"Phosphorus-Containing Polymers and Oligomers"

Vol. 3 PHOSPHORUS-CONTAINING POLYMERS AND OLIGOMERS 447 PHOSPHORUS-CONTAINING POLYMERS AND OLIGOMERS Introduction The only valid generalization about phosphorus polymers is that they tend to be flame retardant (1,2). The flame-retardant effect depends heavily on the phosphorus content. Red (polymeric) phosphorus, despite its combustibility, is a commercial flame-retardant additive. Other features of many phosphorus polymers are adhesion to metals, metal ion-binding characteristics, and increased polarity (2). The flexible P O C linkages tend to impart lower glass-transition temper- atures. Phosphorus polymers with acid groups are used industrially for ion exchange, adhesion, and scale inhibition. Some form water-soluble coatings used as primers in metal protection and for photolithographic plates. The binding properties have led to dental applications. Cellulose phosphates have found some drug and ion-exchange uses. Academic interest has been stimulated by the relationship of certain phosphate polymers to natural products, such as the nucleic acids (see POLYNUCLEOTIDES). This review gives most attention to those phosphorus polymers which have attained commercial use or which have been (or currently are) the subject of seri- ous development efforts. Other reviews encompass phosphorus polymers of mainly academic interest (3,4). The commercial examples tend to be specialty polymers and none have attained large volume usage. One reason is cost. In addition, those polymers having P O links are usually more hydrolyzable than corresponding C O bonded polymers, and moreover the phosphorus acids which are liber- ated tend to catalyze further hydrolysis. Hydrolytically stable phosphine oxide Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved. 448 PHOSPHORUS-CONTAINING POLYMERS AND OLIGOMERS Vol. 3 types are known but are costly. Another hydrolytically stable class, the polyphos- phazenes, is discussed in a separate article. Phosphorus-containing oligomers (low polymers) have been included in this article because they have become more important commercially than high polymers, since they can be used as additives or coreactants to introduce sufficient phosphorus for flame retardancy or some other desired property provided by phosphorus, such as metal binding (5,6). The modification of conventional polymers with small amounts of phosphorus, reactives, or comonomers to impart flame retardancy (1,4,6) or improve other properties has become commercially significant, and is discussed separately below in connection with the polymer class being modified. Polymeric Form of Elemental Phosphorus Phosphorus occurs in several allotropic forms. The principal commercial form is white (or yellow) phosphorus; the molecule consists of four phosphorus atoms arranged in a tetrahedron. Heating white phosphorus at 270–400◦C, preferably with a catalyst, produces red phosphorus, a stable, nontoxic, high melting, insoluble polymeric solid. Red phosphorus (often stabilized by additives) has been used for many decades as the igniting agent for the striking surface of safety matches. It is insoluble, thermally quite stable, and nontoxic; while it can be ignited, it is surprisingly effective as a flame retardant for plastics, and it is now finding commercial use, especially in Europe to flame-retard nylon. Other applications and its mode of action have been reviewed (7). To prevent decomposition of red phosphorus to the toxic and highly flammable white form, it is stabilized with additives (8) and/or encapsulated with a thermoset resin. The structure of red phosphorus is not fully established. It is believed (9) to be a cross-linked polymer with chains having the structure shown in Figure 1. Inorganic Phosphorus Polymers Inorganic polyphosphates are covalently linked polymers (10,11). Chains of re- peating phosphate units are formed by eliminating the elements of water from adjacent orthophosphate units. Such polycondensations take place, for instance, when orthophosphoric acid is heated, producing a broad distribution of linear molecules of various chain lengths corresponding to the general formula HO[P(O)(OH)O]nH. A table of compositions for various weight percentages of P2O5 is given in Reference 10. P Fig. 1. Probable structure of red phosphorus. Vol. 3 PHOSPHORUS-CONTAINING POLYMERS AND OLIGOMERS 449 Polyphosphoric acid itself has found utility mainly as a supported catalyst in the petroleum industry for alkylation, olefin hydration, polymerization, and isomerization, and for syntheses of fine chemicals and dyes. It is used to phospho- rylate alcohol groups, for example in the production of anionic phosphate surfac- tants. Heating alkali metal or alkaline earth metal dihydrogen phosphates produces polymeric salts (cyclic