Mechanism of Chain Propagation

SCHEME 1.2 General mechanism of chain propagation for metallocene-catalyzed propylene polymerization (L = cyclopentadienyl or similar hgand). 

The energetics of each step have been deduced using a variety of computational methods. Propylene uptake (coordination) by a coordinatively unsaturated metallocene alkyl species is favorable ( 10 kcal/mol) and essentially barrierless unless sterically bulky ligands are involved. The barrier to insertion of propylene for a C2-symmetric zirconocene catalyst has been calculated at 2-4 kcal/mol. 

SCHEME 1.3 Four mechanisms for propylene insertion (spectator ligands and charge on metal center not shown P = growing polymer chain = open coordination site half-arrow represents an agostic interaction). (Adapted with permission from Grubbs, R. H. Coates, G. W. Acc. Chem. Res. 1996, 29, 85-93. Copyright 

Ground-State agostic interactions have been observed experimentally in model compounds of active 

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As mentioned in section 4.1, the kinetics of the living polypropylene synthesis have been interpreted in terms of a coordination polymerization mechanism represented by Eq. (22). We discuss here the mechanism of chain propagation on the basis of the structure and stereochemistry of the synthesized polypropylenes. 

Figure 2. Possible mechanisms of chain propagation in heterogeneous polymerizations...

Direct compEirison of the theoretical and experimental yields is complicated by the fact that further reactions of the products have not been taken into account. Even so, some excellent agreements are obtained and in most cases where there is disparity it is clear that this is due to competing propagating steps not embraced by the scheme [78]. The alkylperoxy radical isomerization theory provides a good account, then, of the mechanism of chain-propagation, at least semi-quantitatively. 

From 1952 onwards, Niclause, Combe, and Letort demonstrated that the slow thermal oxidation, at about 100-150°C, of gaseous ethanal and propanal can be interpreted by a mechanism of chains propagated and terminated by the same processes as their photooxidation, but initiated by the thermal process ... 

When dealing with catalysis it is best, however, to classify polymerization reactions according to the mechanism of chain propagation (2). One may distinguish in this way between chain-reaction polymerization and step-reaction (stepwise) polymerization. The essential features of these classes are shown in Table I (15). The diflFerences between the two types of polymerization are also evident from equations of rate (Rp) and average degree of polymerization (DP). For a free-radical polymerization of vinyl compounds (an example of a chain reaction), Rp and DP are functions of monomer and catalyst concentration (Equations 9 and 10) ... 

Chain transfer reactions are considered here irrespective of the exact chemical mechanisms of transfer that are discussed in detail, for example, in the comprehensive review of Glasse [9]. It should be stressed that all calculations were performed bearing in mind anionic polymerization, so the main concern in all theoretical papers reviewed is the absence of kinetic termination rather than the anionic mechanism of chain propagation. Thus, the conclusions drawn are valid for all processes that satisfy this condition. 

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