Mechanistic transformations involving living and

38 Pages·2006·0.84 MB·English

ARTICLE IN PRESS
Prog. Polym. Sci. 31 (2006) 1133–1170
www.elsevier.com/locate/ppolysci
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

Corresponding author. Tel.: [phone].
E-mail address:
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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 can be carried out is critically dependent on the
structure and relative reactivity relationship of the
It is a special privilege to be invited to contribute ionic species and the monomers. In fact, only a few
a short review article to the special issue of Progress monomers are suitable for the preparation of block
in Polymer Science on the occasion of the 50th copolymers by anionic polymerization, as is also
anniversary of Michael Szwarc’s discovery [1] of true for cationic polymerization. Furthermore, the
living anionic polymerization reactions, which synthesis of block copolymers between structurally
provided innovative synthetic strategies for pre- different polymers. i.e., condensation and vinyl
paration of a wide range of polymers. Monodisperse polymers, by a single polymerization method is
polymers, as well as macromolecules with controlled rather difficult due to the nature of the respective
architecture, can be prepared by using the living polymerization mechanisms.
polymerization technique. Macromolecules pre- In recent years, the development of polymeriza-
pared by this technique include block and graft tion processes for a high level of control over molar
copolymers, star polymers, macrocycles, and tele- mass, polydispersity and end-group and molecular
chelic polymers that are useful in step-wise poly- architecture has remained a major challenge. The
merization processes. The discovery stimulated rapid development of metallocene polymerization of
attempts at living cationic and radical polymeriza- olefins and controlled radical polymerization
tions, which were realized at much later dates. strongly reflects this trend. In order to extend the
Living polymerization technique is one of the range of monomers for the synthesis of block
most important synthetic routes for the preparation copolymers, a mechanistic transformation approach
of block copolymers. The disadvantage associated was proposed, by which the polymerization me-
with the traditional methodologies for the prepara- chanism could be changed from one to another
tion of block copolymers is the formation of which is suitable for the respective monomers. The
homopolymer as contaminant. Szwarc et al. [2] first pioneering work on the mechanistic transformation
reported a novel chemical methodology for the was originally reported by Burgess et al. [4–6] three
preparation of polyisoprene-b-polystyrene-b-polyi- decades ago. In fact, the very first conceptual
soprene (PIP-PSt-PIP) triblock copolymers, which approach to mechanistic transformation reactions
are free of homo polystyrene (PSt) and polyisoprene was made by Szwarc [7]. He considered the electron
(PIP). The preparation of triblock copolymers transfer from sodium naphthalenide to styrene (St)
linking hydrophilic and hydrophobic blocks was in the following way, as described in his own words:
first reported by Richards and Szwarc [3]. The ‘‘In my naive thinking I imagined styrene to be
control of the sequence of the blocks and their reduced to a radical anion,
individual chain lengths during synthesis led to
systematic investigations of properties as a function
of chain architecture. However, besides high purity
requirements, this revolutionary technique is limited a species acting as a carbanion on one end, and a
to anionically polymerizable monomers. In fact, genuine radical on its other terminus, both capable
some limitations exist even for anionically poly- of initiating polymerization of styrene but by a
merizable monomers. Whether block copolymeriza- different mechanism. The possibility of a simulta-
tion of two anionically polymerizable monomers neous radical and anionic polymerization intrigued
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mey’’. Electron transfer did take place, but in Transformation reactions can be realized mainly
contrast to Szwarc’s initial thoughts, radical ends in two ways: (i) direct and (ii) indirect transforma-
combined almost instantaneously and the polymer- tions. In direct transformation, a propagating active
ization proceeded only anionically. Even so, his first center is transformed directly to another active
proposal indicates the initiative behind the trans- center with different polarity. This transfer occurs
formation approach, in which different polymeriza- through an electron transfer as shown in Scheme 1
tion mechanisms are combined. Indeed, today for the transformation involving anionic and
simultaneous (dual) polymerizations, involving cationic systems.
structurally different monomers, are a well-estab- The shortcoming associated with the direct
lished method in the transformation tool-kit (see transformation, is the short lifetime of propagating
below). sites, particularly radicals. The active center must
As a consequence of the specific dedication of this have a lifetime sufficient to permit transformation.
