Showing papers by "Marc A. Hillmyer published in 2001"

Polymerization of lactide and related cyclic esters by discrete metal complexes

Abstract: This perspective highlights recent research on the preparation of polyesters by the ring-opening polymerization of cyclic esters employing well-characterized metal complexes. Particular focus is placed on the preparation of polylactide because of environmental advantages: it is biodegradable and its feedstock, lactide, is a renewable resource. A recurring theme is the correlation of precatalyst structure, often by X-ray crystallography, with polymerization activity and selectivity. Through this systematic approach to the deconvolution of catalyst structure/reactivity relationships, improved mechanistic understanding has been attained and key design criteria required for the development of new catalysts that exert control over the molecular parameters of polyesters and related copolymers have been revealed.

Polylactide stereocomplex crystallites as nucleating agents for isotactic polylactide

Abstract: A nucleation efficiency scale for isotactic poly(L-lactide) (PLLA) was obtained with self-nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self-nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D-lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25–15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 300–313, 2001

Polyethylene-poly(L-lactide) diblock copolymers: synthesis and compatibilization of poly(L-lactide)/polyethylene blends

Abstract: A model polyethylene-poly(L-lactide) diblock copolymer (PE-b-PLLA) was synthesized using hydroxyl-terminated PE (PE-OH) as a macroinitiator for the ring-opening polymerization of L-lactide. Binary blends, which contained poly(L-lactide) (PLLA) and very low-density polyethylene (LDPE), and ternary blends, which contained PLLA, LDPE, and PE-b-PLLA, were prepared by solution blending followed by precipitation and compression molding. Particle size analysis and scanning electron microscopy results showed that the particle size and distribution of the LDPE dispersed in the PLLA matrix was sharply decreased upon the addition of PE-b-PLLA. The tensile and Izod impact testing results on the ternary blends showed significantly improved toughness as compared to the PLLA homopolymer or the corresponding PLLA/LDPE binary blends. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2755–2766, 2001

Lactide polymerization activity of alkoxide, phenoxide, and amide derivatives of yttrium(III) arylamidinates

Abstract: In quest of new, single-site catalysts for cyclic ester polymerizations, a series of mononuclear yttrium(III) complexes of N,N'-bis(trimethylsilyl)benzamidinate ([L TMS ] - ) and hindered N,N'-bis-(2,6-dialkylaryl)toluamidinates ([L Et ] - , aryl = Et 2 C 6 H 3 , and [L iPr ] - , aryl = i Pr 2 C 6 H 3 ) were synthesized and characterized by X-ray diffraction: L 2 TMS Y(μ-Cl) 2 Li(TMEDA) (1), L 2 TMS Y(OC 6 H 2 t Bu 2 Me) (2), L 2 TMS Y(OC 6 H 3 Me 2 ) 2 Li(THF) 4 (3), L 2 TMS Y(μ-O t Bu) 2 Li(THF) (4), L iPr Y[N(SiMe 2 H) 2 ] 2 (THF) (5), L 2 Et Y(THF)(Cl)(μ-Cl)Li(THF) 3 (6), and L 2 Et Y[N(SiMe 2 H) 2 ] (7). Coordination numbers ranging from five to seven were observed, and they appeared to be controlled by the steric bulk of the supporting amidinate and alkoxide, phenoxide, or amide coligands. Complexes 2-5 and 7 are active catalysts for the polymerization of D,L-lactide (e.g., with 2 and added benzyl alcohol, 1000 equiv of D,L-lactide were polymerized at room temperature in less than 1 h, with polydispersities less than 1.5). The neutral complexes 2, 5, and 7 were more effective than the anionic complexes 3 and 4. In addition, the presence of the more hindered amidinate ligands [L Et ] - and [L iPr ] - on yttrium-amides slowed the polymerizations (7 < 5 < Y[N(SiMe 2 H) 2 ] 3 ).

