Block copolymers by free radical polymerization

To produce block copolymers by free-radical polymerization, a radical center must be produced at the end of the chain from where fresh chain growth may take place. Two of the ways by which such terminal radicals can be produced are... 

Preparation of Block Copolymers by Free Radical Polymerization... 

To produce block copolymers by free-radical polymerization, a radical center must be produced at the end of the chain from where fresh chain growth may take place. Two of the ways by which such terminal radicals can be produced are (a) decomposition of peroxide groups introduced as an internal part of a polymer chain backbone or as an end group and (b) breaking of C-C bonds in the polymer chain by mechanical means. More recently, the advent of hving or controlled free radical polymerization has opened up a more versatile route to block copolymers by the free radical process. 

Krstina, J., et al. (1995). Narrow polydispersity block copolymers by free-radical polymerization in the presence of macromonomers. Macromolecules, 2i5(15) 5381-5385. 

It is possible to prepare block copolymers by free-radical initiation, as R. B. Seymour, G. A. Stahl, D. R. Owent, and H. Wood discuss in their chapter. Methyl methacrylate macroradicals were made with peroxide and azo initiators in diluents, and different vinyl monomers were polymerized onto them. Block copolymers of two ethylene imines, one having a long (lauroyl) side chain and one with a short (propionyl) side chain were synthesized by M. H. Litt and T. Matsuda in a two-step cationic polymerization process. Block and random copolymers of episulfides were prepared by E. Cernia, A. Roggero, A. Mazzei, and M. Bruzzone using anionic catalysts of metalated sulfoxides and sulfones. 

Graiver, D. Nguyen, B. Hamilton, F. J. Kim, Y. Harwood, H. J. Block Copolymers Containing Silicone and Vinyl Polymer Segments by Free Radical Polymerization. In Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 445-459. 

The homopolymer and block copolymer macromonomers were copolymerized with MMA by free-radical polymerization in benzene at 60 °C using AIBN as an initiator typical concentration were [MMA]=1.2 mol 1 1 and [macromonomer] =0.020 mol l"1. MMA was completely converted in 18 h and the macromonomers conversion reached more than 70% as determined by lH NMR. Incomplete conversion was explained by steric hindrance. Free-radical copolymerization resulted in high MW graft copolymers with PMMA backbone and relatively rigid, nonpolar poly(P-pinene) branches. 

Styrene and butadiene also form copolymers known as high impact polystyrene, or rubber-modified polystyrene, when the content of butadiene is 10%. This type of material has excellent mechanical properties, and it is widely used in practice for the manufacturing of numerous objects, including parts for household appliances, furniture, etc. Rubber-modified polystyrene is commonly used as wood replacement and also for packaging. The synthesis of this material typically is done by dissolving polybutadiene in styrene monomer, followed by free radical polymerization achieved using a peroxide catalyst. This procedure leads to block or graft type copolymers. 

Several synthetic strategies are used to produce block copolymers containing a cationic block. Because charged monomers are not polymerizable by ionic techniques, the synthesis of the required block copolymers can be carried out by free radical polymerization of ionic vinyl monomers using macroinitiators, by modifying one block of a block copolymer and by coupling of two readily synthesized blocks. 

A last, yet much less clearly defined technique of block copolymer preparation, will also be reviewed. Specifically, block copolymers may be prepared by addition of a second vinyl monomer to occluded "living" macroradicals. Thus by use of the proper technique block copolymers have been reported prepared by free radical polymerization. 

Kim et al. [46,47] reported the synthesis of fluorosilicone block copolymers of poly(perfluoroalkylethyl acrylate)-fc-poly(3-[m s(trimethylsilyloxy)-silyl] propyl methacrylates) (PFA-i>-PSiMAs) by a three-step synthetic approach. In the first step, a PFA macromonomer (PFAM) was made by free radical polymerization. Thereafter, a condensation reaction was applied to prepare the PFAM initiator (PFAMI). Finally, the PFAMI and SiMA were reacted to prepare the PFA-i>-PSiMAs block copolymers. In early studies, synthesis of fluorosilicone block copolymers was reported by Boutevin et al. [48-50]. However, two-step hydrosilylation was carried out to prepare the photo-cross-linkahle fluorinated PDMS as reported by Boutevin et al. [48]. In another study, Luo et al. [51] prepared poly(dimethylsiloxane)- -poly(2,2,3,3, 4,4,4-heptafluorobutyl methacrylate- -poly(styrene)... 

Since the preparation of the first identified block copolymer by Melville [26 ] a large variety of A-B and A-B-A block copolymers were prepared by free-radical polymerization by using as well macroinitiators with active chain ends, either peroxide or azo groups, as polyinitiators, for example polyazoesters [20]. These techniques are stiU used at present for the preparation of different types of polyelectrolyte block copolymers, because charge-carrying monomers are in general not directly polymerizable by ionic techniques. 

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