special issue to the 50th anniversary of the discovery Furthermore, a thermodynamic limitation for a
of living polymerizations, this paper does not aim to successful redox process may result from unsuitable
review all published work on transformation reac- redox potentials of the propagating species and
tions, but rather intends to illustrate their broad oxidant and reductant. The only successful example
versatility by concentrating on living and con- of direct transformation involving living polymer-
trolled/living polymerizations. In this connection, ization methods was reported by Endo for the
the reader’s attention is directed to recent reviews preparation of block copolymers of tetrahydrofuran
[8–10] describing mechanistic transformation ap- (THF) with tert-butyl methacrylate (t-BMA),
proaches using various polymerization methods, e-caprolactone (CL) [11] and d-valerolactone (VL)
including condensation and conventional radical [12], as shown in Scheme 2.
polymerizations. Transformation reactions are clas- From the practical point of view, however, indirect
sified on the basis of interconversion between transformation is more attractive because it can be
propagation mechanisms (Fig. 1). It can be seen
that between the main living and controlled/living
polymerization methods, transformations are acces-
sible in both directions. Scheme 1.
Fig. 1. Mechanistic transformation in living/controlled polymerization methods.
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1136 Y. Yagci, M. Atilla Tasdelen / Prog. Polym. Sci. 31 (2006) 1133–1170
n MeOTf CH3 O O+ TfO- 2SmI2
HMPA
O n-1
CH3
m TBMA O
CH3
O n C O m
CH3 SmI2 O-tBu
n O SmI
2
CH3 C O
m CL
n m
O
Scheme 2.
Initiaton Functionalization Termination Functionalization
Mechanism A Mechanism A
Mechanism B
= Initiator for Mechanism A = Monomer 1
= Initiator for Mechanism B = Monomer 2
Fig. 2. Indirect mechanistic transformation.
performed much more easily and uses various poly- mediated polymerizations (NMP) involve reversible
merization modes. Therefore, the following sections termination of the polymer radical with TEMPO
will essentially concentrate on indirect transforma- and chain growth during the lifetime of the
tions. As illustrated in Fig. 2, indirect transformation polymeric radical as shown in Scheme 3.
usually requires multistep reactions. The stable but Besides organo-tin compounds, several other new
potentially reactive functional group for the second initiators can be used to synthesize designed
polymerization mode is introduced at the chain ends, polymers based on poly(e-caprolactone) (PCL)
either in the initiation or the termination steps of the [17–22]. Among them, metallic alkoxides are parti-
polymerization of the first monomer. The polymer is cularly useful to introduce functional groups
isolated and purified, and finally the functional groups selectively at one chain end of PCL [23–25]. Yoshida
are converted to another species. and Osagawa [26] reported the stable radical
functionalization of PCL by using a specially
2. Transformations involving anionic and controlled designed aluminum alkoxide initiator. For this
radical polymerization purpose aluminum tri(4-oxy-TEMPO), prepared
by the reaction of triethylaluminum with three
2.1. Anionic polymerization to controlled radical equimolar amounts of 4-hydroxy-TEMPO, was
polymerization used as an initiator for the anionic polymerization
of CL (Scheme 4).
Following the pioneering work of Veregin et al. PCL with the TEMPO moiety behaved as a
[13], Georges et al. [14], Solomon et al. [15] and polymeric counter-radical for the polymerization of
Rizzardo [16], special attention has recently focused St, resulting in the quantitative formation of poly(e-
on the use of stable nitroxyl radicals such as 2,2,6,6- caprolactone)-b-polystyrene (PCL-b-PSt). The radi-
tetra methylpiperidine-1 oxyl (TEMPO) in order to cal polymerization was found to proceed in
achieve living conditions in conventional radical accordance with a living mechanism without un-
polymerization. In principle, these nitroxide- desirable side reactions. (Scheme 5)
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K
CH2 CH O N CH2 CH + O N
Scheme 3.
O N OH + AlEt3 O N O
3 Al
O O
n
O
O N O C (CH2)5 O H
n
Scheme 4.
1. BPO, m St,
O O
o
95 C, 3.5 h
O N O C (CH2)5 O H
o R CH2 CH O N O C (CH2)5 O H
2. 125 C
n m n
Scheme 5.
