Activated Monomer Mechanism (AMM) in Cationic Ring-Opening Polymerization. The Origin of the AMM and Further Development in Polymerization of Cyclic Esters

Abstract

The origin of the activated monomer mechanism (AMM) in cationic ring-opening polymerization (CROP) is described first. Then, conditions leading to the active chain end (ACE) mechanism and AMM are compared, as well as methods allowing to distinguish between these two mechanisms. These methods are based on the “ion trapping” of the active ionic species using highly basic phosphines or by comparing ACE and AMM kinetics of polymerization. The major factors deciding on the actual mechanism include: basicity of the monomers, ring strain, and the presence of the protic additives in the reaction system. These factors are tabulated for major cyclic ethers and cyclic esters. The historically evolved subsequent steps of AMM in the polymerization of cyclic esters are described: from the first experiments with trialkyloxonium salts, precursors of protonic acids, and added alcohols, via HCl as catalyst, and then CF3S­(O)2OH (polymerizing lactides) to the most popular derivatives of phosphoric acid, like diphenyl phosphate. Conditions allowing to synthesize poly­(ε-caprolactone) (PCL), according to AMM-CROP, with molar mass up to 105 g·mol–1, are described as well as methods to polymerize CL with a protic initiator and acidic catalyst in one molecule. Then various methods enhancing the activity of the polymerizing systems are compared, based predominantly on hydrogen bonding, either to the polymer active end group (usually the hydroxyl group) or to the acid anion. Finally, kinetic equations for ACE and AMM are compared, and it is shown that the majority of the AMM-CROP systems, mostly studied for CL and lactides, proceed as living/controlled polymerizations. Since polymer end groups are hydroxyl groups, then, as it was shown in several papers, any initiator with one or many hydroxyl groups provides macromolecules with the corresponding architecture. The papers on synthetic methods are not discussed in detail.

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“…Competition between these two propagating mechanisms is reminiscent of the situation we previously reported for TMC (Figure S5). − …”

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“…These reactions are proposed to proceed with the typical active chain end mechanism, i.e., there is an onium ion at the chain end. [40] The final formation of poly(1 a-alt-2 a) from the mixture of 1 a and 2 a is considered to be the result of thermodynamical control. "1 a* * poly(1 a)" is a reversible reaction, while the 1 a/2 a alternating copolymerization is irreversible.…”

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“…Therefore, there are three involved reactions during the copolymerization: ROP of 1 a , depolymerization of poly( 1 a ) segments, and 1 a / 2 a alternating copolymerization (Scheme S1). These reactions are proposed to proceed with the typical active chain end mechanism, i.e., there is an onium ion at the chain end [40] . The final formation of poly( 1 a ‐ alt ‐ 2 a ) from the mixture of 1 a and 2 a is considered to be the result of thermodynamical control.…”