Depropagation-propagation equilibria

The value of T and the propagation/depropagation equilibrium constant (A e[ ) can be measured directly by studying the equilibrium between monomer and polymer or they can be calculated at various temperatures given values of AHp and ASP using cq. 11 and 12 respectively. 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

Conversion is limited by propagation-depropagation equilibrium, due to low ceiling temperature... 

The K values eu e the equilibrium constants which describe the position of the propagation-depropagation equilibrium and are equal to the ratio of the propagation rate constant to the depropagation rate constant ... 

In addition to the usual polymer-monomer propagation-depropagation equilibrium that may he present, trioxane polymerization proceeds with the occurrence of a polymer-formaldehyde equilibrium ... 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

Some monomers with no tendency toward homopolymerization are found to have some (not high) activity in copolymerization. This behavior is found in cationic copolymerizations of tetrahydropyran, 1,3-dioxane, and 1,4-dioxane with 3,3-bis(chloromethyl)oxetane [Dreyfuss and Dreyfuss, 1969]. These monomers are formally similar in their unusual copolymerization behavior to the radical copolymerization behavior of sterically hindered monomers such as maleic anhydride, stilbene, and diethyl fumarate (Sec. 6-3b-3), but not for the same reason. The copolymerizability of these otherwise unreactive monomers is probably a consequence of the unstable nature of their propagating centers. Consider the copolymerization in which M2 is the cyclic monomer with no tendency to homopolymerize. In homopolymerization, the propagation-depropagation equilibrium for M2 is completely toward... 

While for many alkene monomers the position of the propagation-depropagation equilibrium is far to the right under the usual reaction temperatures employed (that is, there is essentially complete conversion of monomer to polymer for all practical purposes), there are some monomers for which the equilibrium is not particularly favorable for polymerization. For example, a-methylstyrene in a 2.2 M solution will not polymerize at 25°C and pure a-methylstyrene will not polymerize at 61°C (see Table 6.14). In the case of methyl methacrylate, though the monomer can be polymerized below 220° C, the conversion will be appreciably less than complete. For example, the value of [M]g at 110°C is found to be 0.139 M [3] which corresponds to about 86% conversion of 1 M methyl methacrylate. Since Eqs. (6.195) and (6.196) contain no reference to the mode of initiation, they apply equally well to ionic and ring-opening polymerizations. Thus the lower temperatures of ionic polymerizations often offer a useful route to the polymerization of many monomers that cannot be polymerized by radical initiation because of their low ceiling temperatures. 

Actually, the observed distributions of DP are usually somewhat broader than predicted by Eq. (53). This fact is attributed to the existence of propagation-depropagation equilibrium ... 

Figure 8.2 also shows a plot of Wx vs. x for Voo = 50. It should be noted that the observed distributions would usually be somewhat broader than those shown in Fig. 8.2. This anomaly may be attributed to the presence of propagation-depropagation equilibrium ...

For reversible ring-opening polymerizations, the propagation-depropagation equilibrium can thus be expressed by... 

For monomers with low ceiling temperatures (e.g. a-methylstyrene see Section 2.4.11) the propagation-depropagation equilibrium must be considered when applying Equation (2.68) since an appreciable monomer concentration may exist at equilibrium. 

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