RAFT polymerization block copolymers

A -isopropyl acrylamide (N1PA VI) copolymerization by RAFT 529 JV-isopropyl acrylamide (Ml PAV1) polymerization block copolymers 543, 546 with dithiocarbamale photmnitiaior 465... 

Table 9.9 Block Copolymers Prepared by Macromonomer RAFT Polymerization under Starved-Feed Conditions.380"595...

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

RAFT polymerization has been used to prepare poly(ethylene oxide)-/ /wA-PS from commercially available hydroxy end-functional polyethylene oxide).4 5 449 Other block copolymers that have been prepared using similar strategies include poly(ethylene-co-butylene)-6/oci-poly(S-eo-MAH), jl poly(ethylene oxide)-block-poly(MMA),440 polyethylene oxide)-Moe -poly(N-vinyl formamide),651 poly(ethylene oxide)-Wot A-poly(NlPAM),651 polyfethylene ox de)-b ock-polyfl,1,2,2-tetrahydroperfluorodecyl acrylate),653 poly(lactic acid)-block-poly(MMA)440 and poly( actic acid)-6focA-poly(NIPAM),4 8-<>54... 

I he method of polymerization needs to be chosen for compatibility with functionality in the cores and the monomers to be used. Star block copolymers have also been reported. Mulli(bromo-compounds) may be used directly as ATRP initiators or they can be converted to RAFT agents. One of the most common... 

MA see methyl aery laic MAA see melhacry lie acid inacromonorner RAFT agents 305-8, 501-2. see c/l.w macromonomersi inethacrylic block copolymers symliesis 502 emulsion polymerization 502 transfer constants 307, 502 tnucrtimonomcrs... 

Fig. 16 Various type of elastin-based side-chain polymers (a) ABA block copolymer produced via ATRP, (b) homopolymer produced via ATRP, (c) homopolymer produced via RAFT polymerization...

Synthesis of Block Copolymers by Reversible Addition-Fragmentation Chain Transfer Radical Polymerization, RAFT... 

Water soluble block copolymers consisting of N-isopropylacrylamidc, NIPA, and the zwitterionic monomer 3-[N-(3-methacrylamidopropyl)-N,N-dimethyl]ammoniopropane sulfonate, SPP, were prepared via the RAFT process [82] (Scheme 31). NIPA was polymerized first using AIBN as the initiator and benzyl dithiobenzoate as the chain transfer agent. To avoid the problem of incomplete end group functionalization the polymerization yield was kept very low (less than 30%). The block copolymerization was then performed... 

Fijten MWM, Paulus RM, Schubert US (2005) Systematic parallel investigation of RAFT polymerizations for eight different (meth)acrylates a basis for the designed synthesis of block and random copolymers. J Polym Sci Part A Polym Chem 43 3831-3839... 

The temperature optimization for the RAFT polymerization of EAA revealed an optimum reaction temperature of 70 °C. Block copolymers with a poly(methyl acrylate) (PMA), a poly(n-butyl acrylate) (PnBA), a PMMA, or a poly(A,A-dimethyl aminoethyl methacrylate) (PDMAEMA) first block and a poly(l-ethoxyethyl acrylate) (PEEA) second block were successfully synthesized in an automated synthesizer. The synthesis robot was employed for the preparation of 16 block copolymers consisting of 25 units of the first block composed of PMA (exp. 1 ), PnBA (exp. 5-8), PMMA (exp. 9-13), and PDMAEMA (exp. 13-16) and a second block of PEEA consisting of 25, 50, 75, or 100 units, respectively. The first blocks were polymerized for 3 h and a sample from each reaction was withdrawn for SEC analysis. Subsequently, EAA was added and the reactions were continued for 12 h. The molar masses and PDI values of the obtained block copolymers are shown in Fig. 15. 

NMP is as successful as RAFT polymerization for the construction of block copolymers. A small library of block copolymers comprised of poly(styrene) (PSt) and poly(ferf-butyl acrylate) (FYBA) was designed and the schematic representation of the reaction is depicted in Scheme 10 [49]. Prior to the block copolymerization, the optimization reactions for the homopolymerization of St and f-BA were performed as discussed in this chapter (e.g., see Sect. 2.1.2). Based on these results,... 

Hoogenboom R, Schubert US, van Camp W et al. (2005) RAFT polymerization of 1-ethoxy ethyl acrylate a novel route toward near-monodisperse poly(acrylic add) and derived block copolymer structures. Macromolecules 38 7653-7659... 

Prepared by bulk polymerization, an MIP for the detection of dicrotophos based on the Eu3+ complex has recently been presented [58]. The authors used reversible addition fragmentation chain transfer (RAFT) polymerization followed by ring closing methathesis (RCM) to obtain the star MIP with arms made out of block copolymer. The star MIP containing Eu3+ exhibited strong fluorescence when excited at 338 nm with a very narrow emission peak (half width -10 nm) at 614 nm. This MIP was sensitive to dicrotophos in the range of 0-200 ppb, but showed saturation above this limit. Cross-reactivity of this MIP was evaluated with respect to structurally similar compounds dichlorvos, diazinon and dimethyl methylphosphonate. In these tests no optical response of the polymer was detected even at concentrations much higher than the initial concentration of dicrotophos (>1000 ppb). 

Controlled radical polymerization (CRP) is an attractive tool, because of the resultant controllability of polymerization, and because of it being a versatile method to synthesize of well-defined polymer hybrids. The three main radical polymerization techniques, ATRP, NMP, and RAFT polymerization, have thus been employed. Other techniques, such as the oxidation of borane groups, have also been studied. In general, using CRP techniques, block copolymers can be synthesized from terminally functionalized PO as PO macroinitiator, and block copolymers can be prepared from functionalized PO produced by the copolymerization of olefins with functional monomers. 

Living radical polymerizations in miniemulsions have also been conducted by de Brouwer et al. using reversible addition-fragmentation chain transfer (RAFT) and nonionic surfactants [98]. The polydispersity index was usually below 1.2. The living character is further exemplified by its transformation into block copolymers. 

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