Mechanistic transformations involving living and

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Prog. Polym. Sci. 31 (2006) 1133–1170

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Mechanistic transformations involving living and controlled/living polymerization methods

Yusuf Yagci�, M. Atilla Tasdelen

Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey

Received 2 May 2006; received in revised form 25 July 2006; accepted 27 July 2006

Available online 12 October 2006

Abstract

This review is prepared on the occasion of the 50th anniversary of the historic discovery of living anionic polymerization

by Michael Szwarc. This process enabled preparation, with good control of polymer architecture, of well-defined polymers

such as block and graft copolymers, star polymers, macrocycles, and functional polymers. Transformation reactions

provide a facile route to synthesis of block copolymers that cannot be made by a single polymerization mode. A variety of

transformation reactions involving step-growth, conventional and controlled free radical, cationic, anionic, group transfer,

activated monomer Ziegler–Natta and metathesis reactions are known. In this article, transformation reactions involving

living and controlled/living polymerization methods are reviewed. Other possibilities of combining different polymeriza-

tion methods namely, macromonomer technique, coupling reactions, dual polymerizations and click chemistry are

described. Preparation of star and miktoarm-star block copolymers by using mechanistic transformations is also

presented.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Living polymerization; Controlled polymerization; Transformation reactions; Block copolymers; Graft copolymers

Contents

1. Introduction and historical perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134

2. Transformations involving anionic and controlled radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . . 1136

2.1. Anionic polymerization to controlled radical polymerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1136

2.2. Controlled radical polymerization to anionic polymerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1140

3. Transformations involving cationic and controlled radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . 1140

3.1. Cationic polymerization to controlled radical transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1140

3.2. Controlled radical polymerization to cationic polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145

4. Transformations involving anionic and cationic polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145

5. Transformations involving activated monomer (AM) polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1150

6. Transformations involving metathesis polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1150

7. Transformations involving Ziegler–Natta polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1151

e front matter r 2006 Elsevier Ltd. All rights reserved.

ogpolymsci.2006.07.003

ing author. Tel.: +90212 285 63 86.

ess: [email protected] (Y. Yagci).

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ARTICLE IN PRESSY. Yagci, M. Atilla Tasdelen / Prog. Polym. Sci. 31 (2006) 1133–11701134

8. Transformations involving group transfer polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1154

9. Transformations involving the same polymerization mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155

10. Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155

10.1. Transformations via macromonomer technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155

10.2. Coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156

10.3. Dual polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156

10.4. Combination of polymerization mechanisms by ‘‘click chemistry’’ . . . . . . . . . . . . . . . . . . . . . . . . . . 1157

11. Star and miktoarm-star block copolymers by a combination of polymerization mechanism . . . . . . . . . . . . 1160

12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1160

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161

1. Introduction and historical perspectives

It is a special privilege to be invited to contributea short review article to the special issue of Progress

in Polymer Science on the occasion of the 50thanniversary of Michael Szwarc’s discovery [1] ofliving anionic polymerization reactions, whichprovided innovative synthetic strategies for pre-paration of a wide range of polymers. Monodispersepolymers, as well as macromolecules with controlledarchitecture, can be prepared by using the livingpolymerization technique. Macromolecules pre-pared by this technique include block and graftcopolymers, star polymers, macrocycles, and tele-chelic polymers that are useful in step-wise poly-merization processes. The discovery stimulatedattempts at living cationic and radical polymeriza-tions, which were realized at much later dates.

Living polymerization technique is one of themost important synthetic routes for the preparationof block copolymers. The disadvantage associatedwith the traditional methodologies for the prepara-tion of block copolymers is the formation ofhomopolymer as contaminant. Szwarc et al. [2] firstreported a novel chemical methodology for thepreparation of polyisoprene-b-polystyrene-b-polyi-soprene (PIP-PSt-PIP) triblock copolymers, whichare free of homo polystyrene (PSt) and polyisoprene(PIP). The preparation of triblock copolymerslinking hydrophilic and hydrophobic blocks wasfirst reported by Richards and Szwarc [3]. Thecontrol of the sequence of the blocks and theirindividual chain lengths during synthesis led tosystematic investigations of properties as a functionof chain architecture. However, besides high purityrequirements, this revolutionary technique is limitedto anionically polymerizable monomers. In fact,some limitations exist even for anionically poly-merizable monomers. Whether block copolymeriza-tion of two anionically polymerizable monomers

can be carried out is critically dependent on thestructure and relative reactivity relationship of theionic species and the monomers. In fact, only a fewmonomers are suitable for the preparation of blockcopolymers by anionic polymerization, as is alsotrue for cationic polymerization. Furthermore, thesynthesis of block copolymers between structurallydifferent polymers. i.e., condensation and vinylpolymers, by a single polymerization method israther difficult due to the nature of the respectivepolymerization mechanisms.

In recent years, the development of polymeriza-tion processes for a high level of control over molarmass, polydispersity and end-group and moleculararchitecture has remained a major challenge. Therapid development of metallocene polymerization ofolefins and controlled radical polymerizationstrongly reflects this trend. In order to extend therange of monomers for the synthesis of blockcopolymers, a mechanistic transformation approachwas proposed, by which the polymerization me-chanism could be changed from one to anotherwhich is suitable for the respective monomers. Thepioneering work on the mechanistic transformationwas originally reported by Burgess et al. [4–6] threedecades ago. In fact, the very first conceptualapproach to mechanistic transformation reactionswas made by Szwarc [7]. He considered the electrontransfer from sodium naphthalenide to styrene (St)in the following way, as described in his own words:‘‘In my naive thinking I imagined styrene to bereduced to a radical anion,

a species acting as a carbanion on one end, and agenuine radical on its other terminus, both capableof initiating polymerization of styrene but by adifferent mechanism. The possibility of a simulta-neous radical and anionic polymerization intrigued

ARTICLE IN PRESS

Scheme 1.

Y. Yagci, M. Atilla Tasdelen / Prog. Polym. Sci. 31 (2006) 1133–1170 1135

mey’’. Electron transfer did take place, but incontrast to Szwarc’s initial thoughts, radical endscombined almost instantaneously and the polymer-ization proceeded only anionically. Even so, his firstproposal indicates the initiative behind the trans-formation approach, in which different polymeriza-tion mechanisms are combined. Indeed, todaysimultaneous (dual) polymerizations, involvingstructurally different monomers, are a well-estab-lished method in the transformation tool-kit (seebelow).

As a consequence of the specific dedication of thisspecial issue to the 50th anniversary of the discoveryof living polymerizations, this paper does not aim toreview all published work on transformation reac-tions, but rather intends to illustrate their broadversatility by concentrating on living and con-trolled/living polymerizations. In this connection,the reader’s attention is directed to recent reviews[8–10] describing mechanistic transformation ap-proaches using various polymerization methods,including condensation and conventional radicalpolymerizations. Transformation reactions are clas-sified on the basis of interconversion betweenpropagation mechanisms (Fig. 1). It can be seenthat between the main living and controlled/livingpolymerization methods, transformations are acces-sible in both directions.

Fig. 1. Mechanistic transformation in livin

Transformation reactions can be realized mainlyin two ways: (i) direct and (ii) indirect transforma-tions. In direct transformation, a propagating activecenter is transformed directly to another activecenter with different polarity. This transfer occursthrough an electron transfer as shown in Scheme 1for the transformation involving anionic andcationic systems.

The shortcoming associated with the directtransformation, is the short lifetime of propagatingsites, particularly radicals. The active center musthave a lifetime sufficient to permit transformation.Furthermore, a thermodynamic limitation for asuccessful redox process may result from unsuitableredox potentials of the propagating species andoxidant and reductant. The only successful exampleof direct transformation involving living polymer-ization methods was reported by Endo for thepreparation of block copolymers of tetrahydrofuran(THF) with tert-butyl methacrylate (t-BMA),e-caprolactone (CL) [11] and d-valerolactone (VL)[12], as shown in Scheme 2.

From the practical point of view, however, indirecttransformation is more attractive because it can be

g/controlled polymerization methods.

ARTICLE IN PRESS

O

MeOTfCH3

O

n-1O+

TfO-2SmI2

HMPA

CH3

OSmI2

CH3

O

CH3

OC

O

CH3

C

O-tBu

O

OSmI2

m TBMA

n

n

n

n m

m

m CL

Scheme 2.

