2 mol) of SnCl2 � 2H2O,and 200 mL of ethyl acetate were introduced. Themixture was heated at 80 8C for 1 h and then cooledat room temperature. A sodium dicarbonate solution(5% wt) was added to obtain a basic medium. Theorganic phase was washed with a NaCl saturated sol-ution and dried over anhydrous Na2SO4. The solventwas evaporated to obtain a brown solid (yield ¼ 70%).

1H NMR (200 MHz, CDCl3), d (ppm): 6.9 ppm (d,2H, aromatic H), 6.67 ppm (d, 2H, aromatic H) and2.04 ppm (s, 6H, CH3).

FT-IR: 3200 and 3400 cm�1 NH2 band, 1730 cm�1

C¼¼O (ester).

Synthesis of the Initiator t-Boc-Aminopropyl2-Bromoisobutyrate

In a two-necked round-bottom flask (250 mL) equippedwith a condenser, 10 g (1.3 � 10�2 mol) of 3-amino-1-propanol and 50 mL of methanol were introduced.Then, 29.10 g (1.4 � 10�1 mol) of di-tert-butyl dicar-bonate were added dropwise at room temperature.After the addition, the reaction was heated at 50 8C for4 h. The solvent was evaporated and the product waspurified by a water/ether extraction. The organic solu-tion was then dried over anhydrous Na2SO4 and con-centrated under vacuum to obtain the t-boc-aminopro-panol (yellow viscous liquid, yield ¼ 95%).

1H NMR (200 MHz, CDCl3), d (ppm): 4.80 ppm (s,1H, ��OH), 3.6 ppm (t, 2H, CH2��OH), 3.2 ppm (t,2H, ��CH2��NHBoc), 1.7 ppm (q, 2H, ��CH2��CH2),1.4 ppm (s, 9H, (CH3)3C��).

FT-IR: 3400 cm�1 (OH band), 2400 cm�1 (ure-thane), 2900 cm�1 (CH band).

In a two-necked round-bottom flask (250 mL)equipped with a condenser, 10 g (5.7 � 10�2 mol) of t-boc-aminopropanol, 6 g (6 � 10�2 mol) of triethylamine,and 100 mL of CHCl3 were introduced. Then, 13.1 g(5.7 � 10�2 mol) of bromoisobutyroyl bromide wereadded dropwise, at room temperature for 1 h. After 4 hof reaction under stirring at room temperature, thereaction medium was filtered to eliminate the insolublequaternary ammonium salt. The filtrate was washedwith an aqueous solution of hydrochloric acid (0.1 N)and then with water until pH 7 of the aqueous phasewas achieved. The organic phase was dried over anhy-drous Na2SO4, concentrated by evaporating the solventto recover the pure initiator in high yield (yellow vis-cous liquid, yield 17.6 g, 95%).

1H NMR (200 MHz, CDCl3), d (ppm): 5.00 ppm (s,1H, ��NH��Boc), 4.00 ppm (t, 2H, t-CH2��O), 3.10 ppm(t, 2H, ��CH2��NH��), 1.8 ppm (s, 6H, ��CH3), 1.7 ppm(q, 2H, ��CH2��CH2��), 1.4 ppm (s, 9H, (CH3)3C��).

13C NMR (250 MHz, CDCl3), d (ppm): 172 ppm(��COO), 158 ppm (��COONH), 30 ppm ((CH3)2C��),28 ppm ((CH3)3C��), 80 ppm ((CH3)3C��).

Synthesis of n-Butyl Acrylate Oligomerswith Aromatic Nitro Function

[Monomer]:[Initiator]:[CuBr]:[Ligand] ¼ 100:4:1:1In a 50 mL Schlenk flask, 12.8 g (1 � 10�1 mol) of n-

butyl acrylate, 0.14 g (1 � 10�3 mol) of CuBr, 0.23 g(1 � 10�3 mol) of HMTETA, 1.16 g (4 � 10�3 mol) of p-nitrophenyl 2-bromoisobutyrate, and 12.8 g of anisolewere introduced. The solution was degassed for 10 minwith argon. The Schlenk was then placed in an oil bathpreviously heated at 90 8C. The conversion rate was fol-lowed by GC, using anisole as an internal reference (eq 1).

a ¼Rmonomer

ðtÞRanisoleðtÞ

�Ranisoleð0ÞR

monomerð0Þð1Þ

NOTE 4305

whereRmonomerðtÞ and

Rmonomerð0Þ are respec-

tively the integrals of the monomer peak at t and t¼ 0 and

RanisoleðtÞ and

Ranisoleð0Þ are the integrals

of the peak of the anisole at t and t ¼ 0, respectively.The polymerization was stopped for about 70% of

conversion. The solution was cooled at ambient tem-perature, dissolved in THF, and filtered on silica toeliminate CuBr and ligand. Solvent and non-reactedmonomer were evaporated under vacuum (0.1 mmHgat 90 8C for 2 h).

