Direct Radical Polymerization of Vinyl Ethers: Reversible Addition–Fragmentation Chain Transfer Polymerization of Hydroxy-Functional Vinyl Ethers
2016
Unprecedented controlled radical vinyl polymerization (CRP) of vinyl ethers using reversible addition–fragmentation chain transfer (RAFT) polymerization is reported. In order to overcome the challenge of direct radical polymerization of vinyl ethers, commercial hydroxy-functional vinyl ethers such as 2-hydroxyethyl vinyl ether (HEVE) were subjected to free radical polymerization, generating vinyl polymers without polyacetals obtained by self-polyaddition polymerization. In the case of bulk polymerization using a nonacidic azo-initiator such as dimethyl 2,2′-azobis(2-methylpropionate), conventional free radical vinyl polymerization occurred with sufficiently high number-average molecular weight. For example, poly(HEVE) was produced from the corresponding HEVE monomer with Mₙ = 26 400 in high yield, ≥75%. The resulting polymer was nearly identical to the polymer prepared by living cationic polymerization using the protected monomers except for the steric regularity (meso dyads are 51% and 67% for radical and cationic polymerizations, respectively). Furthermore, cyanomethyl methyl(phenyl)carbamodithioate was found to be an efficient RAFT agent, enabling the CRP of hydroxy-functional vinyl ethers. Under the polymerization conditions, poly(HEVE) macromolecular chain transfer agent (macro-CTA) was prepared. The kinetic studies of RAFT polymerization showed a linear increase of the molecular weight, with up to 50% monomer conversion and relatively low polydispersities (Mw/Mₙ < 1.38). In addition, chain extension experiments including block copolymerization with vinyl acetate and N-vinylpyrrolidone were demonstrated using the resulting poly(HEVE) macro-CTA to confirm the “livingness” of the poly(HEVE).
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