Inorganic Macroraolecules Developments at the interface of inorganic, organic, and polymer chemistry Harry R. Allcock Pennsylvania State University
The study of new high-molecular-weight (high) polymers with inorganic elements in their backbone is an area with broad opportunities for pioneering synthetic research. Fundamental developments in this area are yielding new classes of polymers that are attracting attention as high-technology materials, biomedical polymers, and electrical conductors. The three main areas of modern chemistry are concerned with small molecules, macromolecules, and ordered solids and their surfaces. All three are interconnected, and, increasingly, advances made in one area influence the other two. One of the main goals in chemistry during the past century has been the controlled synthesis of ever more complex molecules with increasingly sophisticated, deliberately tailored properties. The synthesis of macromolecules from simple, small molecules is one of the most spectacular achievements arising from this objective. Today, the field of macromolecules occupies a pivotal position between small-molecule chemistry on the one hand and the science of solids on the other. 22
March 18, 1985 C&EN
The first 50 years of macromolecular synthesis has beeiKjnainly a time oKjaiscovery and development of organic polymers. The reasons for this are clear and logical. Organic polymer synthesis has been nourished by more than a century of impressive discoveries in the field of small-molecule carbon chemistry. In addition, the wide availability of monomers from the petrochemicals industry has provided an economic base on which polymer technology has flourished. Given these facts, what is the future of macromolecular science? What new developments can be expected in the next 10 or 20 years? Is there a justification, scientifically and technologically, for the continuing search for new classes of polymers? The answer to this last question is "yes"—especially if the search extends into relatively unexplored areas of the periodic table. Nearly all polymer chemistry to date has revolved around the chemistry of one element: carbon. If radically different macromolecular systems are needed, they probably will be found among the other 99 or so stable elements. This means that the continued vitality of macromolecular research depends on the establishment of strong connections between the ideas and techniques of polymer chemistry and those of inorganic chemistry and solid-state science—areas that, so far, have developed in almost total isolation from each other. The boundary between organic and inorganic polymer chemistry is an artificial one—a conceptual barrier only. For the most part, the lessons learned from organic polymer chemistry (both synthetic and biological) apply equally well to the inorganic area, and vice versa. New types of inorganic polymers now are being synthesized for several reasons. At the fundamental research level, it is important to find out how long chains of inorganic elements can be assembled and how inorganic macromolecules differ, chemically and physically,
frdm tfrieir organic douriterparts. In this Way the desigri of useful new substcWes can accelerate/But the fulfill-^ ment of these interests runs parallel to the need to solve some challenging problems that historically have not been connected with the science of high polymers. Polymer chain entanglement in the solid state is a key characteristic that distinguishes the properties of macromolecules from those of small molecules. Thus, the development of new flexible or high-strength materials depends on understanding the ways in which large molecules interact and the influence of different chain atoms and side groups on those interactions. This has been and still is the main driving force for polymer research. Other potential uses, still at the exploratory level or just emerging from it, provide a clue to the future development of macromolecular chemistry, both as a science and as technology. The use of polymers as electrical conductors or semiconductors, for example, is an active and rapidly expanding field. The electrical behavior of polyacetylene, poly(phenylene sulfide), and polypyrrole has stimulated broad interest among both fundamental researchers and electrical engineers. In a structural sense, new polymers are needed that have delocalized 7r-electron systems and are capable of forming films or fibers with long-range, three-dimensional ordering, and characterized by ease of doping and stability to the atmosphere. This combination of properties is extremely difficult to achieve with known polymers. Further projected uses of polymers are modeled after examples in biological systems—from the application of macromolecules as // carriers ,, for active sites, as templates for polymer construction, or as information storage systems at the molecular level. Three examples will suffice. First, the linkage of
New uses for polymers arc emerging from research Traditional
Polymers as flexible or high-strength materials