1H NMR (200 MHz, CDCl3), d (ppm): 8.25 ppm (d, 2H,aromatic H), 7.25 ppm (d, 2H, aromatic H), 4.00 ppm(t, 2H, CH2O), 1.5 ppm (t, 2H, CH2), 1.3 ppm (t, 2H, CH2),1.15 ppm (s, 6H, CH3), 0.80 ppm (t, 3H, CH3).

Reduction of the Nitro Function of then-Butyl Acrylate Oligomer

In a two-necked round-bottom flask (250 mL) equippedwith a condenser, 5 g (2.5� 10�3 mol) of nitro functional-

Scheme 3. Simplified scheme of the apparatus used for the study of the photopoly-merization kinetics.

Scheme 4. Synthesis of an aromatic maleimide functionalized poly(n-butyl acrylate)oligomer.


ized n-butyl acrylate oligomer (Mn ¼ 2000 g/mol), 1.65 g(2.5� 10�2 mol) of Zn(0), and 100mL of ethanol were intro-

duced. Then, 20 mL of acetic acid were added dropwise

under nitrogen atmosphere at 0 8C. After the addition,

the mixture was stirred at ambient temperature for

24 h. The solution was filtered on celite, and solvent

and acetic acid were eliminated under vacuum. The

obtained mixture was dissolved in diethyl ether and

washed with a NaOH solution (0.1 N). Then, the organic

solution was washed with a NaCl saturated solution

and dried over anhydrous Na2SO4. The solvent was

eliminated under vacuum to obtain the aromatic amine

functionalized n-butyl acrylate oligomer (yield¼ 70%).1H NMR (200 MHz, CDCl3), d (ppm): 6.9 ppm (d, 2H,

aromatic H), 6.67 ppm(d, 2H, aromatic H), 4.00 ppm (t,

2H, CH2O), 1.5 ppm (t, 2H, CH2), 1.3 ppm (t, 2H, CH2),

1.15 ppm (s, 6H, CH3), 0.80 ppm (t, 3H, CH3).FT-IR (cm�1) : 3300 cm�1 et 3400 cm�1 (NH2 band),

3000 cm�1 (C¼¼C, aromatic), 1730 cm�1 (ester band).

Synthesis of the Aliphatic Amine Functionalizedn-Butyl Acrylate Oligomer

[Monomer]:[Initiator]:[CuBr]:[Ligand] ¼ 100:3:1:1In a 50 mL Schlenk flask, 12.8 g (1 � 10�1 mol) of

n-butyl acrylate, 0.14 g (1 � 10�3 mol) of CuBr, 0.23 g(1 � 10�3 mol) of HMTETA, 0.98 mg (3 � 10�3 mol) oft-boc-aminopropyl 2-bromoisobutyrate, and 12.8 g ofanisole were introduced. The solution was degassedfor 10 min with argon. The round-bottom flask wasthen sealed by a septum and placed in an oil bath at90 8C. The conversion rate was determined using GC.The polymerization was stopped for about 70% of con-version. The solution was cooled at ambient tempera-ture, dissolved in THF, and filtered on silica to elimi-nate CuBr and ligand. The solvent and the non-reactedmonomer were evaporated under vacuum (0.1 mmHg at90 8C for 2 h).

1H NMR (200 MHz, CDCl3), d (ppm): 5.00 ppm (s, 1H,

��NH��Boc), 4.00 ppm (t, 2H,��CH2��O), 3.10 ppm (t, 2H,

Scheme 5. Synthesis of an aliphatic maleimide functionalized poly(n-butyl acrylate)oligomer.

NOTE 4307

��CH2��NH��), 1.7 ppm (q, 2H, ��CH2��CH2��), 1.4 ppm

(s, 9H, (CH3)3C��), 4.00 ppm (t, 2H, CH2O), 1.5 ppm (t, 2H,

OCH2��CH2��CH2), 1.3 ppm (t, 2H, CH2��CH3), 1.15 ppm

(s, 6H, C��(CH3)2), 0.80 ppm (t, 3H, CH3).

Deprotection of the Aliphatic Amine Function of then-Butyl Acrylate Oligomer

In a one-necked round-bottom flask (50 mL), 10 g(4 � 10�3 mol) of oligomer functionalized t-boc amine

Table 1. Mn, DPn, Ip, and a Characteristic Values of the Different n-Butyl Acrylate Oligomers Synthesized

Oligomers Mntheo a (g/mol) DPn

NMR MnNMR (g/mol) Mn

SEC (g/mol) Ip a

NO2aromatic-P(BA) 1756 11 1700 1800 1.20 0.65

NH2aromatic-P(BA) 1640 10 1540 1650 1.20

Maleimidearomatic-P(BA) 2300 15.5 2320 2300 1.14t-boc-NHaliphatic-P(BA) 2010 12.2 1900 2050 1.20 0.55NH2

aliphatic-P(BA) 1900 12 1760 1900 1.19Maleimidealiphatic-P(BA) 2000 15 2270 2100 1.12

a Evaluated by the equation Mntheo ¼ ((nMonomer � a)/ninitiator) � MABu þ Minitiator with a ¼ monomer conversion rate of the poly-

merization.

