Conventional radical polymerization, gelation

ATRP was applied to the copolymerization of a monovinyl monomer and a divinyl cross-linker to study the experimental gelation behavior. The fundamental features of ATRP, including fast initiation and reversible deactivation reactions, resulted in a retarded gelation and the formation of a more homogeneous network in the ATRP process compared to gel formation in a conventional radical polymerization. The experimental gel point based on the monomer conversion in the ATRP reaction occurred later than the calculated value based on Flory-Stockmayer s mean-field theory, which was mainly ascribed to intramolecular cyclization reactions. The dependence of the experimental gel points on several parameters was systematically studied, including the ratio of cross-linker to initiator, the concentration of reagents, reactivity of vinyl groups, initiation efficiency of initiators, and polydispersity of primaiy chains. 

Autoacceleration in the polymerization of MA poses a serious problem [21-23]. Saini et al. [24] attempted to polymerize MA by using /3-PCPY as the initiator with a view to minimize the difficulties experienced due to this phenomenon. The findings led to the conclusion that -PCPY can be used to obtain 19.5% conversion of MA without gelation due to autoacceleration, which is nearly double the conversion obtained by using the conventional free radical initiator (AIBN) in the same experimental conditions. 

A conventional branched polymer results when a dimethacrylate is copolymerized in a random fashion with monofunctional monomers. However it is necessary to keep the dimethacrylate/initiator ratio less than 1/1 in order to prevent gelation. When the dimethacrylate/lnitiator ratio equals or exceeds a 1/1 ratio, there should be an infinite number of branches with each chain connected to at least two other chains, resulting in a gel. Table,1 shows that gels result when a 2/1 dimethacrylate/initiator ratio is used and the dimethacrylate is randomly copolymerized with a monofunctional monomer. This is true whether the polymerization is free radical or GTP. 

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