Exploratory
Polymers as electrical conductors or semiconductors
Polymers linked to transition metals Metal atom
Polymers linked to biomolecules or drugs Bioactiv©
k I 6 h b 6 k k k k 6
Polymers as linear coding systems
March 18, 1985 C&EN 23
Special Report transition metal catalyst molecules to soluble or lightly crosslinked polymers can yield systems that combine the advantages of homogeneous catalysts (reproducible behavior, functional variety) with the attributes of heterogeneous catalysts (stability, ease of recovery). Additional possibilities include the proximal location of different catalyst sites for sequential reactions. Variants include the use of polymer-bound catalysts as electrode mediators in electrochemical reactions. In all cases, the carrier polymer must survive a harsh environment during reaction, a requirement that is difficult to fulfill with most present systems. Second, much interest exists in the linkage of enzymes, whole cells, antigens, or organic reagents to synthetic macromolecules, especially those that are soluble in water or form hydrogels. The function of the synthetic polymer is to separate reactive sites, prevent denaturation of proteins, or allow sequential reactions in neighboring zones. A well-known variant is the use of crosslinked polymers as immobilization supports in automated synthesizers for proteins and oligonucleotides. The broad challenge is to design immobilization substrates that will not inactivate enzymes or other reagents and are water soluble or biologically compatible. These requirements, too, are hard to meet with conventional polymers. Finally, new polymers are needed to control the activity of chemotherapeutic agents. Most chemotherapeutic drugs are small molecules that can readily penetrate semipermeable membranes and be carried from one compartment of the body to another. This mobility is one reason for the side effects experienced by patients during chemotherapy. Water-soluble macromolecules cannot pass easily
Side groups may be modified after inorganic polymers are assembled Ring-opening polymerization
Cyclic starting compound
Unstable reactive polymeric intermediate
Substitution
Substituted polymer Further substitution
Substituted polymer
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March 18, 1985 C&EN
Models help in macromolecular synthesis Model reaction
Macromolecular counterpart
The reaction of the phosphazene cyclic trimer (NPCI2)3 with sodium trifluoroethoxide was worked out in detail first. Then the first successful reactions of the high-molecular-weight polymer (NPCI2)n were achieved using the same reagent to yield the first stable poly(organophosphazene)
through living membranes. Thus, if a drug is linked to such a macromolecule it will be restricted to one region of the body, especially if the polymer also bears a targeting group. If the polymer also can hydrolyze to nontoxic small molecules, it may self-destruct when chemotherapy is complete. Considerable progress has been made in designing therapeutically active polymers. However, very few of the currently available synthetic polymers are entirely appropriate for this use. In biological macromolecules, it is the essential linearity of the long polymer chain that allows them to function as coding systems and templates for replication. These are aspects of synthetic chemistry that have hardly been explored at the present time, and yet the potential is enormous. For the synthetic chemist, the problem reduces to the question of how to construct long polymer chains with a precisely sequenced arrangement of two or more side groups. Use of polypeptide or oligonucleotide synthesizers provides one possible approach, but these methods are at present slow and imperfect. This is clearly an area that will require a radically new approach to macromolecular synthesis. Inorganic or inorganic-organic macromolecules offer a special promise for the solution of these problems. First, many inorganic elements are oxidatively more stable than aliphatic carbon and are less prone to fragment into free radicals at high temperatures. In addition, controlled sensitivity to hydrolysis can be achieved more readily with inorganic systems than with most organic polymers. This is critical if biological behavior is important. Third, the range of synthetic options can be much wider with inorganic systems than with conventional organic macromolecules. Finally, the replacement of carbon atoms in a polymer backbone by inorganic elements alters the skeletal flexibility and generates new solution- and solid-state properties.
The main problem that hinders broad advances in the inorganic polymer field is that the fundamental inorganic chemistry of small covalent molecules, including polymerization chemistry, is still at a relatively primitive stage of development. Efforts are being made to correct this problem. But in the immediate future, the most urgent need is to identify and overcome synthetic hurdles. If this can be done, progress can be expected to accelerate dramatically.