Figure 1. 1H NMR spectrum of the nitro aromatic functionalized poly(n-butylacrylate) oligomer (in CDCl3).


(Mn ¼ 2500 g/mol), 4 g (3.5 � 10�2 mol) of trifluoroaceticacid (TFA), and 25 mL of CHCl3 were introduced. Thesolution was stirred at room temperature for 24 h.

Then, the solvent and TFA were evaporated. The

obtained mixture was solvated in diethyl ether and

washed with a NaOH solution (0.1 N). The organic solu-

tion was then washed with a NaCl saturated solution

and dried over anhydrous Na2SO4. The solvent was

eliminated under vacuum to obtain the aliphatic amine

functionalized n-butyl acrylate oligomer.1H NMR (200 MHz, CDCl3), d (ppm): 5.00 ppm (s, 1H,

��NH2), 4.00 ppm (t, 2H, ��CH2��O), 3.10 ppm (t, 2H,��CH2��NH��), 1.7 ppm (q, 2H, ��CH2��CH2��), 1.4 ppm(s, 9H, (CH3)3C��), 4.00 ppm (t, 2H, ��CH2O), 1.5 ppm(t, 2H, CH2), 1.3 ppm (t, 2H, CH2), 1.15 ppm (s, 6H,C��(CH3)2), 0.80 ppm (t, 3H, CH3).

Imidization of the Amine Terminal Functions of then-Butyl Acrylate Oligomers

In a two-necked round-bottom flask (100 mL)equipped with a condenser and under nitrogen flow,

0.59 g (5.98 � 10�3 mol) of maleic anhydride and20 mL of anhydrous toluene were introduced. Then,10 g of the aliphatic amine functionalized n-butylacrylate oligomer (Mn ¼ 1760 g/mol) were introduceddropwise for 30 min. The solution was stirred for30 min and 0.82 g (6.0 � 10�3 mol) of ZnCl2 wasadded. The mixture was heated at 90 8C for 30 minand a solution of 0.97 g (6 � 10�3 mol.) of HMDS with10 mL of anhydrous toluene were added dropwise tothe solution for 30 min. After 2 h, the solution wascooled at room temperature and filtered on celite. Theproduct was purified by a water/diethyl ether extrac-tion and dried over anhydrous Na2SO4. The solventwas eliminated under vacuum (0.1 mmHg at 70 8C for4 h) to obtain the aliphatic maleimide functionalizedn-butyl acrylate oligomer (yield ¼ 95%).

1H NMR (200 MHz, CDCl3), d (ppm): 6.60 ppm (s,2H,��CH¼¼CH��), 4.00 ppm (t, 2H, ��CH2��O),3.50 ppm (t, 2H, ��CH2��N��), 1.7 ppm (q, 2H,��CH2��CH2��), 1.4 ppm (s, 9H, (CH3)3C��), 4.00 ppm(t, 2H, ��CH2O), 1.5 ppm (t, 2H, CH2), 1.3 ppm (t, 2H,CH2), 1.15 ppm (s, 6H, C��(CH3)2), 0.80 ppm (t, 3H,CH3).

Figure 2. 1H NMR spectrum of the aromatic amine functionalized poly(n-butylacrylate) oligomer (in CDCl3).

NOTE 4309

The same procedure was used to obtain the aro-matic maleimide functionalized n-butyl acrylate oligomer(yield ¼ 94%).

Study of the Kinetics of the Photopolymerization by FT-IR

In a glass tube, 0.50 g (2.2 � 10�4 mol) of the aliphaticamine functionalized oligomer (Mn ¼ 2270 g/mol) and0.20 g of CHCl3 (solvent) were introduced. Then, 20 mg(1.9 � 10�4 mol) of DVE-3 was added using a syringe.The solution was stored under nitrogen atmosphere,safe from the light. For the photopolymerization study,the samples were coated on a polished aluminium plateand protected by a polypropylene (PP) film (thickness16 lm) to prevent air inhibition. The thickness of the

coating was controlled by a polytetrafluroethylene(PTFE) thickness gauges (25 lm) inserted between thePP film and the aluminium plate. The thickness (20–22 lm) was measured after the photopolymerizationreaction, using a Byko-test 7500 apparatus. The UVsource was placed perpendicularly to the sample sur-face (Scheme 3). The UV intensity was measured usinga Solatell apparatus (light intensities ¼ 75 and100 mW/cm2). The conversions of vinyl ether and mal-eimide were determined by IR spectroscopy, using thedisappearance of the bands at 1635 and 1620 cm�1

(elongation of the C¼¼C bond of the DVE-3) and at697 cm�1 (deformation of the CH bond of the malei-mide). The polymerization rate was evaluated usingthe slope of the recorded curves.

Figure 3. 1H NMR spectrum of an aromatic maleimide functionalized poly(n-butylacrylate) oligomer (in CDCl3).


RESULTS AND DISCUSSION

Schemes 4 and 5 report the different routes developed forthe synthesis of maleimide-terminated oligomers. Thesetwo procedures are based on ATRP of n-butyl acrylate,using different initiators.

