2,6-diisopropylphenoxy Ligand Modification The Supported Ziegler-Natta Catalysts

The main purpose of this paper is to modify supported Ziegler-Natta catalysts through their reaction with some specific organic compounds,in order to alter the nature and distribution of the active sites(C^*),thereby control the chemical composition distribution(CCD)of ethylene-α-olefin copolymers(or:linear low density polyethylene,LLDPE).Firstly,an industrial supported Ziegler-Natta catalyst(cat-1)is modified by treating it with 2,6 - diisopropyl phenol,getting the modified catalyst cat-2.The two catalysts were used for ethylene and 1-hexene copolymerization in different comonomer concentration.The copolymers were characterized by ¹³C-NMR and GPC.Experimental study found,comonomer effect of cat-2 is weaker than cat-1. In copolymerization with cat-2,the activity reached the maximum value at relatively low 1-hexene concentration.With increasing comonomer concentration, the molecular weight of copolymer catalyzed by cat-2 declines much slower than cat-1.¹³C-NMR analysis showed that the copolymer catalyzed by cat-2 is more uniform in comonomer distribution than copolymer catalyzed by cat-1,especially the catalytic system activated by MAO.Secondly,a supported Ziegler-Natta catalyst TiCl₄/MgCl₂(cat-3)was prepared, then it was modified by 2,6 - diisopropyl phenol,getting the modified catalyst cat-4.The two catalysts were used for ethylene and 1-hexene copolymerization and copolymerization in the presence of a small amount of hydrogen.The copolymers were characterized by ¹³C-NMR and GPC.The experiment results showed that, comonomer effect of cat-4 is weaker than cat-3,as revealed by ¹³C-NMR analysis of the copolymer's soluble and insoluble fractions in boiling n-heptane,the 1-hexene content in the two fractions is much closer in copolymer catalyzed by cat-4 than cat-3,so the copolymer by cat-4 has more uniform comonomer distribution,especially the catalytic system activated by MAO.The decline of activity with hydrogen amount in the polymerization with cat-4 is slower than with cat-3.In the presence of hydrogen,the copolymer catalyzed by cat-4 also showed a more uniform comonomer distribution than cat-3,especially when the catalytic system was activated by MAO.In general,these changes are beneficial to the synthesis of ethylene-α-olefin copolymers with narrower distribution of composition.1-Hexene homopolymerization catalyzed by cat-3 and cat-4 were also studied. The number of the active(C_p)center in the catalytic systems was determined using a method based on quenching reaction by cinnamoyl chloride.The average polymerization activity of active sites of cat-4 is less than cat-3,maybe some active site in cat-3 catalyst is deactivated by 2,6 - Diisopropyl phenol.When adding a small amount of hydrogen to 1-hexene homopolymerization system,C_p of all the catalytic systems decreased,in which cat-4 reduced more significantly.Based on the main results of this work,the mechanism of catalyst modification by phenoxy ligand was discussed,and two possible models of the interaction between the phenoxy ligand and the active centers were proposed.