Alec B. Scranton

Methods for Synthesis of Hydrogel Networks: A Review

Abstract Hydrogels are macromolecular networks that swell, but do not dissolve, in water. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from crosslinks between network chains. Many materials, both naturally occurring and synthetic, fit the definition of hydrogels. Crosslinked dextrans and collagens are examples of natural polymers that are modified to produce hydrogels. Classes of synthetic hydrogels include poly(hydroxyalkyl methacrylates), poly(acrylamide), poly(N-vinyl pyrrolidone), poly(acry1ic acid), and poly(vinyl alcohol).

A mechanistic investigation of a three-component radical photoinitiator system comprising methylene blue, N-methyldiethanolamine, and diphenyliodonium chloride

Three-component systems, which contain a light-absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usually an iodonium salt), have emerged as efficient, visible-light-sensitive photoinitiators. Although three-component systems have been consistently found to be faster and more efficient than their two-component counterparts, these systems are not well understood and a number of distinct mechanisms have been reported in the literature. In this contribution, photodifferential scanning calorimetry and in situ, time-resolved, laser-induced, steady-state fluorescence spectroscopy were used to study the initiation mechanism of the three-component system methylene blue, N-methyldiethanolamine and diphenyliodonium chloride. Kinetic studies based upon photodifferential scanning calorimetry reveal a significant increase in polymerization rate with increasing concentration of either the amine or the iodonium salt. However, the laser-induced fluorescence experiments show that while increasing the amine concentration dramatically increases the rate of dye fluorescence decay, increasing the DPI concentration actually slows consumption of the dye. We concluded that the primary photochemical reaction involves electron transfer from the amine to the dye. We suggest that the iodonium salt reacts with the resulting dye-based radical (which is active only for termination) to regenerate the original dye and simultaneously produce a phenyl radical (active in initiation) derived from the diphenyliodonium salt.

Polymeric emulsifiers based on reversible formation of hydrophobic units

Emulsions consist of mixtures of immiscible liquids where one liquid is finely dispersed within the continuous phase of another. They are generally not thermodynamically stable: the dispersion tends to separate over time. Aqueous emulsions, widely used in food, pharmaceutical, and many other industries, are often stabilized by block copolymers containing alternating hydrophilic and hydrophobic segments (typically based on ethylene oxide/propylene oxide diblock and triblock systems) that penetrate into the oil and aqueous phase, respectively,. Here we describe a conceptually new type of emulsifier whose hydrophobic blocks are formed spontaneously and reversibly by the complexation of hydrophilic segments, thereby allowing the stabilizing properties of the system to be switched on and off. We illustrate this approach using a comb-type graft copolymer containing a poly(methacrylic acid) backbone and short grafts of poly(ethylene glycol). The uncomplexed polymer is hydrophilic, but acidic conditions induce the formation of hydrogen-bonded hydrophobic complexes between parts of the backbone and the grafts. As a result, the grafted copolymer forms alternating blocks of hydrophilic (uncomplexed) and hydrophobic (complexed) segments that stabilize acidic emulsions. An increase in pH suppresses complex formation and thus leads to the breakup of the emulsion. Emulsion tests show that although the performance of the grafted copolymers is not yet competitive with existing emulsifiers, this approach provides an efficient strategy for the design of fully reversible emulsifiers.

Biomedical applications of polyelectrolytes

Polyelectrolytes are used in a variety of biomedical systems, including dental adhesives and restorations; controlled release devices; polymeric drugs, prodrugs, and adjuvants; and biocompatible materials. This article provides a review of biomedical applications of polyelectrolytes with emphasis on recent developments. For completeness, an overview of the methods for polyelectrolyte synthesis is provided along with a description of the unique properties of polyelectrolyte solutions, gels, and complexes which make them useful in biomedical applications. The discussion of dental materials focuses on the recent developments in glass-ionomer cements and novel organic polyelectrolyte adhesives since these materials are replacing the traditional zinc carboxylates. The section on controlled release applications includes a brief overview of recent developments in the mature areas of coatings, matrices and binders; and provides more in-depth discussions of the advanced responsive, bioadhesive, and liposomal systems that have emerged in recent years. Finally, descriptions of the recent work in polyelectrolytes as biocompatible materials as well as drugs or prodrugs are provided.

Photo-differential scanning calorimetry studies of cationic polymerizations of divinyl ethers

Abstract Photo-differential scanning calorimetry experiments were used to determine effective kinetic constants for propagation and termination for a series of unsteady-state divinyl ether polymerizations at different temperatures and light intensities. For these cationic photopolymerizations the reaction rate and limiting conversion were both found to increase as the reaction temperature was increased. At all temperatures the profile for the propagation rate constant kp exhibited a dramatic increase at the start of the reaction, plateaued at a value between 5 and 301 mol−1 s−1 (depending upon temperature) and then decreased as the reaction reached a limiting conversion owing to trapping of the active centres. The effective termination rate constant kt was very low, with active centre lifetimes approaching 20 min. The overall activation energy for polymerization was found to be 26.5 ± 3.2 kJ mol−1.