Synthesis problems Two fundamental problems dominate the chemistry of inorganic macromolecular synthesis—the question of ring-polymer equilibria and the need for macromolecular substitution reactions. These constitute the lifeline on which the future of the field depends. The molecular diversity of organic polymer chemistry can be traced to the existence of at least three different synthesis routes. These are well known: the addition polymerization of unsaturated monomers like olefins or vinyl compounds; condensation reactions between organic acids and amines or alcohols; and the ringopening polymerization of cyclic ethers, lactams, and other cyclic monomers. This richness of polymerization routes is not yet available in inorganic chemistry. Very few unsaturated inorganic monomers are known—at least in an isolatable form. Relatively few inorganic condensation polymerizations are available and these are restricted mainly to the preparation of mineralogical-type macromolecules such as silicates or phosphates. The main route for polymer construction at present is through the ring-opening polymerization of inorganic cyclic compounds; very few examples of these have been studied in detail. Those that have include the polymerization of cyclic sulfur and selenium, methylcyclosiloxanes, sulfur nitride dimer, and organosilane rings. Ring-opening polymerization is the main point of entry into polyphosphazene chemistry. Surprisingly, very little is known about the ring-opening polymerization or even ring-ring equilibration behavior of other cyclic inorganic compounds. Thus, we face the following dilemma: In principle, a large number of elements could be incorporated into a macromolecular backbone, but only a limited number of options exists for the linkage of short inorganic units into long polymer chains. In addition, even when a ring-opening polymerization is known for one specific compound, no guarantee exists that small structural changes will allow variants of the polymer to be prepared. Nearly all vinyl polymers are prepared by a few well-known techniques, and different polymers can be synthesized simply by changing the side group attached to the vinyl group at the monomer stage. However, ring-opening polymerizations are acutely sensitive to changes in side-group structure. For example, although methylcyclosiloxanes polymerize readily, those with large alkyl or aryl groups often do not. This restriction has stimulated a relatively new approach to polymer synthesis, one that has been only rarely necessary with organic macromolecules. Because so few polymerization routes are available for inorganic
systems, macromolecular diversity must be introduced after the polymer has been assembled. The aim must be to prepare polymers that possess reactive side groups. Replacement of these initial side groups by substitution reactions then allows the controlled introduction of many different side-group systems. Thus, in contrast to the normal organic polymer approach, where the procedure is to prepare stable macromolecules, an objective in the inorganic field should be to deliberately prepare unstable reactive polymeric intermediates and to use these in a wide variety of sidegroup replacement reactions. In this way, a diverse arsenal of reactions for side-group substitution can be mobilized to prepare a broad range of different macromolecules. This approach has been pioneered for inorganic polymers in our laboratory at Pennsylvania State University, and we are convinced that it can be applied to many macromolecular systems.