The first procedure is based on the use of a nitroaromatic functionalized ATRP initiator. In the firststep, nitro functionalized oligomers were obtained byATRP of n-butyl acrylate, using this initiator. The sec-ond step was the reduction of the nitro function toobtain the amine function, using an excess of zinc(Zn(0)) and acetic acid (Scheme 4). The last step of thisprocedure was the imidization in the presence ofmaleic anhydride. The reaction was activated by zincdichloride (ZnCl2) and HMDS.26

The second procedure used an ATRP initiator bear-ing a protected amine function (Scheme 5). Amine-protected functionalized oligomers were obtained byATRP, using this initiator. The second step was thecleavage of the t-boc protection with an excess of TFA.At last, according to the previous procedure, malei-mide functionalized oligomers were obtained by imid-ization of the free amine function.

Synthesis of Aromatic Maleimide FunctionalizedOligomers

Synthesis of the Initiator: p-Nitrophenyl2-Bromoisobutyrate

For this study, the initiator p-nitrophenyl 2-bromoiso-butyrate was synthesized as reported in the litera-ture,16 by reaction of 2-bromoisobutyroyle brominewith p-nitrophenol in the presence of triethylamine indichloromethane and at room temperature. The 1HNMR analysis of the nitro initiator shows the methylproton signal around 1.95 ppm and the aromatic pro-ton signal around 7.25 and 8.25 ppm.

Two routes were possible for the reduction of thenitro function: either before the ATRP reaction (thismethod was used by Haddelton and Waterson16 toobtain aromatic amine-terminated MMA oligomers) orreduction after the polymerization step. These twomethods have been tested. At first, SnCl2 in ethyl ace-tate was used as reducing agent, but the purificationof the crude was not easy. To simplify this reductionstep, the system zinc(0)/acetic acid was selected. Thissystem is not convenient for a reduction beforethe ATRP step. Indeed, this reducing system leads to

Figure 4. 1H NMR spectrum of the t-boc-protected ATRP initiator NPBiB (in CDCl3).

NOTE 4311

a conversion of the C-Br oligomer chain end into aC-H one, and the main advantage of this methodresults in a better efficiency and in an easy purifica-tion procedure. This method was applied after theATRP reaction when the bromine extremity is no longeruseful. Moreover, PMMA oligomers terminated by ahalogen atom are thermally unstable and tend to easilydegrade to give unsaturated compounds. This phenom-enon has been enlightened by different authors.27,28

Synthesis of Nitro Aromatic-TerminatedPoly(n-Butyl Acrylate) Oligomers

The synthesis of nitro functionalized oligomers byATRP was achieved using CuBr/HMTETA as catalyst,in anisole at 90 8C in the following ratio: [Monomer]:[CuBr]:[HMTETA]:[Initiator] ¼ 100:1:1:4. The mono-mer conversion was determined using GC, with ani-

sole as an internal standard. The polymerizationswere stopped for conversion rates around 70%. Thecrudes were diluted in diethyl ether and filtered onsilica to remove the catalyst. The non-reacted mono-mer and the solvent were evaporated under vacuum.The molecular weights were determined by eqs 2 and 3.

DPtheo

n ¼ ½M� � a½initiator�0

ð2Þ

where [M] is the initial monomer concentration, [ini-

tiator]0 is the initial initiator concentration, and a is

the monomer conversion rate

Mn ¼ Mi þMmonomer �DPn ð3Þwhere Mn is the number average molecular weight and

Mi is the initiator molar mass. The experimental and

theoretical values are in good agreement (Table 1).

Figure 5. 1H NMR spectrum of the protected aliphatic amine terminated poly(n-butyl acrylate) oligomer (in CDCl3).


The obtained products were analyzed by SEC and1H NMR (Fig. 1). The aromatic protons signalsaround 7.25 and 8.25 ppm confirm the presence ofthe nitro initiator. The CH in a position of Br (CH,e) appears around 4.2 ppm, but this signal is parti-ally masked by the CH2 signal in a position of themonomer ester function (d, around 4.0 ppm). TheCH3 (h) of the initiator appears in the n-butyl acryl-ate polymer signals. The signals of the butyl esterprotons in the chain appear around 0.8 ppm (CH3, a),1.4 ppm (CH2, b), 1.6 ppm (CH2, c), and 4.0 ppm(CH2, d). The average polymerization degree ðDPnÞwas determined using NMR, by comparing the aro-matic protons signal (i) þ (j) of the nitrophenyl chainend with the ��CO2CH2�� protons signal (e) þ (d) ofthe monomer units (eq 4).

DPn ¼RðdþeÞ�ðiþjÞ

4

2R ðiþjÞ4

ð4Þ

The molecular weights determined by SEC and

NMR analyses (Table 1) are in good agreement with

the theoretical values.