metaphosphates and linear polyphosphates) and cross-linked polyphosphates (ultraphosphates), depending on temperature and the presence of other ingredients (11,12). This complex group of polymers includes materials with crystalline, glass-like, fibrous, or ceramic properties as well as some with thermoplastic and thermoset characteristics; some are useful as binders for metals, ceramics, and dental restorations. Reviews are available on glasses (12,13), crystalline compounds (14), and polyphosphate fibers (15). The water-soluble poly- and metaphosphates exhibit chelating properties. Glassy sodium metaphosphate (DP ca 15–20) is used in water treatment for scale inhibition (16). Long-chain sodium polyphosphates (metaphosphates) are used as preservatives in red meat, poultry,and fish (16). Long-chain sodium and potassium phosphates are added to sausage meat to improve color and texture (16). Polyphosphates are produced naturally in some yeasts, comprising up to 30% of their total phosphate and up to a 600 degree of polymerization (17). These probably provide energy storage for the yeasts. Some of the inorganic phosphate polymers possess properties resembling those of glassy organic plastics. Corning has attempted to commercialize low- melting phosphate-containing glasses as reinforcing fibers with excellent dimen- sional stability (18,19). The fibers can be made in situ by stirring in a high temperature thermoplastic melt. Phosphate glasses can also be molded at 360–400◦C to make high refractive index lenses (20). Heating of ammonium phosphates under an atmosphere of ammonia or in the presence of urea produces ammonium polyphosphate (21,22). At a high degree of polymerization, the product is a water-insoluble solid. This form of ammonium polyphosphate is used commercially as a flame-retardant additive for plastics and as the latent acid component in intumescent paints, mastics, and caulks (23,24). The water resistance can be further enhanced by encapsulation with a resin. Polymeric Phosphorus Oxynitrides and Phosphorus Iminoimides Condensed phosphoramides with linear, cyclic, or cross-linked structures are produced by the reaction of POCl3 with ammonia. The higher molecular weight products are insoluble in water and on further heating are converted to a cross-linked insoluble polymer, phosphorus oxynitride (PON)x (25). Phosphorus oxynitride can be made by prolonged heating of melamine phosphates (26), urea phosphate (26), or ammonium phosphate under conditions where ammonia is retained (27). Phos- phorus oxynitride is an effective flame retardant in those polymers, such as nylon 6, which can be flame retarded by exclusively char-forming condensed-phase means. However, phosphorus oxynitride is ineffective (at least by itself) in those 450 PHOSPHORUS-CONTAINING POLYMERS AND OLIGOMERS Vol. 3 polymers, such as polybutylene terephthalate, which pyrolyze easily to volatile fuel (28–30). An imido analog of phosphorus oxynitride, phospham, (PN2H)x, is made as the exhaustive self-condensation product of aminophosphazenes, but also may be made directly from elemental phosphorus and ammonia or from phosphorus pen- tasulfide and ammonia. Phospham is probably a thermoset phosphazene imide. It can be amorphous or crystalline. It is also an effective and thermally stable flame retardant, especially for high-temperature-processed polyamides (31). It was available for a short time as a development product from Japan. Amorphous polymeric dielectric films of phosphorus nitrides and oxynitrides (“Phoslon”) can be prepared on electronic substrates by chemical vapor deposition (32). Organic Phosphorus Polymers Phosphines. Polymeric phosphines exhibit strong metal-binding properties. Nonpolymeric phosphines, in particular triphenylphosphine, are employed as ligands for cobalt and rhodium in hydroformylation catalysts used in plasticizer manufacture. Extensive efforts have been made to attach phosphine–metal com- plexes to polymers in order to facilitate catalyst recovery and enhance selectivity (33). Problems of cost, catalyst life and activity, heat transfer, and mass transfer seem to have prevented commercialization. Polymers from diarylphosphinylstyrenes have been prepared as ligands (34). Styryldiphenylphosphine monomer, commercially available in laboratory quantities, is easily polymerized or copolymerized (34,35). Copolymers of styryldiphenylphosphine with styrene cross-linked with divinylbenzene are commercially available in laboratory quantities. Some polymeric phosphonium salts have been reported to have advantages in reaction rate or ease of separation relative to monomeric phosphonium salts as catalysts for nucleophilic reactions where the large cation favors nucleophilic reactivity of the anion

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