Model compound approach The substitution reactions of macromolecules are nearly always more complex than those of their smallmolecule analogs. Substitution reactions may be affected
Side groups in silicon-oxygen polymers can be changed by two approaches Disruption of the crosslinks in mineralogical silicates
Incorporation of organic groups before polymer synthesis
Heat, catalytic base
March 18, 1985 C&EN
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Special Report detrimental to the main objectives of the synthetic work. Thus, a strong case can be made for exploring new reactions first with small molecules that mimic the structure and reactivity of the high polymer. Reaction mechanisms, purification methods, and Polysilane structure proofs can all be obtained readily with small-molecule mod. and can contain cyclic or unsaturated side groups els, but the corresponding reactions or macromolecular structures may present formidable problems in the absence of the small-molecule data. This, too, is a key concept that we use repeatedly in our laboratory to probe new methods for macromolecular synthesis. Work with model compounds offers a fairly rapid and inexpensive whereas pyrolysis leads to silicon carbide means for testing reactions, ideas, and principles that may be applied later to the main target—the high polymers themselves. The danger of model-compound studies is that they can easily become a substitute Silicon carbide for the real thing, an end in themselves rather than a means to an end. Few areas of chemistry have been immune to this intellectual trap. Indeed, the flurry by the coiling of long polymer chains in solution. Also, of inorganic polymer activity in the 1950s and early side reactions that would be inconsequential at the 1960s died an untimely death because results from model small-molecule level may wreak havoc with a macrocompounds were not translated into real polymer molecular substitution. work. For example, only one or two crosslinks per chain, generated by unwanted side reactions, could insolubilize The prospects for the field of inorganic polymers have the system before substitution is complete. Skeletal been debated on and off since the discovery of the silicleavage reactions, which might account for, say, 10% of cones in the 1940s. But information exists now that was the reaction pathway, would be acceptable with smallnot available 10 or 15 years ago, and new systems are molecule reactions but would seriously reduce the mobeing studied in detail. lecular weights of all the macromolecules present in the Exploratory probes have been made into the macrosystem. Polymers are valuable systems for studying the molecular chemistry of most of the main-group elements amplification of side reactions, but this is nearly always in Groups III-VI, plus a few of the transition elements. I have selected a few of the most active areas to discuss in detail, especially those that illustrate general principles likely to be important in future work. Most of these Inorganic macromolecules may have systems make use of the elements silicon, phosphorus, sulfur atoms in their backbones or sulfur as an integral component of the main polymer chain. In most cases, organic residues are attached to the skeletal atoms. Thus, some of the most promising systems are hybrid inorganic-organic polymers.
Polysilanes are prepared from diorganodichlorosilanes
Silicates and poly(organosiloxanes) Polymeric sulfur Rhombic sulfur
Poly(sulfur nitride)
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Historically, research on inorganic macromolecules began with the silicates, many of which consist of long chains, sheets, or tubes derived from alternating sequences of silicon and oxygen. Early in this century, investigation of their properties and structure was a topic of some consequence, driven by the practical necessities of the ceramics industry. Glass and ceramics research has made impressive advances since then. Ceramic materials now are known that are extremely stable at high temperatures, have magnetic or semiconducting properties,
Phthalocyanine polymers can be electrical conductors
Polymers of this type are prepared by condensation between dihydroxysilicon-, germanium-, or tin-phthalocyanines. When doped with iodine they conduct electricity along the direction of the macromolecular chain
and are used increasingly as replacements for metals and organic polymers. Optical fiber materials are another recent development. The main problem with polysilicates is that they are insoluble in nonreactive solvents and can be shaped as solids only by inconvenient, high-temperature methods, or as solvent-swollen gels. Although they form fibers like organic polymers, they are not elastic and usually do not absorb impact energy without shattering. Thus, one objective in inorganic polymer research for many years has been to design materials that possess the strength and versatility of organic polymers, plus the low volatility, oxidation resistance, and dimensional stability of silicates. Two approaches have been tried to achieve this end with the use of a silicate-type skeleton. One starts with mineral silicates themselves, the other with an independent synthesis of the siloxane skeleton. These two approaches summarize the options available with a wide variety of inorganic polymer systems. Mineral silicates are insoluble, rigid, and brittle because the siloxane chains are crosslinked by covalent silicon-oxygen bonds or by cations. However, if the crosslinks are broken and replaced by nonpolar, nonionic groups, the properties change to resemble those of organic polymers. Malcolm E. Kenney at Case Western Reserve University has converted the silicate litidionite [NaKCuSi40io]n/ to a waxy ladder polymer by treatment with trimethylchlorosilane. This method has also been used by Brian R. Currell and John R. Parsonage in the U.K. to convert other mineral silicates to poly(organosiloxanes). Denis G. H. Ballard at Imperial Chemical Industries' laboratories in England has reported the delamination of layer silicates, such as vermiculite or montmorillonite, with alkylammonium cations to give materials that resemble organic polymers in some of their physical properties.
