Synthesis of methacrylate derivatives oligomers by dithiobenzoate-RAFT-mediated polymerization

Reversible addition fragmentation chain transfer (RAFT) was used to synthesize methacrylic acid oligomers and oligo(methacrylic acid)-b-poly(methyl methacrylate) (PMAA-b-PMMA) with targeted degree of polymerization≈10. Characterization is by size-exclusion chromatography (SEC) and electrospray mass-spectrometry. SEC data are presented as hydrodynamic volume distributions (HVDs), the only proper means to present comparative and meaningful SEC data when there is no unique relationship between size and molecular weight. The RAFT agent, (4-cyanopentanoic acid)-4-dithiobenzoate (CPADB), produced dithiobenzoic acid as a side product during the polymerization of methacrylate derivatives. Precipitation in diethyl ether proved to be an easy way to remove this impurity from the PMAA-RAFT oligomers. Both unpurified and purified macro-RAFT agent were used to prepare amphiphilic PMAA-b-PMMA copolymers. Diblock copolymer prepared from the purified PMAA homopolymer had a narrower HVD in comparison to those obtained from the equivalent unpurified macro-RAFT agent. This work shows that while cyanoisopropyl-dithiobenzoate or CPADB are good RAFT agents for methacrylate derivatives, they exhibit some instability under typical polymerization conditions, and thus when oligomers are targeted, optimal control requires checking for the degradation product and appropriate purification steps when necessary (the same effect is present for larger polymers but is unimportant).