Long -chain branching in ethylene polymerization using combined metallocene catalyst systems.

摘要

The development of homogeneous and supported metallocene catalysts has revolutionized the polyolefin industry. These single-site catalysts produce ethylene homo- and copolymers with properties that are different from the ones of traditional polyethylenes synthesized with free radical initiators or Ziegler-Natta catalysts. With the advent of monocyclopentadienyl metallocenes (constrained geometry catalysts, CGC), the production of polyolefins with long-chain branches became possible. These catalyst systems can produce polyethylenes with both excellent physical properties and improved melt behavior. The improved processability is believed to be the result of significant amounts of long-chain branching in the polymer.;The concept of combining two metallocene catalysts to manipulate long-chain branching (LCB) in ethylene polymerization was introduced. Using mathematical modeling the feasibility of the idea was investigated and the characteristics of an efficient system were predicted. Polymerization experiments were performed to evaluate the model predictions and to examine the effects of different reaction components on the microstructure of the synthesized polymer. Excellent agreement between experimental data and model predictions was observed. The structure of the branches in CGC polymerization systems was also investigated using a Monte-Carlo model.;The precipitation mechanism in Crystallization Analysis Fractionation (CRYSTAF) was investigated and a mechanism for the separation was proposed. Copolymer chains were simulated using a Monte-Carlo model and the precipitation was simulated according to the proposed mechanism. Good agreement between experimental and simulated CRYSTAF curves verifies the validity of the proposed fractionation mechanism.;A general model for the polymerization of vinylic monomers with free-radical initiators was developed. In this model transfer to polymer as the source of branching with the potential of gelation was considered. The results of this simulation provided information about the distribution of branching and time variations of average molecular weights as well as amounts and molecular weights of chains with different branching contents before, during, and after gelation.