The second approach is used for the well-established synthesis of poly(dimethylsiloxane) elastomers. Cyclosiloxanes are first generated by the hydrolysis of dimethyldichlorosilane and then induced to undergo the now classic ring-opening or ring-expansion polymerization to give a linear high polymer. The difference in properties between poly(dimethylsiloxane)—silicone—and a mineral silicate is striking. Poly(dimethylsiloxane) has a very high skeletal flexibility, a low glass-transition temperature, and is soluble in organic media. The problem with poly(organosiloxanes) prepared by this method is that the organic group is introduced before the polymer chain is constructed. Unfortunately, many organic side groups attached to the cyclic tetramer interfere with the equilibration-polymerization process. Hence, only a restricted number of different organic side groups can be incorporated, which severely limits the range of structures accessible. Moreover, once formed, the poly(organosiloxane) side groups cannot be modified extensively by substitution reactions because the skeletal bonds are generally more reactive than the bonds in the side groups. These limitations are responsible for the restricted number of different polysiloxanes known—compared, for instance, with organic vinyl polymers or poly(organophosphazenes). One approach to bypass this problem is to couple short or long lengths of poly(organosiloxane) chains to organic sequences or other inorganic residues. The siloxane units confer flexible properties on the hybrid polymer, and the organic or inorganic sequences generate new properties, such as mechanical strength, crystallizability, or specific solvent resistance or solubility. Hybrid siloxane-organo polymers have been prepared by Robert W. Lenz at the University of MasMarch 18, 1985 C&EN
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Special Report
Poly(organophosphazenes) are synthesized by polymerization followed by substitution Polyphosphazenes are high-molecular-weight, essentially linear polymers with alternating phosphorus and nitrogen atoms j in the skeleton and two side groups attached to each phosphorus. The starting material is hexachlorocyclotriphosphazene (manufactured commercially from PCI5 and NH4CI), which is j polymerized thermally to high-molecular-weight poly(dichlorophosphazene). This polymer is extremely reactive in solution and is used as a substrate for the replacement of halogen atoms by a wide range of organic and organometallic nucleophiles. The four main classes of poly(organophosphazenes) are prepared by the use of alkoxides or aryloxides, primary or secondary amines, or organometallic reagents (RM) as reactants. Two or more different side groups can be introduced by simultaneous or sequential substitution. This method of synthesis allows an almost unlimited range of different polymers to be prepared from the one substrate, each derivative having different properties. The organometallic substitution route is the least developed, with poly(difluorophosphazene), (NPF2)n, being the preferred substrate in many instances. Use of transition metal anions as nucleophiles is expected to generate a series of hybrid metallophosphazene high polymers.
j j \
j
sachusetts and by James McGrath at Virginia Polytechnic Institute & State University. Hybrid siloxane-carborane polymers (with carborane residues separating siloxane sequences) were developed by workers at Union Carbide's laboratories in Tarrytown, N.Y. These hybrid polymers are more stable thermally than the parent polysiloxane.