Initiaton Functionalization Termination Functionalization

Mechanism A

Mechanism B

= Initiator for Mechanism B = Monomer 2

= Monomer 1

Mechanism A

= Initiator for Mechanism A

Fig. 2. Indirect mechanistic transformation.

Y. Yagci, M. Atilla Tasdelen / Prog. Polym. Sci. 31 (2006) 1133–11701136

performed much more easily and uses various poly-merization modes. Therefore, the following sectionswill essentially concentrate on indirect transforma-tions. As illustrated in Fig. 2, indirect transformationusually requires multistep reactions. The stable butpotentially reactive functional group for the secondpolymerization mode is introduced at the chain ends,either in the initiation or the termination steps of thepolymerization of the first monomer. The polymer isisolated and purified, and finally the functional groupsare converted to another species.

2. Transformations involving anionic and controlled

radical polymerization

2.1. Anionic polymerization to controlled radical

polymerization

Following the pioneering work of Veregin et al.[13], Georges et al. [14], Solomon et al. [15] andRizzardo [16], special attention has recently focusedon the use of stable nitroxyl radicals such as 2,2,6,6-tetra methylpiperidine-1 oxyl (TEMPO) in order toachieve living conditions in conventional radicalpolymerization. In principle, these nitroxide-

mediated polymerizations (NMP) involve reversibletermination of the polymer radical with TEMPOand chain growth during the lifetime of thepolymeric radical as shown in Scheme 3.

Besides organo-tin compounds, several other newinitiators can be used to synthesize designedpolymers based on poly(e-caprolactone) (PCL)[17–22]. Among them, metallic alkoxides are parti-cularly useful to introduce functional groupsselectively at one chain end of PCL [23–25]. Yoshidaand Osagawa [26] reported the stable radicalfunctionalization of PCL by using a speciallydesigned aluminum alkoxide initiator. For thispurpose aluminum tri(4-oxy-TEMPO), preparedby the reaction of triethylaluminum with threeequimolar amounts of 4-hydroxy-TEMPO, wasused as an initiator for the anionic polymerizationof CL (Scheme 4).

PCL with the TEMPO moiety behaved as apolymeric counter-radical for the polymerization ofSt, resulting in the quantitative formation of poly(e-caprolactone)-b-polystyrene (PCL-b-PSt). The radi-cal polymerization was found to proceed inaccordance with a living mechanism without un-desirable side reactions. (Scheme 5)

ARTICLE IN PRESS

N OHO + AlEt3 NO3

Al

NO

O

(CH2)5 O

n

O O

n

O

HO C

Scheme 4.

N OO C

O

(CH2)5 O Hn

1. BPO, m St, 95 oC, 3.5 h

2. 125 oCN OO C

O

(CH2)5 O HCHCH2Rnm

Scheme 5.

NOCHCH2 NOCHCH2 +K

Scheme 3.


The thermal analysis of the block copolymerindicated that the components of PCL and PSt werecompletely immiscible and microphase separated.The incorporation of the TEMPO moiety intopoly(ethylene oxide) (PEO) chain-ends in the radicalform was also achieved [27]. In this case, TEMPO-Na was used as an initiator in living anionicpolymerization of ethylene oxide (EO) (Scheme 6)under conditions such that the stable nitroxylradical at the end of the PEO chain could not bedestroyed.

Again, the resulting PEO with a TEMPO moietyacted as a macromolecular radical trap in NMP ofSt to give poly(ethylene oxide)-b-polystyrene (PEO-b-PSt) with narrow polydispersity. It was found thatPEO of high molecular weight is less efficient at

trapping chain ends, and so can enhance thepolymerization rate.

Polybutadiene-b-polystyrene (PB-b-PSt) [28–30],polydimethylsiloxane-b-polystyrene (PDMS-b-PSt)[31], (PEO-b-PSt) [32] and poly(ethylene oxide)-b-poly(4-vinyl pyridine) (PEO-b-PVP) [33] copoly-mers were synthesized by terminating the corre-sponding living anionic polymerization with asuitable TEMPO derivative and subsequent NMP.

Stable nitroxyl radicals can also be incorporatedinto polymers as side groups. Endo and co-workers[34] copolymerized nitroxyl radical containing ep-oxide with glycidyl phenylether anionically usingpotassium tert-butoxide as initiator (Scheme 7).

The ratio of the nitroxyl radical moiety in theresulting copolymer can be controlled by the feed


ratio. Subsequent polymerization of St in thepresence of this polymer is expected to yield graftcopolymers.

An interesting variation of this approach wasrecently reported by Cianga et al. [35] whodemonstrated that the stable TEMPO radical canundergo a one-electron redox reaction with potas-sium naphthalene. While the TEMPO alcoholate

NO

O

O+

O

Scheme

+N

O

K+

nCH2 CH2 OCH2CHON

mH

Scheme

N O-Na+O O+ n60 °C

NO

NO On

AIBN, m St, 120 °CO (CH2)2 H

Scheme

thus formed does not initiate the polymerization ofSt, the polymerization of EO was readily accom-plished. PEO obtained in this way possessesTEMPO terminal units and was subsequently usedas an initiator for NMP of St to give blockcopolymers (Scheme 8).

Atom transfer radical polymerization (ATRP) is themost widely used controlled radical polymerization in

O

OO

OO

N

O

n m

7.

+N

OK

O

N O CH2 O

n

n

m

CH2 H

8.

NOCHCH2R Onm

On

O (CH2)2 H

O (CH2)2 H

6.

ARTICLE IN PRESSY. Yagci, M. Atilla Tasdelen / Prog. Polym. Sci. 31 (2006) 1133–1170 1139

anion-to-radical transformation methodology. Thefirst example was reported by Acar and Matyjaszews-ki [36] and utilized for the preparation of AB andABA type block copolymers. The macroinitiators, PStand PSt-b-PIP containing 2-bromoisobutyryl endgroups were prepared by living anionic polymeriza-tion and a suitable termination agent. These polymerswere then used as macroinitiators for ATRP toprepare block copolymers with methyl acrylate (PSt-b-PMA), butyl acrylate (PSt-b-PBA), methyl metha-crylate (PSt-b-PMMA), a mixture of styrene andacrylonitrile (PSt-b-P(St-r-AN) and also chain exten-sion with St (PSt-b-PSt) and PSt-b-PIP-b-PSt)(Scheme 9).

Other examples of materials prepared from theanionic polymerization to ATRP are shown inTable 1.

As can be seen from Table 1, the transformationapproach involving the combination of livinganionic polymerization and ATRP has enabled thepreparation of segmented copolymers with anexciting range of structural variety. This way,multiblock copolymers possessing soft segmentsand glassy segments, graft terpolymers, comblikeblock copolymers, stars and dendrimer-like archi-tectures, and polymer ceramic hybrid materials weresuccessfully prepared. A very interesting applicationconcerns the incorporation of a fluorescent dye atthe junction point of poly(methyl methacrylate)-b-poly(butyl acrylate) (PMMA-b-PBA) copolymer.The overall process is depicted in Scheme 10 [40].

Recently, reversible addition–fragmentationtransfer (RAFT) polymerization, another controlledradical polymerization method, has also been usedin this transformation. The mechanism involves thechain transfer of active species such as the radicals

CH2 CH C C

CH3

CH2 CH Li

O

Ph

+

CH2 CH

Ph

CH2 CH O

Ph

C

O

B

CH3

CH3

C

Scheme

stemming from the decomposition of the initiatorand propagating radicals to chain transfer agents(RAFT agents), forming an unreactive adductradical, followed by fast fragmentation to a poly-meric RAFT agent and a new radical. The radicalinitiates the polymerization. The equilibrium isestablished by subsequent chain transfer–fragmen-tation steps. It was shown that PEO containing axanthate end group can be used as a macro-RAFTagent in the polymerization of N-vinylformamide(NVF) to yield poly(ethylene oxide)-b-poly(N-vinylformamide) (PEO-b-PNVF) (Scheme 11) [70].

In another case, hydroxy functionalities of PEOswere converted to dithiobenzoyl groups and used asmacro-RAFT agents in RAFT polymerization of N-isopropylacrylamide (NIPAM) (Scheme 12). De-pending on the functionality of the initial polymers,AB and ABC type block copolymers with well-defined structures were prepared [71,72].

