Living radical polymerization triblock copolymers

Yoshida, E., and Sugita, A. (1998). Synthesis of poly(styrene-b-tetrahydrofuran-b-styrene) triblock copolymers by transformation from living cationic into living radical polymerization using 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl as a transforming agent. J. Polym. Sci.,... 

The establishment of the living radical polymerization of NIPAM encouraged not only many polymer chemists but also polymer physicists to prepare functionalized NIPAM polymers with various controlled sequences and/or shapes, as discussed in this chapter. In the following parts, block, random, or graft copolymers will be simply designated by the acronym A-B for a diblock copolymer, A-B-A for an ABA-type triblock copolymer, A-B-C for an ABC-type triblock copolymer, A-co-B for a random copolymer, A-g-PNIPAM for grafting of NIPAM segments onto the polymer A. For example, PNIPAM-PEO stands for a diblock copolymer of PNIPAM and PEO. 

Narumi A, Matsuda T, Kaga H, Satoh T, Kakuchi T (2002) Synthesis of amphiphilic triblock copolymer of polystyrene and poly(4-vinylbenzyl glucoside) via TEMPO-mediated living radical polymerization. Polymer 43(17) 4835 840... 

Initiation of MMA polymerization by complexes such as (192) was shown to proceed via a bimetallic bis(enolate) intermediate, arising from the dimerization of a radical anion.478" 80 Such a mechanism481,482 explains why efficiencies with such initiators (calculated from polymer molecular weights) are always <50%. Using a similar methodology, the bimetallic bisallyl complex (198) was shown to polymerize MMA in a living fashion (Mw/Mn 1.1) and triblock copolymers with methacrylate and acrylate segments have been prepared. 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

Mayadunne, R.T.A., et al. 2000. Living polymers by the use of trithiocaibonates as reversible addition— fragmentation chain transfer (RAFT) agents ABA triblock copolymers by radical polymerization in two steps. Macromolecules 33(2) 243-245. 

As indicated above, conventional free-radical poljnnerization is not very well suited for the synthesis of advanced polymer architectures. Even the synthesis of di- or triblock copolymers is hardly possible using this technique. The main reason is the continuous initiation and termination of chains. This results in the early generation of dead polymer chains that no longer participate in the polymerization process. The consecutive addition of monomers as employed in living anionic polymerization for the synthesis of eg poly(styrene-6Zoc -butadiene) will only lead to the synthesis of a mixture of two homopolymers. 

Here, we report on the synthesis and characterization of PMMA-poly(/i-butylacrylate)-PMMA (MBuM) symmetric triblock copolymers, as a new family of thermoplastic elastomers. These compounds have be prepared by a novel route based on controlled radical polymerization [4]. Compared to classical anionic living polymerization [5], this new route, sketched in Scheme la, appears particularly appealing since the triblock copolymers are prepared in a two-step process instead of the usual three steps required in anionic polymerization. In the latter case, as butylacrylate cannot be polymerized via a living process, the MBuM symmetric triblocks have to be synthesized by sequential copolymerization of er butylacrylate (tBuA) and MMA followed by the transalcoholysis of the tBu esters with n-butanol... 

Ishizu K, Ochi K (2006) Architecture of star-block copolymers consisting of triblock arms via a Ai -diethyldithiocarbamate-mediated living radical photo-polymerization and application for nanocomposites by using as fillers. Macromolecules 39 3238-3244... 

Many different approaches have been used to synthesize star-block copolymers including anionic, cationic, radical, and condensation polymerization techniques, and even combinations of them [9]. The majority of the molecules produced thus far were prepared by anionic polymerization procedures. The dominant way of preparing star-block copolymers by anionic polymerization is the coupling of preformed diblock or triblock living copolymer chains with a suitable compound to produce the central linking point. In this way divinylbenzene (DVB) was first used in order for a central core to be created [ 10]. This was achieved by adding a predetermined amount of the divinyl compound to a solution of living diblock chains (Scheme 1). 

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