Polysilanes and polysilazanes Polysilanes are a new class of inorganic-organic polymers, with a linear backbone of silicon atoms and two organic side groups attached to each skeletal element. These polymers are interesting in their own right, but industrially they provide an important interface among polymer chemistry, inorganic chemistry, and ceramic technology. Polysilanes are prepared by sodium-induced dechlorination of diorganodichlorosilanes. Heating at 400 28
March 18, 1985 C&EN
to 500 °C under pressure or in the presence of a catalyst brings about a rearrangement to a polymer that has alternating silicon and carbon atoms in the backbone. The initial reaction products can be melt-shaped into fibers and films. Subsequent pyrolysis at 800 to 1250 °C converts the polymer to silicon carbide, while retaining its fibrous character. This method, first reported by Seishi Yajima at Tohoku University in Japan, is used to manufacture silicon carbide fibers by Nippon Carbon Co. In the U.S., the fundamentals of polysilane chemistry have been explored by Robert C. West and his coworkers at the University of Wisconsin. Disruption of the symmetry of the polysilane by the introduction of side groups other than methyl groups enhances its solubility in organic solvents and aids in its purification and fabrication. Photolytic crosslinking of a polysilane reduces the tendency for depolymerization and volatilization during pyrolysis. Overall, about 50 polysilane homopolymers and copolymers have been prepared with different side groups. West also has made interesting polymers that contain cycloalkane and unsaturated aliphatic side groups. This is an active and expanding area, with research being carried out at several U.S. academic and industrial laboratories. Widespread interest in polymers that can be shaped into fibers or films and then pyrolyzed to ceramics has stimulated a reinvestigation of silicon-nitrogen polymer chemistry. Polysilazanes were first studied by Eugene G. Rochow at Harvard University in the 1960s. Compared to poly(organosiloxanes), they proved to be somewhat disappointing, partly because of their hydrolytic sensitivity. However, recent work in Dietmar Seyferth's group at Massachusetts Institute of Technology has focused attention on these polymers as precursors to fabricated silicon nitride fibers and films. Polymers prepared by reactions of dichlorosilane (H2SiCl2) or dialkyldichlorosilanes (R2SiCl2) with ammonia are being investigated for this purpose. This general approach to the preparation of ceramics is focusing attention on long-neglected areas of the periodic table. For example, Leonard Interrante, formerly at the General Electric Research Laboratories in Schenectady, N.Y., but now at Rensselaer Polytechnic Institute, has examined poly(aluminum alkoxides) as precursors to aluminosilicate ceramics. Also, silicon-titanium-carbon fibers prepared by pyrolytic techniques have been described recently by T. Yamamura of Ube Industries in Japan. Much of the chemistry and technology of polymers of this type revolve around the socalled "sol-gel" technique, in which solutions or colloidal suspensions of inorganic precursors are crosslinked to form a gel phase before pyrolysis.
Sulfur systems Polymeric sulfur (plastic sulfur) has been known for many years as a classical inorganic macromolecule. It is made by thermal ring-opening polymerization of Ss rings (rhombic sulfur). Polymeric sulfur is an excellent prototype for studies of ring-polymer equilibria, but it has few practical uses because of its tendency to depolymerize to cyclic small-molecule rings at room tem-
Polyphosphazenes are finding commercial uses Most current technology in polyphosphazene chemistry is based on polymers of structure:
OR" J-N = P— OR_in (n = 10,000-15,000) Depending on the type of side groups present, the polymers may be flexible, film- or fiber-forming materials similar to polyethylene or elastomers with low glass-transition temperatures (Tg = — 80 ° C or higher). Side groups such as methoxy or ethoxy generate elastomeric properties, as do mixtures of two or more different substituents (for example,
CF3CH2O or HCF 2 CF 2 CF 2 CH 2 0). Polymers with only one type of side group, such as CF 3 CH 2 0 or C 6 H 5 0, are microcrystalline materials. Fluoroalkoxy side groups create surface hydrophob i c ^ and solvent resistance. Most polymers of this type resist burning or oxidative breakdown better than single-strand organic polymers. Nearly all are highly stable to water and to a wide range of chemical agents. Uses have been found for these polymers as O-rings; gaskets; fuel line, hydraulic system, and shock absorber components; nonburning insulating materials; and oil pipeline components. Their biostability and biocompatibility have generated interest in their prospective use in biomedical devices.
