Tacticity-Induced Changes in the Micellization and Degradation Properties of Poly(lactic acid)-block-poly(ethylene glycol) Copolymers

Poly(lactic acid)-block-poly(ethylene glycol) copolymers (PLA-b-PEG) featuring varying tacticities (atactic, heterotactic, isotactic) in the PLA block were synthesized and investigated for their micellar stability, degradation, and thermal properties. Utilizing tin(II) bis(2-ethylhexanoate), aluminum salan, and aluminum salen catalysts, the copolymers were synthesized through the ring-opening polymerization of d-, l-, rac-, or a blend of l- and rac-lactide using monomethoxy-poly(ethylene glycol) as a macroinitiator. The critical micelle concentration, which reflects the micellar stability, was probed using a fluorescence spectroscopic method with pyrene as the probe. The copolymers were degraded in a methanolic solution of 1,5,7-triaza-bicyclo[4.4.0]dec-5-ene and the degradation was measured by H-1 NMR spectroscopic and gel permeation chromatographic analyses. Differential scanning calorimetry and thermogravimetric analysis provided information on the thermal properties of the copolymers. Atactic and heterotactic microstructures in the PLA block resulted in lower micellar stability, as well as faster degradation and shorter erosion time compared to polymers with high isotactic enchainment (P-m). By modification of the P-m, micellar stability, degradation, and erosion rates of the copolymers can be tuned to specific biomedical applications. Interestingly, while tin(II) bis(2-ethylhexanoate) and aluminum salan-catalyzed PLA-b-PEG copolymers exhibited similar micellization behavior, the aluminum salen-catalyzed PLA-b-PEG exhibited unique behavior at high micelle concentration in the presence of the pyrene probe. This unique behavior can be attributed to the disintegration of the micelles through the interactions of long isotactic stereoblock segments

Flexible and rigid core molecules in the synthesis of poly(lactic acid) star polymers

This study presents the synthesis and physical characterization of a series of structurally well-defined star-shaped poly(lactic acids). Polymer stars are prepared from a series of multifunctional alcohol cores including flexible polyols pentaerythritol and dipentaerythritol (four-armed and six-armed cores, respectively) and rigid substituted arenes tri(hydroxymethyl)benzene and hexa(hydroxymethyl)benzene. Utilizing a tin(II) octanoate catalyst, arms of 10 monomer units long are built from rac-lactide and l-lactide to form atactic and isotactic star polymers. Polymers were subsequently characterized by means of NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Our results support previous work that suggests that the length of the individual arms, not the total molecular weight, correlates to physical characteristics including glass, melt, crystallization, and decomposition temperatures. In addition, differences between core molecules suggest that the chemical nature of the core can significantly alter the physical properties of the star polymer. Trends in crystallization and glass transition temperatures relative to the core used merit further study and correlate closest to the molecular weight and the number of arms emanating from the star core. It is also clear that the rigidity provided by aromatic cores has a significant effect on the melting temperatures of these macromolecules

An Expanded Range of Catalysts for Synthesizing Biodegradable Polyphosphonates

In this paper we expand the scope of catalysts able to mediate the ring opening polymerisation of the phosphonate monomer 2-methyl-1,3,2-dioxaphosphalone-2-oxide. A range of nitrogen bases efficiently catalyse the reaction, each with pK as of 19 or above; lower pK a bases do not ring open the monomer. Aluminium based catalysts supported by salen and salan ligand frameworks also afford exceptional control, with conversions in excess of 99% and dispersities under 1.1. Together, these studies significantly expand the scope of catalysts to prepare this biodegradable, non-toxic, water soluble polymer. Additionally, we report efforts to expand the monomer scope for these catalysts, showing that altering ring structure and substitution can strongly inhibit productive ring opening. </jats:p

