Phosphazene elastomers

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

In the case of poly(alkoxyphosphazenes) (IV) or poly(aryloxyphos-phazenes) (V) a dramatic change in properties can arise by employing combinations of substituents. Polymers such as (NP CHjCF ) and (NP CgH,).) are semicrystalline thermoplastics (Table I). With the introduction of two or more substituents of sufficiently different size, elastomers are obtained (Figure 4). Another requirement for elastomeric behavior is that the substituents be randomly distributed along the P-N backbone. This principle was first demonstrated by Rose (9), and subsequent work in several industrial laboratories has led to the development of phosphazene elastomers of commercial interest. A phosphazene fluoroelastomer and a phosphazene elastomer with mixed aryloxy side chains are showing promise for military and commercial applications. These elastomers are the subject of another paper in this symposium (10). 

Table I. Properties of Poly(fluoroalkoxy-phosphazene) Elastomers...

The incorporation of polar groups in unvulcanized polymers reduces their solubility in benzene. Thus the copolymer of acrylonitrile and butadiene (NBR), polychlorobutadiene (Neoprene), and fluorinated EP (the copolymer of ethylene and propylene) are less soluble in benzene and lubricating oils than the previously cited elastomers. Likewise, silicones and phosphazene elastomers, as well as elastomeric polyfluorocarbons, are insoluble in many oils and aromatic hydrocarbons because of their extremely low solubility parameters (silicons 7-8 H polytetrafluoroethylene 6.2 benzene 9.2 toluene 8.9 pine oil P.6). 

Elastomers, synthetic -acrylic elastomers [ELASTOMERS, SYNTHETIC - ACRYLIC ELASTOMERS] (Vol 8) -butyl rubber [ELASTOMERS, SYNTHETIC - BUTYL RUBBER] (Vol 8) -chlorosulfonated polyethylene [ELASTOMERS, SYNTHETIC - CHLOROSULFONATED POLYETHYLENE] (Vol 8) -ethylene-acrylic elastomers [ELASTOMERS, SYNTHETIC - ETHYLENE-ACRYLIC ELASTOMERS] (Vol 8) -ethylene-propylene-diene rubber [ELASTOMERS,SYNTHETTC - ETHYLENE-PROPYLENE-DIENE RUBBER] (Vol 8) -fluorocarbon elastomers [ELASTOMERS, SYNTHETIC - FLUOROCARBON ELASTOMERS] (Vol 8) -nitrile rubber [ELASTOMERS, SYNTHETIC - NITRILE RUBBER] (Vol 8) -phosphazenes [ELASTOMERS, SYNTHETIC - PHOSPHAZENES] (Vol 8) -polybutadiene [ELASTOMERS, SYNTHETIC - POLYBUTADIENE] (Vol 8) -polychloroprene [ELASTOMERS, SYNTHETIC - POLYCHLOROPRENE] (Vol 8) -polyethers (ELASTOMERS, SYNTHETIC - POLYETHERS] (Vol 8) -polyisoprene [ELASTOMERSSYNTHETTC - POLYISOPRENE] (Vol 9) -survey [ELASTOMERS, SYNTHETIC - SURVEY] (Vol 8)... 

Two commercial phosphazene elastomers were developed and marketed in the mid-1980s, namely, poly(fluoroalkoxyphosphazene) elastomer (ASTM International designation FZ) and poly(aryloxyphosphazene) elastomer (ASTM International designation PZ) [109]. The structure of the fluorinated product is as follows [110] ... 

Phosphazene elastomers were very successful throughout the 1980s, being used mainly in military and aerospace industry. However, because of their high cost and relatively small volume market, they are not available commercially other than on special orders. 

Thus far, the survey of phosphazene elastomers has been based on the formation and modification of poly(dlchlorophosphazene). Although a large variety of polymers can be prepared by this approach, there are limitations In the preparation of polyphosphazenes with phosphorus-carbon bonds. The reaction of poly(dlchlorophosphazene) with organometalllc agents, such as RMgX or KLl, results mainly In decomposition and not the desired polymers [NPR.]. There are three possible approaches to the preparation ox polyphosphazenes with phosphorus-carbon bonds polymerization of substituted trlmers, poly(dlfluorophosphazene), and thermolysis of small linear molecules. These three approaches will be discussed In turn. 

N-Sllylphosphlnimines. A third approach, which has been Investigated by Nellson (44.45). Involves the synthesis of suitably constructed N-sllyphosphlnlmlnes. Upon heating, compound IX yields poly(dlmethylphosphazene) with a moderate molecular weight. Other N-sllylphosphlnlmlnes have been synthesized, and these may eventually yield new phosphazene elastomers with novel properties (45). 

This paper has reviewed the most recent developments of phosphazene elastomers and touched briefly in some phosphazenes of related Interest. At the time of this writing. Firestone was the supplier of the PNF and APN elastomers. Ethyl Corporation recently licensed the phosphazene technology from Firestone (49). and thus the continued availability of these elastomers Is assured for further commercial development. 

Professor Emeritus, Material Science and Engineering Department, University of Pittsburgh, Pennsylvania Poly(aryloxy)thionylphosphazenes Poly(phosphazene), bioerodible Poly(phosphazene) elastomer Poly(phosphazene), semicrystalline... 

This is illustrated in Scheme I. Occasionally, as when bulky diethylamino units are introduced first, this technique permits the synthesis of polymers in which each phosphorus bears only one bulky unit and one less hindered substituent (non-geminal structure) [22] rather than structures that contain both geminal and non-geminal side group arrangements. Sequential or simultaneous co-substitution reactions are used for the preparation and manufacture of commercial high performance phosphazene elastomers of the type shown in structure 10 [15-17]. 

R.E. Singler, G.L. Hagnauer and R.W. Sicka, "Phosphazene Elastomers Synthesis-Properties-Appli-cations, Part II," to be published in the ACS Symposium Series, 1984. 

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