Then, the nitro function at the chain end wasreduced to obtain the amine function in the presenceof zinc and acetic acid in ethanol. This reductionoccurred at room temperature for 24 h, according toScheme 4. The obtained reaction mixture was filteredon celite and the non-reacted acetic acid and the etha-nol were removed by distillation under vacuum.Finally the product was obtained by diethyl ether/water extraction. The obtained products present aslight brown coloration.

The displacement of the aromatic signals in 1H NMR

confirmed the reduction reaction (Fig. 2). These signals

are shifted from 7.25 and 8.25 ppm before reduction to

6.6 and 6.8 ppm after reaction. When CDCl3 is used as

solvent, the amine function is observed around 3.5 ppm.

The other signals remain unchanged.The poly(ABu) chain analysis shows the disappear-

ance of the ��CHBrCO2Bu (e) characteristic signal andconfirms the bromine elimination during the reduc-tion. The DPn has been evaluated according to eq 5.The nitro function chain end is converted to an aminefunction in the presence of acetic acid and zinc, the

Figure 6. 13C NMR spectrum of functionalized aliphatic amine oligomers beforeand after cleavage of the t-boc protective group (in CDCl3).

NOTE 4313

GPC traces before and after confirmed that the poly-mer molecular weight and its distribution remainsunchanged.

DPn ¼R

d

2Riþj

4

ð5Þ

Synthesis of Aromatic Maleimide FunctionalizedPoly(n-Butyl Acrylate) Oligomers

The last step of this procedure was the obtention ofthe maleimide from the amine-functionalized oligo-mers. The reaction was achieved in the presence ofmaleic anhydride, ZnCl2, and HMDS in toluene at100 8C. This catalyst was essential for the cyclizationof the maleimide function since, when no catalyst wasused, the reaction stopped at the stage of the forma-tion of the amic acid (the latter being observed in 1HNMR as two doublet around 6.4 ppm). After 4 h, thereaction mixture was filtered on celite and the toluenewas removed under vacuum. Then, a diethyl ether/

water extraction made possible the recovering of theexpected compound that was analyzed by 1H NMRand SEC.

The 1H NMR spectrum (Fig. 3) shows the disap-

pearance of the NH2 protons signal (around 3.5 ppm)and the change of the aromatic protons signals at the

chain end (around 7.2 and 7.6 ppm). This spectrumalso shows a new signal around 6.8 ppm, characteristic

of the CH¼¼CH�� protons of the maleimide function.This analysis proved that the reaction was quantita-

tive and the obtained product was slightly brown col-ored. The DPn has been evaluated using the signal ofthe maleimide protons k (6.8 ppm).

DPn ¼R

d

2Rk

2

ð6Þ

The obtained values were slightly higher thanthose of the corresponding amine compounds due tothe elimination of low molecular weight oligomersduring the purification (Table 1).

Figure 7. 1H NMR spectrum of the aliphatic amine terminated poly(n-butyl acrylate)oligomer (in CDCl3).


Synthesis of Aliphatic Maleimide FunctionalizedPoly(n-Butyl Acrylate) Oligomers

Synthesis of Aliphatic Amine FunctionalizedPoly(n-Butyl Acrylate) Oligomers

This synthesis is based on a first ATRP step of n-butylacrylate, using an original initiator bearing a t-bocaliphatic amine-protected group. This initiator syn-thesis was obtained by the reaction of 3-aminopropa-nol with 2-bromoisobutyroyl bromide (Scheme 5).

The esterification step of the 3-aminopropanol withan acyl bromide compound requires to protect theamine function. Furthermore, this protection avoidsside reactions during the polymerization, such as bro-mine substitution reactions by amines or Michaeltype reactions (Scheme 2). The first step of the syn-thesis was the amine protection of 3-aminopropanol.The protection was achieved in methanol at 50 8C byreaction of the primary amine and the di-tert-butyl

dicarbonate. Then the obtained compound was esteri-fied by reaction with 2-bromoisobutyroyl bromine inthe presence of triethylamine in dichloromethane atroom temperature. The obtained product (t-Boc-NPBiB) was analyzed by 1H NMR (Fig. 4) CDCl3: t-butyl protons signals (m) at 1.4 ppm, CH3- protons (h)near the bromine atom at 1.8 ppm, CH2 protons (i)4.0 ppm, CH2 protons (k), and finally, NH proton (l)at 5.0 ppm.

t-Boc-NPBiB was used as the initiator of the ATRPof n-butyl acrylate. The polymerization was performedin the same experimental conditions, as describedpreviously: [Monomer]:[CuBr]:[HMTETA]:[Initiator]¼ 100:1:1:3, in anisole at 90 8C. After polymerization(conversion rate around 70%), the crude was filteredon silica and the non-reacted monomer and the sol-vent were eliminated by evaporation. The obtainedcompound was analyzed by NMR and SEC techni-ques. The SEC analysis shows a relatively narrow dis-

Figure 8. 1H NMR spectrum of aliphatic maleimide functionalized n-butyl acrylateoligomers (in CDCl3).

NOTE 4315

Figure 9. Evolution of the disappearance of the CH band of the maleimide groupduring the photopolymerization (time acquisition: 5 s).