Obviously, the most important step of thesetransformations is the modification of the chainend into a good leaving group. In order to obtainquantitatively functionalized macro-RAFT agentsor ATRP initiators, modification of living anionicpolybutadiene (PB) with diphenylethylene, St andhaloalkanes has been recently investigated [73,74].Lutz and Matyjaszewski [75] utilized the versatilityof combining living anionic polymerization withRAFT to prepare segmented graft terpolymers withcontrolled molecular structure. Anionically pre-pared polylactide (PLL) and poly(dimethylsiloxane)(PDMS) macromonomers were used in RAFTpolymerization of alkyl methacrylates.

A conceptually different transformation reactionwas applied for the preparation of PCL-b-(PMMA-co-PSt)-b-PCL by using iniferter technique in the

H2

Br C

O

C Br

CH3

CH3

ATRP

St, BA, MA, MMA, AN

Block Copolymers

Li

O Li

r

9.

ARTICLE IN PRESS

Table 1

Block copolymers prepared by anionic-to-ATRP transformationa

Anionic segment

(A)

ATRP segment

(B)

Ref.

PSt PVP [37]

PIP-b-PSt PSt [38]

PSt-co-PAN PSt, PtBA, PBA, PGA,

PMMA

[39]

PMA PMMA [38]

PIP PSt [40]

PSt-PB, PSt PMA, PMMA [41]

PFS PMMA [42]

PEO PSt, PHMA, (AB2, AB3, A2B,

A2B2) PSt and (three-arm)

PSt-b-PtBMA star, block

copolymers, PDMAEMA,

PBMA, PMMEMA

[43–48]

PMMA PSt-b-PtBA, PSt-b-PMMA,

PBA-b-PSt

[49,50]

PE-co-PBu PSt, PAcSt [51]

PE-co-PP-b-PEO PHMA [52]

PCL PSt, PDMAEMA, PODMA

block;

PHEMA, PEGMA, PMMA,

PtBA, PMAA star; PMMA,

PHEMA star, block

copolymers, PHEMA (brush

copolymer)

[53–64]

PDMS PSt, POEGMA [65,66]

PLL PSt, PMMA, PtBA dendrimer

based star, PMMA, PtBA,

PBzA (ABA) triblock,

copolymer

[67–69]

aPolymer abbreviations: PSt, polystyrene; PVP, poly(vinyl

pyrolidone); PIP, polyisoprene; PMA, poly(methyl acrylate);

PMMA, poly(methyl methacrylate); PB, polybutadiene; PBA,

poly(butyl acrylate); PFS, poly(ferrocenyldimethylsilanes); PEO,

poly(ethylene oxide); PtBA, poly(t-butyl acrylate); PCL, poly(e-caprolactone); PE, polyethylene; PBu, polybutylene; PAcSt,

poly(4-acetoxystyrene); PP, polypropylene; PHMA, poly(hexyl

methacrylate); PtBMA, poly(t-butyl methacrylate); PDMAEMA,

poly(dimethylamino)ethyl methacrylate); PMAA, poly

(methacrylic acid); PODMA, poly(n-octadecyl methacrylate);

PHEMA, poly(2-hydroxyethyl methacrylate); PEGMA, poly

((ethylene glycol) methacrylate); POEGMA, poly(oligo(ethylene

glycol) methyl ether methacrylate); PDMS, poly(dimethylsilox-

ane); PLL, polylactide; PBzA, poly(benzyl acrylate); PAN,

polyacrylonitrile, PGA, poly(glycidyl acrylate); PMMEMA,

poly(monomethyl ether methacrylate).


controlled radical polymerization step. Substitutedtetraphenylethanes represent a class of thermaliniferters applicable to the radical polymerizationof many monomers in a controlled manner. Theinitiation of anionic coordination polymerization ofCL by aluminum triisopropoxide in the presence of

benzpinacol leads to the formation of polymers withiniferter structure in the middle of the chain [76].The benzpinacolate groups incorporated into thepolymer chain then initiate the polymerizationof St and MMA via a controlled radical mechanismat 95 1C to yield the desired block copolymers(Scheme 13).

2.2. Controlled radical polymerization to anionic

polymerization

The most widely applied controlled radicalpolymerization method for this particular transfor-mation is ATRP. This is mainly because of the factthat hydroxyl and amino groups, potential initiatingsites for the ring-opening anionic polymerization ofcertain monomers, are compatible with the ATRPof vinyl monomers. Examples of such transforma-tions are compiled in Table 2, and the generalconcept is illustrated in Scheme 14 by the exampleof the combination ATRP of vinyl monomers withthe ring opening polymerization of lactides [77].

3. Transformations involving cationic and controlled

radical polymerization

3.1. Cationic polymerization to controlled radical

transformation

Yoshida and Sugita [87,88] described the synth-esis of polytetrahydrofuran PTHF possessing anitroxy radical by terminating the polymerizationof living PTHF with sodium 4-oxy TEMPO. Thepolymer obtained in this way acted as a counter-radical in the polymerization of St in the presence ofa free-radical initiator to yield polystyrene-b-poly-tetrahydrofuran (PSt-b-PTHF) (Scheme 15).

NMP was also extended to azo-containing poly-meric initiators obtained by cationic polymerization[89]. In this case, o-alkoxyamine PTHF wasobtained and upon heating at 125 1C, stablepolymeric nitroxyl radicals were formed. In thepresence of St, the block copolymers produced hadcontrolled molecular weight, since terminationreactions were minimized and the equilibriumbetween dormant and active species allowed con-trolled growth (Scheme 16).

An alternative route for this type of transforma-tion was also reported [90]. The living propagatingchain end was quenched with previously preparedsodium 2,2,6,6-teramethylpiperidin-1-oxylate ac-cording to the reactions in Scheme 17.

ARTICLE IN PRESS

tBu Si

CH3

CH3

O (CH2)3 Li

Ph

Dye

THFtBu Si

CH3

CH3

O (CH2)4 C Li

Dye

Ph

n MMA

LiCl, THF, -78 °C

hydrolysis

ATRP

+

(PMMA)

Br CH

CH3

C

O

Br

PMMA-

tBu Si

CH3

CH3

O (CH2)4 C

Dye

Ph

HO Si

CH3

CH3

O (CH2)4 C

Dye

Ph

C Si

CH3

CH3

O CH2)4 C

Dye

Ph

CHBr

CH3

O

C Si

CH3

CH3

O( (CH2)4 C

Dye

Ph

CH

CH3

O

20 °C

b-PBA

m BA

Scheme 10.

I NVF PNVF n EtO C

S

S

CH3

(CH2)2 C

O

O

CN

+

C C

CH3

(CH2)2 C

O

O PEG

CN

SPNVFn

C

SPNVFn

EtO

C

CH3

(CH2)2 C

O

O

CN

+

C

CH3

(CH2)2 C

O

O

CN

+m NVF PEG b PNVFm

Block Copolymer Formation

Initiation/Propagation

Addition-Fragmentation

C PEG

SPEGS

PEG

Scheme 11.


In the subsequent step, radical polymerization ofSt was carried out with alkoxyamine terminatedPTHF. Although an increase in conversion withpolymerization time was observed and blockcopolymers with polydispersities close to those ofthe prepolymers were readily formed, the initiation

efficiency of o-alkoxyamine PTHF was rather poor.This was attributed to the relatively slow decom-position and initiation of alkoxyamine attached tounsubstituted methylene groups. In a recent articleit was reported that alkoxyamines containingan unsubstituted carbon atom are very slow to

ARTICLE IN PRESS

HO PEG OHO

O O

C SH

S

HOOC CH CH C

O

PEG C

O

CH CH COOH

C

NH

CH(CH3)2

S

CH2

CH

COOHC

S

S

CH2

CH

HOOC C

S

C

O

PEG C

O

C

S

S

CH2

CH

COOH

C

O

PEG C

O

PNIPAM S

S

CH2

CH

HOOC PNIPAM

OAIBN

C

Scheme 12.

R=

OiPr

A lOiPrPriO2 + 3 H O R O H

O-R-OiPr

AlO-R-OiPrPriO-R-O

CL

St, MMAPCL-b-(PSt-co-PMMA)-b-PCL

95°C

Scheme 13.


decompose and that the a-methyl group is essentialfor the conventional radical polymerization toproceed with a truly living character [91].