perature (that is, the system has a thermodynamic "floor temperature" below which the polymer is unstable). So far, it has not proved amenable to significant chemical modification; for example, by the addition of side groups. By contrast, poly(sulfur nitride), (SN) n , has generated great interest as a covalent metal and as an electrical conductor and superconductor. This polymer was first prepared in 1910 by Frank P. Burt in London and in 1925 by Francis L. Usher in Bangalore by passing the hot vapor of cyclic (SN) 4 through heated silver gauze or quartz wool. The condensate, now known to be the cyclic dimer (SN)2, polymerizes in the solid state to (SN) n , a fibrous, gold-colored material. Margot Becke-Goehring at Heidelberg in the 1950s first commented that (SN) n is an electrical conductor. Later Mortimer M. Labes and his coworkers at Temple University in Philadelphia showed that (SN) n is a metal down to liquid-helium temperature. And in 1975 Richard L. Greene, G. Bryan Street, and Laurance J. Suter at IBM discovered that the polymer becomes a superconductor at temperatures below 0.26 K. In the same year, Alan G. MacDiarmid, Alan J. Heeger, and their coworkers at the University of Pennsylvania prepared crystals of (SN) n large enough for x-ray structure analysis and suggested a mechanism for the conduction. It now seems clear that electrical conductivity can take place along the electron-delocalized chains and between chains. Hence, the ordered crystalline nature of the polymer, as well as its molecular structure, is responsible for its electrical behavior. Poly(sulfur nitride) oxidizes rather readily, losing its color and electrical conductivity. It cannot be shaped into films or long fibers because it is insoluble and does not melt without decomposition. However, this polymer was the stimulant that focused attention on other macromolecules as potential electrical conductors. Thus, later
Most of the technology has been developed at Firestone Tire & Rubber's laboratories, Horizons Research, the Army Materials & Mechanics Laboratory and, recently, by Ethyl Corp. Soviet, Japanese, French, and Italian research and development groups are also active in this area. A number of alternative phosphazene polymers have been developed, based on the linear or matrix linkage reactions of the trimer (NPCI2)3 or on the organic polymerization reactions of vinyl-type phosphazenes. Recent work by Christopher W. Allen at the University of Vermont, by research workers at the National Aeronautics & Space Administration, and in India, Japan, Poland, and the Soviet Union has focused on these aspects.
work with polyacetylene, poly(phenylene sulfide), and polypyrrole can be traced to the pivotal role played by poly(sulfur nitride). The behavior of this polymer also is stimulating a search for new conjugated unsaturated inorganic macromolecules. The area of sulfur-nitrogen polymers is one that has important prospects for the future. Cyclic trimers are known that have repeating units such as —S(F)=N— and —S(C1)(0)=N—. If high polymers could be made with these same repeating structures, they would be excellent starting points for polymer substitution reactions.
Phthalocyanine polymers. Nearly all the polymers mentioned so far possess backbones derived from nonmetallic main-group elements, such as silicon or sulfur. However, Case Western Reserve's Kenney, Kenneth J. Wynne at the Naval Research Laboratory, and Tobin J. Marks at Northwestern University have pioneered the synthesis of an unusual class of inorganic-organic polymers in which a metallic or metalloid element (such as aluminum, gallium, silicon, germanium, tin, or chromium) alternates in the polymer backbone with an element such as oxygen or dicoordinate fluorine. Each of the metallic or metalloid elements forms the central atom of a phthalocyanine system, and these stack together to form a rigid, crystalline-type sequence. These polymers are electrical conductors after treatment with dopants such as iodine or tetracyanoquinodimethane (TCNQ). Phthalocyanine polymers with a covalent Si-O, Ge-O, or Sn-O backbone are prepared by hydrolysis of a phthalocyanine containing a metallic or metalloid dihalide unit. Subsequent condensation results in loss of water and formation of the skeleton, which can contain 100 or more repeating units. Such polymers have high chemical and thermal stability. The polysiloxane polyMarch 18, 1985 C&EN
29
Special Report mer can be dissolved in concentrated sulfuric acid and recovered unchanged. A significant advantage of the polymeric phthalocyanines over their nonpolymeric (that is, crystal stacked) counterparts is that the central polymer chain virtually guarantees repetitive stacking of the phthalocyanine rings, a key requirement for electrical conductivity. Moreover, the polymeric character allows these materials to be fabricated into films and fibers.
