Termination of polymerization

Termination of polymerization, 173 Tetrafluoromethane, 32 Tetrahydroquinone quadrupole spectrum, 195... 

The termination of polymerization of substituted ethylenes is by an internal Friedel-Crafts reaction, whereas that of the substituted benzyl chlorides is by the reaction with chloride ions. 

Of key importance in the Lewis acid promoted living anionic polymerization of methacrylic esters with aluminum porphyrin is how to suppress the undesired reaction between the nucleophile (2j ) and the Lewis acid, leading to termination of polymerization (Fig. 11). As mentioned in previous sections, one of our approaches was to make use of sterically crowded Lewis acids such as methyla-luminum bis(ort/zo-substituted phenolates). This section focuses attention on the steric bulk of the nucleophile component (2 ), by using strategically designed aluminum porphyrins and some other methacrylates, for the purpose of understanding the scope and limitation of this method (Fig. 12). 

Evidence for such reactions in methylmethacrylate polymerizations was obtained by termination of polymerizations with acetic acid followed by measurements of methanol formed (119, 120). Fig. 6 shows typical results obtained. It is assumed that the methanol found corresponds to lithium methoxide in the reaction mixture. Some methanol might in fact be produced only in the termination reaction from pseudo-cyclized species (cf. p. 82). In addition the tertiary alkoxides formed by attack of the initiator or growing polymer chains on the carbonyl group of the monomer might not eliminate lithium methoxide immediately but would do so on termination with acetic acid. In any case much of the methanol formed corresponds to actual alkoxide in the reaction mixture and the results give a minimum value of the concentration of species inactive in polymerization. For brevity it will be referred to as lithium methoxide. 

While the active end of a growing polymeric radical forms a single entity, the active end of an ionically growing polymer involves two species, a charged terminal atom or terminal group of the polymer and, associated with it, an oppositely charged counter-ion. The interaction between these two entities may lead to termination of polymerization, and this event may take place in a variety of ways. 

Once least squares values of the /3 s were obtained, it was desirable to extract from them as much information as possible about the original parameters. To do so, we make one further statement concerning the relations between the rate constants for mutual termination of polymeric radicals of different size. It has been shown (2) that termination rates in free radical polymerizations are determined by diffusion rates rather than chemical factors. The relative displacement of two radicals undergoing Brownian motion with diffusion coefficients D and D" also follows the laws of Brownian diffusion with diffusivity D = D -J- D" (11). It... 

The eventual collision between H30+ and a negative center regenerates then the water molecule and destroys the negative ion. Hence, (H20)2 or higher agglomerates react with carbonium ions as Lewis bases, alcohol is formed, and one water molecule is destroyed. Therefore, the termination of polymerization proceeds simultaneously with the destruction of the terminator. However, at very low water concentration, the single H20 molecule eventually reacts as a Bronsted base—i.e., as a proton acceptor (Reaction 4). In this process the polymerization is terminated, but the terminator is not destroyed. This accounts for the experimental results—viz., the last trace of moisture cannot be removed from the monomer by prolonged irradiation (24, 37). 

The occurrence of these reactions is always determined by thermodynamic factors. Oxirane has a large ring strain. Its polymerization around room temperature exhibits AGp<0. For 1,4-dioxane under the same conditions, AGp > 0. In other words, polyoxirane will split off 1,4-dioxane because the Gibbs energy of its depolymerization is negative. Actually the polymer should depolymerize completely. That this is not the case, is caused by kinetic factors. Termination of depolymerization need not coincide with termination of polymerization. 

There are several ways in which block copolymers can be made. The three main methods are (1) sequential addition of monomers, (2) the preparation of a functionalized polymer followed by the use of the functionalized polymer as a macroinitiator or chain-stopper for initiation or termination of polymerization of the second monomer, and (3) use of a multiple-headed initiator. The purity of the block copolymers produced in these processes is dependent upon the livingness (lack of side reactions that lead to termination) of the chemistry used to make them. If the integrity of the chain-ends is maintained throughout the polymerization because all possible termination mechanisms are absent or eliminated, then pure block copolymers can be produced. If, however, impurities get into the process or if there are side reactions that lead to chain termination, the resulting block copolymers are contaminated with some homopolymer. Depending upon the application, some contamination of homopolymer in the block copolymer may be acceptable. 

The kinetic scheme with constant reaction of the polymer/monomer droplet increases fairly quickly with conversion, and the mobility of the polymer chains rapidly falls below the mobility of the monomer. The reduced diffusion of live polymer chains in the droplet will reduce the rate of termination of polymerization. The associated increase in the number of radicals will cause a rapid increase in the polymerization rate. This phenomenon is well known as the Trommsdorf or gel effect [8,9]. The gel effect causes a growth of the polymer chain length and widening of the molecular weight distribution (Figure 9.5). 

Termination of polymerization in micelles finally occurs, since the expelled polymer nuclei absorb monomer and the resultant mixed nucleus adsorbs on to its surface a monolayer of soap. Eventually this process reduces the soap concentration below the critical value for micelle formation, and further initiation practically ceases. This generally occurs at about 13 to 20% yield of polymer, but if as much soap as 6% be used as catalyst, micelles can persist up to the highest yields. 

What has been shown in Figure 3 is that increasing the rates of all of the bimolecular reactions, proportionately and together, causes a net increase in the induction time (time to failure). This result probably reflects the importance of the bimolecular termination of polymeric... 

R-CHjCHy + CH2CH2-R R-CH2CH3 + H2C=CH-R Fig. 3.5 Termination of polymerization by disproportionation. 

Another method is based on introduction into polymerization of compounds (quenching agents) labeled by a radioactive isotope and by termination of polymerization in such a manner that this compound or its part joins a growing polymer chain (QR method). In the case of olefin polymerization on ZN catalysts, alcohol labeled with in the hydroxyl group was used as a quenching agent (QR RO H method) ... 

For a monodisperse polymer sample, d = 1. The ranges of d values change drastically with the different mechanisms of polymerization. The values of d are 1.01-1.05 in living polymerization (anionic, cationic, living free radical, etc.), around 1.5 in condensation polymerization or coupling termination of polymerization, around 2 in disproportionation reactions on polymerization, 2-5 for high-conversion olefins, 5-10 in self-acceleration on common free radical polymerization, 8-30 in coordination polymerization, and 20-50 in branching reactions on polymerization. 

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