Cation-to-ATRP or reverse ATRP to form ABand ABA type block copolymers was also per-formed [92,93]. One or two bromopropionyl endgroups were introduced on PTHF by using func-tional initiator and termination approaches in thering- opening polymerization of tetrahydrofuran(Scheme 18).

Bromo-functionalized PTHFs obtained this waywere used as initiators in ATRP of St, MMA andMA to yield AB and ABA type block copolymers.Notably, in the case of St and MA, the formation oftriblock copolymers was significantly slower.

It was also reported [94] that PSt with chlorinetermini, synthesized by living cationic polymeriza-tion without any additional reaction, was anefficient macroinitiator for living ATRP of St,MMA, and MA (Scheme 19).

ARTICLE IN PRESS

Table 2

Block copolymers prepared by ATRP-to-anion transformationa

ATRP segment Anionic segment Ref.

Poly(acrylate ethyl

lactose octaacetate)

Poly(benzyl-L-

glutamate)

[78]

Poly(acrylate ethyl

lactose)

Poly(L-alanine N-

carboxyanhydride)

[79]

PMMA PLL, PCL [80,81]

Polyacrylates PCL [82]

PSt PEO, PLL, PPp, g-benzyl-L-glutamate

N-

carboxyanhydride

or PtBA

[83–86]

aPolymer abbreviations (see Table 1): PPp, polypeptide.

HOCH2CH2OH+

St

ATRPC C O

CH3

CH3

CH2CH2O

O

PSt

C C OH

CH3

CH3

O

Br

ATRP MMA Sn(OC

PSt-b-PMMA PSt-b-PLL

PSt-b-PMMA-b-PLL

Sn(OCt)2

LL

LL

Scheme

O(CH2)4On-1

+ NNaO O

Scheme


With some variations in the initiator design, morecomplex structures such as block, graft andmiktoarm-star block copolymers having PTHF[95–100] chains as the cationic segment weresynthesized.

Cationic to ATRP transformation was also usedin the synthesis of triblock copolymers with poly-isobutylene (PIB) as the middle sequence. Thesematerials are particularly useful as thermoplasticelastomers. In this case, a few units of St were addedto living difunctional PIB after the isobutylene hadreacted. The isolated PIBs could act as bifunc-tional macroinitiators for ATRP [101]. A similarstrategy was used by Chen and Batsberg [102] forthe synthesis of block copolymers of isobutylene

H

C C O

CH3

CH3

CH2CH2OH

O

Br

t)2

14.

nOO(CH2)4 N

m St,125°C

OO(CH2)4 N O CH CH2n m

BPO

O

15.

ARTICLE IN PRESS

O AgOTf OO

O

Br

Br

O

BrO

O

Br

O(CH2)4 OHn

O

O(Tf)2 OO

ONa

O

Br

2

O

O

Br

O(CH2)4 O

O

Br

n

Scheme 18.

n

NO

(CH2)4O N O(CH2)4n

NOO(CH2)4n

CH2O(CH2)4 CH O Nmn

N

125 °Cm St

Scheme 16.

O(CH2)4On-1

+n

NNa O NO

NO NHOSodium

ascorbate

Sodium hydride

O(CH2)4

Scheme 17.


ARTICLE IN PRESS

ClCl

n-1

+ n

SnCl4 / nBu4NCl

-15 °C

St MA MMA

PSt b PSt PSt b PMA PSt b PMMA

ATRP

Scheme 19.


with p-acetoxystyrene (PIB-b-PAcSt) or styrene(PIB-b-PSt).

In a more recent study [103], chlorine end groupsof PIB were quantitatively converted to bromoestergroups to facilitate ATRP from end-positionedactivated ester groups (Scheme 20). This way thecapping with short blocks of PSt observed in theearlier method could be avoided. A similar strategygave rise to polyisobutylene-b-poly(methyl metha-crylate) PIB-b-PMMA [104].

The concept was further extended to the prepara-tion of poly(t-butyl acrylate)-b-polyisobutylene-b-polystyrene (PtBA-b-PIB-b-PSt) (ABC) triblock[105] and amphiphilic pentablock copolymers basedon PIB [106]. Using the same combination, seg-mented graft copolymers could also be obtained.For example, Hong et al. [107] prepared PIB-g-PMMA and PIB-g-PSt by using partially bromi-nated polyisobutylene-b-poly(p-methoxystyrene)PIB-co-P(p-MeSt) as a macroinitiator in ATRP ofthe respective monomers (Scheme 21).

Phase behavior and dynamic mechanical proper-ties were strongly affected by the composition and/or the side-chain architectures. Moreover, theproperties of the segmented graft copolymers werecontrolled over a wide range, leading to toughenedglassy polymers or elastomers.

3.2. Controlled radical polymerization to cationic

polymerization

In parallel with the recent advances in controlledradical polymerizations, many transformations ofATRP, NMP and RAFT to cationic polymerizationhave been reported (Table 3).

A typical example of this transformation isillustrated by the preparation of PSt-b-PTHFinvolving a mechanistic change from ATRP tocationic ring opening polymerization (Scheme 22)[112].

4. Transformations involving anionic and cationic

polymerizations

Anion-to-cation or reverse transformation reac-tions were successfully employed to prepare blockcopolymers. The particular advantage of thesetransformations is that both anionic and cationicblocks can be prepared under living polymerizationconditions. In this connection, prominence must begiven to the pioneering work of Burgess et al. [4–6],demonstrating the great versatility of the transfor-mation reactions. They prepared bromine-termi-nated PSt by direct reaction of excess bromine orxylene dibromide with living PSt (Scheme 23).

It was found that competing Wurtz couplingreactions (Scheme 24) may be prevented [119] byusing a Grignard intermediate. The desired bromo-functionalized PSt was obtained with up to 95%efficiency.

Bromo-functionalized polymer was employed toprepare block copolymer upon generating carboca-tions by reacting suitable silver salts (Scheme 25).

Although block copolymers with narrow poly-dispersity were obtained, quantitative transforma-tion efficiency was not achieved, even at lowtemperatures, because of the b-proton eliminationreactions (Scheme 26). Termination by b-protonelimination may be avoided by using xylenedibromide in the halogenation process. However,

ARTICLE IN PRESS

C PIB C Cl

CH3

CH3

Cl

CH3

CH3BuOK

THF BF3 OEt2/Hexane

OH

C PIB C

CH3

CH3CH3

CH3

OHHO

CH3CH(Br)COCl

CH2Cl2, DMAPC PIB C C

CH3

CH3CH3

CH3

OO

O

CH CH3C

O

CHCH3

Br Br

ATRP

PSt-b-PIB-b-PSt

St

2

Scheme 20.

CH2 C

CH3

CH3

CH2 CH CH2 CH

CH2Br CH3

x y z

ATRP

St or

MMA

Graft copolymers

Scheme 21.

Table 3

Transformations of controlled radical polymerization to cationic polymerizationa

Controlled radical polymerization Cationic polymerization Type of segmented copolymer Ref.

ATRP Carbocationic PIB-b-PSt-b-PMMA-PSt-b-PIB [108,109]

ATRP Ring opening S-(PSt)2-(PDOP)2 miktoarm star [110]

ATRP Vinyl PMVE-b-PtBu, PMVE-b-PAA [111]

ATRP Ring opening PSt-b-PTHF, PTHF-b-PSt-b-PTHF [112]

ATRP Ring opening PTMO-b-PS and PTMO-b-PSt-b-PMMA [113]

ATRP Promoted cationic PSt-b-PCHO [114]

DPE Promoted cationic PSt-b-PCHO and PMMA-b-PCHO [115]

DPE Carbocationic PMMA-b-PSt-g-PIB [116,117]

NMP Ring opening PSt-g-PEI comb polymer [118]

aPolymer abbreviations: see Table 1; PIB, polyisobutylene; PDOP, Polydioxapane; PMVE, poly(methyl vinyl ether); PtBu, poly(t-butyl

acrylate); PTHF, polytetrahydrofuran; PTMO, poly(trimethylene oxide); PCHO, poly(cyclohexene oxide); PEI, poly(ethylene imine).


it does not improve the overall transformationefficiency.

