Polymerization quasi-living

The formation of polymer can be considered as a quasi-living polymerization. After the polymerization is complete, it can be reinitiated with the addition of more monomer to the unquenched polymer. However, the degree of polymerization cannot be predicted by the monomer/initiator molar ratio, the polydispersity is 1.5-2.0, and water, or even carboxylic acids, act as inhibitors and do not terminate the polymerization [10]. 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

In conclusion, styrene can be considered as an ideal monomer in photo-chemically induced polymerization and can be used successfully in quasi living polymerizations. 

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) ... 

Active centers should be formed instantly at interaction of the catalyst with cocatalyst and the time necessary for formation of the active centers should be less than the time of quasi-living polymerization... 

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). 

M.D.C. Topp, I.H. Leunen, RJ. Dijkstra, K. Tauer, C. Schellenberg, J. Feijen, Quasi-living polymerization of N-isopropylacrylamide onto poly(ethylene glycol). Macromolecules, 33, 4986-8, 2000. 

Figure 1. Moleclar weights during quasi-living polymerization of propene.

A new method called stopped-flow polymerization was used to observe a quasi-living polymerization state of propene. It was found that molecular weight distribution remained constant from the very beginning (0.1 s) of the polymerization. Kinetic parameters were dependent on the catalyst preparation methods. 

In order to precisely evaluate the values of kp, etc. by means of Egs. (1) and (2), observation of the transitional change of molecular weight in a quasi-living polymerization state, where the transfer reaction is negligibly small, is needed. 

Here, we apply a new method called stopped-flow polymerization. A quasi-living polymerization state (0.1 - 1.0 s) of propene polymerization with typical MgCl2-supported high yield catalysts was observed in order to discuss the effect of the catalyst preparation method on some kinetic parameters by using the above method. 

In general, several factors have to be taken into accoimt in order to attain a living or a quasi-living polymerization of... 

---