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SYNTHESIS AND PROCESSING: GENERAL METHODS 11 2 Synthesis and Processing: General Methods MOLECULAR SYNTHESIS The focus of much of present polymer chemistry is on improving synthesis methods for better control of the composition and structure of existing as well as new materials. The basic premise is that specificity in molecular structure rules molecular self-assembly and morphology, which in turn dictates properties. Precise control of the composition and structure of novel macromolecules (molecular weights of 104 to 106) to achieve a well-defined synthesis of a particular macromolecule is extremely difficult. Normally, only average quantities are specified: chain length, composition, end groups, stereoregularity, branching, and, most importantly, sequencing. In the mid-1950s the discovery of certain catalysts that controlled the tacticity (stereoregularity) of -olefins permitted the introduction of isotactic polypropylene, currently a 100-million-pound product worldwide. Previous production methods resulted in a mixed-tacticity material with low melting point and poor physical and mechanical properties. Another example is anionic polymerization, which enabled the width of the molecular weight distribution to be significantly narrowed and afforded the polymer chemists standard reference materials for determining a host of physical properties that were known to depend on molecular weight (but not precisely how they depended). Recent advances include group-transfer polymerization (Webster et al., 1983), aluminum- catalyzed ring-opening polymerization (Aida and Inoue, 1981), and cationic polymerization of vinyl ethers (Higashimura, 1986). These techniques provide versatile chemistry to synthesize well-defined macromolecules and efficient routes to the rich varieties of molecular architecture (e.g., diblocks, stars, multiblock, and functionalized end groups) available with macromolecules.
SYNTHESIS AND PROCESSING: GENERAL METHODS 12 The ultimate goal of synthetic chemists is to approach the specificity of nature in controlling the macromolecules of biology, which have extremely well-defined, yet complex, composition and stereoregularity and accomplish a wide variety of functions in a most elegant manner. As an example, consider the bacteriorhodopsin molecule (Figure 1). This macromolecule provides for light-activated proton transport across the cell membrane as part of the synthesis process of adenosine triphosphate (ATP). Every chain possesses just the correct group of residues to provide the necessary structure and chemical environment for its function. Researchers in synthetic polymer chemistry are only just beginning to utilize biotechnological approaches to produce synthetic polymers of heretofore never-achieved specificity. Thus far, molecular biologists have directed their efforts exclusively toward natural products. An initiative to focus the tremendous power of biotechnological techniques in the area of synthetic macromolecules is quite promising. Figure 1 Schematic of the bacteriorhodopsin macromolecule, which provides light-dependent proton transport from the inside to the outside of the cell (Khorana, 1987). Once a macromolecule has been synthesized, consolidation into a material depends, of course, on the detailed processing history. Polymers have characteristic relaxation times that are orders of magnitude slower than other substances, such that they have a long memory of their previous deformation
