Kinetic studies of the injection of comonomers during polymerization of ethene and propene with MgCl₂‐supported Ziegler‐Natta catalysts

Abstract

The effects of injecting ethene, propene, 1-hexene, 2-methylpropene, 3,3-dimethyl-Ibutene, cis-2-butene, trans-2-butene, 2,3-dimethyl-2-butene, cyclopentene, styrene, 1,3butadiene and 1,4-pentadiene during polymerizations of ethene and propene have been investigated. Catalysts used were a supported MgC12/EB/TiC14 ballmilled catalyst and precipitated MgCl2/2-EH/TiCl4/DIBP catalysts activated with triethylaluminium in presence of EB. (EB = ethyl benzoate, 2-EH = 2-ethylhexanol, DIBP = diisobutyl phthalate).The polymerizations were performed at 50°C, in a heptane slurry at 1 atm (1.00 -lo5 Pa) total pressure.The instantaneous polymerization rate was observed by continuous monitoring of the monomer consumption, the time resolution of the measurements being less than one second. Two different kinetic effects can be observed after injection of the comonomer. First there is an immediate rate change, usually a rate reduction. Then a secondary effect may follow, which is a slow, and sometimes significant and persistent, activation. The two effects obviously have different causes, and for none of these we found single causes that can explain all the observations. The strongest immediate effect was observed after addition of dienes and indicates a direct association of the comonomer to the active center. A marked nonlinearity in the response may hint to inhomogeneity in the structural environment of the active centers. Addition of a-olefins during ethene polymerization gave the strongest secondary effect. In general the immediate effects were stronger during propene polymerization than during the polymerization of ethene. A significant difference in the effects of cis-2-butene and trans-2-butene in the polymerization of propene indicates that steric effects may be important.

Multicenter nature of titanium‐based Ziegler–Natta catalysts: Comparison of ethylene and propylene polymerization reactions

“…In these experiments, one of the monomers was polymerized for a significant period of time; then it was removed from the reactor and replaced with another monomer. This technique is often used to evaluate the effects of a monomer change (or the use of monomer mixtures) in polymerization kinetics 20, 35–43. The main rationale of the sequential polymerization technique for our purposes is based on the finding that various centers greatly differ in stability (Fig.…”

Condensed Mode Cooling for Ethylene Polymerization: Part II. The Effect of Different Condensable Comonomers and Hydrogen on Polymerization Rate

“…In their review of the relevant literature of the time, Karol et al noted that this “comonomer effect” seemed to be wide spread and observed for many different types of catalysts, including titanium and vanadium‐based Ziegler–Natta catalysts, chromium catalysts, as well as metallocenes. Several explanations have been offered for these observations, including disintegration of the particles, increased monomer diffusion, displacement of absorbed or complexed molecules, activation of dormant sites by comonomer, an increase in the propagation rate, or formation of new active sites by comonomer …”

“…Several explanations have been offered for these observations, including disintegration of the particles, increased monomer diffusion, displacement of absorbed or complexed molecules, activation of dormant sites by comonomer, an increase in the propagation rate, or formation of new active sites by comonomer. [16][17][18][19][20][21][22][23][24] The complexity of the situation can be understood by considering solution polymerization as an example. The comonomer effect has been shown to be a function of the type, quantity, and chain length of the α-olefi n in question.…”

Influence of support friability and concentration of α‐olefins on gas‐phase ethylene polymerization over polymer‐supported metallocene/methylaluminoxane catalysts

“…11,12 The comonomer enhancement has been attributed to physical and chemical effects. The physical effects include the fracturing of the catalyst particle to expose new active sites [13][14][15] and the enhanced diffusion of monomer molecules through the semicrystalline ethylene/a-olefin copolymer encapsulating the catalyst particles. 16 Chemically, a-olefins are thought to participate in the formation of new catalyst sites and/or the activation of dormant sites 17,18 and in the increase of the propagation rate constant.…”