Three-Dimensional Network Formation in the Radical Copolymerizations of tert-Butyl Methacrylate with Polyethylene Glycol Dimethacrylates

As part of our continuing studies concerned with the elucidation of reaction mechanism and the control of the network formation in the radical monovinyl-divinyl copolymerizations, this article deals with the radical copolymerizations of tert-butyl methacrylate (tBMA) with various polyethylene glycol dimethacrylates [CH₂=C (CH₃) CO (OCH₂CH₂) _nOCOC (CH₃) =CH₂ (n= 1, 2, 3, 4, 9, and 23) (PEGDMA); the effect of bulky tert-butyl groups on the three-dimensional network formation processes was examined in detail by comparing with our previous results of methyl methacrylate (MMA) - PEGDMA copolymerizations. Thus, in MMA - PEGDMA copolymerization systems, the primary chain length of the resulting prepolymers increased significantly with an increase in the oxyethylene unit number, n, of PEGDMA⁷⁾ , while the primary chain length dependency on n was quite small in tBMA-systems as the highest value was only 1.8 times of the one obtained at n=1. Actual gel points were observed at higher conversions in tBMA-PEGDMA copolymerizations ; apparently, gelation was delayed. The deviation of actual gel point from theoretical one became greater in tBMA-systems. The remarkably enhanced deviation with an increase in n was observed in MMA-systems, while the tBMA-systems showed a reversed tendency as was a bit reduced deviation. These results are ascribed to a suppressed occurrence of intramolecular cyclization accompanied by the loop structure formation⁷⁾ , intermolecular crosslinking, and intramolecular crosslinking as a reflection of a lessened flexibility of polymer main chain and a steric effect due to the bulky tert-butyl groups. Some supporting evidences are obtained from the correlation curve between molecular weight and elution volume and the conversion dependence of molecular weight distribution curves determined by SEC-MALLS, and also the correlations of both r.m.s. radius of gyration and second virial coefficient with molecular weight by light scattering measurements.