高分子化学浙江大学polymer2-2PolymerizationRate.ppt

* For bulk polymerization at low conversion a plot of the Xn-1against the rate of polymerization Rp (the rate of polymerization can easily varied by the concentration of the initiator), yields: * Termination by disproportionation For ?=1, For disproportionation without chain transfer So we got * 6.2.1 Transfer to monomer For vinyl chloride, CM is around 10-3, so (Etr-Ep) is around 17~63 KJ/mol, so CM increases with temperature It is important to notice that the transfer to monomer reaction cannot be decreased via a decrease in [M], because Xn is independent on the [M] for the transfer to monomer step. The only transfer reaction that cannot be avoided is transfer to monomer. Thus, the maximum upper limit of the MW that can be reached under a given set of reaction conditions is given by the transfer to monomer reaction, assuming the absence of all other transfer events * * 6.2.2 Transfer to initiator For bulk polymerization In addition, the effects on the overall polymerization kinetics and on the molecular weight distribution are small, if low concentrations of initiator are employed. Also unavoidable is the transfer to the initiator molecule used to induce polymerization. * * 6.2.3 Transfer to solvent For solution polymerization There are a few solvents that show significantly higher transfer constants, such as CCl4. The transfer to a solvent molecule is of considerable importance, because solvents are used in high concentrations in most industrial polymerization processes. * 6.2.4 Transfer to polymer At higher monomer conversion, transfer processes to the formed polymer are becoming significant. Interestingly, the transfer to polymer rate constants are considerably higher (by a factor of 10) than those observed for the corresponding monomer. Intermolecular transfer to polymer resulting in so-called long-chain branching. Intramolecular transfer to polymer, (backbiting) leading to short-chain branching (in ethylene polymerizations). * 6.2.5 Transfer to additives T