Polymer Chemistry

Molecular Templates Do the Trick

Cana l comp lexes act as j igs t o ho ld monomer molecules in p lace w h i l e p o l y m e r i z a ­t ion goes o n

Nature makes a 4 Λ Λ ACS w h o l e host of I ΐ 4NATIONAL s t e r e o s p e c i f i c I V V M E E T I N G polymers - pro­

t e i n s , n u c l e i c acids, cellulose are b u t a few examples. To do this, biological

systems use molecular templates—en­zymes—to synthesize large, ordered polymer molecules. But polymers with ordered structures are almost unknown among man-made products.

However, in the past three years, a number of stereospecific olefin polymers have been made using stereospecific polymerization catalysts, such as Zieg-ler catalysts. These new polymers— isotactic polypropylene, synthetic "nat­ural" rubber, for instance—are much stronger and tougher than conventional polyolefins.

"Now, it appears that we have du­plicated nature's way of synthesizing stereospecific polymers," General Elec-tric's John F . Brown and Dwain M. White told the Division of Polymer Chemistry. Here's how: Erect a jig to hold the monomer while polymeriza­tion takes place. For the jig, Brown and White use a canal complex; they polymerize by h igh energy electron beam irradiation. Finely , the com­plex former is dissolved, leaving the polymer.

• Tight Grip. Canal complexes are solid addition compounds in which the major component (complex former) forms a crystal lattice with long holes or canals in it, Brown explains. Mol­ecules of the minor component, the monomer, are confined in the canals. When polymerization is started by brief exposure to a high energy elec­tron 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 have com­pletely 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 the right size hole for butadiene. To form the complex, urea and buta­diene are mixed and allowed to stand for a while in the cold. Polymeriza­tion is started by exposing the complex to a 1 m.e.v. electron beam. After polymerization is over, water washing removes the urea, leaves the polybuta­diene. This is a hard, tough, crystal­line solid. By contrast, ordinary poly­butadiene is a rubbery material.

Using canal complexes, Brown has polymerized monomers such as vinyl chloride, vinylklene chloride, cyclo-hexadiene, 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

'Nature's W a y " to O r d e r e d Polymers

Schematic repre­sentation of a di-ene polymeriza­tion in a thio-u r e a c a n a l . Drawing is about to scale, shows the diene mole­cules and thio­urea molecules in an edge view and the relative mo­lecular positions before and after polymerization

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(high energy electron beam)

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work for only a few monomers. How­ever, molecular templates in biological systems—enzymes, for example—work the same way. This is the reason Brown thinks he has duplicated nature's method of synthesizing ordered poly­mers.

Big advantages of the method: It is simple and it works every time. Ac­cording to Brown, this is an absolutely sure way to get completely stereospe­cific polymers; there is no way it can go wrong. "Either it goes right or it doesn't go at all,"

New Nitro Compounds Dinitroethylat ion react ion opens new f i e ld in po l y -n i t ro chemistry, b u t security clouds most uses

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Here's a reac-4 Λ Λ ACS ^ ο η t n a t opens a 1 A A NATIONAL new field in ali-1 V V M E E T I N G _ phatic polynitro

chemistry. It's called dinitro­ethylation—a way to put a gem-à\-nitroethyl group

in an organic molecule and make com­pounds such as polynitroalcohols or esters. 'But its significance is clouded in military security. For this reason, commercial prospects for dinitroethyl­ation products are not known now, but they likely fit into the solid propellant fuel picture.

Dinitroethylation was discovered at Aerojet-General, a firm long associated with the solid and liquid propellant field. Aerojet's M. B. Frankel told the Division of Organic Chemistry that dinitroethylation was discovered when potassium 2,2,4,4-tetranitrobutyl ace­tate was made from 2-bromo-2,2-di-nitroethyl acetate and potassium iodide. Frankel thought this reaction would make potassium 2,2-dinitroethyl ace­tate. This salt was not found, explains Frankel, since it probably decomposed to 1,1-dinitroethyJene and potassium acetate. l ie adds further that the 1,1 compound could not be isolated, either, and is probably a highly active chemical that condensed (via a Michael con­densation ) with potassium 2,2-dinitro­ethyl acetate to form potassium 2,2,4,4-tetranitrobutyl acetate.

This reaction mechanism would be true, continues Frankel, if metallic salts of organic and inorganic compounds which have labile hydrogen atoms

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