Termination and chain transfer processes

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). These groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8.2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

Termination and Transfer Processes. Clear evidence about the mechanism of termination and chain transfer processes can be obtained from the polymer chain end structure. One polymer chain end is controlled by the initiation mechanism, and the second one is controlled by termination and/or chain transfer. The chemical structure of the end groups has been studied in a few cases only (J ). Several peculiarities of these reactions will be outlined here. 

Ethene copolymerizations have been thoroughly studied during recent years. A lot of work remains to be done in this area, however, in particular on termination and chain-transfer processes. The progress in these investigations is correlated to improvements achieved in the quality of copolymer MWD analysis. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

In the kinetic analysis, the fact that termination and chain transfer are second and first order processes with respect to the radical concentration, respectively, can then be exploited. The scaling of the number MWD now has to be done such that the radical concentration versus time that is derived from this distribution, is in agreement with the amount of material that is found to originate from chain transfer to monomer. Hence, the degree of freedom in the scaling of the number MWD is limited by a second kinetic condition. It should be noted, however, that knowledge of the ratio of termination by combination over disproportionation is needed for that. 

As PHTP is a chiral compound, polymerization of l,3-//-a 5-pentadiene in optically active PHTP gave an isotactic polymer [67]. Inclusion polymerization may be a living polymerization, in which termination and chain-transfer reactions are suppressed or reduced. Farina et al. [68] succeeded in preparing block copolymers by adding different monomers into the previously irradiated PHTP. Inclusion polymerization proceeds by a radical process, which has been studied by ESR spectroscopy [69]. Crystal structures of urea-monomer and urea-polymer inclusion compounds were studied by Chatani et al. [70-73] and those of PHTP complexes were studied by Colombo and Allegra [74]. 

The immediate result of the intervention of the chain transfer process indicated in the first step is the termination of a growing chain and the reactivation of a polymer molecule, which then adds monomer to gener-... 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

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