Aliphatic polyester polymer stars: synthesis, properties and applications in biomedicine and nanotechnology

abstract: A critical review: the ring-opening polymerization of cyclic esters provides access to an array of biodegradable, bioassimilable and renewable polymeric materials. Building these aliphatic polyester polymers into larger macromolecular frameworks provides further control over polymer characteristics and opens up unique applications. Polymer stars, where multiple arms radiate from a single core molecule, have found particular utility in the areas of drug delivery and nanotechnology. A challenge in this field is in understanding the impact of altering synthetic variables on polymer properties. We review the synthesis and characterization of aliphatic polyester polymer stars, focusing on polymers originating from lactide, epsilon-caprolactone, glycolide, beta-butyrolactone and trimethylene carbonate monomers and their copolymers including coverage of polyester miktoarm star copolymers. These macromolecular materials are further categorized by core molecules, catalysts employed, self-assembly and degradation properties and the resulting fields of application (262 references)

Controlled radical polymerization of vinyl acetate mediated by a vanadium complex

Initiation of the polymerization of vinyl acetate with azobis(isobutyronitrile) in the presence of a vanadium bis(iminopyridine) complex generates vanadium-capped dormant polymer chains with excellent correlation between molecular weight and conversion and good molecular weight distributions.JID: 9610838; 2010/02/17 [aheadofprint]; 2010/03/09 [epublish]; ppublishSource type: Electronic(1

Tacticity Control in the Synthesis of Poly(lactic acid) Polymer Stars with Dipentaerythritol Cores

The synthesis of a family of polymer stars with arms of varied tacticities is discussed. The effect of polymer tacticity on the physical properties of these polymer stars is presented. Dipentaerythritol cores support six poly(lactic acid) (PLA) arms. Lewis acidic tin and aluminum catalysts control the polymerization to afford polymer stars of variable tacticity. The analysis of these polymers by NMR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and differential scanning calorimetry reveals the effects of tacticity control on the physical properties of the polymer stars. Preliminary decomposition studies suggest that the biodegradation profile of a polymer star may also be tuned by stereochemical control. This is the first systematic altering of tacticity in PLA polymer stars, showing that polymer tacticity can have a great impact on star properties.PT: J; UT: BIOSIS:PREV20110005905

Morphology of Poly(styrene-co-butadiene) Random Copolymer Thin Films and Nanostructures on a Graphite Surface

We studied the morphology of poly­(styrene-<i>co</i>-butadiene) random copolymers on a graphite surface. Polymer solutions were spin coated onto graphite, at various concentrations and molecular weights. The polymer films and nanostructures were imaged using atomic force microscopy. Above the overlap concentration, thin films formed. However, total wetting did not occur, despite the polymers being well above their <i>T</i>_g. Instead, dewetting was observed, suggesting the films were in a state of metastable equilibrium. At lower concentrations, the polymers formed networks, nanoislands, and nanoribbons. Ordered nanopatterns were observed on the surface; the polymers orientated themselves due to π–π stacking interactions reflecting the crystalline structure of the graphite. At the lowest concentration, this ordering was very pronounced. At higher concentrations, it was less defined but still statistically significant. Higher degrees of ordering were observed with poly­(styrene-<i>co</i>-butadiene) than polystyrene and polybutadiene homopolymers as the copolymer’s aromatic rings are distributed along a flexible chain, which maximizes π–π stacking. At the two lowest concentrations, the size of the nanoislands and nanoribbons remained similar with varying molecular weight. However, at higher concentrations, the polymer network features were largest at the lowest molecular weight, indicating that in this case, a large proportion of shorter chains stay on top of the adsorbed ones. The contact angles of the polymer nanostructures remained mostly constant with size, which is due to the strong polymer/graphite adhesion dominating over line tension and entropic effects

Organometallic mediated radical polymerization

Controlled radical polymerization has become increasingly important over the past decade and a half, allowing for the facile synthesis of specific macromolecular architectures with excellent control over the chemical and physical properties. This article presents an organized and detailed review of one particular CRP technique, organometallic mediated radical polymerization (OMRP), focusing on the individual catalysts developed, their efficacy and monomer scope. Rhodium, cobalt, molybdenum, osmium, iron, palladium, titanium, chromium and vanadium mediated radical polymerizations are presented alongside organo-main group mediated reactions. A separate section reviews the types of copolymers which have been synthesized using OMRP techniques. An attempt is made to unify the many disparate names which have previously been used for OMRP by virtue of the common mechanistic aspects displayed by the different catalyst systems. A mechanistic discussion highlights the similarities and differences between these systems and examines the interplay between reversible termination and degenerative transfer OMRP and competing 1-electron redox processes