The thermal analysis of the block copolymer trapping chain ends, and so can enhance the
indicated that the components of PCL and PSt were polymerization rate.
completely immiscible and microphase separated. Polybutadiene-b-polystyrene (PB-b-PSt) [28–30],
The incorporation of the TEMPO moiety into polydimethylsiloxane-b-polystyrene (PDMS-b-PSt)
poly(ethylene oxide) (PEO) chain-ends in the radical [31], (PEO-b-PSt) [32] and poly(ethylene oxide)-b-
form was also achieved [27]. In this case, TEMPO- poly(4-vinyl pyridine) (PEO-b-PVP) [33] copoly-
Na was used as an initiator in living anionic mers were synthesized by terminating the corre-
polymerization of ethylene oxide (EO) (Scheme 6) sponding living anionic polymerization with a
under conditions such that the stable nitroxyl suitable TEMPO derivative and subsequent NMP.
radical at the end of the PEO chain could not be Stable nitroxyl radicals can also be incorporated
destroyed. into polymers as side groups. Endo and co-workers
Again, the resulting PEO with a TEMPO moiety [34] copolymerized nitroxyl radical containing ep-
acted as a macromolecular radical trap in NMP of oxide with glycidyl phenylether anionically using
St to give poly(ethylene oxide)-b-polystyrene (PEO- potassium tert-butoxide as initiator (Scheme 7).
b-PSt) with narrow polydispersity. It was found that The ratio of the nitroxyl radical moiety in the
PEO of high molecular weight is less efficient at resulting copolymer can be controlled by the feed
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ratio. Subsequent polymerization of St in the thus formed does not initiate the polymerization of
presence of this polymer is expected to yield graft St, the polymerization of EO was readily accom-
copolymers. plished. PEO obtained in this way possesses
An interesting variation of this approach was TEMPO terminal units and was subsequently used
recently reported by Cianga et al. [35] who as an initiator for NMP of St to give block
demonstrated that the stable TEMPO radical can copolymers (Scheme 8).
undergo a one-electron redox reaction with potas- Atom transfer radical polymerization (ATRP) is the
sium naphthalene. While the TEMPO alcoholate most widely used controlled radical polymerization in
60 °C
O N O-Na+ + n O O N O (CH2)2 O H
n
AIBN, m St, 120 °C
O N O (CH2)2 O H R CH2 CH O N O (CH2)2 O H
n m n
Scheme 6.
O
O N O O O
+ n m
O O O
O
N
O
Scheme 7.
+ +
K +
N
N
OK
O
n
O
N O CH CH2 CH2 CH2 O H N O CH2 CH2 O H
m n n
m
Scheme 8.
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anion-to-radical transformation methodology. The stemming from the decomposition of the initiator
first example was reported by Acar and Matyjaszews- and propagating radicals to chain transfer agents
ki [36] and utilized for the preparation of AB and (RAFT agents), forming an unreactive adduct
ABA type block copolymers. The macroinitiators, PSt radical, followed by fast fragmentation to a poly-
and PSt-b-PIP containing 2-bromoisobutyryl end meric RAFT agent and a new radical. The radical
groups were prepared by living anionic polymeriza- initiates the polymerization. The equilibrium is
tion and a suitable termination agent. These polymers established by subsequent chain transfer–fragmen-
were then used as macroinitiators for ATRP to tation steps. It was shown that PEO containing a
prepare block copolymers with methyl acrylate (PSt- xanthate end group can be used as a macro-RAFT
b-PMA), butyl acrylate (PSt-b-PBA), methyl metha- agent in the polymerization of N-vinylformamide
crylate (PSt-b-PMMA), a mixture of styrene and (NVF) to yield poly(ethylene oxide)-b-poly(N-vinyl
acrylonitrile (PSt-b-P(St-r-AN) and also chain exten- formamide) (PEO-b-PNVF) (Scheme 11) [70].
sion with St (PSt-b-PSt) and PSt-b-PIP-b-PSt) In another case, hydroxy functionalities of PEOs
(Scheme 9). were converted to dithiobenzoyl groups and used as
Other examples of materials prepared from the macro-RAFT agents in RAFT polymerization of N-
anionic polymerization to ATRP are shown in isopropylacrylamide (NIPAM) (Scheme 12). De-
Table 1. pending on the functionality of the initial polymers,
As can be seen from Table 1, the transformation AB and ABC type block copolymers with well-
approach involving the combination of living defined structures were prepared [71,72].
anionic polymerization and ATRP has enabled the Obviously, the most important step of these
preparation of segmented copolymers with an transformations is the modification of the chain
exciting range of structural variety. This way, end into a good leaving group. In order to obtain
multiblock copolymers possessing soft segments quantitatively functionalized macro-RAFT agents
and glassy segments, graft terpolymers, comblike or ATRP initiators, modification of living anionic
block copolymers, stars and dendrimer-like archi- polybutadiene (PB) with diphenylethylene, St and
tectures, and polymer ceramic hybrid materials were haloalkanes has been recently investigated [73,74].
