Catalytic chain transfer reaction

It should be noted that the cobalt catalyst cannot be used to effectively regulate the polymer molecular weight in conventional emulsion polymerization. The cobalt catalyst molecules are primarily present in the emulsified monomer droplets initially. A small proportion of the catalyst may also reside in the monomer-swollen micelles (if present). Transport of the cobalt catalyst molecules from the monomer droplets to the growing latex particles stemming from micellar nucleation or homogeneous nucleation is prohibited during polymerization. Thus, the probability for the chain transfer of a polymeric radical to the cobalt catalyst is greatly reduced. 

---

To control molecular mass, CTAs may be used. The chemically controlled transfer reaction of dodecyl mercaptan in homo- and copolymerizations of styrene and MMA is not influenced by the presence of up to 40 wt.% C02. On the other hand, the diffusion-controlled catalytic chain-transfer reaction of Co complexes was enhanced by up to 1 order of magnitude depending on the reaction conditions, in particular on CO2 content amd the Co complex used. ... 

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

Catalytic chain transfer has now been applied under a wide range of reaction conditions (solution, bulk, emulsion, suspension) and solvents (methanol, butan-2-one, water). The selection of the particular complex, the initiator, the solvent and the reaction conditions can be critical. For example ... 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

Figure 33 The catalytic mechanism for the production of borane-terminated isotactic polypropylene (z-PPs) via in situ chain-transfer reaction by a styrene/hydrogen consecutive chain-transfer reagent allowing the utilization of MAO cocatalyst (50). (Adapted from ref. 74.)...

The Zr-FI catalyst selectively forms PE even in the presence of ethylene and 1-octene, while the Hf complex affords amorphous copolymers, resulting in the catalytic generation of PE- and poly(ethylene-c6>-l-octene)-based multiblock copolymers through a reversible chain transfer reaction mediated by R2Zn. The development of an FI catalyst with extremely high ethylene selectivity as well as a reversible chain transfer nature has made it possible to produce these unique polymers. Therefore, both Ti- and Zr-FI catalysts are at the forefront of the commercial production of polyolefinic block copolymers. 

The kinetics of the above reported chain transfer reactions seem to be also catalytically affected by the titanium compound present in the reaction system. In fact we have observed (Table III) that both the numbers of ethyl groups and aluminum atoms bound to the polymeric chains decrease with decreasing amount of titanium compound in the catalytic system. 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

When using, however, two-component catalysts alcohols also react with inactive metal-polymer (aluminium-polymer) bonds which are formed in the chain transfer reactions with a cocatalyst. It is expedient to use the alcohol method only for catalytic systems and polymerization conditions for which the number of inactive metal-polymer bonds is low. Such a case is the polymerization of 4-methyl-1-pentene on vanadium trichloride activated with various organoaluminium compounds. For this system the influence of catalyst composition and polymerization conditions on Cp and kp was determined by quenching the polymerization with tritiated alcohol. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

A corresponding principle applies to controlled radical polymerisation performed in quite a number of modes such as nitroxide-mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP), reversible addition fragmentation chain transfer (RAFT) or catalytic chain transfer (CCT) reactions. All of these variants of controlled radical polymerisation lead to well-defined architectures with the particular advantage that a much larger number of monomers are suitable and the reaction conditions are much less demanding than those of living ionic polymerisation reactions. 

Alkyl acrylates were for the first time polymerized in a living fashion with the aid of the unique catalytic action of rare earth metal complexes [4]. Since these monomers have an acidic a-H, termination and chain transfer reactions occur so frequently that their polymerizations generally do not proceed in a living manner. By taking advantages of the living polymerization ability of both MMA and alkyl acrylate, ABA or ABC type tri-block copolymerization was performed to obtain thermoplastic elastomers. 

The steps of initiation and propagation are common to all free-radical reactions and to many other types of catalysis. The termination step may be temporarily suppressed in some reactions. There are sometimes chain transfer reactions in which the catalytic site is switched from one molecule to another. See Problem 2.16 and Chapter 13 for examples. 

---