Backbone-assisted reactions of polymers. VII. Kinetics of isomerization of poly[(chloromethyl)thiirane] in the absence of solvent

The kinetics of the repeating unit isomerization of poly[(chloromethyl)thiirane] have been determined at four temperatures in the absence of solvent. The reaction can be treated as a reversible first-order process with rate constants and activation energies similar to those observed in analogous reactions of β-chlorosulfides of low molecular weight. These observations are consistent with a mechanistic scheme that involves rate determining cyclization to a thiiranium ion intermediate, followed by rapid ring opening by chloride ion attack.     

Assembly of triple-stranded beta-sheet peptides at interfaces

A 30-residue peptide, BS30, which incorporates two proline residues to induce reverse turns, was designed to form a triple-stranded beta-sheet monolayer at the air-water interface. To discern the structural role of proline, a second peptide, BS30G, identical to BS30 but with glycine residues replacing proline, was prepared and examined in parallel fashion. Surface pressure-molecular area isotherms indicated a limiting area per molecule (ca. 460 A(2)) for BS30 that corresponds well to that estimated from the known dimensions of crystalline beta-sheet monolayers (492 A(2)). Comparable measurements on BS30G yielded a smaller molecular area (380 A(2)). Grazing incidence X-ray diffraction measurements performed on the BS30 monolayer at nominal area per molecule of 500 A(2), exhibited two Bragg peaks corresponding to 4.79 and 34.9 A spacings, consistent with formation of triple-stranded beta-sheet structures that assemble into two-dimensional crystallites at the air-water interface. Visualized by Brewster angle microscopy, BS30 monolayers displayed uniform, solidlike domains, whereas BS30G appeared to be disordered.     

Self-assembly of model DNA-binding peptide amphiphiles

Peptide amphiphiles combine the specific functionality of proteins with the engineering convenience of synthetic amphiphiles. These molecules covalently link a peptide headgroup, typically from an active fragment of a larger protein, to a hydrophobic alkyl tail. Our research is aimed at forming and characterizing covalently stabilized, self-assembled, peptide-amphiphile aggregates that can be used as a platform for the examination and modular design and construction of systems with engineering biological activity. We have studied the self-assembly properties of a model DNA-binding amphiphile, having a GCN4 peptide as the headgroup and containing a polymerizable methacrylic group in the tail region, using a combination of small-angle X-ray scattering, small-angle neutron scattering, and cryo- transmission electron microscopy. Our results reveal a variety of morphologies in this system. The peptide amphiphiles assembled in aqueous solution to helical ribbons and tubules. These structures transformed into lamella upon DNA binding. In contrast with common surfactants, the specific interaction between the headgroups seems to play an important role in determining the microstructure. The geometry of the self-assembled aggregate can be controlled by means of adding a cosurfactant. For example, the addition of SDS induced the formation of spherical micelles.     

Protein-based materials,toward a new level of structural control

Through billions of years of evolution nature has created and refined structural proteins for a wide variety of specific purposes. Amino acid sequences and their associated folding patterns combine to create elastic, rigid or tough materials. In many respects, nature's intricately designed products provide challenging examples for materials scientists, but translation of natural structural concepts into bio-inspired materials requires a level of control of macromolecular architecture far higher than that afforded by conventional polymerization processes. An increasingly important approach to this problem has been to use biological systems for production of materials. Through protein engineering, artificial genes can be developed that encode protein-based materials with desired features. Structural elements found in nature, such as beta-sheets and alpha-helices, can be combined with great flexibility, and can be outfitted with functional elements such as cell binding sites or enzymatic domains. The possibility of incorporating non-natural amino acids increases the versatility of protein engineering still further. It is expected that such methods will have large impact in the field of materials science, and especially in biomedical materials science, in the future.     

