Kinetics And Mechanism Of Olefin Polymerization With Supported Z-N Catalyst In The Induction Stage

Polyolefins are the most renowned material in these days because of its great advantages over the other synthetic polymers in words of cost performance and efficiency of large-scale production.It is extensively used in our daily routine,and now it is considered a necessary part of our lives.The substances which enable polyolefin production are Ziegler-Natta catalysts,among which supported TiCl4/MgCl2 type Ziegler-Natta catalyst is the most important and widely used class.A lot of studies have been done more than three decades on fundamental area there are still many unsolved problems related to mechanism of the catalysis process.Among these problems,the formation or initiation of catalytic centers in the first few minutes of olefin polymerization is especially important,as it will profoundly influence kinetics,particle morphology and polymer chain structure of the whole polymerization process.Mechanistic studies on comonomer effects,namely,influences of comonomer on catalytic activity and polymer structure by adding a comonomer into olefin polymerization system is important for olefin copolymerization processes,but the past results showed strong debates.This dissertation mainly contains the following sections.Chapter 1 is a review on active centers and kinetics of olefin copolymerizations via MgCl2 supported Ziegler-Natta catalyst.There are many related research reports in literature,very few of them made direct micro-kinetic studies based on experimental tracing of the active center concentration([C*]),owing to lack of reliable and convenient methods of counting the active centers and their dynamic changes in the polymerization process.Chapter 2 describes the kinetics of ethylene polymerization with a TiC14/MgCl2-type Ziegler-Natta catalyst.Time-dependent changes of polymerization activity and concentration of active centers([C*])in the first 5 min were determined.Initiation of the active centers was found to proceed in two stages.In the first stage,[C*]/[Ti]quickly rose to about 1%in less than 30 s and then remained stable in the subsequent 60 s.Then the value started to increase again,forming the second buildup stage.The polymerization activity was found to change roughly in parallel with the change in[C*]/[Ti].Changes in the polymer/catalyst particle morphology and polymer molecular weight distribution with polymerization time were also analyzed.Moreover,a mechanistic model was proposed to explain the two-stage kinetics:initiation of active sites on the outer surface of catalyst particles takes place in the first stage,and initiation of active sites buried inside the particles takes place in the second stage.These buried sites are released when the catalyst particles are fragmented by the expanding polymer phase.Chapter 3 is about copolymerization of ethylene-propylene with a TiC14/MgC12 type Z-N catalyst conducted for different durations from 30 to 600 s,and changes of polymerization rate,concentration of active centers([C*])and copolymer chain structure with time were traced.The copolymerization rate decayed with time,but[C]/[Ti]increased in the same period.This was attributed to release of more active sites through disintegration of catalyst particles by the growing polymer phase.Ethylene content of the copolymer quickly decreased in the span of 30-90 s,meaning that the active centers activated in the reaction process have stronger ability of incorporating propylene than those activated at the very beginning.The copolymer samples were fractionated into two parts,namely n-heptane soluble fraction(random copolymer)and insoluble fraction(segmented copolymer with high ethylene content).With continuation of the copolymerization,active centers producing the random copolymer chains increased much faster than active centers producing the segmented copolymer chains,and became the dominant centers after 120 s.Consequently,proportion of the soluble fraction sharply increased with time.All these results indicate that the active centers located on the external surface of catalyst particles are highly different from those buried inside the particles.Chapter 4 describes kinetic study on short duration propylene polymerization with a TiCl4/MgCl2 type Z-N catalyst.By tracing change of[C']/[Ti]with time in the polymerization period of 0-1200 s,two induction stages can be roughly divided.The[C]/[Ti]increase can be explained by gradual release and activation of active sites that are buried inside the particles.These buried sites are released in catalyst fragmentation by the expanding polymer phase.Study on the morphology of nascent polymer particles showed that the particles produced after no more than 120 s polymerization contain large number of tiny pores and cracks,but longer polymerization time lead to filling up of most tiny pores and cracks.The apparent chain propagation rate constant(kp)decreased with time in the period of 120-1200 s,which can be explained by increasing diffusion barrier caused by morphology changes of the polymer/catalyst particles.The kp value increased with time sharply in the initial 120 s of polymerization,meanwhile the polymer molecular weight also increased evidently.Chapter 5 describes the ethylene-propylene copolymerization at different[E]/([E]+[P])monomer feed ratios with a TiCl4/MgCl2 type Z-N catalyst for very short duration(30 s).Changes in copolymerization rate,[C*],propagation rate constants of both ethylene and propylene insertions and the copolymer composition with the monomer feed ratio were determined.Mechanism of comonomer effects was studied by comparing the copolymerization results with the kinetic parameters of ethylene and propylene homopolymerizations with the same catalyst and at the same conditions.The number of active centers([C*]/[Ti])was found to gradually decrease from 0.98%of the ethylene homopolymerization to 0.13%of the propylene homopolymerization system when[E]/([E]+[P])ratio was gradually lowered from 1 to 0.In other words,[C*]/[Ti]of the copolymerization systems fell in between those of the two homopolymerization systems.kp of ethylene insertions was remarkably increased by introducing small amount of propylene in the polymerization system,and kept at high level when[E]/([E]?[P])was lowered from 0.82 to 0.19.On the other hand,k.p of propylene insertions was continuously enhanced when[E]/([E]+[P])was raised from 0.19 to 0.82.The effect of ethylene on propylene reactivity followed a different mode as the effect of propylene on ethylene reactivity.Melting temperature of the copolymer increased with increase of propylene content when[E]/([E]+[P])decreased from 0.82 to 0.49,meanwhile randomness of the copolymer continuously decreased with decrease of[E]/([E]+[P])ratio.With increase of[E]/([E]+[P])ratio from 0.49 to 0.82,molecular weight distribution of the copolymer shifted to the high molecular weight side,and the polydispersity index became narrower.The low molecular weight shoulder peak in MWD of ethylene homopolymer disappeared in MWD of the copolymers produced at[E]/([E]+[P])= 0.49-0.82.Theoretical models for the catalyst structure,kinetics of the catalysis process and nature of the comonomer effects have been proposed and discussed based on the unprecedented experimental evidences collected in this work,which can promote more reasonable and comprehensive understanding of olefin polymerization with MgCl2?supported Z-N catalyst.