Abstract
The preparation of renewable polyesters with good barrier properties is highly desirable for the packaging industry. Herein we report the synthesis of high molecular weight polyesters via an innovative use of an in situ drying agent approach and the barrier properties of the films formed from these polyesters. High number average molecular weight (Mn) semiaromatic polyesters (PEs) were synthesized via alternating ring-opening copolymerization (ROCOP) of phthalic anhydride (PA) and cyclohexene oxide (CHO) using a salen chromium(III) complex in the presence of 4-(dimethylamino)pyridine (DMAP) cocatalyst. The use of a calcium hydride (drying agent) was found to enhance the number Mn of the synthesized PEs, which reached up to 31.2 ku. To test the barrier properties, PE films were prepared by solvent casting approach and their barrier properties were tested in comparison poly(lactic acid) films. The PE films showed significantly improved water vapor and oxygen barrier properties compared to the commercial poly(lactic acid) (PLA) film that suggests the potential use of these PEs in in the food packaging industry.
Highlights
Polyesters (PEs), especially those derived from renewable resources, have attracted great interest because they are considered as an alternative to the current petrochemical-based plastics [1]
In order to increase the molecular weight of PE synthesized via the ring-opening polymerization method, many different metal-salen ligand catalysts and cocatalysts have been developed for the ring-opening copolymerization (ROCOP) of epoxides and anhydrides, including zinc [6], magnesium [7], chromium [8], cobalt [9], manganese [10], iron [11], aluminum [12], and nickel complex [13], most of which showed significantly higher activity in the presence of a nucleophilic cocatalyst, such as 4-dimethylaminopyridine (DMAP), bis(triphenylphosphine)iminium salts ([PPN]X), phosphines and ammonium salts
The oxygen permeability of the PE film was 544 cc-mil/(m2 -day-atm), which was considerably lower than that of the poly(lactic acid) (PLA) film 1528 cc-mil/(m2 -day-atm). This 62% reduction in oxygen permeability observed for the PE film compared to PLA could be attributed to relatively strong secondary interaction strengthened by the presence of aromatic benzene rings on PE chains as compared to PLA, which should theoretically have weak secondary interactions
Summary
Polyesters (PEs), especially those derived from renewable resources, have attracted great interest because they are considered as an alternative to the current petrochemical-based plastics [1]. A benefit of ring-opening polymerizations is that they can be performed at mild temperatures without any need for the vacuum removal of the methanol/water molecules prepared during a condensation polymerization This reaction provides a greener synthetic approach for polyester synthesis compared to the polycondensation method. In order to increase the molecular weight of PE synthesized via the ring-opening polymerization method, many different metal-salen ligand catalysts and cocatalysts have been developed for the ROCOP of epoxides and anhydrides, including zinc [6], magnesium [7], chromium [8], cobalt [9], manganese [10], iron [11], aluminum [12], and nickel complex [13], most of which showed significantly higher activity in the presence of a nucleophilic cocatalyst, such as 4-dimethylaminopyridine (DMAP), bis(triphenylphosphine)iminium salts ([PPN]X), phosphines and ammonium salts. To the best of our knowledge, the barrier properties of the PEs prepared by the ROCOP approach have never been reported
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