RAFT polymerization transformation

The thiocarbonylthio group can be transformed post-polymerization in a variety of ways to produce end-functional polymers or it can be removed. The presence of the thiocarbonylthio groups also means that the polymers synthesized by RAFT polymerization are usually colored and they possess a labile end group that may decompose to produce sometimes odorous byproducts. Even though the color and other issues may be modified by appropriate selection of the initial RAFT agent, these issues have provided further incentive to develop effective methods for treatment of RAFT-synthesized polymer to transform the thiocarbonylthio groups post-polymerization. 

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. 

Transformations can be achieved not only between different polymerization methods, but also by the same mechanism, using different initiating systems. For example, ATRP can be combined with RAFT polymerization, as both are controlled radical polymerization methods (Scheme 11.41) [170-172]. 

Scheme 30.27 Preparation of macrocyclic PNIPAM by RAFT polymerization of Nl-PAM from an azide-functional chain transfer agent followed by transformation of the polymer chain end with propargyl acrylate...

In addition to ATRP and NMRP, RAFT has also been used in this transformation. As mentioned previously, block copolymers with desired properties can be prepared by careful selection of the CTA, initiator, and monomers. A novel methodology employing this transformation for the synthesis of well-defined AB diblock and ABA triblock copolymers of poly (vinyl alcohol) (PVA) and PEO was recently reported. For this purpose, mono- or difimctional PEOs with xanthate end group were synthesized by anionic polymerization and employed as CTAs in the RAFT polymerization of vinyl acetate (VAc) to yield well-defined PEO-b-PVAc and PVAc-l -PEO-l7-PVAc. Eventually, direct hydrolysis of acetate groups by sodium hydroxide in methanol solution and... 

Living cationic polymerization can be also transformed to RAFT polymerization to synthesize well-defined block copolymers. Very recently, PIB-CTA was prepared by site transformation of hydoxyl-terminated PIB, which was obtained by living cationic polymerization of IB using the TMPCl/TiCU initiation system and subsequent conversion of chlorides into hydroxyl groups. Consequently, RAFT polymerization of MMA or St was mediated by the PIB-CTA resulting in AB block copolymers with narrow polydispersities (Scheme 55). In a very similar report, the PIB-CTA was prepared via click chemistry rather than esterification reaction and used in the polymerization of NIPAM yielding PIB-b-PNIPAM. " ... 

RAFT agent molecule is transformed into the end group of one polymer chain. Hence, the final chain lengths of the polymeric material can easily he controlled via vaiying the ratio of RAFT agent concentration to conversion. It should he noted that the free radical concentration—at least in principle—is not reduced in the RAFT process. Hence, the overall rate of polymerization is imchanged in an ideal RAFT polymerization compared to a conventional free radical polymerization. 

A common feature of all polymers prepared by RAFT polymerization is that they bear a thiocar-bonylthio group at one chain end or both. This limits the industrial application of the RAFT process because the thiocarbonylthio end group gives color and instability (especially under basic conditions) to the polymer. Discuss and compare the various methods that can be used for removal or transformation of the thiocarbonylthio end group. 

The use of the base-catalyzed thiol-isocyanate reaction in the quantitative preparation of m-end functional polymers was also recently reported (Li et al, 2009). Here, RAFT polymerization was used to prepare poly (A, N- diethylacrylamide), the dithioester end group of which was transformed into a thiol moiety by aminolysis. Subsequent end-group modification of the resulting thiol-terminated polymer was achieved in an overnight, ambient temperature and oxygen-free reaction with one of a collection of commercially available isocyanates in the presence of triethylamine (Figure 2.11a). 

For SFRP and RAFT polymerizations, directly using the polymers in the next cationic ring-opening polymerizations without any modification of the terminal group is impossible since the terminal groups are nitroxide and dithioester groups, respectively. Several transformation reaction steps are then required, and low transformation efficiency results. 

The presence of withdrawing groups on Z, which lead to higher transfer coefficients, increases the likelihood of side reactions such as hydrolysis or aminolysis and participation in cycloaddition reactions such as the hetero-Diels Alder reaction with diene monomers and 1,3-dipolar cyclo-addition. This is an important consideration in some RAFT agent syntheses, can be critical to the choice of RAFT agent for specific polymerization conditions e.g., in aqueous media or in emulsion polymerization), and determines the ease of end group transformation processes that may be required post-RAFT polymerization. 

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. 

Low molecular weight or polymeric ATRP initiators have been converted to dithiobenzoate RAFT agents by reaction with phcnylcthyl dithiobenzoate RAFT agent441,655 or by reaction with bis(thiobenzoyl) disulfides under ATRP conditions.483 It is likely that ATRP initiators can be transformed to other forms of RAFT agent by similar methods. 

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. 

RAFT is effective with a wide range of monomers, but distinguishes itself from SFRP and ATRP in that it can polymerize carboxylic acid-containing monomers such as methacrylic acid [46]. The polymerizations are performed at temperatures of 100 °C or less with typical polydispersities in the 1.1 1.25 range under either bulk, solution or emulsion conditions. Initially formed homopolymers can readily be chain extended or transformed into block copolymers by reaction with a second monomer [47]. 

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