Preparation and polymerization of the two isomeric (chloromethyl)oxetanes

2-(Chloromethyl)oxetane was prepared in 8% overall yield from 2-propen-1-ol by protection of the alcohol with dihydropyran, epoxidation, ring expansion with dimethylsulfoxonium methylide, acid deprotection, and chlorination with triphenylphosphine in CCl₄. 3-(Chloromethyl)oxetane was prepared in 22% overall yield by hydroboration-oxidation of 3-chloro-2-chloromethyl-1-propene followed by base-catalyzed cyclization. Each of the oxetanes was converted to the corresponding elastomeric homopolymer by treatment with a triethylaluminum–acetylacetone–water mixture. Poly[2-(chloromethyl)oxetane] was found to be similar to polyepichlorophydrin in reactivity toward benzoate ion, whereas poly[3-(chloromethyl)oxetane] is more reactive by a factor of 2.     

Functional polymers. VI. Preparation and polymerization of methyl 3-vinylsalicylate,methyl 3-vinylacetylsalicylate, 3-vinylsalicylic acid,and 3-vinylacetylsalicylic acid

The title compounds were synthesized and their homopolymerizations and copolymerizations with styrene and a number of acrylic monomers were investigated. Methyl 3-vinylacetylsalicylate was prepared in a five-step synthesis from 2-ethylphenol in an overall yield of 40%. Methanolysis of this compound gave methyl 5-vinylsalicylate in 63% yield. Hydrolysis of methyl 3-vinylsalicylate gave a nearly quantitative yield of 3-vinylsalicylic acid which could be acetylated to 3-vinylacetylsalicyclic acid (3-vinyl aspirin). 3-Vinylsalicylic acid derivatives were readily homopolymerized and copolymerized with styrene, methyl methacrylate, and methacrylic acid, and 3-vinylsalicyclic acid was copolymerized with a number of vinyl and acrylic monomers. Copolymer compositions were determined by examination of ¹H-NMR spectra.     

Radical copolymerization of 2-ethylacrylic acid and methacrylic acid

Copolymers of 2-ethylacrylic acid (EAA) and methacrylic acid (MAA) were prepared in bulk and in N,N-dimethylformamide (DMF). Although precipitation of the copolymers was observed in bulk, the reaction mixtures remained apparently homogeneous in DMF. Best-fit terminal-model reactivity ratios were determined by a nonlinear least squares technique to be r_MAA = 1.14 and r_EAA = 0.23 in bulk, and r_MAA = 1.91 and r_EAA = 0.09 in 50% DMF solution. Examination of ¹³C-NMR spectra provided convincing evidence for the formation of statistical copolymers. Copolymerizations richer in MAA provided copolymers of higher molecular weights.     

Infrared study of the kinetics and mechanism of adsorption of acrylic polymers on alumina surfaces

In this paper, we studied the kinetics of the adsorption of poly(methyl methacrylate), PMMA, onto native aluminum oxide surfaces by X-ray photoelectron spectroscopy and reflection-absorption infrared spectroscopy, with the intent of tracking the various changes observed in the infrared spectrum of the adsorbed polymer layer as a function of adsorption time. Specifically, we utilized the relative changes in the absorption bands of the carbonyl, carboxylic acid, and carboxylate groups to determine the sequence of events that culminate in the formation of bonds between carboxylate groups on hydrolyzed PMMA and specific sites on the aluminum oxide surface. We have shown that the adsorption process involves the hydrolysis of a fraction of the methoxy groups of the PMMA to generate COOH groups. Unlike previous assumptions, the formation of COOH groups on the PMMA chains does not constitute a sufficient condition for the actual chemisorption of the polymer chains onto the metal oxide surface. To promote bonding, the acid groups must undergo dissociation to form the carboxylate groups, followed subsequently by actual bond formation, that is, active anchoring, on the surface. This process is mediated by the aluminum oxide sites on the surface in the presence of water. Hence, the adsorption process occurs via a two-step mechanism, in which the first step, that is, the hydrolysis step, is a necessary but insufficient condition and the second step, that is, the anchoring step, is largely dependent on the type of interfacial chemistry possible for a particular polymer-metal oxide surface, the polymer conformation, the molecular weight, and the flexibility of the adsorbing molecules.