Scheme 6. Reactional scheme of the copolymerization of vinylic ethers with malei-mide terminated oligomers.

tribution and a good agreement between experimentaland theoretical values of molecular weights (Ip ¼ 1.20,Mn

theoretical ¼ 2010 g/mol, Mn ¼ 2050 g/mol). The ini-tiation of the polymer chains by the t-Boc-NPBiBcompound was proved by 1H (Fig. 5) and 13C NMR(Fig. 6). The t-boc CH3 proton signal (m) appearsaround 1.4 ppm (with the CH2 protons signal (b) of

the lateral chain), the CH2 protons signal (k) near theurethane function around 3.2 ppm, the CH2 protonsof the initiator in ester a position (i) around 4.15 ppm(under the signal of the O-CH2 protons (d) of themonomer), and the NH proton signal (l) around5.00 ppm. The 13C NMR confirmed the presence oftert-butyl group (signal at 28 ppm) and urethane car-bonyl (signal at 162 ppm).

The DPn was evaluated by NMR (eq 7), taking intoaccount protons (i) and CHBr (e), which are located inthe same zone as those of the monomer units in thepolymer chain (d).

DPn ¼RðdþeþiÞ�

3

Rk

2

2Rk

2

¼R ðdþ eþ iÞ � 3

Rk

2Rk

ð7Þ

where,Reþ i ¼ 3

Rk

2 .Then, the cleavage of the t-boc protection was

achieved in acidic medium. Two different procedureswere tested. The first one used the p-toluene sulfonicacid (APTS) and the second one used the TFA. WhenAPTS was used, the reaction was performed at 100 8Cfor 24 h. A light brown coloration was observed after

Figure 10. FT-IR spectrum before and after photopolymerization of a maleimide/DVE-3 mixture.

Figure 11. Comparison of the conversion yield ofmaleimide and vinyl ether functions for an initialequimolar mixture (intensity ¼ 75 mW/cm2) [^: malei-mide (MI), n: vinyl ether (VE)].

NOTE 4317

elimination of the solvent that was ascribed to a par-tial degradation of the polymer. We used TFA indichloromethane for 24 h, at room temperature, whichgave no colored product. After reaction, the excess ofTFA and the solvent were removed under vacuum.The expected product was then purified by extractionof the organic layer dissolved in diethyl ether, bybasic water. To confirm the cleavage of the t-boc groupreaction, 13C NMR analysis was performed before andafter reaction (Fig. 6). The t-butyl group signal of thet-boc (around 28 ppm) and the urethane carbonyl sig-nal had totally disappeared after reaction with TFA.

The 1H NMR (Fig. 7) also shows the disappearanceof the t-butyl protons signal (m) (at 1.37 ppm). TheCH2 protons signal in a position of the urethane func-tion (k) at 3.2 ppm before reaction now appears at2.8 ppm, after cleavage of the t-boc protection, that is,characteristic of the CH2 protons in a position of anamine function.

These results were confirmed by titration of theamine functions of the polymer dissolved in a THF/water solution (50/50 %vol), using a HCl solution(M ¼ 0.01 mol/L). The functionality was calculatedusing both molecular weights obtained by SEC andresults of the titration of the amine functions (eq 8).

A functionality around 0.95 was obtained for the twosamples.

f ¼ namine

noligomer¼ VðeqÞ � C0

moligomer

Mn

ð8Þ

where namine and noligomer are respectively the molenumber of amine functions determined by titrationand the mole number of oligomers determined bySEC, V(eq) is the volume at the equivalent point, C0

is the concentration of the HCl solution, moligomer isthe mass of oligomer used for the titration, and Mn isthe number average molecular weight obtained bySEC.

The FT-IR spectroscopy analysis confirmed thepresence of the primary aliphatic amine (3300 and3400 cm�1). The SEC analysis did not show any sig-nificant modifications of the molecular weight distri-bution, which proved that no alteration of the poly-mer occurred during these different steps. The laststep was the imidization of the aliphatic amine func-tions according to the same experimental conditionsthat was applied previously in the case of aromaticamine one.

The 1H NMR shows the presence of maleimidefunctions (Fig. 8). The CH2 protons signal in a posi-

Scheme 7. Mechanism of the photoinitiation induced by UV light without usingfree initiator.

Figure 13. Comparison of vinyl ether conversionyield for different maleimide compounds (radiationintensity equal to 75 mW/cm2) (~: aromatic malei-mide, n: aliphatic maleimide).

Figure 12. Maleimide conversion yield determinedaccording to the disappearance of the CH¼¼CH band(698 cm�1) (n: intensity ¼ 75 mW/cm2, l: intensity200 mW/cm2, ^: intensity ¼ 1000 mW/cm2, malei-mide/divinyl ether equimolar mixtures).


tion of the amine function shifted from 2.8 to3.5 ppm, which corresponds to a conversion of theamine function into a maleimide one. The CH protonsignal of the double bond of the maleimide functionappears around 7.5 and 6.7 ppm.

