Factors affecting rate termination

Pilati, F., Toselli, M., Messori, M., Manzoni, C., Turturro, A. E. and Gat-tiglia, E. G., On specific factors affecting the crystallization of PET the role of carboxyl terminal groups and residual catalysts on the crystallization rate, Polymer, 38, 4469 (1997). 

The efficiency of a particular amine must depend not only on the rate of the initial hydrogen abstraction, but also on the nature and subsequent reactions of the radical produced. The free radical produced by H transfer may well be stabilised by resonance and may be insufficiently reactive to start a new oxidation chain [40], particularly when the amino group is surrounded by bulky substituents [9]. If the radical does react, then the subsequent rate and nature of the reaction will depend upon the intermediates and on the relative importance of chain termination and chain transfer reactions. Some formal grouping of the factors affecting the efficiency of a given inhibitor and the kinetics of the inhibited reaction is possible. 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

The state of aggregation of the polymerising system represents another important factor which may affect the kinetics of polymerisation. It is well known (96,97) that many radical polymerisations are enhanced by increase in the viscosity of the po-lymerisingsystem, and this phenomenon was explained by a decrease in the rate of termination step which may become diffusion-controlled. In fact, the effect of viscosity should be observed at any stage of radical polymerisation, and this problem has been discussed recently by Benson and North (98, 99). Of course, this type of acceleration cannot be observed when the growth involves living polymers and therefore such an explanation does not apply to polymerisation of NCA, particularly since no termination resulting from active end-active end interaction takes place in these processes. 

Surface termination can be demonstrated conclusively by experiment. Alteration of the size, shape and nature of the surface affect the rate, and systematic variation of these factors enables the contribution made by the surface termination to be assessed. 

---