Vinyl acetate polymerization with ATRP

Depending on the monomer, one needs to adjust the components of the system as well as reaction conditions so that radical concentrations are sufficiently low to effectively suppress normal termination. The less reactive monomers, such as ethylene, vinyl chloride, and vinyl acetate, have not been polymerized by ATRP. Acidic monomers such as acrylic acid are not polymerized because they interfere with the initiator by protonation of the ligands. The car-boxylate salts of acidic monomers are polymerized without difficulty. New ATRP initiators and catalysts together with modification of reaction conditions may broaden the range of polymerizable monomers in the future. 

There are numerous combinations of transition metal and ligand that can be used to tailor the ATRP catalyst system to specific monomers. The ATRP systems are tolerant of many impurities and can be carried out in the presence of limited amount of oxygen and inhibitors [216,217]. This approach is so simple that it has been proposed as undergraduate experiments to prepare block copolymers [218,219]. However, the ATRP catalyst can be poisoned by acids, but the salts of methacrylic and vinylbenzoic acids have been polymerized directly in aqueous media [206]. Also, the use of protecting groups [206], followed by a deprotection step to yield the acids, has been successful in organic media [220]. While ATRP cannot be used to prepared well-defined polymers of vinyl acetate [221 ] as of yet, these goals may be realized with further catalyst development. 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

Other functionalized monomers that were polymerized by ATRP are shown in Table HI. Using ATRP, N-vinylpyrrolidone and hydroxypropyl methacrylamide were successfully homopolymerized when a cyclam was used as the ligand instead of bipy. Methyl aciylate and vinyl acetate were copolymerized and a random copolymer with narrow molecular weight distribution was obtained. Copolymerization of isobuter and acrylonitrile monomers was also successful to prepare alternating copolymers. M alternating copolymer was also obtained when styrene and N-(cyclohexyl)maleimide... 

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... 

A higher value of kgg indicates a high free radical concentration in the system, which can result in a higher degree of chain termination whereas a low kgg indicates a low radical concentration that will slow down the rate of polymerization. Under optimized ATRP conditions, a relatively lower kgg value will be normally maintained, which practically eliminates the undesired chain termination and results in the formation of polymers with narrow molecular weight distributions. A variety of functional monomers such as styrene, acrylates, acrylamides, vinyl acetate, vinyl pyridine, and vinyl pyrrolidone can be conveniently polymerized using this technique [79, 80]. 

In a later work, copolymers of PVC with various grafted chains like polyCbutyl acrylate), PMMA, PS, and poly(methyl acrylate) were synthesized (43). The backbone was a random copolymer of vinyl chloride and vinyl acetate. The carbonyl substituted alkyl halide groups were used as the initiator for the polymerization of these monomers by ATRP. 

The homopolymerization of vinyl acetate with the Atom Transfer Radical Polymerization Method (ATRP) has not yet been successful [267],... 

An advantage of initiating an ATRP with an PH as the transferable group, over conducting a standard RAFT polymerization, is that no new chains are formed by added radical initiators and higher MW copolymers can be prepared. A similar process also allowed preparation of copolymers with vinyl acetate. The procedure also provides the possibility to conduct both ATRP and RAFT reactions concurrendy or sequentially with ATRP initiators containing pseudohalide as one of the transferable groups. 

Indeed, it is now possible to extend the range of monomers incorporated into a block copolymer by conducting sequential RAFT and ATRP polymerizations employing a dual functional bromoxanthate iniferter (initiator-transfer agent-terminator). Poly(vinyl acetate)-1 -PS, poly(vinyl acetate)- 7-poly (methyl acrylate), and poly(vinyl acetate)- -PMMA block copolymers with low polydispersity 1.25) were prepared... 

ATRP is successfully employed in the polymerization of a large variety of vinyl monomers such as styrenes, methacrylates, acrylates, acrylonitrile, and some others [2,9-15]. However, at present, available catalytic systems seem to be unsuitable for the less reactive monomers such as ethylene, olefines, vinyl chloride, and vinyl acetate. In the polymerization of monomers with strong electron-donating groups such as /7-methoxy styrene, some side reactions arising from the involvement of cationic intermediate are observed. Acrylic and methacrylic acids are also not prone to ATRP because they form Cu(II) carboxylates, which are inefficient deactivators. However, hydroxy derivatives such as hydroxyethyl acrylate and hydroxyethyl methacrylate can be polymerized by ATRP. 

The living R-ROP of cyclic ketene acetals was achieved with nitroxy-mediated polymerization (NMP) (29), ATRP (30), and RAFT (31) methods to afford the polyesters with low polydispersities. Recently, it has been reported that the block and random copolymers with vinyl monomers showing low polydispersities could also be obtained by living radical ring-opening copolymerizations (32, 33). 

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