Molecular Weight Bimodality of Ethylene/1-Butene in A Two-step Polymerization Process: Effects of Polymerization Conditions

The molecular weight and molecular weight distribution were adjusted by controlling polymerization process to meet bimodal poly(ethylene-co-1-butene) using Ziegler-Natta catalyst. The process was carried out through two-step poly- merization in a semi-batch reactor where in the first step, a low molecular weight homopolymer of ethylene was produced in the presence of hydrogen and in the next step, the high molecular weight copolymer of ethylene with 1-butene was produced after the hydrogen was removed from the reactor. The effects of hydrogen and comonomer concentrations and the polymerization time for the first and second steps were investigated. Although, the second step was run in the absence of hydrogen, the results indicate that shifting the reaction from the first step to the second step leaves potentially some effects on the active centres. The higher hydrogen concentration decreased the rate of polymerization and molecular weight in the first step while, it indirectly raised the rate of polymerization and consequently the yield of reaction at the second step considerably. Increased comonomer concentration in the second step reduced the average molecular weight and increased the melt flow rate while slightly increased the rate of polymerization. It is found that the effect of comonomer concentration on the rate of copolymerization in the second step is lowered by the increase of hydrogen concentration at the first step.