No abstract

A pulsed-gradient spin-echo NMR technique was used to measure the self-diffusion coefficients (D) of water and several different solute probe molecules in ternary poly(vinyl alcohol) (PVA)−water−solute systems, as a function of PVA concentration (up to 0.35 g/mL). The self-diffusion coefficient of water decreased with increasing PVA concentration, in a manner consistent with the Mackie−Meares obstruction effect model, and was independent of the polymer molecular weight, degree of hydrolysis, or the presence of the solute probes. The self-diffusion coefficients of the solute probes (methanol, tert-butanol, formamide, acetic acid, trimethylamine, tetramethylammonium cation, and poly(ethylene glycol) of molecular weight 400 and 4000) decreased with increasing PVA concentration and increasing probe size. The free volume theory could be used to describe the self-diffusion of solute probes only. The dependence of all the probe self-diffusion coefficients on polymer concentration could also be described using a stretched exponential function of the form D = D0 exp(−αc ν) proposed by Phillies. With increasing PVA molecular weight, the scaling parameter ν generally decreased while the scaling parameter α generally increased. The analysis permitted an estimate of the correlation length (ξ), corresponding to the mesh size of the polymer network, which is found to decrease from 20−30 Å in the semidilute regime to 4−6 Å in the moderately concentrated regime and was independent of PVA molecular weight.

Despite the tremendous progress made in the field of biomedical engineering, many challenges still remain to be addressed-especially in the design of new materials. The biodegradable synthetic polymers currently used for biomedical applications are almost exclusively based on short aliphatic moieties, such as lactides, glycolides, e-caprolactone, and sebacic acid, which all display relatively "hard" mechanical properties that adjust poorly to those of tissues, and therefore cause considerable stress mismatches at the interface responsible for necrosis or abnormal regeneration. [1] Materials based on bile acids show great promise for drug delivery [2] and controlled release [3] applications, and their rigid steroidal backbone and amphiphilicity appear to make them candidates of choice for fine-tuning the mechanical and interfacial properties of synthetic degradable polymers. However, reports on main-chain bile acid based polyesters, polyamides, and polyurethanes are still scarce in the literature, and their synthesis constitutes a real challenge, especially when higher molecular weights are required. Most techniques used to date rely on the use of toxic coupling agents.Ring-opening polymerization (ROP) is a very versatile technique that has been applied to the synthesis of polyesters with a controlled molecular weight. In virtually all cases, small strained cycles (3-8-membered rings) are used and enthalpy drives the polymerization. However, macrocycles, including those based on esters and alkenes, could be polymerized, and afforded appreciably high molecular weights, depending on the conditions.[4] In these cases, polymerization is driven by entropy, as described by the Jacobson-Stockmayer theory for ring-chain equilibria. [5] Entropy-driven ring-opening polymerization (ED-ROP) therefore appears to be a method of choice for the synthesis of high-molecular-weight polyesters based on bile acids. Furthermore, the use of this technique means that the use of large amounts of coupling agents often required in polycondensations can be avoided, thus considerably lowering the toxicity of the material. Furthermore, metathesis chemistry has proved to be a powerful tool for both the preparation of macrocycles [6] and the polymerization of alkenes. [3a, 7] We describe here the synthesis of novel macrocycles by the simple ring-closure metathesis (RCM) of two flexible chains attached to a bile acid core through ester bonds, and their entropy-driven ring-opening metathesis polymerization (ED-ROMP) using ruthenium-based Grubbs catalysts. The polymers obtained show typical rubberlike elasticity, with elongation moduli that favorably compare to those of soft tissues and elastin [1c, 8] (Figure 1) and constitute, to the best of our knowledge, the first example of degradable thermoplastic amorphous elastomers.Cyclic bile acids 1 (38-membered ring) and 2 (35-membered ring) were synthesized (Scheme 1) in relatively high yields (73 and 59 %, respectively) from their corresponding dienes, at high dilution.[9] Cyclic oligomers were ...

