RESEARCH
New Coordination Polymers Are Inorganic One group of polymers is based on octahedral units with monomers in a spiral; other group is based on tetrahedral units Chemists at Pennsalt Chemicals have synthesized a new family of inorganic double-bridge coordination polymers. One group is based on octahedral units, the other on tetrahedral units (C&EN, July 16, page 3 3 ) . The monomer in the octahedral series is di-/x-diphenyrphosphinatoacetylacetonatometal ( I I I ) . Although most of the Pennsalt work was done with chromium, other metals such as iron, cobalt, and aluminum may be used. In the tetrahedral system, the monomer is di-/x-diphenylphosphinatometal ( I I ) . In this series the metal may be zinc, cobalt ( I I ) , or beryllium. Dimethyl or phenylmethyl may be substituted for diphenyl. The octahedral family occurs with the repeating units arranged in a spiral that contributes to the flexibility and elasticity of the polymer's backbone.
The tetrahedral polymer, by contrast, doesn't occur as a spiral, but its backbone also contains eight-membered flexible rings that are similar to those in the octahedral series. According to Dr. Gerhard Barth-Wehrenalp and Dr. B. Peter Block of Pennsalt research and development, some members of the tetrahedral series resist decomposition up to 500° C. Both the octahedral and tetrahedral polymers are the early results of a program to make heat resistant inorganic polymers that might have uses as adhesives, coatings, wire insulation, and structural materials. But the evolution of such polymers is still in too early a stage for Pennsalt to have firmed any plans for their commercial applications. Although there has been much activity in the field of coordination polymers, the materials studied have partly
STRUCTURE. Pennsalt's Dr. B. Peter Block (standing), Dr. G. Barth-Wehrenalp (seated, left), and Dr. George McCoy study structure of the inorganic polymers. Most of the work was done with chromium, but other metals can be used, too 52
C & E N J U L Y 30, 1962
organic backbones. Pennsalt's polymers, though, are the first doublebridge coordination polymers with inorganic backbones ever prepared, Dr. Barth-Wehrenalp and Dr. Block say. Basic Unit. One group of polymers has di-/x-diphenylphosphinatoacetylacetonatochromium(III) as a basic building block [/ACS, 84, 1749 (1962)], and is made by a substitution - addition polymerization. Several methods may be used to make the polymers, Dr. Block says. The best way found so far is the reaction between 1 mole of chromium (III) acetylacetonate and 2 moles of diphenylphosphinic acid at 175° to 250° C. in nitrogen, he adds. The solid reaction product is fractionated by successive extractions, first with ethanol, then with benzene, and finally with chloroform. The insoluble chromium compound is an amorphous green powder. Some of the insoluble chromium polymers swell in benzene. And some members of the soluble fractions can be made into film by evaporating the solvent. The highest molecular weight polymers are probably in the insoluble fraction left from the final chloroform extraction, Dr. Block says. But, he adds, no satisfactory technique for measuring molecular weights of these insoluble materials is available. However, quantitative and qualitative analyses of the insoluble fractions prompt the Pennsalt workers to suggest that the insoluble fractions contain larger polymers of the same type found in the soluble fractions. Ebullioscopic measurements in benzene give molecular weights of up to 10,870 for the chloroform-soluble fractions, and between 1940 and 2633 for the fractions that are soluble in benzene. The fusion temperature controls the balance between the insoluble and soluble fractions. At lower temperatures, almost no benzene-insoluble fraction is formed. At 250° C , though, about 40% of the resulting product is the insoluble fraction.
The original reaction of this series led to the formation of the dimer of di - μ - diphenylphosphinatoacetylacetonatochromium(III). In this reaction, 1.5 moles of chromium (III) acetylacetonate and 1 mole of diphenylphosphinic acid are fused at 300° C. to yield the dimer. The dimer, in turn, can be polymerized at 200° C. in an inert solvent by reacting it with more diphenylphosphinic acid. Isolation and characterization of the dimer jus tify assuming the existence of double • diphenylphpsphinate bridges between chromium atoms in the polymer back bone, Dr. Block says. There are other inorganic backbone polymers, Dr. Barth-Wehrenalp notes. Among these are the silicones. The silicones, however, have a tendency to form cyclic, low molecular weight com pounds when heated to high tempera tures. Ladder-type silicon polymeric struc tures containing aluminum or titanium in addition to silicon and oxygen are thermally stable to about 500° C, work in the U.S.S.R. shows. Rupture of one bond of a double-bridge system does not necessarily lead to a break down of the polymer chain, but little is known about the mechanical properties of such polymers, Dr. Block notes. Another class of coordinationbonded inorganic polymers includes palladium dichloride and molyb denum iodide. Little or nothing is known about the mechanical proper ties of these polymers, Dr. Barth^ Wehrenalp says. Most inorganic chemists consider these compounds to be coordination polymers, he notes. The metal centers are surrounded with more atoms or groups than might appear obvious on the basis of the metal's valence. Palladium in palladium dichloride, for example, has a coordination number of four. The compound exists as par allel chains within which the palla dium atoms are attached to each other through double bridges of chlorine. And molybdenum has a coordination number of six; thus, in molybdenum triiodide, the molybdenum ions along the chain's length are attached through triple bridges of iodine. The diammoniate of cadmium chlo-
Spiral Contributes to Potential Elasticity
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