A conceptually similar approach was used byMuhlbach and Schulz [120] to prepare stryreneand 1-azabicyclo[4.2.0]octane block copolymer. Byend-capping living PSt with EO and subsequentreaction with bromoacetyl bromide, they obtained a

polymer with bromoacetyl groups. This polymertogether with AgClO4 acted as a macroinitiator forliving polymerization of the cyclic monomer(Scheme 27). Very little homopolymer was formed.Two distinct glass transitions at 10 and 94 1Cwere observed with the block copolymer cor-responding to poly(1-azabicyclo[4.2.0]octane) and

ARTICLE IN PRESS

CH2 CH Li + Br2 CH2 CH Br-LiBr

CH2 CH Li BrCH2

CH2Br+ -LiBr CH2

CH2BrCHCH2

Scheme 23.

CH2 CH Li CH2 CH Br+ CH2 CH CH2CH-LiBr

Scheme 24.

ATRPCH2 CH Br CH2 CH

AgClO4+ AgBr

O

PSt-b-PTHF

ClO4

Scheme 22.

CH2 CH Br + AgClO4 CH2 CH ClO4 AgBr+

Scheme 25.

CH2 CH + HClO4CH2 CH ClO4

Scheme 26.


PSt sequences respectively, which indicates that theblocks are incompatible and phase separated.

Cationic polymerization of cyclic amines is wellknown [121–123]. Low molecular weight initiatorssuch as ethyltosylate induce polymerization of cyclicamines such as 1-tert-butylaziridine. The idea ofusing a macroinitiator having a tosylate end groupto polymerize cyclic amines prompted Kazamaet al. [124] to attempt to polymerize 1-tert-butylaziridine using PDMS having a terminal tosylate

group. However, no polymerization occurred whenmacroinitiator was used; but this clearly indicatesthe initiative behind the study of the transfor-mation reaction between anionic and cationicpolymerizations.

Anionic polymerization is one of the bestmethods to prepare end-functionalized polymers.Vinyl polymers with haloalkyl groups at one chainend were prepared by anionic polymerizationfollowed by termination of the living anion with


an excess of 1,2-dichloroethane or 1.4-dibromobu-tane [125]. These polymers served as macroinitiatorsfor the polymerization of MeOZO and aromaticvinyl monomers (Scheme 28).

CH2 CH2Na CH CH NaX (CH2)n X

n=1, X= Cln=2, X= Br

KIMeOZO

CH(CH2)nN

COCH3

CH2CH2 CH2

X

Scheme

OAnionic

PolymerizationHO CH2CH2On

LiO CH2CH2O

BuLi

CHN

C

R

CH2CH2 m

O

Scheme

CH2 CH CH2 CH2 O

O

CH2 Br + AgClO4

N

CH2 CH CH2 CH2 O

O

CH2 N CH2 CH2 m

m

n

nC

C

Scheme 27.

Block copolymers consisting of poly(N-acylethy-leneimine) and PEO chains were prepared byinitiating the polymerization of 2-methyl-2-oxazo-line (MeOZO) or 2-ethyl-oxazoline with a,o-dito-sylated or mesylated PEO [126] (Scheme 29). Theblocking efficiency was close to 100%.

Simionescu et al. [127] used poly(ethylene oxideadipate) having tosylate groups at both ends asmacroinitiators for cationic polymerization ofMeOZO to produce ABA type block copolymers.Miyamoto et al. [128] further explored the conceptand prepared block copolymers consisting of PPOand PMeOZO by using poly(propylene oxide)-p-nitrobenzene sulfonate as a macroinitiator for thecationic polymerization of MeOZO (Scheme 30). Asthe conversion to the sulfonate functionality wasquantitative, the polymerization of MeOZO by themacroinitiator produced a mixture of AB and ABAtype block copolymers.

CH2 CH (CH2)n N

COCH3

CH2 CH2

CH(CH2)n CH2 CH2 CH (CH2)n X

28.

n TsO CH2CH2O Tsn

Lin

TsCl

Ts=p-CH3C6H4SO3

Cationic Polymerization

of MeOZO

2CH2O CH2 CH2

C

R

n m

HTSCl/DAP

N

O

29.


An interesting application of new polymerarchitectures by combination of anionic and catio-nic polymerization was described by Deffieux andSchappacher [129] and Schappacher and Deffieux[130]. In this study, quantitative formation ofcomblike polymers was obtained by grafting poly-styryl lithium onto poly(chloroethyl vinyl ether)(PCEVE) (Scheme 31).

The advantage of the system arises from the factthat since the PCEVE backbone and PSt grafts canbe prepared by both living cationic and anionicpolymerization, it is possible to synthesize graftcopolymers possessing both a backbone with con-trolled dimensions and an adjustable number ofbranches of precise length and narrow molecularweight distribution. Star and hyperbranched poly-mers were also prepared by following the samestrategy.

Many examples of active site transformationsfrom cation to anion to prepare block and graftcopolymers based on PIBs have been reported(Table 4).

The recent report by Feldthusen et al. [143] onthis type of transformation illustrates the versatilityof the method to obtain new and unique polymerarchitectures. In this work, living PIB chains werequantitatively captured with 1,1-diphenylethylene,

HO CHCH2O CHCH2 OCH2CH OH

Me MeMe

n m

SO2 CHCH2O CHCH2NO2

Me Me

n

MeOZOCH2CH2N

Me M

C

Me

np

+

O

CHCH2O

Scheme

+ CH2 CH

OCH2CH2Cnm-1

CH2 CH CH2 CH Li

Scheme

leading to diphenylmethoxy and diphenylvinyl endgroups (Scheme 32).

The stable macroanions obtained by the subse-quent metalation of the end groups, according tothe reactions shown in Scheme 33 were used toinitiate living anionic polymerization of t-BMAyielding PIB-b-PtBMA block copolymers with al-most quantitative efficiency [144].

The hydrolysis of the ester groups of the acrylatesegment further enabled the preparation of amphi-philic polyisobutylene-b-poly(methacrylic acid)(PIB-b-PMAAc). A series of linear and starcopolymers consisting of PIB and PMMA werealso prepared [144].

The process was further improved by replacingdiphenylethylene with thiophene in the end-cappingprocess [145]. The advantage of this modificationwas related to the quantitative functionalization ofliving PIB with thiophene and the possibility ofmetalation of the thiophene end groups with n-BuLi. This is an important improvement forindustrial processes since lithiation by n-BuLi ismuch more convenient than metalation with Na/Kalloy.

Graft copolymers consisting of a PMMA back-bone and poly(isobutyl vinyl ether) (PIVBE) sidechains were also prepared by combined anionic and

NO2 SO2Cl2,6-Lutidine

OCH2CH SO2 NO2

Me

m

CHCH2 OCH2CH NCH2CH2

Mee

C

Me

m q

O

30.

-LiCl

l

CH2 CH

OCH2CH2 CH CH2

n

m

31.

ARTICLE IN PRESS

Table 4

Block copolymers via cation-to-anion transformation reactionsa

Block copolymers Ref.

PIB-b-PB [131]

PIB-b-PMMA [132]

PIB-b-PS [133]

PIB-b-PMMA [134]

PIB-b-PCL [135]

PIB-b-MeSt-g-PPV [136]

PIBVE-b-PCL [137]

PIB-b-PtBMA, PIB-b-PMAA [138]

PEtOZO-b-PLL, PEtOZO-b-PCL [139]

PTHF-b-PMMA [140,141]

PTHF-b-PSt, PTHF-b-PIP [142]

aPolymer abbreviations (see Table 1): PIB, polyisobutylene;

PS, polysiloxane; PMeSt, poly(methyl styrene); PPV, polypiva-

lactone; PIBVE, poly(isobutyl vinyl ether); PtBMA, poly(t-butyl

methacrylate); PEtOZO, poly (2-ethyl-2-oxazoline); PTHF,

polytetrahydrofuran; PIP, polyisoprene.

CH2 C Ti2Cl9

CH3

CH3

C Ti Cl9CH2 C

CH3

CH3

CH2

CCH2 C

CH3

CH3

CH2CCH2 C

CH3

CH3

CH2 OCH3

CH3OH 2) CH3OH/NH3

1) CH3OH

Scheme 32.


cationic polymerizations based on the homopoly-merization of a bifunctional monomer [146].

5. Transformations involving activated monomer

(AM) polymerization

Polymerization by the AM mechanism hasopened up promising pathways for transformationreactions. Kubisa [147], Penczek et al. [148] andBrzezinska et al. [149] reported that cationicpolymerization of oxiranes may proceed in thepresence of hydoxyl containing compounds by theAM mechanism according to Scheme 34.

