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.
Highlights
This viewpoint describes the origin of the activated monomer mechanism (AMM) in cationic ring-opening polymerization (CROP) and further development of this method in the synthesis of biodegradable polyesters
Starting in the 1990s, the AMM with organocatalysts became the preferred method in CROP of cyclic esters
Addition of protic compounds in CROP of THF leads to the reversible chain transfer and, at certain conditions, to the coexistence of the active chain end (ACE) and AMM mechanisms.[10]
Summary
This viewpoint describes the origin of the activated monomer mechanism (AMM) in cationic ring-opening polymerization (CROP) and further development of this method in the synthesis of biodegradable polyesters. There are a large number of extensive general review papers concerning ROP19,20 and recent developments in the application of organocatalysts[21,22] as well as describing syntheses, properties, and applications of PLA and PCL.[23] Starting in the 1990s, the AMM with organocatalysts became the preferred method in CROP of cyclic esters. The AMM in CROP of cyclic ethers and esters usually provides living and/or controlled processes These terms, as well as the definition of the activated monomer, are given by IUPAC.[4] in some papers on ROP the basic terms are described differently. ORIGIN OF THE ACTIVATED MONOMER MECHANISM IN THE CATIONIC RING-OPENING POLYMERIZATION It has been known for a long time that in CROP of cyclic ethers undesirable oligocyclics are formed, via a process called “backbiting” (bb). Side reactions can be avoided reaction of the AM with a polymer chain was observed.[39]
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