We have measured the self-diffusion coefficients of a series of solute probes, including ethylene glycol and its oligomers and polymers in aqueous solutions and gels of poly(vinyl alcohol) (PVA) using the pulsed-gradient spin−echo NMR techniques. In an effort to link the diffusion properties of small and large molecules in polymer systems, we have selected this group of diffusant probes with various molecular weights, ranging from 62 to 4000. The self-diffusion coefficients of the solute probes decrease with increasing PVA concentrations (from 0 to 0.38 g/mL) and with increasing molecular size of the probes. The temperature dependence of the self-diffusion coefficients has also been studied for ethylene glycol and poly(ethylene glycol)s of molecular weights 600 and 2000. Energy barriers of 30.0, 36.5, and 39.0 kJ/mol have been calculated respectively for the probes, in the temperature range 23−53 °C. The experimental data are used to fit a new physical model of diffusion (Petit et al. Macromolecules, 1996, 29, 6031), which is shown to be successful in describing the effects of polymer concentration, temperature, and molecular size of the diffusants on the self-diffusion coefficients of small and large molecular probes in the polymer system.

In dilute heteropolymer solutions, a limited number of the chains, just like proteins, could collapse and associate to form a stable/metastable mesoglobular phase between single-chain globules and macroscopic precipitates. Recently, we found that inserting more hydrophobic comonomers into a thermally sensitive chain backbone surprisingly led to the formation of smaller mesoglobules in water. This apparent contradiction to our conventional wisdom can be satisfactorily explained in terms of the overlooked viscoelastic effect; namely, hydrophobic association inside each mesoglobule increases the chain relaxation time (τ e). When it becomes much longer than the interaction time (τc) of two colliding mesoglobules, each mesoglobule behaves like a tiny nonadhesive "glass" ball. This stabilization mechanism is completely different from thermodynamic consideration in which one normally tries to make the particle surface hydrophilic so that τ c is reduced.

We propose a new physical model for the interpretation of the diffusion of solvent and other solute molecules in polymer solutions. In this model, the polymer solution is regarded as a network where the diffusing molecules have to overcome periodic energy barriers of equal magnitude, where the distance between the barriers corresponds to the correlation length in polymer solutions as defined in de Gennes' scaling theory. We demonstrate that this model applies to the diffusion of small molecules in polymer matrices, such as ternary aqueous systems of poly(vinyl alcohol) (PVA) and binary organic solutions of poly(methyl methacrylate) (PMMA). The model successfully interprets the diffusion of solute molecules and water in aqueous polymer solutions and that of solvent molecules in organic polymer solutions. In particular, the effects of polymer concentration and temperature on diffusion can be predicted. An energy barrier of 21 kJ/mol is calculated from the variable temperature studies of self-diffusion in the range 23−53 °C carried out on a PVA−water−tert-butyl alcohol ternary system.

Reversible addition−fragmentation chain transfer (RAFT) polymerization was used to prepare a series of homo- and copolymers of N-alkyl-substituted acrylamides. The acrylamide monomers have similar chemical structures, but they all exhibit difference in reactivities, especially between N-monosubstituted and N,N-disubstituted acrylamides during the RAFT process. Results from size exclusion chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry indicate chain transfers to monomers are easier to occur for N-monosubstituted polyacrylamides, with negative deviations of the molecular weights from the theoretical values. The high transfer activity makes them good macro-chain transfer agents (CTAs). The stronger electron-donating conjugative effect renders the disubstituted acrylamides more reactive, meaning that they can react more readily with monosubstituted polyacrylamide−CTAs to form a sequent block. Tri- and tetrablock copolymers with multiple thermosensitivity have been successfully prepared and tested following these guiding principles.

Oxygen inhibits free radical polymerization and yields polymers with uncured surfaces. This is a concern when thin layers of resin are being polymerized, or in circumstances where conventional means of eliminating inhibition are inappropriate. In this study, we tested the hypothesis that viscosity, filler content, and polymerization temperature modify oxygen diffusion in the resin or the reactivity of radical species, and affect the degree of conversion near the surface. Confocal Raman micro-spectroscopy was used to measure monomer conversion from the surface to the bulk of cured resins. Increased viscosity was shown to limit oxygen diffusion and increase conversion near the surface, without necessarily modifying the depth of inhibition. The filler material was shown to increase, simultaneously, oxygen diffusivity and the viscosity of the resin, which have opposite effects on conversion. Polymerization at a temperature above approximately 110 degrees C was shown to eliminate oxygen inhibition.