Moreover the 13C NMR shows the CH¼¼CH bond at134 ppm. The amine signal disappearance on the FT-IR spectrum confirms the imidization reaction andthe SEC analysis did not show any significant molec-ular weight changes.

Study by FT-IR Spectroscopy of the Maleimide-Terminated Oligomers Copolymerization

The maleimide functions can copolymerize with vinylethers without any added photoinitiator. A lot of stud-ies concerning the maleimide photopolymerizationhave been described in the literature.4,7,9,10,29 Gener-ally in these studies, the maleimide compounds usedare small molecules, whereas in this work, we studiedthe reactivity of maleimide-terminated oligomers.

The reactivity of these oligomers was studied by aUV photoinitiated copolymerization of a mixture con-taining maleimide functionalized oligomers, a divinylether (DVE-3), a solvent, and in some cases, a photoini-tiator. After reaction, the obtained compounds weretotally cross-linked polymers (Scheme 6). The disap-pearance of the DVE-3 vinyl functions (C¼¼C bond at1635 and 1620 cm�1) and maleimide functions (CH ofCH¼¼CH�� bond around 698 cm�1) was observed by theFT-IR spectroscopy and made possible to evaluatethe kinetics of the reaction (Figs. 9 and 10).

In a first part, we studied the reactivity of the mal-eimide-terminated oligomers (aromatic and aliphaticmaleimide) with DVE-3, with or without any addedphotoinitiator. Cross-linked copolymers obtained start-ing from maleimides and vinyl ethers are well-known toexhibit a good thermostability and make them suit-able for composite materials resistant to high thermalstresses.30

Many studies showed that the maleimide groupscan initiate a photopolymerization reaction.31–34 Twomechanisms of photoinitiation were proposed for theacceptor/donor systems like maleimide/vinyl ether.For the first one, the excitation of the maleimide func-tions by UV radiation allows to extract protons byintra- or intermolecular mechanism which gives thephotoinitiation step.35 The second one is based on thereaction of maleimide/vinyl ether complexes in a fun-damental or excited state.

In a first part, an equimolar mixture of maleimidesand vinyl ether functions was studied. For the ali-phatic maleimide, after 30 s of UV irradiation (I ¼ 75mW/cm2), the conversion was higher than 62% formaleimide functions and equal to 56% for vinyl etherfunctions. At the end of the reaction 30% vinyl etherfunctions remained free (Fig. 11). A higher conversionrate of the maleimide functions in comparison withthat of the vinyl ether one seems to enlighten sidereactions like maleimide homopolymerization30 orcycloaddition.31

Moreover, this study confirmed that aliphatic mal-eimide-terminated oligomers allow the photoinitiationof the reaction. According to Morel et al.,29,35 the mal-eimide compounds can generate radicals by extractionof labile hydrogen atoms when they are in an excitedstate under the action of UV radiation10,29,32,36

(Scheme 7).The maleimide structure determines the initiation

efficiency. The hydrogen donor character of the malei-mide seems to be one of the main factor that influen-ces the rate of copolymerization.29 In our case, the ali-phatic maleimide-terminated oligomers present differentlabile hydrogen atoms near the maleimide function(N��CH2��CH2��CH2��O��).

Furthermore, the UV radiation intensity influencesthe propagation rate of the photopolymerization, andthe higher the radiation intensity is, the faster thepolymerization reaction (Fig. 12). The aromatic malei-mide reactivity is very different in comparison withthe aliphatic maleimide (Fig. 13).

Furthermore, after 40 min of UV irradiation, 50%of the vinyl ether and maleimide functions did notreact. This low reactivity for the aromatic maleimidecan be explained by the absence of labile protons near

Table 2. Evolution of the Propagation Rate as aFunction of the Darocur Concentration

Vp/M0 (s�1) Darocur (% wt)

4.10�4 01.10�2 0.51.15.10�2 1

Experimental conditions: [MI]/[VE] ¼ 1; Intensity ¼ 75mW/cm2.

Figure 14. Comparison of the conversion yield ver-sus time reaction for aromatic maleimide/DVE-3 mix-tures with or without photoinitiator (^: with 0.5%Darocur, ~: without Darocur).

NOTE 4319

the maleimide function. Indeed, Morel et al.29 showedthat the hydrogen donor character of the maleimidedetermines the polymerization reactivity.29

To increase the conversion rate of the aromaticmaleimide, the light intensity was increased and aphotoinitiator was added. A strong variation of thepolymerization rate was observed when the UV radia-tion intensity raised from 75 to 1000 mW/cm2 (then,the aromatic maleimide conversion rate was equalto 65% after 30 s). The addition of Darocur 1173(Hydroxyalkyl acetophenolamine) (0.5% wt) raisedby a factor of 25 the polymerization rate (Fig. 14,Table 2).

The polymerization mechanism of maleimides withvinyl ethers has been largely studied by numerousauthors,36–42 and the mechanism has been describedas an acceptor/donor type copolymerization, depicted

as a homopolymerization of an acceptor/donor com-plex or copolymerization by crossed propagation(Scheme 8).

