Diblock copolymers using RAFT

Triblock copolymers can be prepared from diblock copolymers by a third monomer addition. They can also be prepared using a bis-funetional NMP or ATRP initiator or a bis-RAFT agent (for examples, see Table 9.13). Symmetrical trithiocarbonates (Table 9.15) should also be considered as bis-RAFT agents in... 

To make further use of the azo-initiator, tethered diblock copolymers were prepared using reversible addition fragmentation transfer (RAFT) polymerization. Baum and co-workers [51] were able to make PS diblock copolymer brushes with either PMMA or poly(dimethylacrylamide) (PDMA) from a surface immobihzed azo-initiator in the presence of 2-phenylprop-2-yl dithiobenzoate as a chain transfer agent (Scheme 3). The properties of the diblock copolymer brushes produced can be seen in Table 1. The addition of a free initiator, 2,2 -azobisisobutyronitrile (AIBN), was required in order to obtain a controlled polymerization and resulted in the formation of free polymer chains in solution. 

Bouhamed, IL. Boufi, S., and Magnin, A., Dispersion of alumina suspension using comb-like and diblock copolymers produced by RAFT polymerization of AMPS and MPEG, J. Colloid Interf.I Sci., 312, 279, 2007. 

It is of obvious interest to explore the use of other polymerization techniques that, being more tolerant to the experimental conditions and monomers, can produce amphiphilie azobenzene BCPs with no need for post reactions. Notably, Su et al. have reeently reported the synthesis of such an amphiphilic diblock copolymer with PAA as the hydrophilic block using reversible addition-fragmentation transfer (RAFT) polymerization (structure d in Fig. 6.2) (Su et al., 2007). Using RAFT, they prepared PAA capped with dithiobenzoate and used it as the macro-RAFT transfer agent to polymerize the hydrophobic azobenzene polymer successfully. It ean be expected that more amphiphilic azobenzene BCPs will be synthesized using the eontrolled radical polymerization techniques (ATRP and RAFT) because of their simplicity, versatility, and efficiency. 

A combination of RAFT and cationic ROP was used to synthesize a series of (poly(methyl methacrylate)](poly (l,3-dioxepane)](polystyrene) 3-miktoarm star terpoly-mers. The synthetic approach involved the sjmthesis of PS functionalized with a dithiobenzoate group, using RAFT polymerization, and subsequent reaction of this group with hydroxyethylene dnnamate in THF (Scheme 73). The hydroxyl group served as the initiating site for the cationic ROP of 1,3-dioxepane in the presence of triflic acid. Finally, the diblock copolymer with the dithiobenzoate group situated between the two blocks was used for the RAFT polymerization of MMA. The... 

The emergence of controlled living radical polymerization techniques has opened a novel era for the synthesis of a vast variety of diblock copolymers consisting of blocks that could not be linked together by using the ionic polymerization techniques. The control of chain transfer reactions and the suppression of unwanted termination are now possible in free radical polymerization techniques through ATRP, RAFT, and NMRP. 

In the case of AB diblock copolymers prepared by the RAFT technique, the order of monomer addition must be taken into account. A characteristic example of such a block copolymer synthesis is demonstrated in Scheme 19. Initially, a poly(N, N-dimethylacrylamide) (PDMA) macro-CTA was prepared, followed by the use of PDMA-CTA as an initiator to polymerize successfully the second monomer N,N-dimethyl vinyl benzy-lamide (DMVBA). The final diblock copolymer is not contaminated with homopolymer. It has been discovered that the reverse approach is impossible, probably due to the slow fragmentation of the intermediate radical or due to the slow initiation efficiency of the intermediate radical (styrenic macroradical). 

Rieger and coworkers [332] reported a surfactant free RAFT emulsion polymerization of butyl acrylate and styrene using poly(A,iV-dimethylacrylamide) trithiocarbonate macromolecular transfer agent. They observed that the polymerizations were fast and controlled with molar masses that matched well the theoretical values and low polydispersity indexes. Monomer conversions close to 100% were reached and the polymerizations behaved as controlled systems, even at 40% solids contents. The products were poly(lV,lV-dimethyl acrylamide)-b-poly(/i-butyl acrylate) and poly(Al,Al-dimethylacrylamide)-b-polystyrene amphiphilic diblock copolymers formed in situ. 

More recently controlled free radical polymerization methodologies have been employed for the preparation of novel smart AB diblock copolsrmers. Nitroxide-mediated polymerization was utilized for the synthesis of sodiiun 4-styrenesulfonate-block-sodium 4-vinylbenzoate block copolymer (133). These strong acid/weak acid species exhibit reversible pH-induced self-assembly, with the sodium 4-styrenesulfonate residues remaining ionized and thns permanently hydrophilic over the useful pH range whereas the sodium 4-vinylbenzoate block can be reversible protonated (the carboxylate residue has a pa s 4.0). The same block polymers can also be prepared via RAFT, albeit with somewhat more control. Other workers reported the preparation of such AB diblock copolymers as well as some analogous amine-based styrenic diblock copolymers, (48) shown in Figure 54. 

H NMR spechoscopy was used by Grignard et al. [175] to prove the controlled features of the RAFT (co)polymeri-zation of 17/,17/,27/,27/-heptadecafluorodecyl acrylate (ACS) with 2-hydroxyethyl acrylate (HEA). Monomer conversion, polymer molecular weight, and the copolymer composition (in the case of the random copolymers) versus time were determined by H NMR analysis of the polymerization medium. The livingness of the PAC8 chains was confirmed by the synthesis of PAC8-(i-PHEA diblock copolymers. 

Use of a monohydroxy PDMA maaoinitiator to initiate the TU-amino-catalyzed ROP of LA allowed the authors to prepare PDMA-b-PLA diblock copolymers (Scheme 17). Double-headed initiators were also used for the purpose of block copolymer synthesis, by combining controlled radical polymerization (RAFT or NMP) and organocatalysis by TUs. 

Chong and coworkers reported the preparation of AB type diblock copolymers by the RAFT process [44,45]. The results show the versatility and convenience of this process. Diblock copolymers PMMA-Zi-PMAA can be prepared directly from MAA monomer using PMMA-S-C(S)Ph as a macrotransfer agent. 

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