Molecular Templates Do the Trick C a n a l c o m p l e x e s a c t as jigs t o h o l d m o n o m e r molecules in p l a c e w h i l e p o l y m e r i z a tion goes o n N a t u r e makes a host of stereospecific polymers - pro teins, nucleic Polymer acids, cellulose Chemistry are b u t a few examples. T o do this, biological systems use molecular templates—en zymes—to synthesize large, ordered polymer molecules. But polymers with ordered structures are almost unknown among man-made products. However, in t h e past three years, a number of stereospecific olefin polymers have been m a d e using stereospecific polymerization catalysts, such as Ziegler catalysts. T h e s e new polymers— isotactic polypropylene, synthetic "nat ural" rubber, for instance—are much stronger and tougher than conventional polyolefins. "Now, it appears that we have du plicated nature's way of synthesizing stereospecific polymers," General Electric's John F . Brown and Dwain M. White told the Division of Polymer Chemistry. Here's how: Erect a jig to hold the m o n o m e r while polymeriza tion takes place. For the jig, Brown and White use a canal complex; they polymerize by h i g h energy electron beam irradiation. F i n e l y , the com plex former is dissolved, leaving the polymer. 4 Λ Λ ACS I ΐ 4NATIONAL IVVMEETING
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• Tight Grip. Canal complexes are solid addition compounds in w h i c h the major component (complex former) forms a crystal lattice with long holes or canals in it, Brown explains. Mol ecules of the minor component, the monomer, are confined in the canals. When polymerization is started by brief exposure to a high energy elec tron beam, the canals hold the monomer molecules in a fixed position relative to one another. As a result, the growing polymer chain can form no branches because it is confined by the jig. The polymer chain can grow in just one way—head to tail addition of monomer molecules. Polymers produced this way h a v e com pletely ordered, frans-1,4-structures and are hard, tough materials, Brown says. To make ordered polybutadiene, Brown uses urea as the complex former. Reason: Urea forms a canal complex with t h e right size hole for butadiene. To form the complex, urea and buta diene are mixed and allowed to stand for a while in the cold. Polymeriza tion is started b y exposing the complex to a 1 m.e.v. electron beam. After polymerization is over, water washing removes the urea, leaves the polybuta diene. This is a hard, tough, crystal line solid. By contrast, ordinary poly butadiene is a rubbery material. Using canal complexes, Brown has polymerized monomers such as vinyl chloride, vinylklene chloride, cyclohexadiene, and acrylonitrile. As a complex former, he uses urea or thiourea. Big problem in using this method, Brown says, is that the size and shape of the monomer molecules must match the size of the canals very precisely. Result: Any one complex former will
' N a t u r e ' s W a y " to O r d e r e d P o l y m e r s
Schematic repre sentation of a diene polymeriza tion in a thiourea canal. Drawing is about to scale, shows the diene mole cules and thio urea molecules in an e d g e view and the relative mo lecular positions before and after polymerization
/ POLYMERIZATION (high energy electron beam)
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