Polymerization reaction dynamics of ethylene glycol methacrylates and dimethacrylates by calorimetry

Abstract Profiles of reaction rate as a function of time were obtained for free radical crosslinking polymerizations of ethylene glycol methacrylates and dimethacrylates using differential scanning calorimetry. Copolymerizations of ethylene glycol monomethacrylates with small amounts of dimethacrylate crosslinking agents were studied in addition to homopolymerizations of dimethacrylates. In the former case, the effects of the crosslinking agent, initiator, solvent, and the pendent ethylene glycol chain length were investigated. Typically, a sharp increase in the reaction rate was observed which was attributed to autoacceleration. The time period before the onset of autoacceleration increased and the maximum reaction rate decreased as the solvent concentration or the pendent chain length were increased or the crosslinking agent concentration was decreased. These results were explained by an increase in the mobility of the growing polymer leading to a decrease in the magnitude of the gel effect. Studies of dimethacrylates yielded similar results for the effect of solvent. However, the maximum reaction rate increased when the number of repeating units between the methacrylate functionalities was increased from one to four, but decreased when the number of repeating units was increased from four to nine. Photopolymerization experiments indicated that radicals with significantly different lifetimes might be present in dimethacrylate homopolymerizations.

A mechanistic investigation of the three‐component radical photoinitiator system Eosin Y spirit soluble, N‐methyldiethanolamine, and diphenyliodonium chloride

Photo-DSC and in situ, time-resolved, laser-induced, steady-state fluorescence spectroscopy were used to study the initiation mechanism of the three-component system: Eosin Y spirit soluble (EYss), N-methyldiethanolamine, and diphenyliodonium chloride. Kinetic studies based on photo-DSC revealed that the fastest polymerization occurred when all three components were present (the next fastest was with the dye/amine pair, and the slowest was with the dye/iodonium pair). However, the laser-induced fluorescence experiments showed that the pairwise reaction between the eosin and iodonium bleaches the dye much more rapidly than does the reaction between the eosin and amine. We concluded that although a direct eosin/amine reaction can produce active radicals in the three-component system, this reaction is largely overshadowed by the eosin/iodonium reaction, which does not produce active radicals as effectively. We proposed that the amine reduces the oxidized dye radical formed in the eosin/iodonium reaction back to its original state as well as the simultaneous production of an active initiating amine-based radical. Because of the difference in the pairwise reaction rates for eosin/amine and eosin/iodonium, it is likely that this regeneration reaction was the primary source of active radicals in the three-component eosin/amine/iodonium system.

The effect of oxygen on the three‐component radical photoinitiator system: Methylene blue, N‐methyldiethanolamine, and diphenyliodonium chloride

In this article, we extend our mechanistic study of the three-component radical photoinitiator system, consisting of methylene blue (MB), N-methyldiethanolamine, and diphenyliodonium chloride, by investigating the influence of oxygen on the rate of the consumption of MB dye. The mechanism involves electron transfer/proton transfer from the amine to the dye as the primary photochemical reaction. Oxygen quenches the triplet state of the dye, leading to retardation of the reaction. We used time-resolved steady-state fluorescence monitoring to observe the MB concentration in situ in both a constant oxygen environment and a sealed reactor as the dye is consumed via photoreaction. In the sealed reactor, we observed a retardation period (attributed to the presence of oxygen) followed by rapid exponential decay of the MB fluorescence after the oxygen was depleted. On the basis of the impact of the amine and iodonium concentrations on the fluorescence intensity and the duration of the retardation period, our proposed mechanism includes an oxygen-scavenging pathway, in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism. The iodonium salt is an electron acceptor, acting to reoxidize the neutral dye radical back to its original state and allowing it to reenter the primary photochemical process.

Equilibrium swelling of poly(methacrylic acid-g-ethylene glycol) hydrogels: Effect of swelling medium and synthesis conditions

Responsive hydrogel networks comprising of poly(methacrylic acid) (PMAA) backbone and oligomeric ethylene glycol (PEG) grafts were synthesized by free-radical solution polymerization and their equilibrium swelling properties were characterized in aqueous solutions of a homologous series of alcohols. These hydrogels are known to exhibit swelling transitions in response to external stimuli which lead to formation or disruption of hydrogen-bonded complexes between the backbone and the grafts. Swelling studies performed in aqueous mixtures of methanol, ethanol and propanol revealed that the effectiveness of an alcohol in breaking the PMAA/PEG complexes increased as the aliphatic segment length of the alcohol was increased. These results confirm the importance of hydrophobic interactions for stabilizing the complexes. Studies performed to determine the effect of the synthesis conditions on the equilibrium swelling properties revealed that the equilibrium degree of swelling increased as the solvent fraction during synthesis was increased. Finally, molecular stimulations revealed that it is sterically possible to form complexes with a 1:1 stoichiometry between chains of poly(methacrylic acid) and poly(ethylene glycol) with essentially no additional bond strain.

CONSUMPTION OF THE MOLECULAR OXYGEN IN POLYMERIZATION SYSTEMS USING PHOTOSENSITIZED OXIDATION OF DIMETHYLANTHRACENE

ABSTRACT It is well known that the presence of oxygen in free radical polymerization systems leads to an inhibition period and a lowered ultimate conversion. In this contribution, we report a method for consuming molecular oxygen photochemically before the polymerization takes place, thereby allowing the reaction to proceed in an oxygen-free environment. The method is based on the generation of singlet oxygen by reaction of the ground state oxygen with the excited triplet state of the singlet oxygen generator (a porphyrin, Znttp). The singlet oxygen is then consumed by reaction with a second compound (the singlet oxygen trapper, dimethylanthracene). The possible factors that might affect the efficiency of the singlet oxygen generation/trapping processes were discussed and the effectiveness of a dimethylanthracene/Znttp combination for consumption of oxygen was investigated in two acrylate systems of different viscosity.