Molecular weight study lipase-catalyzed polymerization

Russell et al. [43] studied lipase-catalyzed polymerizations of activated diesters and fluorinated diols. The effects of reaction time, continuous enzyme addition, enzyme concentration, and diol chain length were studied to determine factors that might limit chain growth. Potential limiting factors considered were enzyme inactivation, enzyme specificity, reaction thermodynamics, hydrolysis of activated esters and polymer precipitation. The polymer molecular weight at 50°C steadily increased and then leveled off after 30h at Mw 1773. 

Detailed studies on the lipase-catalyzed polymerization of divinyl adipate and 1,4-butanediol were performed [41-44]. Bulk polymerization increased the reaction rate and molecular weight of the polymer however, the hydrolysis of the terminal vinyl ester significantly limited the formation of the polyester with high molecular weight. A mathematical model describing the kinetics of this polymerization was proposed, which effectively predicts the composition (terminal structure) of the polyester. 

A time course study of 11-hydroxydecanoic acid polymerization catalyzed by Candida cylindracea lipase was reported by O Hagan and Zaidi [28]. The authors revealed that oligomers are formed relatively rapidly and then later condense to generate higher molecular weight polyesters. After 7 days, they reported formation of a polyester with molecular weights up to Mw = 35000. 

While the polymerization of optically inactive AA-BB and AB monomers under DKR conditions leads to chiral polyesters, these approaches always result in limited molecular weights since a condensation product is formed that needs to be effectively removed. A solution for this would be to use the eROP of lactones, where no condensation products are formed during polymerization. In principle, the eROP of lactones can lead to very high MW polyesters (>80kgmol 1) [57]. Addition of a methyl substituent at the ro-position of the lactone introduces a chiral center. Peeters et al. conducted a systematic study of substituted e-caprolactones which revealed that monomers with a methyl at the 3-, 4-, or 5-position could be polymerized enantioselectively while a methyl at the 6-position (a-methyl-e-caprolactone, 6-MeCL) could not [58]. The lack of reactivity of the latter monomer in a Novozym 435-catalyzed polymerization reaction was attributed to the formation of S-secondary alcohol end-groups. These cannot act as a nucleophile in the propagation reaction since the lipase-catalyzed transesterification is highly R-selective for secondary alcohols. 

Recently Gross et. al. reported that polymerization of e-CL catalyzed by Novozyme-43S (immobilized lipase B from Candida Antarctica) was improved by reaction in toluene because of higher log P (76). In situ NMR measurement in toluene-dg was employed to study die effect of reaction temperature on propagation kinetics and average molecular weight. The polymerization conducted at 90 °C gave the most rapid polymerization rate. 

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