David G. Rethwisch

Enzymatic and chemoenzymatic approaches to synthesis of sugar-based polymer and hydrogels

We have prepared a variety of sugar-based polymers using enzymatic and chemoenzymatic synthetic methods. In enzymatic synthesis, the regioselectivity of enzymes was used to copolymerize a sugar and a diester to make poly(sucrose adipate). Sugars including sucrose and glucose derivatives such as α-methyl glucoside have been used. In chemoenzymatic synthesis, the regioselectivity of enzymes was used to make the sugar-based monomers, while standard free radical or condensation methods were used for polymerization. The sugars could be in the backbone (poly(sucrose adipamide)) or as a pendant group (poly(sugar acetylene)s, poly(sugar acrylate)s, and poly(sugar methacrylate)s). We also prepared poly(sugar acrylate) and poly(sugar methacrylate) hydrogels and their copolymers with acrylic acid. The degree of swelling of these gels was studied as a function of pH and ionic strength. Potential applications of these materials include water absorbents, food additives and drug delivery systems.

Sucrose diacrylate : A unique chemically and biologically degradable crosslinker for polymeric hydrogels

A series of degradable hydrogels based on different vinyl monomers such as acrylamide, sucrose-1′-acrylate, and acrylic acid were synthesized using sucrose-6,1′-diacrylate (SDA) as a crosslinking agent. SDA was prepared by enzymatic transesterification of vinyl acrylate with sucrose in pyridine. Base catalyzed hydrolysis of SDA in aqueous solution was studied as a function of pH. As expected, hydrolysis of SDA was faster at higher pHs such that poly(acrylamide), poly(sucrose 1′-acrylate), and poly(acrylic acid) hydrogels underwent substantial degradation at and above pH 7, 9, and 13, respectively. The degradation was characterized by changes in the swelling ratios of the hydrogels indicating breakage of the crosslinking agent. Degradation of the hydrogels at their chemically stable pHs was studied in presence of enzymes. Enzymes, including pepsin and a fungal Lipase, were able to degrade the poly(acrylamide) hydrogel at pH 4 and 5, respectively. Poly(acrylic acid) hydrogel was degraded in presence of a fungal protease at pH 7.8.

Role of metallic promoters in the direct synthesis of methylchlorosilanes

Overall reaction rates and selectivities for dimethyldichlorosilane were determined for the direct reaction of silicon with methyl chloride to produce methylchlorosilanes. Copper was used as the catalyst, and zinc, tin, and aluminum were investigated as promoters. To unambiguously determine effects of the promoters, high-purity reagents were used for contact mass sample preparation. Others have reported that tin significantly increases both reaction rate and selectivity for samples prepared using technical grade silicon. However, promotion of high purity samples with tin alone (i.e., SiCuSn) significantly decreased both the reaction rate and selectivity. These conflicting results are explained in terms of the aluminum impurities present in technical grade silicon. Tin is suggested to promote this reaction synergistically with zinc and aluminum by lowering the melting points and surface tensions of the catalyst/promoter alloys which form on the surface.

Photoresponsive control of ion-exchange in leucohydroxide containing hydrogel membranes

Abstract Polyacrylamide hydrogels containing bis-[4-{dimethylamino}phenyl] {4-vinyl-phenyl} methyl leucohydroxide (TPMLH) have been studied as ion selective membranes. An understanding of the factors influencing the ionic diffusion rates based on photochromism is presented. The observed increase in methyl orange (anionic) flux after UV irradiation was mainly due to the photoinduced generation of fixed cationic charges in the membrane as opposed to a combined effect of fixed charge generation and membrane structural changes. Methyl orange fluxes in a fully activated membrane (in pH 4) were close to an order of magnitude higher than UV activated membranes. The flux of 4-dimethylamino pyridine (a neutral species) in both systems was essentially unchanged by irradiation. Modeling of the predicted fluxes based on the Nernst–Planck and Donnan equilibrium equations using no adjustable parameters gave good correlation with the observed results.

