Crosslinking Kinetics in Free-Radical Copolymerization

Abstract

<p>A new kinetic theory for free radical copolymerization with long chain branching and crosslinking is proposed. This kinetic theory accounts for the history of the generated network structure as well as for all of the important elementary reactions in free radical polymerization. The present theory can be used to make calculations of various important properties such as onset of gelation, weight fraction of sol and gel, number- and weight-average chain length of sol fractions, crosslinking density in sol and gel fractions, etc. Since free radical polymerization is kinetically controlled, each primary polymer molecule experiences a different history of crosslinking and cyclization. The present theory proves the existence and permits the calculation of the crosslinking density distribution, although all statistical models which assume an equilibrium system inevitably employ the assumption that the crosslinking density is the same for all chains. The existence of a crosslinking density distribution with a significant variance is an important feature of the present kinetic theory stating that polymer networks synthesized by free radical polymerization are inherently inhomogeneous on a microscopic scale. This theory reduces to the Flory/Stockmayer theory under Flory's simplifying assumptions and may therefore be considered a general mean-field theory.</p> <p>The present theory was successfully applied to the copolymerization of methyl methacrylate/ethylene glycol dimethacrylate, and acrylamide/N,N'-methylene-bis-acrylamide. In real systems it was found that the effect of cyclization (intramolecular reactions) and the decreased reactivity of pendant double bonds relative to the monomeric double bonds are important.</p> <p>This new kinetic theory should assist one to design superior quality network polymer systems and it can also be used to control various polymerization processes. It was found that if branches are formed by chain transfer to polymer the crosslinking density is always higher in continuous stirred tank reactors (CSTR) than in batch reactors, however, this is not true in general for vinyl/divinyl copolymerizations. The variance of the crosslinking density distribution in CSTR's is large due to their broad residence time distributions.</p>