Norbornenyl-based RAFT agents for the preparation of functional polymers via thiol-ene chemistry

A synthetic platform for the preparation of various norbornenyl (Nb) containing reversible addition fragmentation chain transfer (RAFT) agents has been developed. The design of the chain transfer agents (CTAs) is based on the desymmetrization of alpha,alpha'-dibromo-p-xylene by monosubstitution with an alkoxide anion and subsequent replacement of the residual bromine atom in the benzylic bromide to create several series of RAFT CTA including trithiocarbonate, xanthate and dithiobenzoate CTAs, allowing for the possibility to introduce other functional groups besides Nb, such as an allyl group. While a norbomene functionality was chosen as most reactive functional group toward thiols in radical-mediated thiol-ene chemistry, an ally] group was introduced for the sake of direct comparison of the double bond reactivity in the thiol-ene reaction. Control of the radical polymerization of acrylates, styrene and vinyl acetate has been achieved by using this novel family of CTAs. The results indicate that the Nb group remained intact at low monomer conversions (e.g., below 50% for styrene and vinyl acetate, below 30% for acrylates) and at optimal reaction temperatures (e.g., 70 degrees C for styrene and vinyl acetate, 62 and 65 degrees C for 1-ethoxyethyl acrylate and methyl acrylate, respectively), while the monomer-to-CTA ratio was kept high. Polymers with high end-group fidelity were modified with a series of thiol-containing compounds, leading to alpha-semitelechelics with different chain-end structures. While allyl-containing polymers exhibited a significantly lower reactivity, modification of the Nb-containing semitelechelics was rapid and fully accomplished under the same reaction conditions. However, for the given conditions, dodecanethiol and benzyl mercaptan showed a lower reactivity toward Nb-containing polymers, as evidenced by the obtained modification efficiency of 70% and 45%, respectively