Propagation Kinetics and Thermodynamics

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). t he reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerization 

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This chapter is primarily concerned with the chemical microstructure of the products of radical homopolymerization. Variations on the general structure (CHr CXY) are described and the mechanisms for their formation and the associated Tate parameters are examined. With this background established, aspects of the kinetics and thermodynamics of propagation are also considered (Section 4.5). 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

The goal of these experiments is the measurement of the front propagation of one of the two stable stationary states into the other, see Chap. 5 particularly Sects. 5.1.3 and 5.1.4 and Figs. 5.2 5.7. From such measurements we can determine equistabUity conditions for the two stationary states where the front propagation velocity is zero. We thus obtain kinetic and thermodynamic conditions for the coexistence of the two stationary states. 

The selectivity exhibited by an olefin metathesis catalyst for the production of -olefins or Z-olefins is a result of both kinetic and thermodynamic factors. Kinetic selectivity results from preferential formation of either syn- or anri-metallacyclo-butanes following olefin binding iy -metallacycles will undergo a cycloreversion to produce Z-olefins, whereas -olefins are derived from anfi-metallacycles. Thermodynamic selectivity arises as a result of secondary metathesis processes, in which the product olefins continue to react with the propagating catalyst. 

The low tendency of 1,2-disubstituted ethylenes to polymerize is due to kinetic considerations superimposed on the thermodynamic factor. The approach of the propagating radical to a monomer molecule is sterically hindered. The propagation step is extremely slow because of steric interactions between the P-substituent of the propagating species and the two substituents of the incoming monomer molecule ... 

Initial correlations of this type are self-propagating. Similarly if at some initial time (which may be taken in the remote past) the particle description p was valid, it will be valid for all times later. To insure the validity at the initial time it would be sufficient to assume that then the system was formed by stable particles only. It is interesting that two widely different questions such as the validity of the kinetic (or thermodynamic) description of many-body systems and the particle description may be related to self-propagating initial correlations. 

Worsfold and Bywater 212) have proposed that propagation through non-aggregated chains is kinetically (and not thermodynamically) controlled and yields only the carbanion having the cis conformation this can isomerize to the trans form unless the geometry is locked in by a further act of propagation ... 

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