In the last decades, biodegradable polymers have received much attention, and aliphatic polyesters are among the most interesting candidates for biodegradable fibres, nonwovens, films, sheets, bottles and injection-moulded products. High molar masses are an essential requisite in order to achieve good mechanical properties in polymeric materials, but there is an intrinsic difficulty in synthesising high molar mass aliphatic polyesters through a conventional polycondensation process.
During the polycondensation, high reaction temperature and long reaction time are necessary in order to make up for the decrease in the concentration of reactive end groups as the polymerisation reaction proceeds and the molar mass increases. Furthermore, decomposition may occur due to the high temperature during the polymerisation, thus imposing a restriction on the molar mass. The use of chain-extending agents, of heat stabilizers, as well as the technique of direct polycondensation in a solvent has been found useful to obtain high molar mass aliphatic polyesters.
We have performed the synthesis and the structural characterisation of a set of aliphatic co-polyesters, obtained from 1,4-butandiol and dimethyl esters of succinic, adipic and sebacic acids. High molar mass aliphatic homo- and co-polyesters (Mw from 62000 up to 210000) were synthesised by melt polymerisation starting from dimethyl esters and 1,4-butanediol, using an excess of 10% of 1,4-butanediol, in the presence of titanium (IV) butoxide as esterification catalyst (Scheme n.1: Synthesis of high molar mass homo- and co-polyesters)
The use of 10% excess of 1,4-butanediol with respect to the monomeric dimethyl ester in the polymerisation reaction was found essential to increase the polymer viscosity and consequently to obtain materials showing good filmability. Polyester materials were characterised by 1H NMR, SEC, DSC, X-ray and viscosity. Good filmability was achieved for all the polymers.
Unfortunately, biodegradable polyesters do not cover optimal thermal, mechanical and processing properties and this reduces in general their industrial applications.
The incorporation of aromatic units provides acceptable melting temperatures, such as in EASTAR BIO, a copolyester based on 1,4-butanediol, adipic acid and terephthalic acid. Correspondingly, the introduction of amide groups in the main chain, giving rise to strong intermolecular hydrogen-bond interactions, can significantly improve thermal and mechanical properties, combining good end-use, processing facilities and suitable biodegradability, even at relatively low molecular weight.
Several poly(ester amide)s have been recently proposed and their promising performances are at present under evaluation. A commercial series of poly(ester amide)s has been circulated with the trade name BAK, based on adipic acid, caprolactone and hexamethylene diamine as the amide components and 1,4-butanediol and ethylene glycol as the ester components.
Recently, we have performed the synthesis and the structural characterization of a set of poly(ester amide)s, obtained from sebacic acid and amino alcohols, with and without the use of a chain extender. High molecular weight poly(ester amide)s were obtained using a single-step polycondensation procedure, in the presence of tin(II) 2-ethylhexanoate, otherwise by means of a two step polymerization method, including melt polycondensation and chain extension reactions.
Related papers:
1. Giorgio Montaudo, Paola Rizzarelli
Synthesis and enzymatic degradation of aliphatic copolyesters
Polymer Degradation and Stability 70 (2000) 305-314
2. Paola Rizzarelli, Concetto Puglisi, Giorgio Montaudo
Soil burial and enzymatic degradation in solution of aliphatic co-polyesters
Polymer Degradation and Stability 85 (2004) 855-863
Top