Ring-opening polymerizations equilibration reactions

Several important assumptions are involved in the derivation of the Mayo-Lewis equation and care must be taken when it is applied to ionic copolymerization systems. In ring-opening polymerizations, depolymerization and equilibration of the heterochain copolymers may become important in some cases. In such cases, the copolymer composition is no longer determined by die four propagation reactions. 

If desired, the linear oligosiloxanes, and indeed any linear polydimethylsiloxane, can be converted into cyclosiloxanes by base-catalyzed pyrolysis. If this reaction is carried out under equilibrating conditions and the products are fractionally distilled with removal only of the most volatile compound, D3, the entire mixture can be converted to this valuable intermediate. This procedure is frequently used to obtain pure D3 and D4, useful for polymer synthesis by ring-opening polymerization. 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

These ring-opening polymerizations are referred to as equilibration reactions. Since a variety of Interchange reactions can take place, a quantitative conversion of the tetramer to high polymer Is not achieved and there Is, at thermodynamic equilibrium, a mixture of linear and cyclic species present. Scheme III shows examples of the types of redistribution reactions thought to be occurring In these systems. It Is generally convenient to use D" to refer to a dlfunctlonal slloxane unit and M to refer to a monofunctional slloxane unit. Thus, D4 represents the cyclic slloxane tetramer and MM represents the linear hexamethyldlslloxane. 

Catalysts are often needed to facilitate ring-opening polymerization reactions. Although the Sg-S equilibration is believed to be a free-radical process, most ringopening polymerizations follow an ionic pathway ... 

J/° K-mole),3 these reactions do not proceed to 100% completion, and monomer recoveries of 5-30% are typical. The removal of unreacted monomer may be a limitation if a finished part is to be formed by ring-opening polymerization. Virtually all ring-opening polymerization reactions which involve ring-chain equilibration form measurable quantities of monomeric or oligomeric cyclics." ... 

Like all other chemical reactions, polymer syntheses may be subdivided into two categories, namely into kinetically controlled (KC) polymerizations and thermodynamically controlled (TC) polymerizations. KC polymerizations are characterized by irreversible reaction steps, equilibration reactions are absent, and the reaction products may be thermodynamically stable or not. TC polymerizations involve rapid equilibration reactions, above all formation of cyclics by back-biting of a reactive chain end (see Formula 5.1), and the reaction products represent the thermodynamic optimum at any stage of the polymerization process. Borderline cases also exist, which means that a rapid KC polymerization is followed by slow equilibration. This combination is typical for many Ring-opening polymerizations (ROPs). 

In two-step polymerization, initially the polyamic acid is formed from an equimolar mixture of dianhydride and diamine in a polar aprotic solvent such as A,A-dimethylacetamide (DMAc) or A,A-dimeth-ylformamide (DMF). The reaction pathway for the formation of poly(amic acid) involving intermediates is presented in Scheme 3.2. The reaction mechanism involves nucleophilic attack of the amino group to the electrophilic carbonyl carbon of the anhydride group. This opens the anhydride ring to form an amic acid group. Formation of the poly(amic acid) is an equilibration reaction in which the forward reaction... 

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