typically involve movement of polar functional groups from the surface to greater depths and are driven by a decrease in surface energy. The functional groups do not migrate far and in some cases can be forced to return to the surface by contact with a polar medium. This mobility can effect desired surface properties and likely plays an important role in further chemistry of these surfaces.
Certainly significant advances have been made in this field in the past decade, but with respect to the highly evolved disciplines of organic polymer synthesis and synthetic organic chemistry, the field is in its infancy in terms of elegance and versatility. Few modified polymer surface systems have been studied with any degree of detail, and there is not currently a model system that can be used for comparisons. Key questions concern the applicability of the tenets of solution organic chemistry to surfaces, the utility of functionalized surfaces, correlations between surface structure and properties, and the new types of materials that can be prepared with surface synthetic techniques.
Organisms carry out an astonishing array of complex chemical transformations via coupled enzymatic reactions. The fact that enzymes operate selectively and under mild conditions of temperature, pressure, and solvent has generated justifiable interest in the use of enzymes in polymer synthesis. Work to date has addressed three issues: (1) in vitro use of isolated enzymes to catalyze polycondensations, (2) use of organisms (either wild type or genetically altered) to produce monomers that are subsequently converted to polymers by conventional methods, and (3) use of organisms to produce polymers directly.
The in vitro use of enzymes to catalyze polycondensations has been shown to offer several advantages. Enantioselective polymerizations have been reported, and polymerizations of monomers containing reactive functional groups (e.g., epoxy diesters) have been accomplished without destruction of the reactive functionality. Limitations of the method arise in part from the fact that many of the polymerizations of interest are best run in solvents for which the enzyme is poorly suited (i.e., in nonaqueous systems). As a result of this—and perhaps other—factors, molecular weights of the polymers prepared in this way are modest—generally Mn < 15,000. Recent developments in nonaqueous enzymology, including the use of protein engineering to improve enzymatic activity in organic solvents, offer promise for a solution to this problem.
Several intriguing reports have discussed the use of microorganisms to produce polymer intermediates. For example, 4,4'-dihydroxybiphenyl, an intermediate in the manufacture of engineering thermoplastics, has been produced in yields of greater than 95 percent in a one-step fermentation process. Microbial syntheses of long-chain dicarboxylic acids (useful in the production of polyamides and polyesters), and of 5,6-dihydroxy-1,3-cyclohexadiene (an intermediate in the preparation of polyparaphenylene) have also been reported. The opportunities