Control of branching vs. cross-linking in conventional free radical copolymerization of MMA and EGDMA using CoBF as a catalytic chain transfer agent

Solution copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) by conventional free radical methodology using a cobalt oxime (CoBF) as a catalytic chain transfer agent allows synthesis at high conversion of fully soluble branched poly (methyl methacrylate)s. Using CoBF levels of 8-32 ppm, mole ratios of MMA/EGDMA up to greater than or similar to 100/10 can be employed in producing branched products without any macrogelation or microgel formation. Higher levels of EGDMA require higher levels of CoBF to inhibit cross-linking, but the reactions are difficult to control and have poor reproducibility. An H-1 NMR analysis confirms the incorporation of EGDMA at a level comparable to that of the comonomer feed and typically 50-70% of these residues are present as branching segments rather than as pendent partially reacted residues. Double detection (DD) and triple detection (TD) size exclusion chromatographs (SEC) have been used to evaluate the molar mass and branching character of the products. Molar mass data from both instruments show good agreement, and products with molar mass distribution curves very similar to those of previous products prepared using dodecanethiol (DDT) as a stoichiometric chain transfer agent can be produced. The maximum level of EGDMA that can be incorporated using CoBF is, however, lower than is the case when DDT is used. The analytical data from the TDSEC instrument allows the slice-by-slice generation of intrinsic viscosity (IV) data and corresponding data for the Zimm branching factor, g', across the molar mass distribution. At a given molar mass of 130,000 g/mol, g' correlates well with the level of EGDMA used in the polymerization feed, and is consistent with the H-1 NMR data showing efficient incorporation of this branching comonomer.