Perfectly alternating copolymer of lactic acid and ethylene oxide as a plasticizing agent for polylactide

Abstract: The ring-opening polymerization of 3-methyl-1,4-dioxan-2-one (MDO) mediated by a catalytic system composed of Y[N(TMS)2]3 and benzyl alcohol (BnOH) led to a new polymer (PMDO) comprised of perfectly alternating lactic acid and ethylene oxide repeat units. Samples of PMDO were characterized by NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). In a series of MDO polymerizations ([MDO]0 = 3.0 M in toluene, between −30 and 60 °C), the equilibrium monomer concentrations were measured. The thermodynamic parameters for the polymerization reaction were determined: ΔHp° = −12.1 ± 0.5 kJ mol-1 and ΔSp° = −42 ± 2 J mol-1 K-1. The glass transition temperature (Tg) of PMDO was determined to be ≈−24 °C by differential scanning calorimetry and found to vary little over the molecular weight range studied (3−21 kg/mol). Mixtures of low-molecular-weight PMDO and atactic polylactide (PLA) were prepared by solution casting and subsequent anne...

A simple and mild route to highly fluorinated model polymers

Abstract: We have developed a simple and mild method for the fluorination of polybutadiene based on the addition of perfluoroalkyl iodides (RfI) to carbon−carbon double bonds. Triethylborane (Et3B) was utilized to initiate this free radical addition to model polybutadiene (PBD) homopolymers and a polystyrene−polybutadiene block copolymer at room temperature. Optimized reaction conditions led to consumption of more than 95% of the double bonds and preservation of narrow molecular weight distribution after the modification. We propose that the reaction undergoes a cyclization pathway rather than open-chain addition and that five-member ring structures are formed during the addition of RfI to 1,2-PBD. From 1H NMR spectroscopy, we estimate that 83% of the double bonds in the 1,2-PBD cyclized with their neighbors. This agrees well with a theoretical prediction by Flory for random irreversible cyclization between neighboring polymer repeat units. We also demonstrate the selective fluorination of the 1,2-PBD block in a po...

A study of the Lewis acid-base interactions of vinylphosphonic acid-modified polybutadiene-coated zirconia.

Abstract: Polybutadiene-coated zirconia (PBD-ZrO2) is very useful for reversed-phase separations under a wide variety of conditions. Its excellent chemical (pH = 1−13) and thermal (up to 150 °C) stability distinguish it from silica-based reversed phases. Just as with silica-based phases, zirconia's surface chemistry significantly influences the chromatography of certain classes of analytes. Zirconia's hard Lewis acid sites can be chromatographically problematic. Analytes such as carboxylic acids strongly interact with these sites on PBD-ZrO2 and do not elute. Addition of phosphate or other strong, hard Lewis bases to the eluent brings about elution, but the resulting peak is often tailed and broad. Typically, cationic solutes are more retained in the presence of phosphate or fluoride due to adsorption of the Lewis base additives and the concomitant development of a negative charge on the surface. This Coulombic interaction can be used to optimize selectivity, but the reversed-phase−cation-exchange retention can pro...

Adhesion of polymer-inorganic interfaces by nanoindentation

Abstract: Nanoindentation combined with atomic force microscopy was applied to measure the fracture toughness of polystyrene/glass interfaces. Film delamination occurs when the inelastic penetration depth approximately equals or exceeds the film thickness. The delamination size was accurately measured using atomic force microscopy. Using multilayer indentation and annular-plate analyses, the interfacial fracture toughness was then assessed. The values obtained from the two analyses are in good agreement with the fracture toughness of the interface being approximately 350 mJ/m2. By appropriate fracture surface characterization, it was shown that fracture occurs along the polystyrene/glass interface. Crack arrest marks were observed, and their possible cause discussed. On the basis of the morphology of the fracture surface, the fracture toughness was also evaluated using a process zone analysis. The result agrees well with those obtained from the other two analyses.

Block Copolymer Synthesis

Abstract: The efficient synthesis of structurally well-defined block copolymers is exciting and important area of research. This article describes several methods for the preparation of di- and triblock copolymers. Examples are taken from the literature and include anionic, cationic, free-radical and metal-catalyzed polymerizations that are useful for the preparation of these hybrid materials. In addition, examples that combine these polymerization mechanisms are described.