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High molecular weight poly(ε-caprolactone)s (PCL) were synthesized via zwitterionic polymerization of ε-caprolactone initiated with N-heterocyclic carbenes. Ring-opening polymerization of ε-caprolactone (1 M) with carbenes 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (1) or 1,3,4,5-tetramethylimidazol-2-ylidene (3) affords cyclic PCL with molecular weights up to Mn= 114 000 g/mol. Crystallization of the cyclic PCL was investigated by synchrotron small-angle X-ray scattering experiments and differential scanning calorimetry. High molecular weight cyclic poly(ε-caprolactone) crystallizes with a similar lamellar thickness and long period spacing as linear poly(ε-caprolactone), but the crystallization of cyclic poly(ε-caprolactone) is faster than that of linear poly(ε-caprolactone) for molecular weights greater than 75 000 g/mol. These results imply that the polymer topology does not have a significant influence on the crystal structure or morphology but can have a significant influence on the rate of crystallization from the melt. [ABSTRACT FROM AUTHOR]

The zwitterionic ring-opening polymerization of lactide initiated by N-heterocyclic carbenes generates cyclic polylactides with well-defined molecular weights between Mn = 5000 and 30 000 g/mol with narrow polydispersities (Mw/Mn ≤ 1.31). These zwitterionic polymerizations are extremely rapid (kp = 48.7 M-1 s-1), but also exhibit exceptional control of molecular weight and molecular weight distribution. The unusual kinetic features of these zwitterionic polymerizations are illuminated with kinetic and mechanistic investigations, which implicate a mechanism that involves a slow initiation step (second order.in [M]), a propagation step (first order in [M]) that is much faster than initiation (kj = 0.274 M-2 s-1), cyclization (kc = 0.0575 s-1), and depropagation (kd = 0.208 s-1). Numerical and stochastic simulations of the kinetic data provide a kinetic rationale for the evolution of molecular weight with monomer conversion: the molecular weights increase with increasing monomer conversion, exhibit a nonzero intercept near 0% monomer conversion, and are relatively insensitive to the initial monomer-to-initiator ratio. The observed narrow molecular weight distributions are due to a high rate of propagation relative to cyclization and chain transfer. Kinetic simulations define the kinetic criteria under which the active zwitterions remain in solution; these simulations were substantiated by chain-extension experiments, which provide experimental evidence for chain extension of the zwitterions and reinitiation by the N-heterocyclic carbenes liberated upon macrocyclization. The kinetic model rationalizes some of the unique features of zwitterionic ring-opening polymerization and provides a useful mechanistic framework to optimize these polymerizations as a strategy to generate well-defined cyclic polyesters. [ABSTRACT FROM AUTHOR]

The article discusses the ring-opening polymerization (ROP) of lactones using organometallic catalysts. According to the authors, the living of ROP of lactide yields a linear relationship between monomer conversion and molecular weight and poly(lactide) with a narrow polydispersity (PDI). They add that the benefits of living ROP have enabled the controlled synthesis of block, graft and star polymers. They also mention the characteristics of a living chain-growth polymerization.

The article discusses a study that reveals that saturated carbene 1,3-dimesitylimidazol-2-ylidene mediates the ring-opening polymerization of four-membered lactones to give cyclic polyesters through a novel mechanism involving the reversible collapse of zwitterionic intermediates to neutral imidazolidine spirocycles. Further efforts are directed toward expanding the synthetic utility of spirocycles and synthesizing macrocycles with functional groups and topology.

We previously reported on a rheological study of Langmuir films of poly(tert-butyl methacrylate) (PtBMA), and this work describes further studies on this system. Here, surface pressure−area (−A) isotherms and interfacial shear rheology experiments are performed to better understand the effects of two modifications:  varying the temperature between 5 and 40 °C and introducing small amounts of carboxylic acid groups by partially hydrolyzing the polymer. Increased temperature produced isotherms with lower plateau surface pressures, p, and values shifted to lower areas above p. Film properties transition from being primarily viscous to being dominated by elasticity as pis crossed for all temperatures studied, even as the value of pchanges. Increasing the hydrolysis fraction leads to isotherms shifted to lower areas throughout the curve and higher pvalues. Both temperature variation and chemical modification are believed to affect the relative importance of polymer−polymer and polymer−subphase interactions. [ABSTRACT FROM AUTHOR]

Background: Telbivudine has been suggested to induce hepatitis B surface antigen (HBsAg) decline to the similar degree as pegylated interferon. We aimed to investigate whether telbivudine could further decrease HBsAg titer in patients who maintain undetectable serum hepatitis B virus (HBV) DNA after initial entecavir treatment. Methods: In this open-label trial, patients who had serum HBsAg and HBV DNA levels ≥1,000 IU/mL and <60 IU/mL, respectively, following entecavir (0.5 mg/day) treatment for HBeAg-positive chronic hepatitis B were randomized to either switch treatment to telbivudine (600 mg/day, n = 47) or continue entecavir (n = 50) for 48 weeks. Results: The baseline characteristics were comparable between groups including HBsAg levels (median, 3.41 log10 IU/mL). All patients had undetectable HBV DNA and normal alanine aminotransferase level. At week 48, the mean change in serum HBsAg levels was not significantly different between the telbivudine and entecavir groups (-0.03 log10 IU/mL vs. -0.05 log10 IU/mL; P = 0.57). No patient experienced HBsAg seroclearance or HBsAg decline >0.5 log10 IU/mL. Eleven patients (23.4%) in the telbivudine group, but none in the entecavir group, experienced virologic breakthrough (P < 0.001). Seven patients (14.9%) exhibited genotypic resistance mutations (M204I +/- L180M) during the virologic breakthrough. Conclusion: Sequential therapy with entecavir followed by telbivudine resulted in a high rate of virologic breakthrough and drug-resistance without any beneficial effect on HBsAg decline. These results do not support the use of low genetic barrier drugs as a switch treatment strategy in patients who achieve virologic response with high genetic barrier drugs. Trial Registration: NCT01595685 (date of trial registration: May 8, 2012). [ABSTRACT FROM AUTHOR]