The use of 13 carbon nuclear magnetic resonance (NMR) spectroscopy in the molecular characterization of macromolecules has advanced our knowledge into structural areas that have been nearly impossible to measure by other spectroscopic techniques. Innovative applications have led to determinations of polymer configurational distributions, comonomer sequence distributions, average sequence lengths, structure and distribution of short chain branches, and analyses of nonreactive end groups. As a result, the importance of 13C NMR to the field of polymer science cannot be overemphasized. The key to the success of 13C-NMR studies in defining polymer molecular structure has been a structural sensitivity which encompasses more than just a few functional groups or carbon atoms. A sensitivity to polymer repeat unit sequences of lengths from two to as many as five, seven, and even nine contiguous repeat units [1,2] has been observed. Of course, any structural technique that senses a unique response from as f...

A review of high resolution liquid 13carbon nuclear magnetic resonance characterizations of ethylene-based polymers

Rate coefficients of free-radical polymerization deduced from pulsed laser experiments

Fluoroelastomers: synthesis, properties and applications

A new and promising tool in membrane research is the detergent-free solubilization of membrane proteins by styrene-maleic acid copolymers (SMAs). These amphipathic molecules are able to solubilize lipid bilayers in the form of nanodiscs that are bounded by the polymer. Thus, membrane proteins can be directly extracted from cells in a water-soluble form while conserving a patch of native membrane around them. In this review article, we briefly discuss current methods of membrane protein solubilization and stabilization. We then zoom in on SMAs, describe their physico-chemical properties, and discuss their membrane-solubilizing effect. This is followed by an overview of studies in which SMA has been used to isolate and investigate membrane proteins. Finally, potential future applications of the methodology are discussed for structural and functional studies on membrane proteins in a near-native environment and for characterizing protein-lipid and protein-protein interactions.

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The styrene-maleic acid copolymer: a versatile tool in membrane research

Polycarbonate urethanes (PCNUs) have been used as a replacement for traditional biomedical polyether-urethanes due to their reported resistance to oxidative biodegradation. However, relatively little is known about their hydrolytic stability in the presence of inflammatory derived enzymes. This has in part motivated the current study relating to the effect of hard segment chemistry and the microdomain structures generated by such chemistry, on the cholesterol esterase (CE) catalyzed hydrolysis of PCNUs. The bulk structures of the studied materials were characterized using gel permeation chromatography (GPC), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR) for their bulk structures, and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for their subsurface structures. 14C-labeled PCNUs were incubated with CE (400 units/mL), for a period of 10 weeks (pH 7.0 at 37 degrees C), and radiolabel release was used to monitor the degradation. The results showed that all of the polymers synthesized in this study were susceptible to CE-catalyzed hydrolytic degradation, and that the extent of degradation was highly dependent on the nature of hard segment interactions within the polymer and at the surface. More specifically, the degree of phase separation and soft segment crystallinity were found to be less important in comparison to the hydrogen bonding among the carbonate and urethane linkages. The rank of the different chemical groups' susceptibility to hydrolysis was as follows: nonhydrogen bonded carbonate > nonhydrogen bonded urethane > hydrogen bonded carbonate > hydrogen bonded urethane. The findings suggest that the degree of hydrogen bonding, when processed into a polyurethane material could be an important parameter to consider in the design of new biostable polyurethane products.

Enzyme-induced biodegradation of polycarbonate-polyurethanes: dependence on hard-segment chemistry.

The copolymerization of styrene and acrylonitrile in bulk at 60 °C has been investigated by measurement of the copolymer/comonomer composition relationship and of the monomer sequence distributions using C NMR. Alternative models for the mechanism of the copolymerization have been evaluated in the most general forms, with allowance for nonzero conversions, by deriving reactivity ratios from composition data and then comparing predicted and experimental sequence distributions. The system shows significant deviations from the terminal model. Compositions cannot differentiate between penultimate and complex participation models, both of which give significant improvement over the terminal model. The measured sequence distributions are quite close to the predictions of the penultimate model but substantially different from those of the complex participation model, showing clearly that the penultimate model is the most appropriate of the models considered. We have obtained r = 0.22, r = 0.03, r = 0.63, and r = 0.09. No significant improvement was observed with the antepenultimate model.

Analysis of the mechanism of copolymerization of styrene and acrylonitrile

The DEPT technique for the generation of CH, CH, and CH C NMR subspectra has enabled the resolution and assignment of broad and overlapping aliphatic resonances in copolymers. For styreneacrylonitrile copolymers, the CH resonances gave dyad fractions that were in agreement with the previously established copolymer structure. For styrene-maleic anhydride copolymers, three broad resonances were observed in the methylene subspectrum. These resonances were assigned to SSS, SSM and MSS, and MSM triads in preference to SS, trans-MS, and cis-MS dyads on the basis of a variety of evidence. This sequence information can be applied to the study of the mechanism of copolymerization. Partial esterification of the anhydride unit in some styrene-maleic anhydride copolymers was detected by generating the CH subspectrum. A modification of the DEPT sequence to obtain partially relaxed subspectra enabled T and T relaxation times of both C and H nuclei to be measured for each of the resolved resonances.

Applications of DEPT experiments to the carbon-13 NMR of copolymers: poly(styrene-co-maleic anhydride) and poly(styrene-co-acrylonitrile)

The role of donor-acceptor complexes in polymerization

The mechanism of copolymerization of styrene and maleic anhydride in bulk at 60 °C has been investigated by measurements of copolymer compositions using 13C NMR and of monomer sequence distributions using DEPT pulse programs to distinguish CH2 subspectra, which have been attributed to styrene-centered triads. The strong tendency to alternation observed by most previous workers was confirmed. Nonlinear, least-squares analysis of the compositions of copolymers prepared over a wide comonomer composition range showed that the penultimate model gave a better fit to the data than the terminal model with a confidence level >95%. There was only a penultimate effect for styrene-terminated chain radicals. A similar analysis of the composition data of Bamford and Barb for comonomer mixtures with high styrene contents gave reactivity ratios in good agreement with the values derived from our composition data. The complex-participation and complex-dissociation models could also be fitted to the composition data but there was a strong correlation between different reactivity ratios, indicating extensive minima on the hypersurface. The monomer sequence distributions were in good agreement with the predictions of both the penultimate and complex-participation models and could not distinguish between them.

Analysis of the mechanism of copolymerization of styrene and maleic anhydride

The first results are reported for the quantitative application of ESR spectroscopy to emulsion polymerization kinetics. When this technique, in conjunction with dilatometric rate measurements, was applied to the seeded emulsion polymerization of methyl methacrylate at 50 °C, the dependence of the propagation rate coefficient (k) on the weight fraction of polymer (W) was found to be k = k° (0.33 ≤ w≤ 0.84) and k exp{-29.8[w - 0.84]} (0.84 ≤ w≤ 0.99), where k° = 790 ± 300 dmmol s. These results reflect the passage of the latex particles through their glass transition point (at w= 0.84), leading to the propagation step being controlled by the diffusion of monomer to the propagation site. The observed decrease in kfor w> 0.84 appears to be less dramatic than that predicted by current free volume theories.

P.W. O'Sullivan

Papers

Analysis of the mechanism of copolymerization of styrene and acrylonitrile

Applications of DEPT experiments to the carbon-13 NMR of copolymers: poly(styrene-co-maleic anhydride) and poly(styrene-co-acrylonitrile)

The role of donor-acceptor complexes in polymerization

Analysis of the mechanism of copolymerization of styrene and maleic anhydride

Propagation rate coefficients from electron spin resonance studies of the emulsion polymerization of methyl methacrylate