FREE-RADICAL CROSS-LINKING COPOLYMERIZATION - GELATION BEHAVIOR OF MACROMONOMERIC AZOINITIATORS VERSUS MACROCROSSLINKERS

In order to evaluate the gelation behavior of macromonomeric azoinitia tors (MIMs), the free radical polymerization of MIM and macrocrosslink ers (MCRs) have been compared. MIMs were synthesized by the reaction b etween 4,4'-dicyano-4,4'-azovaleryl chloride and poly(ethylene glycol) s PEG-400, -1000, -1500, -2000 and 4-vinylbenzyl chloride. MCRs were o btained by vinylation of both hydroxyl ends of PEG samples with 4-viny lbenzyl chloride. Bulk copolymerization of styrene with MIMs or MCRs i n the concentration range between 2.91 and 31.50 wt.-% at 70 degrees C for 3 h gave PSt-PEG networks. The gelation behavior of MIM was remar kably different from that of MCR. In each case, gel fractions and swel ling ratios of the gels depend on the molecular weights of the PEG seq uences. Critical minimum concentrations for the onset of gelation were determined as 5, 8, 10 and 10 wt.-% for MIMs containing PEG units wit h 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 abo ve. Below those concentrations no network is formed. The critical mini mum values for the onset of gelation were also obtained from home-and copolymerization of partially vinylized MIMs with styrene.