硅烷类外给电子体对丙烯-丁烯序贯聚合的调控作用

Regulation of Silanes as External Electron Donors on Propylene/butene Sequential Polymerization

外给电子体(ED)作为负载型Ziegler-Natta催化剂的一个重要组分,在影响α-烯烃的催化活性及聚合物的立构规整性方面发挥着重要作用.本文研究了4种不同结构及电子密度的硅烷类外给电子体[二甲基二甲氧基硅烷(D1)、二丁基二甲氧基硅烷(D2)、二苯基二甲氧基硅烷(D3)及二环戊基二甲氧基硅烷(D4)]对丙烯均聚及丙烯(一段)-丁烯(二段)序贯聚合的影响.结果表明,ED对烯烃聚合的催化活性、活性中心数及活性中心定向能力都具有显著的影响.密度泛函理论(DFT)模拟计算表明,随着ED的空间位阻和电子密度增加,ED在MgCl_(2)表面的吸附能降低,吸附稳定性降低;ED的空间位阻和电子密度增加有利于提高丙烯聚合活性中心的定向能力,当n(D4)/n(Ti)=20时,合成的聚丙烯(PP)中的等规聚丙烯(iPP)组分含量达到92.8%.当n(ED)/n(Ti)=15时,丙烯聚合的聚合速率常数达到最大值;具有更大空间位阻和电子密度的ED使得丙烯-丁烯序贯聚合的活性中心具有更强的定向能力,ED对丁烯(二段)聚合活性及聚丙烯/聚丁烯合金(PBA)中等规聚丁烯(iPB)组分的熔点影响更显著.

The influences of four kinds of silanes as external electron donor(ED)with different structure and electron density[dimethyl dimethoxysilane(D1),dibutyl dimethoxysilane(D2),diphenyl dimethoxysilane(D3),dicyclopentyl dimethoxysilane(D4)]on propylene homopolymerization and propylene(first stage)-butene(second stage)sequential polymerization were investigated.The results showed that ED had significant effects on the catalytic activity,number of active centers{[C*]/[Ti]}and the stereoregularity of active centers in olefin polymerization.Density functional theory(DFT)simulations showed that with the increase in steric hindrance and electron density of ED,the adsorption energy of ED on the surface of MgCl_(2) decreased,so the adsorption stability decreased.The increase in steric hindrance and electron density of ED was conducive improving the stereoregularity of the active centers.When n(D4)/n(Ti)=20,the content of isotactic polypropylene(iPP)fractions in polypropoylene(PP)reached as high as 92.8%.When n(ED)/n(Ti)=15,the rate constant for chain propagation of propylene polymeriza⁃tion reached the maximum value.The ED with greater steric hindrance and electron density enabled the active center of the sequential polymerization of propylene-butene with greater stereoregularity,ED had more significant effect on the polymerization activity of butene(second stage)and the melting point of the isotactic polybutene(iPB)fractions in polypropylene/polybutene alloys(PBA).