Polymerization of ε-Caprolactone Using Bis(phenoxy)-amine Aluminum Complex: Deactivation by Lactide

Abstract

Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.

Random copolymerization of l‐lactide and ε‐caprolactone by aluminum alkoxide complexes supported by N2O2 bis(phenolate)‐amine ligands

“…However, the catalyst became much less active for rac ‐LA. We have shown that the chelation of the ring‐opened LA provided a more crowded metal center and blocked the coordination of incoming monomer . However, although lactide chelation hindered the next incoming LA or ε‐CL monomer, the propagation may still proceed at elevated temperature.…”

“…However, after the ROP of the first LA monomer, the ester group on LA can coordinate to the metal center creating a stable five‐membered metallacycle product. Therefore, high energy was required for the propagation step resulting in slower polymerization rate than those of ε‐CL …”

“…From our previous work, we have been fascinated by the N 2 O 2 ligand framework where the N 2 O 2 bis(phenolate)‐amine ligand can create a bowl‐shaped structure around the metal center. In the aluminum complexes supported by N 2 O 2 bis(phenolate)‐amine ligands having pyridine, dimethylamine and diethylamine as a side arm, the rates of the homopolymerization of LA were faster for the N 2 O 2 complex containing pyridine while the rates of homopolymerization of ε‐CL were faster when the side arm was diethylamine . These results suggested that, through some variations of the amine side arms, the catalytic activities toward LA and ε‐CL could be adjusted that could lead to random copolymerization of LA and ε‐CL.…”

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

“…Apparently, specific ligand structure with donor side-arm allows to come close to the target of the synthesis of random lactone/lactide copolymers. Phomphrai et al [167] carried out a study of the catalytic behaviour of the complex 63 and came to the conclusion that κ 2 -lactate fragment is undergoing transformation to κ 1 -alkoxy fragment at elevated temperatures (Figure 41). Specifically to complex 70 , such process can be complicated by the side reaction of alkyl migration to nitrogen atom.…”

Comparative Experimental and Theoretical Study of Mg, Al and Zn Aryloxy Complexes in Copolymerization of Cyclic Esters: The Role of the Metal Coordination in Formation of Random Copolymers

“…The formation of random copolymers with the catalysis by metal chelates was usually attributed to higher inhibition of the coordination of LA relative to εCL due to higher steric hindrance for LA coordination [ 50 , 79 ]. This interpretation appears to be incomplete bearing in mind a higher donor number of lactones in comparison with lactides [ 80 , 81 ], a considerable advantage of LA over εCL in coROP can be attributed to the formation of highly stable chelate metal complexes after LA ring-opening that inhibit subsequent ring-opening of εCL [ 82 , 83 ]: this suggestion was confirmed experimentally and theoretically for aryloxy complexes of Mg [ 47 ].…”