Ring-opening polymerization acid-initiated mechanism

Both protonic and Lewis acids initiate ring-opening polymerization of siloxanes. In spite of the practical importance [240] and the very extensive studies of this system, the mechanism of the polymerization is not completely understood. Even the nature of ionic propagating species is still a matter of controversy. The silicenium [241] or oxonium [242,243] ion structure has been postulated. 

Acheson process, 566 Acid-initiated ring-opening polymerization comparison widi base-initiated process, 79 kinetics, 79-80 mechanism, 79-81 rate retardation by water, 81 relative reactivities of cyclosiloxanes, 79 step-growth process, 79 Acyclic oligosilanes, photochemical transformations, 432/-433/ Acylsilanes... 

The most effective, and commercially applied, method to produce polylactide is via the ring-opening polymerization of lactide. This process is initiated by metal complexes and proposed to occur via a coordination-insertion mechanism, as illustrated in Fig. 2. The most common initiators for this polymerization are Lewis acidic metal alkoxide or amide complexes. Key initiator criteria are sufficient Lewis acidity to enable binding and activation of the lactide unit and a labile metal alkoxide (or amide) bond so as to enable efficient insertion. 

Ring-Opening Polymerization. Polyamides can be formed by ringopening reactions as in the polymerization of caprolactam. The polymerization may be carried out at high temperature with water, amino acid, or amine carboxylate salt as Initiator. The predominate ring-opening mechanism with these initiators is carboxyl-catalyzed aminolysls. In the case of water initiation, hydrolysis occurs (Reaction 10) to form small amounts of the amino acid. The amino acid then reacts with lactam amide groups (Reaction 11). 

If the initiator is a Lewis acid or a proton-donating acid, epoxides are polymerized by a cationic mechanism Polymerization reactions that involve ring-opening reactions, such as the polymerization of propylene oxide, are called ring-opening polymerizations. 

The absence of linear oligomers is due to rapid reactions of the hydroxy and oxonium ion end groups. This mechanism is quite general for ring-opening polymerizations of cyclic ethers initiated with strong protonic acids. Substituted tetrahydrofurans generally resist polymerizations. 

Many aldehydes trimerize in acidic conditions. Paraldehyde and many other trimers fail to undergo further polymerizations to high molecular weight, linear polymers. The trimer of formaldehyde, trioxane, however, is unique. It polymerizes by a mechanism of ring opening polymerization. This is discussed in Chap. 5. To avoid trimer formations, low dielectric solvents must be used at low temperatures. In addition, the initiator must be carefully selected. 

It is believed that the controlled cationic ring-opening polymerization proceeds by an activated anionic mechanism as suggested by Penczek [91]. According to his suggestion, the acid activates the cyclic ester and the alcohol subsequently initiates the polymerization. 

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