Olefin monomers propagation/termination activation

Metal complexes bound to a polymer support most frequently induce ionic polymerization of olefins, dienes and acetylenes, and less commonly radical polymerization of vinyl-type monomers, acting at all reaction stages initiation, chain propagation and termination. Active sites for the addition of monomer molecules to the growing polymer chain can in many cases be regenerated yielding new polymer chains (catalysis via a polymer chain). 

The polymerization model most commonly adopted for olefin copolymerization is the terminal model, particularly for studies of polymerization kinetics. In the terminal model, only the last monomer molecule added to the chain end influences polymerization and transfer rates. Besides the fact that it is logically expected, there is also significant experimental evidence supporting the terminal model for olefin polymerization. Since monomer propagation and chain-transfer reactions take place by insertion between the chemical bond formed by the metal in the active site and the polymer chain end, it is certainly reasonable to assume that both the nature of the active site and the type of monomer last added to the chain will affect these reactions. On the other hand, higher-order models such as the penultimate and pen-penultimate models have not found widespread use in coordination polymerization. 

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. 

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section... 

The terminal cross metathesis (CM) reaction, as depicted in Figure 3.5, is probably the most straightforward synthetic method to introduce complex molecular fragments or functional groups to a ROMP polymer chain end. The propagating ruthenium carbene complex typically reacts with an acyclic olefin in a CM reaction. The newly generated carbene complex is still metathesis-active, and can in principle undergo secondary metathesis reactions or initiate the polymerization of the residual monomer. 

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