Thus propagation involves the reaction of aprotonated (activated) monomer molecule with a

nucleophilic site in the neutral growing macromole-cule. This procedure can be adapted to transforma-tion reactions However, most of the reportedtransformations were achieved from AM to conven-tional radical polymerization by using thermal orphotochemical activation. For example, 4,40-azo-bis(4-cyanopentanol) was used in AM polymeriza-tion of epichlorohydrin (ECH) to produce polymerswith azo linkages in the main chain [150]. Polymer-ization was conducted under typical conditions, i.e.,by slow addition of ECH to the solution of initiatorcontaining catalyst. Reaction was considerablyslower than in the presence of simple diols (e.g.ethylene glycol) and only 28% conversion wasobtained under conditions sufficient to reach com-plete conversion in the polymerization initiated byethylene glycol. Poly(epicholorohydrin) PECH pre-pared this way, was used in the polymerization of Stto produce block polymer (Scheme 35). The use ofTEMPO in the free-radical step as describedpreviously for azo-linked PTHF would providecontrol over the second block.

The reverse mode of this transformation is alsoaccessible. For instance, Steward [151] has shownthat by using a free radically prepared hydroxy-terminated PB as a macrocatalyst in AM polymer-ization of ECH, block copolymers are produced inquantitative yield.

Polymers with a variety of photochromophoricgroups have been extensively used as precursors forblock and graft copolymers [152]. More recently,benzoin derivatives containing hydroxy groups wereused as initiators of AM polymerization of ECH.The resulting polymers contain photoactive benzointerminal groups [153].

Polymerization of acrylates and cyclic ethers canbe photochemically initiated by a benzoin moietyincorporated in the polymers (Scheme 36). In thelatter case organic oxidants such as onium salts areessential for converting the originally formedradicals to cations to afford quasi-living cationicpolymerization of CHO [154].

It is clear that further transformation reactionsinvolving controlled/living polymerization methodscan be realized by using suitable selected hydroxylcompounds in AM polymerization.

6. Transformations involving metathesis

polymerization

Ring-opening metathesis polymerization is aconvenient route to well-defined polymers

ARTICLE IN PRESS

CCH2 C

CH3

CH3

CH2

C MtCH2 C

CH3

CH3

CH2 C LiCH2 C

CH3

CH3

CH2

Mt=K, Na, Cs

LiCl

Mt Mt-MtOCH3

CCH2 C

CH3

CH3

CH2 OCH3

Scheme 33.

H + O OHR OH

OO RH + H

Scheme 34.


[155–157] initiated by Ti, Mo, W, Ta, Re, Rucomplexes [158–163]. However, this type of poly-merization is limited to cycloolefins such as norbor-nene, norbonadiene, dicyclopentadiene and otherstrained polycyclics for which ring opening isessentially irreversible [164]. It therefore seems thatthe transformation reactions involving metathesispolymerization allow extension of the range ofattainable block copolymers. The first reportedexample [165,166] involves block copolymerizationof St and cyclopentene. When a polystyryl anionwas used in conjunction with tungsten hexachloride,the propagating anion was transformed to acovalent species, and propagating centers for thepolymerization of the second monomer might havea bridged structure as shown in Scheme 37.

The same catalyst system was re-examined, butinstead of a copolymer, only dimers and oligomersof the polystyryl co-catalyst were isolated [167].With different catalysts, namely ruthenium andmolybdenum initiators, various block and graftcopolymers were also prepared via anionic to ring-opening metathesis polymerization [168–170].

Tritto et al. [171] and Risse et al. [172] reportedtwo independent transformation reactions for blockcopolymer synthesis. The first report [172,173]involves changing the mechanism from living me-tathesis polymerization of cycloalkene to grouptransfer polymerization of silyl vinyl ether. Secondly,they prepared block copolymers of norbornene and

ethylene by transforming metathesis polymerizationto Ziegler–Natta polymerization [174]. The reversetransformation is also possible [175].

More recently, Coca et al. [176] and others[177,178] reported general methods of transforma-tion of living metathesis polymerization into ATRPand anionic polymerization for the preparation ofblock copolymers. In this approach polynorborneneand poly(dicyclopentadiene) with terminal bromidewere synthesized by end-capping the correspondingliving chain ends with benzyl bromide (Scheme 38).

These polymers served as efficient macroinitiatorsfor the homogeneous ATRP of St and MAaccording to the mechanism described earlier (seeabove). Several other examples [179–181] of thistype of transformation, including those used for thepreparation of liquid crystalline block [182] andgraft copolymers [183] were reported.

7. Transformations involving Ziegler–Natta

polymerization

Ziegler–Natta polymerization is well known toinvolve a two-stage process. In the first stage analuminum alkyl such as trialkyl aluminum is reactedwith TiCl4 in order to give active b-TiCl3. Alkylradicals, also produced in this reaction, are termi-nated by coupling and give inert products. Subse-quent alkylation of b-TiCl3 then occurs to generatethe titanium species capable of initiating polymer-ization of olefins such as ethylene (Scheme 39).

As the polymerization results in the incorporationof alkyl ligand in the final product, polymericaluminum compounds may conveniently be em-ployed instead of small-molecule analogs to affect

ARTICLE IN PRESS

W Cl CH2 CH Li

W

Cl

Li

CH2 CH

W

Cl

H

Li

CH2 CH+

Scheme 37.

OH

CH2Cl+ HO N N OH

ECH

AMPECH N N PECH

St

PECH PSt PECH

Polymerization

Scheme 35.

OH

CH2Cl+

ECH

AMPhCCHHO

Ph

O

PhCCHO

O

CH2CHOH

CH2Cl

n

hv

+ PhC

O

nCHO

Ph

CH2CHOH

CH2Cl

O

OCH3

O

CHO

Ph

CH2CHOH

CH2 Cl

CH2 C

CH3

C O

OCH3

nCHO

Ph

CH2CHOH

CH2Cl

On mm

Polymerization Ph

Oniumsalts,

Scheme 36.


an anionic-to-Ziegler–Natta transformation processyielding novel block copolymers [184–187]. Byfollowing this strategy aluminum compounds havebeen successfully synthesized by successive alkyla-tion of aluminum halide with living anionic poly-mers as shown in Scheme 40. The method has beenused to prepare block copolymers of St withethylene and acetylene. Notably, extremely lowtransformation efficiencies were obtained. However,the purified block copolymers have potential uses aselectroactive polymers [188].

Agnuri et al. [189] achieved another useful active-site transformation from Ziegler–Natta to radicalpolymerization. Using this transformation, theyprepared alkenic-vinylic type block copolymerscontaining crystalline and amorphous sequences.Ziegler–Natta polymerization was induced withdiethylzinc as the transition complex and peroxygroups were incorporated into crystalline polymerthrough oxidation of the carbon zinc bond.Thermolysis of macroinitiator formed this wayresulted in the generation of a pair of radicals.

ARTICLE IN PRESS

MoO

N

O

MoO

N

O n

MoO

N

O

On

Br

+

+

O

Br

ATRPSt, MA

Block copolymers

Scheme 38.

TiCl4 R3Al+ RTiCl3 R2AlCl+ β TiCl3 R+

+ RTiCl2 R2AlCl+

RTiCl3 CH2 CH2+

Coordination

Insertion

RTiCl2C2H4

RCH2CH2TiCl2

β TiCl3 R3Al

Scheme 39.

3 AlCl3+ ( )3 Al 3 LiCl+Li

Scheme 40.

O2Et M1 ZnEt Et M1 O O Zn O O Et

Et M1 O O EtZnO2 ++

Block copolymer Homopolymer

M2M2

Scheme 41.


Both radicals were capable of initiating the poly-merization of vinyl monomers such as MMA.Therefore, block copolymer formation was accom-panied by the formation of homopolymer arisingfrom the ethoxy radicals (Scheme 41).

Doi and Keii [190] and Doi et al. [191] havedemonstrated the living coordination polymeriza-tion of propylene (P) by a soluble Ziegler–Nattacatalyst composed of vanadium acetylacetonate anddiethylaluminum chloride. The living polypropylene(PP) chain end can be transformed to iodide bytreatment with a solution of iodine in toluene. Incombination with AgClO4 the polymer containingiodide generates a carbocation which initiatescationic polymerization of THF at 0 1C as shownin Scheme 42. However, the blocking efficiency wasnegligible at 20 1C.

