Other Phosphorus-Containing Polymers

Elemental phosphorus itself exists in several polymeric forms. If the white allotropic form, which consists of P4 tetrahedral molecules, is put under high pressure, preferably at elevated temperatures, it can be catalytically converted to other modifications.41 It first becomes red, then violet, then black as the degree of polymerization increases. These materials are very difficult to characterize because of branching and the formation of cyclics. In the extreme limit, the structure approaches that of graphite, and shows good electrical conductivity.42 No evidence exists at all for the formation of high-molecular-weight, linear chains of elemental phosphorus. 

Like other elements important in inorganic chemistry, phosphorus is used mainly in combination with other elements in a polymeric chain 43,44 Small amounts of phosphorus in a polymer can be greatly advantageous, imparting flame retardancy, improved adhesion to metals, and ion-binding characteristics. Phosphorus-containing polymers are therefore extensively used as flame retardants for fabrics, as adhesives, and as ion-exchange resins. 

This problem has been partially overcome by elimination of the phosphorus-oxygen bonds, as, for example, in the poly(phosphinoisocyanates), which have the structure shown in 6.47.42 It is also possible to form poly(metal phosphinates) with repeat unit -M(0PR20)2- by allowing a metal alkoxide to react with a phosphinic acid.43 Typical metal atoms are aluminum, cobalt, chromium, nickel, titanium, and zinc.43 Polymeric phosphine oxides can be prepared by the reactions 

The materials obtained are extremely stable,43 because the electron pair of the phosphine structure has been donated to an oxygen atom. A final series of chains of this type are the poly(phosphorylamides). An example, the poly(phosphoryldimethylamide) chain, is shown in 6.48. These polymers have interesting elastomeric properties, but presumably are hydrolytically sensitive.42 

As discussed in Chapter 3, some other inorganic polymers also contain phosphorus atoms.47 They are derived from the basic phosphazene structure described in Chapter 3, and are obtained by the ring-opening polymerization of heterocyclic compounds in which one of the disubstituted phosphorus atoms is replaced by another moiety. Specifically, introduction of a carbon atom can yield poly(carbophosphazenes), with the repeat unit shown in 6.49. Alternatively, replacement with a sulfur atom can yield a poly(thiophosphazene) (6.50). Relatively little is known about these polymers at the present time 47 

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Chaubal MV, Gupta AS, Lopina ST, Bruley DF. Polyphosphates and other phosphorus-containing polymers for drug delivery applications. Crit Rev Ther Drug Carrier Syst 2003 20(4) 295-315. 

Although no other phosphorus-containing polymers synthesized cationically have been commercialized, they exhibit interesting properties due to their composition, i.e., low flamability, flame retardancy and high efficiency to complex heavy metal ions. 

Other phosphorus containing polymers that have been the subject of thermal stability studies include alkyl phosphate-type polyols [2] and poly(diethyl-2-(methacryl-oyloxy) ethyl phosphate [3]. 

This new area of chemistry is still at a very early stage of development with most of the breakthroughs occurring in the last couple of years. The future holds promise for more exciting developments in the use of P=C bonds in polymer science and it is very possible that apphcations may be found for these new types of materials. In addition, an exciting prospect for the future is the further expansion of these methodologies, which are so common for C=C bonds, to other phosphorus-containing multiple bonds and other p-block elements. 

Phosphorus-containing polymers, which can exhibit biological activity, have aroused considerable interest. Other such polymers are excellent fire retardants. 

Other phosphorus-containing organic compounds found in soils and waters include phospholipids, nucleic acids (DNA and RNA) and their nucleotide residues (Newman and Tate, 1980 Condron et al., 1990), phosphorylated polymers of mannose produced by yeasts (Harrison, 1987), and tei-choic acids that form the cell walls of many Gram-positive bacteria (Zhang et al., 1998 ... 

Generally, however, the incorporation as additive of a phosphorus-containing polymer appears to be a valuable approach to impart flame retardancy to a polymer matrix. Chang et al. have synthesized a phosphorus-containing polymeric flame retardant. The phosphorus content in the polymeric flame retardant was 13.9 wt%. The LOI increased from 20.4 to 35.4% when the polymeric flame retardant was incorporated into the epojgr resin at a content of 8 wt%. The UL-94 rating was VO, with only 0.7 wt% of phosphorus. Similar results were found for other polymer matrices (PET and unsaturated polyester). 

For flame-retardant polyester fibres the copolymerisation of phosphorus retardants is the most common method. However, a serious difficulty is that the phosphorus-containing polymer is easily hydrolysed. Work by the Toyobo Company Limited in Japan has shown that two identical PET fibres can offer differing properties depending on where the phosphorus compound is situated within the pol5mier chains. One has the addition as a side chain and the other an identical phosphorus compound in the polymer backbone. Both fibres had almost the same physical and flame retardant properties, yet the main-chain type hydrolysed around twice as fast as the side-chain type, and led to an immediate drop in toughness. 

PMMA - Red Phosphorus System. The initial reaction that was investigated was that between PMMA and red phosphorus (4-51. Phosphorus was chosen since this material is known to function as a flame retardant for oxygen-containing polymers (1 2). Two previous investigations of the reaction of PMMA with red phosphorus have been carried out and the results are conflicting. Raley has reported that the addition of organic halides and red phosphorus to PMMA caused moderate to severe deterioration in flammability characteristics. Other authors have reported that the addition of chlorine and phosphorus compounds are effective flame retardant additives (12). 

Polycarbonate exhibits a relatively high oxygen index of 26. It may be further flame retarded by the addition of flame-retardant additives, including tetrabromo-BPA polymer, oligomer, or other bromi-nated additives, alkali metal salts,polytetra-fluoroethylene, phosphorus-containing additives, or silicones.Glass fillers or other inert fillers may also provide improved performance in Underwriters Laboratories (UL) flame testing. 

To achieve effective char formation in hydrocarbon-like polymers, we are beginning to see combinations of phosphorus compounds with other molecules, and we are beginning to see phosphorus containing molecules combining structural elements that will induce char formation. Several recent examples of the blend approach which is really an extension of FR technology... 

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