极少量1,4-戊二烯对典型Ziegler-Natta催化剂催化的丙烯聚合反应及聚丙烯性质的影响

Influence of Tiny Amounts of 1,4-Pentadiene on Propylene Polymerization and Polymer Properties Catalyzed by a Classical Ziegler-Natta Catalyst

以9,9-二(甲氧基甲基)芴(BMMF)为内给电子体的MgCl2/TiCl4催化剂与三乙基铝(TEA)组成典型Ziegler-Natta催化剂体系,催化丙烯本体聚合,考察极少量1,4-戊二烯作为共聚单体对聚合反应行为的影响.结果发现,在[1,4-戊二烯]/[丙烯]共单体摩尔比非常低(<0.1%)的范围内,随着1,4-戊二烯浓度增大,催化剂活性迅速降低,[1,4-戊二烯]/[丙烯]比为0.08%时仅为约初始的1/4.但由特性黏数得到的聚合物分子量对1,4-戊二烯浓度呈现不规则变化,1,4-戊二烯并未使聚合物分子量单调降低,而在极低浓度时反而使其有所提高.凝胶渗透色谱(GPC)表征结果不但进一步确认了聚合物分子量的变化规律,还表明1,4-戊二烯并未使聚合物链发生支化.动态剪切流变测试结果显示极少量的1,4-戊二烯使聚合物的剪切黏度上升,模量提高,松弛时间延长;聚合物的拉伸黏度在拉伸流变测试中呈现相同的规律.通过1,4-戊二烯/氢气在聚合反应中协同作用的探讨,提出了对MgCl2/TiCl4/BMMF-TEA催化的丙烯/1,4-戊二烯反常共聚合行为的解释.

MgCl2/TiCl4 catalyst with 9,9-bis （methoxymethyl）fluorine （BMMF） as internal electron donor has been paired with triethylaluminum （TEA） and used to catalyze propylene polymerization in bulk,to which tiny amounts of 1,4-pentadiene （PD） are added and their influence on the polymerization behavior and polymer properties are investigated. Gel permeation chromatography （GPC） , differential scanning calorimetry （DSC） and dynamic shear rheology are used as the main polymer characterization tools. The results indicate that,within a very low [ PD ] / [ propylene ] molar ratio range （ [ PD ] / [ propylene ] 〈 0.1% ）, the catalyst activity decreases sharply following PD concentration increasing. At a [ PD ]/[ propylene ] of 0.08% , the activity is barely a quarter of that when PD is absent. However, it is found that PD does not cause a monotonous decrease in viscosity-average molecular weight （Mv） as its concentration increases. Matter-of-factly, at the lowest PD concentration an abrupt increase in M is observed for the copolymer as compared to the homopolymer. GPC results further confirm that increase and indicate no branching has occurred to the PP chains due to PD. Results from dynamic shear rheology tests reveal that tiny amounts of PD raise polymer shear viscosity and modulus and prolong the relaxation time, so does the tensile viscosity in extensional flow tests. We argue that a synergism may exist between PD and H2 that is used in the polymerizations as a polymer molecular weight controller （ chain transfer agent） , of which a too low concentration of PD may presume ＂ chain extension＂ for polymer chain growth,while higher PD concentrations will inevitably slow down the rate of chain growth and cause reduction in polymer molecular weight.