Mechanistic investigations of lipase-catalyzed degradation of polycarbonate in organic solvents.

The biodegradation of an engineering thermoplastic, poly (bisphenol-A carbonate) (BPAPC), was carried out using three different lipases from Candida antarctica (CAL), Candida rugosa (CRL) and porcine pancreas (PPL) in water-miscible (tetrahydrofuran) and water-immiscible (chloroform) solvents for 10 days. The degradation was monitored by gel permeation chromatography and Fourier transform infrared spectroscopy. Maximum degradation (ca. 60% reduction in M(n)) of BPAPC was observed in THF with PPL when compared to the control without the enzyme. The degradation products in all the experiments were bisphenol-A and 4-α-cumyl phenol suggesting that the lipases act through an end-chain scission on the polymer. The degradation of BPAPC in THF was in the order of PPL>CAL>CRL, while in CHCl(3) it was CRL>CAL>PPL. To understand this disparity, and to probe the mechanistic aspects of degradation, molecular dynamics investigations were performed on the lipases with model BPAPC in both the solvents. The results also suggested that catalytic triad (Ser, His, Asp/Glu) was involved in the hydrolysis of carbonate bond leading to release of bisphenol-A. These data provide us the basic understanding of the degradation mechanism and a novel methodology for degrading polycarbonate.

Biodegradation of physicochemically treated polycarbonate by fungi.

Two fungal strains isolated from soil and a commercial white-rot fungus, Phanerochaete chrysosporium NCIM 1170 (SF2), were tested for biodegradation of untreated, UV-, and thermal-treated bisphenol A polycarbonate (PC). The isolated strains based on 18S rDNA analysis were characterized as Engyodontium album MTP091 (SF1) and Pencillium spp. MTP093 (SF3). About 5.4% weight loss and 40% reduction in M(n) were observed for UV-treated polycarbonate in one year with SF2 strain. An increase in surface energy and oxygen content and a reduction in methyl index indicated oxidation of PC during this period. PC exposed to the SF1 strain showed a 15 degrees C decrease in glass transition temperature, indicating an increase in the number of chain ends and, hence, an increase in the free volume of polymer. No bisphenol A, the monomer of PC, was detected during the study. NMR and FTIR spectra showed the formation of methyl groups due to pretreatments. EDAX analysis exhibited surface oxidation of the PC. The current study advocates that biodegradation of PC can be enhanced by pretreatments.

Biodegradation of aliphatic and aromatic polycarbonates.

Polycarbonate is one of the most widely used engineering plastics because of its superior physical, chemical, and mechanical properties. Understanding the biodegradation of this polymer is of great importance to answer the increasing problems in waste management of this polymer. Aliphatic polycarbonates are known to biodegrade either through the action of pure enzymes or by bacterial whole cells. Very little information is available that deals with the biodegradation of aromatic polycarbonates. Biodegradation is governed by different factors that include polymer characteristics, type of organism, and nature of pretreatment. The polymer characteristics such as its mobility, tacticity, crystallinity, molecular weight, the type of functional groups and substituents present in its structure, and plasticizers or additives added to the polymer all play an important role in its degradation. The carbonate bond in aliphatic polycarbonates is facile and hence this polymer is easily biodegradable. On the other hand, bisphenol A polycarbonate contains benzene rings and quaternary carbon atoms which form bulky and stiff chains that enhance rigidity. Even though this polycarbonate is amorphous in nature because of considerable free volume, it is non-biodegradable since the carbonate bond is inaccessible to enzymes because of the presence of bulky phenyl groups on either side. In order to facilitate the biodegradation of polymers few pretreatment techniques which include photo-oxidation, gamma-irradiation, or use of chemicals have been tested. Addition of biosurfactants to improve the interaction between the polymer and the microorganisms, and blending with natural or synthetic polymers that degrade easily, can also enhance the biodegradation.