Termination reactions, radical structures

The final three chapters cover termination reactions. The structural dependence of the rate of oxy-radical terminations, the technique of spin trapping, and pyridinyl radicals are discussed. 

Any understanding of the kinetics of copolymerization and the structure of copolymers requires a knowledge of the dependence of the initiation, propagation and termination reactions on the chain composition, the nature of the monomers and radicals, and the polymerization medium. This section is principally concerned with propagation and the effects of monomer reactivity on composition and monomer sequence distribution. The influence of solvent and complcxing agents on copolymerization is dealt with in more detail in Section 8.3.1. 

When the termination involves only combination, the polymerization gives a polymer with two initiator fragments at its chain ends. Because termination in the bulk polymerization of St with AIBN at a moderate temperature occurs by combination, the polymer obtained has two initiator fragments at both chain ends. In the radical polymerization of most monomers, however, termination by disproportionation and chain transfer reactions occur it is therefore impossible to control these termination reactions, i.e., the chain-end structure. Therefore, the number of initiator fragments per one molecule is always less than two. 

North, A. M., The Influence of Chain Structure on the Free Radical Termination Reaction, Chap. 5 in Reactivity, Mechanism and Structure in Polymer Chemistry, A. D. Jenkins and A. Ledwith, eds., Wiley-Interscience, New York, 1974. 

In termination reactions, all mesomeric structures may contribute. Cases in point, where one would not immediately expect this to play a significant role are the a-carboxymethyl radicals (Wang et al. 2001). For some of the nucleobase radicals also more than one mesomeric structure maybe written (Chap. 10), and it is not unlikely that also here this aspect has to be taken into account. 

Chain growth differs from step growth in that it involves initiation and usually also termination reactions in addition to actual growth. This makes its kinetic behavior similar to that of chain reactions (see Chapter 9). However, the chain carriers in chain-growth polymerization need not be free radicals, as they are in ordinary chain reactions. Instead, they could be anions, cations, or metal-complex adducts. While the general structure of kinetics is similar in all types of chain-growth polymerizations, the details differ depending on the nature of the chain carriers. 

When the oxygen pressure is high, the termination reaction almost exclusively followed Eq. (14). In the solid state, when sufficient oxygen concentration cannot be maintained in the system, the termination reaction (Eq. 15) becomes significant. The polymer radicals may be coupled mutually (Eq. 16) and form crosslinks with polymer radicals. These processes are dependent on the chemical and physical structure of irradiated polymers. 

It is possible to exactly identify and characterize the radical species and chain structures of the reaction intermediates, which are determined by their different reactive or unreactive chain ends. The reactive intermediates are best described by diradical (DR), asymmetric carbene (AC) and dicarbene (DC) oligomer molecules of different lengths. The respective singlet (S = 0), triplet (S = I) or quintet (S = 1) states and their roles in the polymerization process are investigated in detail by solid state spectroscopy. A one-dimensional electron gas model is successfully applied to the optical absorption series of the DR and AC intermediates as well as on the different stable oligomer SO molecules obtained after final chain termination reactions. 

From spectroscopic data, presented in the following, we conclude that the mechanism of polymerization is described by three series of intermediate states differing by the number of reactive radical or carbene chain ends these are the diradicals DR , the dicarbenes DC , and the asymmetric carbenes AC . Via a final chain termination reaction an additional series of reaction products is obtained. These are the stable oligomers SO with two unreactive chain ends. The schematic structures of the DR, DC, AC, and SO molecules are shown by example of the trimer in Table 2. The lengths of the dimer-, trimer-, tetramer-... units are characterized by the numbers n = 2, 3,4,... of the respective monomer molecules. The symbols and the schematic structures as well as the notation of the optical and the ESR absorption lines, are summarized in Table 2. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

Table IV shows the reactivity ratios rG and r, derived from the probabilities in Table III in accord with a first-order Markov model (2), where it is assumed that the more likely propagating terminal radical structure is 1 (—CHF-) and not 0 (—CH2). This assumption is consistent with gas phase reactions of VF with mono-, di-, and trifluoromethyl radicals, which add more frequently to the CH2 carbon than to the CHF carbon (20). The reactivity ratio product is unity if Bernoullian statistics apply, and we see this is not the case for either PVF sample, although the urea PVF is more nearly Bernoullian in its regiosequence distribution. Polymerization of VF in urea at low temperature also reduces the frequency of head-to-head and tail-to-tail addition, which can be derived from the reactivity ratios according to %defect — 100(1 + ro)/(2 + r0 + r,). Our analysis of the fluorine-19 NMR spectrum shows that commercial PVF has 10.7% of these defects, which compares very well with the value of 10.6% obtained from carbon-13 NMR (13). Therefore the values of 26 to 32% reported by Wilson and Santee (21) are in error.

In the propagation steps, a site-specific radical R is generated from an organic substrate by removal of the Z group. In Scheme 1 the structure [RZMR 3] represents a reactive intermediate or a transition state. The radical R then reacts with the hydride generating the reduced product and fresh R 3M radicals. The chain reactions are terminated by radical combination or disproportionation. 

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