Free radical crosslinking copolymerization. Gelation behavior of macromonomeric azoinitiators versus macrocrosslinkers

In order to evaluate the gelation behavior of macromonomeric azoinitiators (MIMs), the free radical polymerization of MIM and macrocrosslinkers (MCRs) have been compared. MIMs were synthesized by the reaction between 4,49-dicyano-4,49-azovaleryl chloride and poly(ethylene glycol)s PEG-400, -1000, -1500, -2000 and 4-vinylbenzyl chloride. MCRs were obtained by vinylation of both hydroxyl ends of PEG samples with 4-vinylbenzyl chloride. Bulk copolymerization of styrene with MIMs or MCRs in the concentration range between 2.91 and 31.50 wt.-% at 708C for 3 h gave PSt-PEG networks. The gelation behavior of MIM was remarkably different from that of MCR. In each case, gel fractions and swelling ratios of the gels depend on the molecular weights of the PEG sequences. Critical minimum concentrations for the onset of gelation were determined as 5, 8, 10 and 10 wt.-% for MIMs containing PEG units with molecular weight 400, 1000, 1500 and 2000, and 6, 12, 18 and 19 wt.-% for MCRs containing PEG units with the same molecular weights as above. Below those concentrations no network is formed. The critical minimum values for the onset of gelation were also obtained from homoand copolymerization of partially vinylized MIMs with styrene.