successfully prepared. A very interesting application Lutz and Matyjaszewski [75] utilized the versatility
concerns the incorporation of a fluorescent dye at of combining living anionic polymerization with
the junction point of poly(methyl methacrylate)-b- RAFT to prepare segmented graft terpolymers with
poly(butyl acrylate) (PMMA-b-PBA) copolymer. controlled molecular structure. Anionically pre-
The overall process is depicted in Scheme 10 [40]. pared polylactide (PLL) and poly(dimethylsiloxane)
Recently, reversible addition–fragmentation (PDMS) macromonomers were used in RAFT
transfer (RAFT) polymerization, another controlled polymerization of alkyl methacrylates.
radical polymerization method, has also been used A conceptually different transformation reaction
in this transformation. The mechanism involves the was applied for the preparation of PCL-b-(PMMA-
chain transfer of active species such as the radicals co-PSt)-b-PCL by using iniferter technique in the
CH2 CH Li + CH2 CH C CH2 Li
CH3
O CH3
Ph Br C C Br
CH3
O CH
2 CH O Li
Ph
O CH3
ATRP
CH2 CH O C C Br Block Copolymers
St, BA, MA,
Ph CH3
MMA, AN
Scheme 9.
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Table 1 benzpinacol leads to the formation of polymers with
a
Block copolymers prepared by anionic-to-ATRP transformation iniferter structure in the middle of the chain [76].
The benzpinacolate groups incorporated into the
Anionic segment ATRP segment Ref.
(A) (B) polymer chain then initiate the polymerization
of St and MMA via a controlled radical mechanism
PSt PVP [37]
at 95 1C to yield the desired block copolymers
PIP-b-PSt PSt [38]
(Scheme 13).
PSt-co-PAN PSt, PtBA, PBA, PGA, [39]
PMMA
PMA PMMA [38] 2.2. Controlled radical polymerization to anionic
PIP PSt [40] polymerization
PSt-PB, PSt PMA, PMMA [41]
PFS PMMA [42]
The most widely applied controlled radical
PEO PSt, PHMA, (AB2, AB3, A2B, [43–48]
polymerization method for this particular transfor-
A2B2) PSt and (three-arm)
PSt-b-PtBMA star, block mation is ATRP. This is mainly because of the fact
copolymers, PDMAEMA, that hydroxyl and amino groups, potential initiating
PBMA, PMMEMA
sites for the ring-opening anionic polymerization of
PMMA PSt-b-PtBA, PSt-b-PMMA, [49,50]
certain monomers, are compatible with the ATRP
PBA-b-PSt
of vinyl monomers. Examples of such transforma-
PE-co-PBu PSt, PAcSt [51]
PE-co-PP-b-PEO PHMA [52] tions are compiled in Table 2, and the general
PCL PSt, PDMAEMA, PODMA concept is illustrated in Scheme 14 by the example
block;
of the combination ATRP of vinyl monomers with
PHEMA, PEGMA, PMMA, [53–64]
the ring opening polymerization of lactides [77].
PtBA, PMAA star; PMMA,
PHEMA star, block
copolymers, PHEMA (brush 3. Transformations involving cationic and controlled
copolymer) radical polymerization
PDMS PSt, POEGMA [65,66]
PLL PSt, PMMA, PtBA dendrimer [67–69]
3.1. Cationic polymerization to controlled radical
based star, PMMA, PtBA,
transformation
PBzA (ABA) triblock,
copolymer
Yoshida and Sugita [87,88] described the synth-
a
Polymer abbreviations: PSt, polystyrene; PVP, poly(vinyl
esis of polytetrahydrofuran PTHF possessing a
pyrolidone); PIP, polyisoprene; PMA, poly(methyl acrylate);
nitroxy radical by terminating the polymerization
PMMA, poly(methyl methacrylate); PB, polybutadiene; PBA,
poly(butyl acrylate); PFS, poly(ferrocenyldimethylsilanes); PEO, of living PTHF with sodium 4-oxy TEMPO. The
poly(ethylene oxide); PtBA, poly(t-butyl acrylate); PCL, poly(e- polymer obtained in this way acted as a counter-
caprolactone); PE, polyethylene; PBu, polybutylene; PAcSt, radical in the polymerization of St in the presence of
poly(4-acetoxystyrene); PP, polypropylene; PHMA, poly(hexyl
a free-radical initiator to yield polystyrene-b-poly-
methacrylate); PtBMA, poly(t-butyl methacrylate); PDMAEMA,
tetrahydrofuran (PSt-b-PTHF) (Scheme 15).