Selective flux of organic liquids and solids using nanoporous membranes of polydicyclopentadiene

Membranes were fabricated from the ring opening metathesis polymerization of dicyclopentadiene with the Grubbs first generation catalyst, and the permeability of twenty-one molecules through them was studied. Both polar and apolar molecules with molecular weights from 101 to 583 g mol−1 permeated these membranes with values for flux of 10−5 to 10−6 mol cm−2 h−1 but selected molecules did not permeate them and had flux 104 to 105 times slower. The difference in flux was large between molecules that permeated and those that did not permeate, but no trend was observed that correlated flux with molecular weight or hydrophobicity. Rather, molecules that did not permeate the membranes had large cross-sectional areas that led to low rates of diffusion within the highly cross-linked polydicyclopentadiene membranes. The degree of cross-linking within the polydicyclopentadiene membranes was measured using infrared spectroscopy and approximately 84% of the dicyclopentadiene monomer had reacted to form cross-links. These are the first organic solvent nanofiltration membranes that separate molecules with molecular weights from 100 to 600 g mol−1 based on cross-sectional areas.

Quantification of 13C in solids using CPMAS-DD-NMR spectroscopy

Abstract Quantitative analysis of several standard materials is used to evaluate various techniques for analysis of 13 C cross polarization magic angle spinning dipolar decoupling (CPMAS-DD)-NMR spectra. These results suggest that uncertainty in the kinetics of the initial cross-polarization process may lead to systematic errors in the fitting of CPMAS-DD-NMR data. Methods are suggested to minimize the effects of this potential source of error. These results indicate that 13 C CPMAS-DD-NMR can give quantitatively reliable results.

The direct synthesis of methylchlorosilanes I. Steady-state and transient reaction kinetics

Abstract Reaction kinetics under steady-state and transient conditions were used to study the effects of the catalyst (copper) and promoters (zinc and tin) on the direct reaction of methyl chloride with silicon to produce methylchlorosilanes. Use of infrared spectroscopy allowed rapid analysis of the gas-phase product composition. This allowed the accurate measurement of the apparent activation energy and the apparent reaction order for each product. An improved method for calculating direct reaction rates is also introduced. Addition of Zn as a promoter to Cu did not change the overall reaction rate but increased the selectivity for more methylated silanes. Tin promotion increased the selectivity for more chlorinated silanes, while having little affect on the overall reaction rate. Copromotion with zinc and tin increased the overall reaction rate; however, the selectivity for dimethyldichlorosilane (DMDC) was dependent on the concentration of tin. A trace amount of tin (ca. 0.005-0.01 wt%) produced the highest selectivity for DMDC; however, increasing the amount of tin to 0.2 wt% decreased the selectivity. We propose that the primary role of zinc is to direct the dissociative adsorption of McCl such that the methyl group is attached to the Si and the Cl group is attached to the Zn.

Gas solubility in glassy polymers: predictions from non-equilibrium EoS

Abstract Extension of well established EoS for fluid mixtures to the description of thermodynamic properties of non-equilibrium glassy polymeric phases is considered and corresponding calculations for pseudo-equilibrium gas content are performed. Results obtained using three different “tangent hard spheres chain” EoS are discussed and compared with experimetnal data for low-pressure gas solubility in glassy polycarbonate. While the overall result allows to conclude that tools developed herein may be confidently used for pure predictions of gas and vapor solubility in polymeric material below the glass transition temperature, interesting differences among the performance of different models were observed which can be attibuted to the different form of interaction potential considered.

USE OF ALCOHOLS AS COSOLVENTS IN ENZYME-FACILITATED TRANSPORT OF ORGANIC ACIDS THROUGH A LIQUID MEMBRANE

Abstract Further applications and developments were studied for the use of enzyme-facilitated liquid membranes (EFLM). The use of ethanol as a cosolvent in the aqueous phase of the EFLM enhanced the facilitated transport of phenyl-acetic acid by more than 13 times relative to the case where water with low butanol concentrations was used, and more than 1000-fold relative to non-facilitated transport in the butanol/water system. This transport enhancement is proposed to result from a favorable shift in the equilibrium between phenylacetic acid and its ester (ethyl phenylacetate) which is the species transported across the liquid membrane. The enhancement goes through a maximum with increasing ethanol content that is proposed to result from the observed denaturing of the enzyme at high ethanol concentrations. A simple model is proposed which qualitatively describes the observed enhancement by ethanol blends.

Infrared spectroscopic determination of methyl-chlorosilanes from the direct process reaction

Abstract Infrared spectroscopy was used to determine the distribution of products from a flow reactor. Gas-phase methylchlorosilanes, which were formed in the copper-catalyzed reaction of methyl chloride with silicon, flowed through a gas cell used for the infrared measurements. The silanes could be determined simultaneously at pressures of 1–100 Torr in the presence of methyl chloride at atmospheric pressure. The calculated results were similar to those obtained by standard gas chromatographic techniques; however, the infrared method required only 15 s for data collection.