Cationic coordination polymerization propagation

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

Free-radical polymerization. No matter whether the propagating centers are free radicals, anionic, cationic, or coordinated, the propagation rate is equal to the sum of the consumption rates of the two monomers, given by eqns 10.93 ... 

Because of the nature of the active species, coordination polymerization has been classified as ionic polymerization, which follows the polyaddition mechanism s characteristic steps, in the growing of the polymeric chain initiation, propagation, and termination. As for the initiation step, the ionic active species is produced by the reaction between the catalyst and cocatalyst. Usually, the catalysts are actually precursor catalysts or precatalysts, which become the real cationic active species after the activation or reaction with the cocatalyst (Fig. 5.8). 

Figure 5.9 outlines the steps for the chain polyaddition mechanism involved in the coordination polymerizations for any kind of active species initiated through different cocatalysts. The counteranion species was suppressed for practical representation of the active site. Once the cationic species is created, it starts the growth of the polymeric chain through continuous addition of monomer. The propagation step is forward described in Figure 5.9 according to the most accepted reaction cycle proposed by Cossee and Arlman, which is known as the Cossee-Arlman mechanism [51]. 

This TOO polymerization with the chelate catalyst no doubt involves coordinate propagation and is the first such case of coordinate propagation of an oxetane. Heretofore, oxetanes have been cationically polymerized. There are several reasons for concluding that this chelate catalyst system is a coordination polymerization. First, the very high molecular weight obtained at elevated temperature with a low rate is characteristic of coordination polymerization rather than of cationic polymerization. Also, Vandenberg s work on the cis- and trans-2,3-epoxy-... 

Polyepichlorohydrin is manufactured by a continuous solution polymerization process using aluminum-based catalysts with base monomers of epichlorohydrin (ECH), ethylene oxide (EO), and AGE. A cationic coordination mechanism is suggested as the chain propagation mechanism. The commercial polymer is 97%-99% head-to-taU and has been shown to be stereo random and atactic. The CO, GCO, ECO, and GECO polymers are amorphous and linear [1]. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

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