poly(dimethylamino)ethyl methacrylate); PMAA, poly
(methacrylic acid); PODMA, poly(n-octadecyl methacrylate); NMP was also extended to azo-containing poly-
PHEMA, poly(2-hydroxyethyl methacrylate); PEGMA, poly meric initiators obtained by cationic polymerization
((ethylene glycol) methacrylate); POEGMA, poly(oligo(ethylene [89]. In this case, o-alkoxyamine PTHF was
glycol) methyl ether methacrylate); PDMS, poly(dimethylsilox-
obtained and upon heating at 125 1C, stable
ane); PLL, polylactide; PBzA, poly(benzyl acrylate); PAN,
polymeric nitroxyl radicals were formed. In the
polyacrylonitrile, PGA, poly(glycidyl acrylate); PMMEMA,
poly(monomethyl ether methacrylate). presence of St, the block copolymers produced had
controlled molecular weight, since termination
reactions were minimized and the equilibrium
between dormant and active species allowed con-
controlled radical polymerization step. Substituted trolled growth (Scheme 16).
tetraphenylethanes represent a class of thermal An alternative route for this type of transforma-
iniferters applicable to the radical polymerization tion was also reported [90]. The living propagating
of many monomers in a controlled manner. The chain end was quenched with previously prepared
initiation of anionic coordination polymerization of sodium 2,2,6,6-teramethylpiperidin-1-oxylate ac-
CL by aluminum triisopropoxide in the presence of cording to the reactions in Scheme 17.
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Ph
CH3 Ph CH3
THF
tBu Si O (CH2)3 Li + tBu Si O (CH2)4 C Li
20 °C
CH3 Dye CH3 Dye
CH3 Ph
n MMA
tBu Si O (CH2)4 C
LiCl, THF, -78 °C
CH3 Dye (PMMA)
CH3 Ph
hydrolysis
HO Si O (CH2)4 C
O
CH3 Dye
Br CH C Br
O CH3 Ph
CH3
Br CH C Si O CH2)4 C
CH3 CH3 Dye
O CH3 Ph
m BA
CH C Si O( (CH2)4 C
ATRP
CH3 CH3 Dye
PMMA- b-PBA
Scheme 10.
Initiation/Propagation
S CH3 O
I NVF PNVF n + EtO C S C (CH2)2 C O PEG
CN
Addition-Fragmentation
PNVFn S CH3 O PNVFn S CH3 O
C S + C (CH2)2 C O PEG C S C (CH2)2 C O PEG
CN
EtO CN
Block Copolymer Formation
CH3 O
m NVF + C (CH2)2 C O PEG PEG b PNVFm
CN
Scheme 11.
In the subsequent step, radical polymerization of efficiency of o-alkoxyamine PTHF was rather poor.
St was carried out with alkoxyamine terminated This was attributed to the relatively slow decom-
PTHF. Although an increase in conversion with position and initiation of alkoxyamine attached to
polymerization time was observed and block unsubstituted methylene groups. In a recent article
copolymers with polydispersities close to those of it was reported that alkoxyamines containing
the prepolymers were readily formed, the initiation an unsubstituted carbon atom are very slow to
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O O O O
O
HO PEG OH HOOC CH CH C PEG C CH CH COOH
O O
CH2 C PEG C CH2
C SH CH CH
S C S COOH
HOOC S C
S S
AIBN C O
NH
CH(CH3)2
O O
CH2 C PEG C CH2
CH CH
C S PNIPAM COOH HOOC PNIPAM S C
S S
Scheme 12.
OiPr
O-R-OiPr
A l
2 PriO OiPr + 3 H O R O H Al
PriO-R-O O-R-OiPr
CL
R=
St, MMA
PCL-b-(PSt-co-PMMA)-b-PCL
95°C
Scheme 13.
decompose and that the a-methyl group is essential Bromo-functionalized PTHFs obtained this way
for the conventional radical polymerization to were used as initiators in ATRP of St, MMA and
proceed with a truly living character [91]. MA to yield AB and ABA type block copolymers.
Cation-to-ATRP or reverse ATRP to form AB Notably, in the case of St and MA, the formation of
and ABA type block copolymers was also per- triblock copolymers was significantly slower.
formed [92,93]. One or two bromopropionyl end It was also reported [94] that PSt with chlorine
groups were introduced on PTHF by using func- termini, synthesized by living cationic polymeriza-
tional initiator and termination approaches in the tion without any additional reaction, was an
ring- opening polymerization of tetrahydrofuran efficient macroinitiator for living ATRP of St,
(Scheme 18). MMA, and MA (Scheme 19).