Combination termination polymerization reactions

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

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. 

GPC has become especially useful in polymerization reaction engineering since it permits comparatively rapid and precise determination of MWD. As shown in Figure 5, Xw obtainable from the MWD in a batch reactor can be used to determine the initiation rate constant as well as kp2/kt assuming the pseudo-steady state, the absence of chain transfer, and termination by combination. Similar relationships with CSTR s are available. 

The most important free-radical chain reaction conducted in industry is the free-radical polymerization of ethylene to give polyethylene. Industrial processes usually use (/-Bu())2 as the initiator. The t-BuO- radical adds to ethylene to give the beginning of a polymer chain. The propagation part has only one step the addition of an alkyl radical at the end of a growing polymer to ethylene to give a new alkyl radical at the end of a longer polymer. The termination steps are the usual radical-radical combination and disproportionation reactions. 

POLYESTERAMIDE PREPOLYMER In the prepolymer reaction, the multifunctional acyllactam which normally acts as initiator for caprolactam polymerization, also functions to combine the polymeric polyol moeities. An excess of acyllactam is used so that the resulting prepolymer is terminated by acyllactam. The reaction occurs slowly with heat (13), but in the presence of an alkaline catalyst is completed within seconds. The prepolymer may be prepared in mass or in the presence of inert organic solvents, or in caprolactam as part of the total copolymerization reaction. See Reaction B. 

The main point of concern in this synthesis is the termination reaction. Only the combination reaction is allowed. Termination in polymerization is mostly the reaction of two macroradicals. In the formation of telechelics, the reaction with primary radicals cannot be ignored. The ratio of combination to disproportionation reactions may be different in both cases. Even if primary radicals react with each other by combination and macroradicals react by combination, the cross-reaction can be a complete disproportionation ( ). 

The chain lengthening of the polymeric radical is the propagation step. The polymer can be formed as any combination of 1,2-, 1,4-cis or 1,4-trans additions the polymer can be a result of one or all of the three addition processes. The termination step of a polymerization reaction puts a stop to the growing polymer. In free radical polymerization, the termination step rids the growing polymer of its free electron. This generally proceeds by any one of three different methods dimerization, disproportionation and abstraction. Dimerization involves the joining of two growing polymer radicals. It can be shown as ... 

The kinetics of dilute solution polymerization of butyl acrylate were studied by Melville and Bickel (15). They measured the propagation and termination coefficients at 25 c kp 13(i/mole sec) and kt 1.8 x 10 ( /mole sec). Melville and Bickel also concluded that the termination reaction at 25 C occurred mostly throu combination, thus theoretically increasing the probability to produce a carboxyl terminated PnBA with an ideal ftuiction-allty of f=2 when the polymerization reaction is initiated with ABCVA. 

The initiator, I-I, is homolyticaUy broken into two free radicals by either absorption of a photon, hv, or by collision with another molecule, B. In either case there must be sufficient energy to break the center bond of the initiator. Once formed, each free radical, written as I, can start a polymerization reaction by adding monomer molecules. The first step leads to the free radical monomer, I-A , also written as Mj. The next step of the reaction produces a free radical called Mj. Each step of the chain reaction has up to this point its own specific rate constant. Addition of further monomers goes, however, substantially with the same rate, so that one can write during the main growth period of the molecule for each step the fourth reaction equation in Fig. 3.23, where x is the degree of polymerization. Ultimately, the reaction may be stopped by a termination reaction of the chain. The example of the termination reaction shown is a combination of two free radicals. The special case for... 

The preparation of chain-end functionalized polymers has been largely limited to living polymerization techniques, used in combination with a controlled initiation or termination (functionalization) reaction. Unfortunately, there are only a few transition metal coordination catalysts that exhibit living polymerization behavior, and the utility of most of these is limited to the preparation of polyethylene. Furthermore, living polymerization only produces one polymer chain per initiator, which presents a very low rate of catalyst activity for a typical polyolefin preparation. 

The degree of polymerization x is equal to twice for combination termination and equal to for disproportionation terminator. Deviations that occur are due to chain transfer reactions in which the following occurs ... 

In treatments of polymerization reactions that concentrated on a single feature, the effect of molecular weight upon the termination rate constant has been deduced, the relative rates of initiation of two monomers in a copolymerization have been assessed, constants for chain transfer to monomer have been obtained in an emulsion copolymerization, the relative amounts of chain termination by combination and disproportionation have been discovered from a molecular weight distribution, and the rate constant for long-chain branch formation in the free-radical polymerization of ethylene has been found by fitting a probalistic model. ... 

Polymerization reactions are very complex and polymeric materials are difficult to characterize (Paquette and Kupranycz van Voort, 1987). During thermal oxidation, intra- and intermolecular reactions of alkoxyl, alkyl and peroxyl radicals may lead to the formation of dimers, trimers and large molecular weight polymers with C-O-C and C-O-O-C crosslinks (Nawar, 1984, 1985). Dimers are a major component of non-volatile products formed in oxidized and heated fats/oils (Nawar, 1984 Taub, 1984). As is shown in reaction (11.7), by combining two radicals to form a non-radical dimer, the free radical chain reaction is thus terminated. 

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