The objective of this study was to develop a mathematical model that describes and even predicts the hydrolytic degradation of aliphatic polyesters. From literature, it is known that the main process of degradation of aliphatic polyesters is the autocatalytic hydrolysis of ester bonds. Because of this hydrolysis, polymer chains are cleaved and the molecular weight will decrease. With time, the molecules become small enough for the system to start losing weight, since the small molecules dissolve in the aqueous medium. In addition, the crystallinity can change during degradation and influence the degradation rate of the polymer and the monomer ratio of a copolymer. In order to test the model several aliphatic polyesters were synthesized. The degradation behavior of these polymers was investigated by placing them in an aqueous environment (pH ) 7.4) at 37 °C. At certain time intervals, samples of the polymers were taken and analyzed. A mathematical model was developed based on the autocatalytic hydrolytic degradation mechanism of aliphatic polyesters and verified with the measured results. Up to now, no models are known that include the autocatalytic hydrolysis behavior of aliphatic polyesters. The calculations of the new developed model are compared with measured results. It shows that it describes the concentration change of all polymer chains present as function of the degradation time. Hence, the change in molecular weight distribution, the decrease of the average molecular weight and the mass loss of the polymer as function of the degradation time can be predicted. This is a major advancement with respect to any earlier developed model. The only unknown input parameter for the model is the hydrolysis rate constant. The mathematical model is valid for semicrystalline as well as amorphous polymers and for copolymers.
The autocatalytic equation derived in this study describes and even predicts the evolution of the number average molecular weight of aliphatic polyesters upon hydrolytic degradation. The main reaction in the degradation of aliphatic polyesters is autocatalytic hydrolysis of ester bonds, which causes the molecular weight to decrease. During hydrolysis of the ester bonds in the main chain of the polyester, the chains are cleaved and the end group concentrations will rise. The fundamentals of this equation are based on that principle. To validate the derived equation, the hydrolytic degradation of poly(4-methylcaprolactone), poly(epsilon-caprolactone), poly(d,l-lactide), and two different poly(d,l-lactide-co-glycolide) copolymers was monitored after immersion in a PBS buffer (pH = 7.4) at 37 degrees C. The number average molecular weight, mass loss, and crystallinity were determined after different time intervals. The experimental results confirm that hydrolytic degradation of aliphatic polyesters is a bulk erosion process. When comparing the M(n), calculated with the new autocatalytic equation, with the experimental results, it was found that the new model can predict the decrease of the M(n) upon hydrolytic degradation for semicrystalline and amorphous polymers, as well as for copolymers, without the need for complicated mathematics and excessive input parameters. This is a major improvement with respect to earlier proposed models in literature.
The chemoenzymatic synthesis of block copolymers from a bifunctional initiator using enzymatic ring-opening polymerization (eROP) and ATRP in two consecutive steps was investigated. First, a polycaprolactone (PCL) macroinitiator was obtained via an enzymatic ring-opening polymerization initiated by the bifunctional initiator. By carefully managing the water activity in the system, the amount of PCL not initiated by the bifunctional initiator was reduced to <5%. Moreover, comparison of the results from 1 H NMR and MALDI-ToF of PCL obtained from different bifunctional initiators revealed an influence of the initiator structure on the initiation behavior in the enzymatic reaction. Block copolymers were obtained in a subsequent ATRP. The combination of various characterization techniques such as GPC, GPEC, and DSC provided clear evidence of the block structure of the polymers.
The synthetic parameters for the chemoenzymatic cascade synthesis of block copolymers combining enzymatic ring-opening polymerization (EROP) and atom transfer radical polymerization (ATRP) in one pot were investigated. A detailed analysis of the mutual interactions between the single reaction components revealed that the ATRP catalyst system could have a significant inhibiting effect on the enzyme activity. The inhibition of the enzyme was less pronounced in the presence of multivalent ligands such as dinonyl bipyridine, which thus could be used in this reaction as an ATRP catalyst. Moreover, the choice of the ATRP monomer was investigated. Methyl methacrylate interfered with EROP by transesterification, whereas t-butyl methacrylate was inert. Block copolymers were successfully synthesized with this cascade approach by the activation of ATRP after EROP by the addition of the ATRP catalyst and, with lower block copolymer yields, by the mixing of all the components before the copolymerization. A detailed kinetic analysis of the reactions and the structure of the block copolymers showed that the first procedure proceeded smoothly to high block copolymer yields, whereas in the latter a noteworthy amount of the poly(t-butyl methacrylate) homopolymer was detected. V V C 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: [4290][4291][4292][4293][4294][4295][4296][4297] 2006
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