Abstract
Substituted polyglycolides having two asymmetric centers are attractive alternatives to materials derived from petroleum because of their biocompatibility and biodegradability. The conformational behavior of various substituted polyglycolides has been investigated by both quantum mechanical and molecular dynamics approaches. Polymethylglycolide (polylactide) and polyphenylmethylglycolide in RS or SR forms are predicted to adopt 27 ribbon type structures with φ, ψ values of ±30, ±50 or +30 or +50 respectively stabilised by carbonyl-carbonyl interactions. Isopropylglycolide and isobutyl-glycolide having branching at β & γ positions respectively in their side chains can be realized in all SS form with φ, ψ values lying in right handed helical region. In addition to carbonyl-carbonyl interactions, the hydrophobic interactions between the side chains in isopropylgly-colide the C-H-O interactions also contributes to the stability. With cyclic side chains directly attached to Cα of backbone, polyphenylglycolide (polymandelide) and polycyclohexylglycolide are found to adopt left handed helical structure without hydrogen bonds in RR form, stabilised by stacking interactions and hydrophobic interactions respectively. In all the forms of polyphenylglycolide & polycyclohexylglycolide, the cyclic side chains are found to be locked into unfavourable gauche plus conformation. The stability of substituted polyglycolides has been analyzed in terms of various interactions. The carbonyl-carbonyl interactions in all the conformations of all forms of substituted polyglycolides are found to be of highly shielded parallel motif with only one short carbon-oxygen interaction. Simulation studies of substituted polyglycolides in water give a good insight of the approach of water molecules to the backbone.
Highlights
Conventional polymers such as polyethylene, polypropylene and polystyrene persist for many years after disposal due to their resistance to degradation which creates serious environmental problems
It is obvious from the results that substituted polyglycolide with smallest side chain i.e. polymethylglycolide is found to be most stable in the alternate R, S or alternate S, R forms with φ, ψ values in the left & right or right & left helical region
The conformational behavior of substituted polyglycolides has been found to depend on the nature of side chains
Summary
Conventional polymers such as polyethylene, polypropylene and polystyrene persist for many years after disposal due to their resistance to degradation which creates serious environmental problems This has lead to the search of biodegradable polymers which degrade by the action of microorganisms such as bacteria, fungi and may be broken down by non-enzymatic processes such as chemical hydrolysis and the degraded products should be non-toxic to living organisms directly or indirectly. Polylactides/polymethylglycolide (PLA), as one of commercially available and biomass-derived polymers, has been attracting much attention from both academic and industrial view points. It has tremendous potential in both traditional and non-traditional applications where thermoplastics are employed. Modern and emerging production technologies have lowered its production costs [22,23, 30,31] as it can be obtained from a renewable
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