Synthesis And Characterization Of Short-Chain Branched Polyethylene With Controlled Structure

In this dissertation,short chain branched polyethylene with controlled structure was synthesized by molecular design method.The molecular weight and short chain content were mainly controlled and systematic structure characterization of the obtained samples was done. Conclusions were summarized as follows:1.A series of polybutadienes with controlled molecular weight and low molecular weight distribution were synthesized via anionic polymerization using sec-BuLi as initiator. The 1,2-structure content was controlled by THF.The molecular weight and micro-structures were investigated by GPC,FT-IR and ¹H-NMR measurements.It was found that the molecular weight ranged from 10000 to 140000 and the molecular weight distribution was less than 1.15.The 1,2-structure content varied from 7.0 to 29.7 mol%.Then the obtained polybutadienes were hydrogenated by p-toluenesulfonyl hydrazide(TSH) under atmospheric pressure and resulted in model ethylene-butene copolymers.The molecular parameters were investigated by ¹H-NMR,¹³C-NMR,GPC and FT-IR.The results showed that the hydrogenation degree approached 100%without side reactions under proper conditions,but the M_w tested from high-temperature GPC was lower.The effects of M_w and branch content on crystallization properties were studied by DSC.It was showed that the melting point and crystallinity of the hydrogenated samples reduced either by increasing the branch content or by increasing the M_w.Finally,high 1,4-structure content polybutadiene was also hydrogenated to synthesize model linear polyethylene.2.Ethylene was copolymerized with 1-hexene via metallocene catalyst Et[Ind]₂ZrCl₂.By regulating the polymerization conditions,such as temperature,1-hexene concentration and Al/Zr ratios,two series of LLDPEs were obtained:(1) M_w was about 100000 and branch content ranged from 0 to 5.5 mol%;(2) M_w was about 50000 and branch content ranged from 7.1 to 15.1 mol%.The molecular parameters and crystallization properties were investigated by FT-IR,¹H-NMR,¹³C-NMR,GPC and DSC techniques.It was found that 1-hexene unit was isolated between ethylene blocks,and when the 1-hexene content was more than 8.7 mol%,HH diads appeared.The melting point and crystallinity decreased with increasing the 1-hexene content of series(1). 3.Two kinds of short chain branched polyethylenes,the blends of metallocene homopolyethylene and ethylene/1-hexene copolymers were tested by rheological characterization.No dependence of the molecular structure and the rheolocical properties was found for the hydrogenated polybutadienes because of the limitation of the samples.The storage modulus,loss modulus and complex viscosity of the ethylene/1-hexene copolymers decreased both as the 1-hexene content of the copolymer increased and the molecular weight decreased.No dependence of the molecular structure and the rheolocical properties was found for the PE blends.