Quasi-living polymerizations chain transfer

A new and more effective and reliable variant of the kinetic method is the stopped flow method (SF method), which has been offered by Keii and Terano [153] for determination of the number of active centers and the propagation rate constant in olefin polymerization on ZN catalysts. The main feature of this method is determination of Cp and k values in conditions of quasi-living polymerization, when transfer reactions of a polymer chain practically do not proceed and linear dependences of molecular weight of formed polymer and yield of polymer on polymerization time are observed. It has been shown that these conditions are obtained for propylene polymerization on supported titanium-magnesium catalysts (TMC) at low temperature (30°C) and at times of polymerization less than 0.2 s in these cases, values of Cp and can be calculated from Eqs. (14) and (15) ... 

In the literature, various reasons for formation of polymers with broad MMD on heterogeneous ZN catalysts are discussed. Convincing evidence has been obtained using the SF method that the reasrni is heterogeneity of the active centers on a surface of the catalyst [186]. In conditions of quasi-living polymerization there are no transfer reactions of the growing polymer chain and polymer is formed on the surface of catalyst in very small quantities. This polymer cannot cause diffusion restrictions, but nevertheless polymer with broad MMD (Mw/M = 3.2-4.3) is formed. The further increase in time of polymerization does not influence the width of MMD (M ,/M = 3.6). 

Quasi-living Carbocationic Polymerization. Recently, Kennedy et al. (165. 173) developed polymerization systems in which under well-defined conditions (a special manner of continuous mixing of monomer with initiating systems), chain termination and chain transfer to monomer are reversible or avoidable, and for all practical purposes the system behaves as if Rj. and are equal to zero. Fast R was achieved by premixing the ingreaients of the initiating systems. 

Since the metal alkyl is only involved in initiation of chain growth while subsequent propagation and chain transfer (usually involving chain transfer to monomer via hydride abstraction ) are usually very rapid, it is generally not possible to determine the fate of the metal alkyl in such polymerizations. End-group analyses provide evidence for a cationic propagation and chain-transfer mechanism, but the initiator moiety is not detected unless the process is at least quasi-living under the conditions studied. 

The quasi-steady-state approximation (QSSA) is commonly made for the moments of living polymer chains since, for most practical situations, an equilibrium is achieved instantaneously between chain initiation and chain transfer, fc,C [M] = ( cpR + daclMo- This equilibrium results from the fast dynamics of the initiation and transfer reactions compared to that of the overall polymerization rate. In this case, an even simpler system of equations is obtained than the one listed in Table 2.6. 

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