Mulhaupt et al. [192,193] have reported severalstudies related to the preparation of block copoly-mers from thiol, maleic acid and hydroxy-functionalPP prepared by a metallocene catalyst.

A facile and inexpensive reaction process for thepreparation of PP-based graft copolymers contain-ing an isotactic PP main chain and severalfunctional polymer side chains was recently de-scribed by Zou et al. [194] and Cao et al. [195](Scheme 43). In this case, a Ziegler–Natta—ATRPtransformation was applied

The same research group also reported thetransformation of metallocene-mediated olefin poly-merization to anionic polymerization by a novelconsecutive chain transfer reaction for the prepara-tion of PP-based block copolymers [196].

The metallocene-ATRP route has been success-fully followed by Matsugi et al. [197] to producepolyethylene-b-poly(methyl methacrylate) (PE-b-PMMA). The block copolymers obtained exhibitedunique morphological features that depended on thecontent of PMMA segment. Moreover, the block


copolymers effectively compatibilized the corre-sponding homopolymer blend at a nanometer level.

A relatively new coordination olefin polymeriza-tion method, degenerative transfer coordinationpolymerization, was recently combined with ATRPto prepare block and graft copolymers with linearPE segments [198–201].

8. Transformations involving group transfer

polymerization

Group transfer polymerization is another[202,203] method for producing well defined acrylicpolymers by silyl ketene acetal activation usingnucleophilic or electrophilic catalysts. This type ofpolymerization involves generation of ketene acetalat the chain end for each monomer unit addition(Scheme 44).

In contrast to anionic living polymerization,group transfer polymerization can be performed at

I2

O On-1

CH2 CH V3+

CH3

CH2 CH

CH

CH2 CH

CH3

O ClO4

Scheme

CH2 CH

CH3

CH2 CH

CH3

CH2x

+

MgCl2/DIBP/

TiCl4

AlEt3/DDS

HCl

80 °C,

1,1,2,2-C2H2Cl4

Scheme

room temperature or above. Preparation of blockcopolymers by utilizing only the group transferpolymerization mechanism is limited to methacry-late type polymers. Both batch process and sequen-tial monomer addition techniques have been used.Block and graft copolymers of methacrylates withnonacrylic monomers using group transfer poly-merization in conjunction with other polymeriza-tion routes can be realized via the transformationapproach. In principle, any polymer, independent ofpolymerization mode, possessing terminal or side-chain silyl ketene acetal groups can act as amacroinitiator for group transfer polymerization.Ester groups are readily converted to silyl keteneacetal macroinitiators by sequential treatment withlithium diisopropyl amide (LDA) and chlorotri-methylsilane [204,205] (Scheme 45).

Typical examples of this procedure were reportedby Ruth et al. [204] and Verma et al. [206]. Theseauthors prepared PIB or poly(alkyl vinyl ether)

AgClO4 ClO4CH2 CH

CH3

I

3

CH2 CH

CH3

O (CH2)4m n

42.

CH2 CH

CH3

CH2 CHx y

CH

CH CH3

Cl

y

ATRPGraft copolymers

St, MMA

43.


oligomer by living cationic polymerization. Thehydroxyl end groups were first converted to estergroups and subsequently the above describedchemistry was applied to afford the silyl keteneacetal macroinitiator. Reaction of the polymericsilyl ketene acetal with MMA in the presence oftetrabutyl ammonium dibenzoate produced a blockcopolymer (Scheme 46).

Although MMA conversion was quantitative, theauthors reported a significant amount of unreactedPIB; hence there are contamination problems. Thecoupling reactions of the two corresponding livinghomopolymers also yield block copolymers withlimited success [207]. Coupling efficiencies up to80% could be obtained. However, both studiesconfirm the potential of transformation reactionsfor preparation of thermoplastic elastomers.

Jenkins et al. [205] applied this approach toprepare St, MMA graft copolymers. In this case,PSt containing a minor portion of residues withfunctional groups served as macroinitiator for thesubsequent group transfer polymerization.

O

O

CH

CH3

CH3

O

C

CH3 CH3

1. LDA

2. (CH3)3SiCl

O C

CH3

CH3

(CH3)3SiO

O

C

CH3 CH3

(CH3)3SiO

C C

O

Scheme 45.

O

OMe

OSiMe3

OMe

PMMA OSiMe3

OMe

+

Scheme 44.

CH2 PIBCHCH2OCC

CH3

CH3

OSi(CH3)3

TBABB

PMMA- b-PIB

Scheme

PMMA–Br samples prepared by group transferpolymerization and subsequent bromination wereused to initiate sequential conventional free radical[208] and ATRP [49] polymerization to yieldtriblock copolymers.

9. Transformations involving the same

polymerization mechanism

Transformations can be achieved not only be-tween different polymerization methods, but also bythe same mechanism using different initiatingsystems. For example, ATRP can be combined withNMP, both being controlled radical polymerizationmethods [209–211]. It should be noted that trans-formation within the same polymerization process isnot limited to controlled radical polymerization.For example, transformations involving vinyl andring-opening anionic or cationic polymerizationsare also possible (Scheme 47, Table 5).

10. Other methods

10.1. Transformations via macromonomer technique

The transformation approach can be applied tothe preparation of segmented graft copolymers viathe macromonomer technique. Since Milkovich[227–229], again from Szwarc’s school, demon-strated the syntheses and applications of a varietyof macromonomers, it has been established that themacromonomer technique is one of the best for thepreparation of well-defined graft copolymers. To-day, such macromonomers can be prepared notonly by living anionic polymerizations but also bynewly developed controlled/living polymerizationmethods which extend the application of thetransformation approach to a wide range ofmonomer couples. In these cases, macromonomerswere prepared by one mode of polymerization and

CH2 CH CH2 O C

CH3

CH3

(CH3)3SiO

MMA

-b-PMMA

C

46.


(co)polymerized by another mechanism as depictedin Fig. 3.

Macromonomer synthesis is usually carried outby initiation or termination by a reagent possessing

Anionic vinylpolymerization

1.BuLi, cyclohexane, 40 °C2.Ethylene oxide, 25 °C3.H+

OH

n

Anionic ring openingpolymerization

1.AlEt3, toluene, 25°C or 70 °C2. D,L or L-Lactide, 70°C 3. H+

OO

O

O

O

n m

Scheme 47.

Table 5

Examples of polymers obtained by transformations involving the

same type of polymerization mechanismsa

Transformation Reference

NMP-ATRP [209–215]

Cobalt mediated—ATRP [216]

DT-ATRP [201]

Anionic vinyl-AROP [217–223]

Cationic vinyl-CROP [224–226]

aAbbreviations: NMP, nitroxide-mediated radical polymeriza-

tion; ATRP, atom transfer radical polymerization; DT, degen-

erative transfer polymerization; AROP, anionic ring opening

polymerization; CROP, cationic ring-opening polymerization.

Initiaton Functionalization Te

Mechanism A

Mechanism B

= Mon

= Mon

I

M

Fig. 3. Mechanistic transformation

a polymerizable end group, provided the propagat-ing species of mechanism A is unreactive towardsthis end group. In some cases, however, thepropagating ends are first terminated by an appro-priate reagent and than converted to polymerizableend groups by organic reactions to exclude thereaction of reactive species with these groups. Themacromonomer and the added low molar massmonomer should be polymerizable by mechanism B.Recent examples of such transformations have beenreported for various living and controlled/livingpolymerization mechanisms (Table 6).

10.2. Coupling reactions

Direct coupling of preformed living blocks (usuallycation and anion or group transfer) also enables theformation of block copolymers (Table 7). A typicalexample of such a coupling process between oppo-sitely charged macroions is presented in Scheme 48for the preparation of polystyrene-b-poly(ethyl vinylether) PSt-b-PEVE.

10.3. Dual polymerizations

Another synthetic scheme to produce blockcopolymers comprised of monomers that are poly-merizable by different polymerization mechanismsis concurrent polymerization in one step. In thisconcept, a single initiator (also called bifunctional,dual, or double-headed initiator) is used to performtwo mechanistically distinct polymerizations, with-out the need of intermediate transformation oractivation steps. Sogah et al. first reported thesynthesis of multifunctional initiators possessing

rmination Functionalization

omer functionality for mechanism B

omer functionality for mechanism A

echanism A

I

via macromonomer technique.


initiating sites for different types of polymerizationand their use in the synthesis of block and graftcopolymers [118,256].

