Depropagation ring-opening polymerization

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... 

Polyformaldehyde can also be prepared by polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane polymerizes by ring opening polymerization and cationic initiators are the only effective initiators. Formaldehyde is always present when trioxane is polymerized because the growing polyoxymethylene chains by depropagation may lose one monomer unit, which is formaldehyde not trioxane. In spite of the fact that formaldehyde plays an (as yet incompletely understood) role in trioxane polymerization, which is a cyclic ether polymerization like dioxolane or tetrahydrofurane [5], trioxane will not be discussed in this review. 

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. 

For reversible ring-opening polymerizations, the propagation-depropagation... 

Write the rate expression for propagation in ring-opening polymerizations where there is an equilibrium between propagation and depropagation. 

It is known that the ring-opening polymerization of lac-tides is an equilibrium process and some monomers are present in the reaction mixture even at the very end of polymerization. Depropagation (like side reactions if they do occur) results in broadening of the molecular weight distribution. Thus, the value of the dispersity close to 1 indicates that at the moment when the above-described dispersion polymerization of D,L-lactide was stopped, the system was still far from equilibrium. 

Polymerization of spiroorthoesters proceeding with single ring opening is a reversible process the possibility of participation of the chain unit (other than participation of the terminal unit in depropagation) in the process was not considered. Polymerization of spiromonomers has been reviewed [207],... 

Chain Scission with Depropagation. Many carbon-chain polymers and other simple chains, such as acetal resins (polyethers), are produced by chain reaction polymerization, either via double bonds or by ring opening. Such polymerizations involve repeated addition of a monomer molecule to an active center, which may be a radical, an ion, or a coordination complex. 

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