|240. Szablan, Z.; Ah Toy, A..; Terrenoire, A.; Davis, T. P.; Stenzel, M.H.; Müller, A.H.E.; Barner- Kowollik, C.: Living/Controlled Free Radical Polymerization of Sterically Hindered Monomers: Improving Understanding of 1,1-Disubstituted Monomer Systems, J. Polym. Sci. Part A: Chem. 44, 3692 (2006)|
The sterically hindered, 1,1-disubstituted monomers di-n-butyl itaconate (DBI), dicyclohexyl itaconate (DCHI) and dimethyl itaconate (DMI) were polymerized using reversible addition fragmentation chain transfer (RAFT) free radical polymerization and atom transfer radical polymerization (ATRP). Cumyl dithiobenzoate (CDB), cumyl phenyl dithioacetate (CPDA), 2-Cyanoprop-2-yl dithiobenzoate (CPDB), 4-4-azo-bis(4-cyanopentanoic acid) (CPADB), and S-Methoxycarbonylphenylmethyl dithiobenzoate (MCPDB) have been employed as RAFT agents to mediate a series of polymerizations at 60°C yielding polymers ranging in number-average molecular weight, Mn, from 4500 to 60,000 g mol-1. The RAFT polymerizations of the above hindered monomers displayed hybrid living behaviour (between conventional and living free radical polymerization) of varying degrees depending on the molecular structure of the initial RAFT agent. In addition, DCHI was polymerized via ATRP using a CuCl: methyl benzoate: PMDETA: cyclohexanone system at 60°C. Both the ATRP and RAFT polymerization of the hindered monomers displayed living characteristics, however broader than expected molecular weight distributions were observed for the RAFT systems (PDI ranging from 1.15 - 3.35). To assess the cause of this broadness, chain transfer to monomer constants for DMI, DBI and DCHI were determined (CDMI = 1.4*10-3, CDBI = 1.3*10-3 and CDCHI = 1.0*10-3) at 60°C. Simulations carried out using the PREDICI® program package suggest that chain transfer to monomer contributes to the broadening process. In addition, the experimental results indicate that viscosity has a pronounced effect on the broadness of the molecular weight distributions.