Diblock copolymers, RAFT

Table 9.28 Diblock Copolymers Prepared by RAFT Polymerization1...

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. 

ATRP is a powerful synthetic tool for the synthesis of low molecular weight (Dp < 100-200), controlled-structure hydrophilic block copolymers. Compared to other living radical polymerisation chemistries such as RAFT, ATRP offers two advantages (1) facile synthesis of a range of well-defined macro-initiators for the preparation of novel diblock copolymers (2) much more rapid polymerisations under mild conditions in the presence of water. In many cases these new copolymers have tuneable surface activity (i.e. they are stimuli-responsive) and exhibit reversible micellisation behaviour. Unique materials such as new schizo-... 

Recent attempts to prepare 26 by RAFT, however, failed [153]. Double hydrophilic block copolymers of NIPAM and 23e [154], as well as of N,N-diethylacrylamide and 23b [155], were prepared with the CTA benzyl dithiobenzoate, and exhibit LCST and UCST behavior in water. The new polymer 51 is also part of amphiphiUc di- and triblock copolymers [152]. Diblock copolymers with poly(ethylene glycol) methyl ether acrylate, dimethylacry-lamide, or 4-vinylstyrene sulfonate are macrosurfactants with a switch-able hydrophobic block. Triblock copolymers containing additionally 4-vinylbenzoic acid differ in the nature of the hydrophilic part [152]. Near-monodisperse block copolymers of N,N-dimethacrylamide and 49a were synthesized in different ways via macro-CTAs of both monomers as the first step. Such sulfobetaine block polymers form aggregates in pure water but are molecularly dissolved after addition of salt [152,156,157]. 

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. 

Liu, O.Y. Wang, L.S. Schork, F.J. Synthesis of well-defined statistical and diblock copolymers of acrylamide and acrylic acid by inverse miniemulsion raft polymerization. Macromol. Chem. Phys. 2010, 211 (18), 1977-1983. 

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). 

Transformation of free radical polymerization to cationic polymerization is also possible and has been applied to all controlled radical polymerizations, namely, ATRP, NMP, and RAFT (Table 5). The most representative example of this approach is summarized in Scheme 56. A dual initiator containing active sites for both CROP and ATRP is employed first in ATRP generating a macro initiator for the cationic polymer-ization. In this case, PSt macroinitiator was synthesized via ATRP of St initiated by 2-hydroxylethyl 2 -bromobutyrate and consequently utilized in cationic ring polymerization of 1,3-dioxepane (DOP). The AB-type diblock copolymers (PSt-Z -PDOP) that resulted, with narrow polydispersity, indicated that the polymerizations were controlled. 

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. 

Similarly, such AB diblocks may be prepared based on the acrylamido family of monomers. For example, the synthesis of novel AB diblock copolymers comprised of the two anionic monomers sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and sodium 3-acrylamido-3-methylbutanoate (AMBA) have been reported (42,44). By analogy with the styrenic block copolymers, these AMPS-AMBA species also exhibit reversible pH-induced self-assembly by virtue of the fact that the AMBA residues may be reversible protonated, switching the residues from a hydrophilic (high pH) to a hydrophobic (low pH) state. Similar AB diblocks of AMPS with sodium 6-acrylamidohexanoate which also exhibit pH-induced micellization have been reported by Yusa and co-workers (45) RAFT has... 

Most recently the core cross-linking approach was adopted for the preparation of novel nanoassemblies derived from pH-responsive AB diblock copolymers prepared via RAFT (222). Here the A block was the permanently hydrophilic DMA species with the timably hydrophilic/hydrophobic DMBVA forming the B block. In aqueous media, at high pH, these block copolymers form core-shell structures with the DMA residues forming the corona and the DMVBA chains in the core. Addition of a hydrophobic, difimctional, alkyl halide namely 1,4-bisbromomethylbenzene results in this species being sequestered into the hydrophobic core of the micelles where it reactions with the tertiary amine residues via the Menshutkin reaction to yield the core-cross-linked species. Successful core cross-linking was... 

Figure 11.37 General synthetic approaches for the generation of AB diblock copolymers by the RAFT process. (Adapted from Barner, Davis, Stenzel, and Barner-Kowollik, 2007.)...

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