We tried to determine the mechanism in the caseof our maleimide-terminated oligomers. A constantconcentration of aromatic maleimide functions waschosen when varying concentrations of vinyl etherfunctions (Table 3). The propagation rate of the aro-matic maleimide was determined for each concentra-tion of the vinyl ether functions. This rate dependson the propagation mode: a linear evolution as afunction of the concentration of vinyl ether functionscorresponds to a homopolymerization of a maleimide/vinyl ether complex. On the other hand, a linearevolution with square root of the concentration ofvinyl ether functions corresponds to a crossed poly-merization.

Scheme 8. Possible mechanisms for an acceptor-donor type copolymerization.

Table 3. Evolution of the Propagation Rate

[MI]/[VE][VE]0mol/L

[MI].[VE](mol2/L2)

MI conversion(after 30 s)

Vp/M0

(s�1)

1/1 0.208 0.040 35% 0.0101/2 0.416 0.083 55% 0.0181/3 0.624 0.129 84% 0.0281/4 0.832 0.173 90% 0.0361/5 0.104 0.210 94% 0.046

Experimental conditions: constant concentration of aromatic maleimide function ([MI]) andvariable concentration of vinyl ether functions ([VE]); Darocur (0.5% Wt); Intensity ¼ 75 mW/cm2; [MI]0 ¼ 0.208 mol/L.


Crossed propagation:

D� þ A �!k12 �DA�

A� þD �!k21 �AD�

D� þ �A� �!kt �AD�

ð9Þ

where D and A stand respectively for donor andacceptor type monomers.

Vp can be expressed as a function of the concentra-tions of maleimide ([MI]) and of vinyl ether functions[VE] (eq 10):

Vp ¼ k21k12kt

� �1=2

V1=2a ½MI�1=2½EV�1=2 ð10Þ

Homopolymerization of an acceptor/donor complex:

AD� þ ½AD� �!kp ADAD�

2� AD� �!kt ADDA

Vp can be expressed as a function of the concentra-tions of maleimide ([MI]) and of vinyl ether functions[VE] (eq 11):

Vp ¼ K ½MI�½EV� ð11Þ

where K is a constant.To determine the photopolymerization mechanism,

the kinetics of initiation has to be constant during allexperiments. This condition is observed since the con-centration in aromatic maleimide in the different runs

is constant. Figure 15 shows a linear evolution of thepolymerization rate with the concentration of vinylether functions. This proves a mechanism based onthe homopolymerization of an acceptor/donor complex.

This result is in agreement with those obtained byBianchi42 for the photopolymerization of the t-butylmaleimide (t-BuMI) with the hydroxybutylvinyl ether(HBVE) in the presence of Lucirin TPO (2,4,6 trime-thylbenzoyldiphenylphosphine oxide). Our values ofthe conversion rate after a period of 30 s are close tothose observed for other acceptor/donor systems suchas the t-butyl maleimide/hydroxybutyronitrile systemstudied by Bianchi.42

CONCLUSION

The synthesis of maleimide (aliphatic or aromatic)terminated poly(n-butyl acrylate) oligomers based onthe ATRP method has been developed. The oligomerswith an aromatic maleimide-terminated function havebeen obtained, starting from an initiator bearing anaromatic nitro function. The nitro function was thenreduced and the imidization of the obtained aminefunction gave the expected maleimide-terminatedoligomers. To obtain oligomers with an aliphatic mal-eimide function, a new initiator with an aliphaticamine function was used for the ATRP step. Becauseof its high reactivity, it has been necessary to protectthe amine function by a t-boc group to avoid side reac-tions. After the ATRP reaction, the cleavage of the t-boc group by an acid treatment gave the aliphaticamine functionalized oligomer. The last step of imid-ization gave the corresponding maleimide-terminatedoligomer.

The obtained maleimide-terminated oligomers werecopolymerized with divinyl ether under UV irradia-tion with or without photoinitiator. The FT-IR studyof these copolymerizations showed a really higherreactivity of the aliphatic maleimide. The addition ofDarocur as photoinitiator with the aromatic malei-

Table 4. Conversion Rates According to DifferentSystems of Photoinitiation

Initial Mixture

ConversionRate

(after 30 s)

MIaliphatic-P(BA) 62%MIaromatic-P(BA) 2.5%MIaromatic-P(BA) þ Darocur (0.5 %) 35%t-BuMI/HBVE* 86%

Experimental conditions: [HM]/[VE] ¼ 1 and intensity¼ 75 mW/cm2.

* System using t-BuMI (t-butyl maleimide) and HBVE(hydroxybutylvinylether) studied in the literature30 for anintensity ¼ 60 mW/cm2.

Figure 15. Evolution of the polymerization rateaccording to [MI][VE].

NOTE 4321

mide was necessary to make this system enoughcross-linked. The photopolymerization mechanismwas determined to be a homopolymerization of anacceptor/donor complex.

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Documents

Synthesis of maleimide-terminatedn-butyl acrylate oligomers by atom transfer radical polymerization: Study of their copolymerization with vinyl ethers