This concept was further developed by Hawker etal. [257] who performed dual living polymerizationsfrom a single initiating molecule without therequirement of additional reaction. The compat-ibility of either NMP or ATRP with living ring-

Table 6

Transformations via macromonomer techniquea

Mechanism A Mechanism B Ref

AROP ATRP [81,230–238]

AROP RAFT [230,232,239,240]

AROP ROMP [170,179,241–246]

AROP CROP [146,247]

CROP ATRP [248,249]

CROP ROMP [250]

ROMP ATRP [198,251]

aAbbreviations (see Table 5): ROMP, ring opening metathesis

polymerization; RAFT, reversible addition-fragmentation trans-

fer polymerization.

Table 7

Summary of coupling reactionsa

Type of coupling Block copolymer Ref.

Anionic–cationic PSt-b-PTHF [252]

Anionic–cationic PSt-b-PEVE [253]

Anionic–cationic PMMA-b-PTHF [254]

Anionic–cationic Poly(glycopeptide)-b-PMeOZO [255]

Group transfer–cationic PMMA-b-PBVE [206]

aPolymer abbreviations (see Tables 1 and 3): PEVE, poly(ethyl

vinyl ether).

CH2 C Li

CH3

sBu

CH2 CH

O

C2H5

HI/ZnI2

toluene

Scheme

opening polymerization of CL was demonstrated bythe synthesis of a variety of well-defined blockcopolymers. The basic strategy followed for the dualpolymerization is demonstrated in Scheme 49.

It is interesting to note that block copolymerswith low molecular weight distributions were pre-pared with either sequence, i.e., living radicalpolymerization or living ring-opening polymeriza-tion, first. Similarly, hydroxy-functionalized ATRPinitiators can be used as bifunctional initiators forthe polymerization of both CL and a variety of vinylmonomers (Scheme 50).

The novel block copolymers obtained had lowpolydispersities and controllable molecular weightsfor both blocks.

Lim et al. [258] used a palladium complex for thecationic polymerization of THF and the ring-opening metathesis polymerization of norbornene.They also demonstrated that even condensation andchain polymerization can be performed simulta-neously in one step. This was achieved by the use ofunimolecular compounds which can simultaneouslyact both as an initiator for chain polymerization andas an end-capper for condensation polymerization.The method provides a simple way to combine ring-opening polymerization, NMP, or ATRP with acondensation polymerization to yield interestingand useful block copolymers [259].

10.4. Combination of polymerization mechanisms by

‘‘click chemistry’’

‘‘Click chemistry’’ has recently been introduced asa new way of categorizing organic reactions that arehighly efficient, modular and selective, and occur

CH2 CH

O

C2H5

PSt- b- PEVE

48.

ARTICLE IN PRESS

HO CH2 CH O N

m St, 125 °Cn CL

O CH2 CH O NCO(CH2)5

O

H

CH CH2 CH O NCH2OCO(CH2)5

O

H

m St, 125 °C n CLAl(OiPr)3

CH CH2CH O NCH2

HOn

n m

m

Al(OiPr)3

Scheme 49.

m MMA, 75 °Cn CL

O CH2CO(CH2)5

O

H CBr3

n CL

n m

HO CH2 CBr3

Et3Al

CBr2 CH2 C BrCH2HO

OCH3

CH3

O

Et3Alm MMA, 75 °C NiBr2/(PPh3)2

NiBr2/(PPh3)2

m

OCO(CH2)5

O

H CBr2 CH2 C BrCH2

OCH3

CH3

On

Scheme 50.


with simple work-up procedures [260]. The 1,3-dipolar cycloaddition of azides to alkynes hasbecome the most popular click reaction, and iswidely used in biology, chemistry and materialscience. By using the efficient click reaction,different architectures of macromolecules such as

dendrimers [261,262], dendronized linear polymers[263], hydrogels [264], supra-polymers [265] andnovel conjugated polymers [266] can be created.Click chemistry strategy has also been implementedfor the preparation of segmented copolymers ofmonomers polymerizable by different mechanisms.

ARTICLE IN PRESS

O

O

Sn(OTf)2

EtOH

n m

O OOH

OO

NN

NO

O

n m

p

N3O

O

p

O

O

O OOH

OO

Scheme 51.

n

NO

O

O

O

O

O

BrO

m

n

mO

BrO

NO

O

O

O

O

+

Toluene reflux

Scheme 52.

Table 8

Star and miktoarm-star block copolymers prepared by mechan-

istic transformationsa

Method A Method B Ref

AROP ATRP [43,45,46,60–62,68,77,83,273–281]

AROP NMP [282]

ATRP NMP [209–213,273,283,284]

CROP ATRP [95,100,110,112,285–287]

CROP RAFT [288]

Cationic ATRP [104]

Anionic Cationic [289]

aAbbreviations (see Tables 5 and 6).


For example, Opsteen and van Hest [267] success-fully prepared PEO-b-PMMA and PEO-b-PSt byusing azide and alkyneend-functionalized homopo-lymers as click reaction components. Here, both PStand PMMA homopolymers were obtained byATRP and postmodification reactions. Emrick etal. [268]. reported the grafting of PEO end-cappedby an azide group onto pendant acetylenic groupsof PCL prepared by ring opening polymerization ofthe corresponding functional monomer with CL(Scheme 51).

In a subsequent work by Riva et al. [269], thefunctional groups of the system were reversed.

ARTICLE IN PRESS

LL,Sn(Oct)2

MMA or StCu(I)Br/PMDETA ATRP

AROP

dendrimer

Scheme 53.


Thus, azide pendant groups of the aliphaticpolyesters, PCL and PLL were reacted with theacetylenic end groups of the PEO to yield corre-sponding graft copolymers. The latter methodologyallowed the click reaction to be conducted undermild conditions and in organic solvents—which isparticularly important for the derivatization orgrafting of unstable PLL.

Another strategy in click chemistry is theDiels–Alder (DA) reaction, [4+2] system, basedon the coupling of a diene and a dienophile[270–272]. The DA reaction has been successfullyapplied to prepare block [270–272] and graftcopolymers [270–272] with PEO segments and vinylpolymers prepared by ATRP or NMP. Thus, ABC-type miktoarm-star terpolymer [270–272] with PEO-PSt-PtBA arms, PEO-b-PSt and PEO-b-PMMA,and PSt-g-PEO via DA reaction of maleimide- andanthracene-end functionalized polymers. The over-all strategy is represented in Scheme 52 by thepreparation of PEO-b-PSt.

11. Star and miktoarm-star block copolymers by a

combination of polymerization mechanism

Star and miktoarm-star block copolymers havearoused interest due to their unique physicochem-ical properties [267]. Star-shaped homopolymerswith well-defined arms can be synthesized usingmultifunctional initiators in living anionic andcontrolled radical polymerizations, especiallyATRP. More recently, the combination of various

living polymerization techniques, such as ATRPand anionic ring opening polymerization (AROP)to synthesize novel star and miktoarm-star blockcopolymers, has attracted much interest since thesecombinatorial methods not only enrich the types ofpolymerizable monomers available but also enablevariable compositions, architectures, and propertiesto be combined in one polymeric structure. Anonexhaustive list of this class of polymers ispresented in Table 8.

A typical example of such systems is representedin Scheme 53 for the preparation of star blockcopolymers from dendrimer initiators by combiningAROP and ATRP [68].

12. Conclusions

Today, uniform polymers with tailored size, blockand graft copolymers, functional polymers, and starand comb-shaped polymers can be produced byliving and controlled/living polymerization techni-ques. By invoking transformation reactions, i.e.,combining different polymerization mechanisms, itis possible to synthesize complex but well-definedpolymeric materials from new and existing mono-mers. We have summarized recent achievements intransformation reactions, with special attention tochemistry using living and controlled/living poly-merization methods to synthesize structurally well-defined block and graft copolymers, including starand miktoarm-star block copolymers By manipula-tion of the methods described in this article, it is


possible to prepare new polymers with controlledsize, which are useful for a variety of applications.In fact, the design and synthesis of structurally well-defined complex macromolecules has become amajor research direction in the field of materialsscience. However, from a practical viewpoint, manyof the transformation methods so far availableexhibit the disadvantage of being multistep synth-eses, which greatly reduces their practical applica-tions. Future research must target the simplificationof transformation reactions that have the potentialto achieve materials with desired structures andproperties.

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Mechanistic transformations involving living and