Catalytic chain transfer mechanism

Macromonomers such as 66, 68 and 94 are themselves catalytic chain transfer agents (Section 6.2.3.4) and transfer to macromonomer is one mechanism for chain extension of the initially formed species. The adduct species in the case of monomeric radical adding dimer (100) may also react by chain transfer to give 101 which is inert under polymerization conditions (Scheme 6.25). Polymerizations to... 

Various Co111 cobaioximes (90-92) have also been used as catalytic chain transfer agents.133 148"149 To be effective, the complex must be rapidly transformed into the active Co11 cobaioximes under polymerization conditions. The mechanism of catalytic chain transfer is then identical to that described above (6.2.5.1). 

The molecular weight values of poly(TMC) obtained from oxetane and C02 in the presence of a (salen)CrCl/n-Bu4NN3 catalyst system were generally lower than the theoretical values (Mn = 11050 Mn (theoretical) = 85 000). This situation was most likely due to a chain transfer mechanism arising from the presence of trace amounts of water in the system. However, when the catalytic runs were carried out under rigorously anhydrous conditions, the molecular weights more closely tracked the predicted values. 

The mechanism proposed for catalytic chain transfer" is showm in Scheme... 

Figure 1 Proposed mechanisms for catalytic chain transfer polymerization.

Figure 2 Generally accepted catalytic chain transfer polymerization mechanism.

The chemical mechanisms of transition metal catalyses are complex. The dominant kinetic steps are propagation and chain transfer. There is no termination step for the polymer chains, but the catalytic sites can be activated and deactivated. The expected form for the propagation rate is... 

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.)...

Independent of the ligand system, two different activation methods have been used in performing the propylene polymerization experiments. In both cases, the catalytic activities and molecular weights of the polymers are a sensitive function of the aluminum content provided by the activators. This dependence suggested an additional reversible chain transfer to aluminum when activating with MAO. As lower contents of A1 are provided in the polymerization system in the case of in situ activation with TIBA/borate, the only mechanism occurring is the chain back-skip. Furthermore, the differences in the polymer microstructures prepared with MAO and borate as cocatalysts are reflected. They sustain the proposed reversible chain transfer. 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

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