Polyphosphazenes From a synthetic point of view, phosphazene polymers are the most highly developed of all the inorganic polymer systems, far surpassing the silicones in structural variety. With their backbone of alternating phosphorus and nitrogen atoms and with two inorganic or organic side groups attached to each phosphorus, they have a range of physical and chemical properties previously found only in organic backbone polymers. More than any other system, the polyphosphazenes illustrate the ideas and principles mentioned earlier for the probable development of other inorganic polymer systems. A ring-opening polymerization leads to the formation of a reactive, high-polymeric intermediate. This then functions as a substrate for a wide variety of side group replacement reactions with organic nucleophiles that yield stable organophosphazene high polymers. Each different side group or combination of side
Three routes could yield poly(organophosphazenes)
Polymerize
Reactive polymer
Replace CI with organic ^ group Ft
groups confers different physical and chemical properties on the final polymer. The overall synthesis pathway was discovered and developed in my research group, initially in collaboration with Robert L. Kugel and later through the work of Daniel P. Mack, Timothy J. Fuller, Karen M. Smeltz, Angelo G. Scopelianos, John P. O'Brien, Dennis B. Patterson, Thomas L. Evans, and numerous other coworkers. It has so far been used to prepare more than 250 chemically different polyphosphazenes in our laboratories and elsewhere; some are being used commercially. Other groups active in this area in recent years have been those led by Robert Singler, Gary Hagnauer, and Nathaniel Schneider at the U.S. Army Materials & Mechanics Laboratory; David P. Tate at the Firestone Tire Laboratories; V. V. Kireev in the Soviet Union; and a number of other groups in Japan, France, and Italy. Researchers at Ethyl Corp. in Baton Rouge have recently begun work in this field. In addition, physico-chemical studies on polyphosphazenes have been carried out in a number of academic laboratories. The polyphosphazene system fulfills one other key requirement mentioned earlier: the existence of smallmolecule analogs of the high polymer for use as reaction and structural models. Thus, the cyclic trimer (NPCl2)3 or the tetramer ( N P C ^ k is an excellent reaction model for the high polymer (NPCWn- Other cyclic species, such as (NPF2)3 and various trimers that contain both organic and halogen substituents, function in the same way. In addition, short-chain linear phosphazenes, such as 0=PC1 2 —(N=PC1 2 ) X —N=PC1 3 , are good structural models for (NPCl2)n/ i n nuclear magnetic resonance or x-ray diffraction studies, for example. Because high-polymeric phosphazene chemistry is a very broad and complex field, I will focus on only a few key aspects and concepts that can be expected to provide channels for future fundamental and technological advances.
Polymerization
Poly (organophosphazene) Reactive trtmer
Replace CI with organic group R
Polymerize
Heat
Slfylmonophosphaitfie Note; So far, the polymerization step In Route 2 Has failed to yield h^h-molecttrar-weightDolymers*
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March 18, 1985 C&EN
Two general methods are available for the construction of high-polymeric phosphazene chains—ringopening polymerization and condensation. Of these, the ring-opening polymerization routes yield a much wider range of different polymers with higher molecular weights, although future developments with the condensation route may increase its versatility. In principle, there are three alternative polymerization methods that could yield poly(organophosphazenes). The first is the main synthesis route for the preparation of nearly all the known polyphosphazenes. The crucial step in this process is the controlled polymerization of (NPCl2)3 to a high polymer that will dissolve in organic solvents and will undergo complete substitution. The history of how this process was discovered illustrates the way one or two key fundamental experiments can lead to the development of an entirely new field of chemistry. The cyclic trimer ( N P C ^ h has been known for more than 100 years. H. N. Stokes reported in the 1890s that, w h e n heated, the trimer first melts and then is transformed to a rubbery material. This substance, known as
Phosphazene trimers having both organic and halogen side groups polymerize
Heat
Heat
Heat
"inorganic rubber/' remained a laboratory curiosity for the next 70 years, unstudied in any detail mainly because it was insoluble in all solvents and hydrolyzed to phosphate, ammonia, and hydrochloric acid in a moist atmosphere. Our initial assumption was that the hydrolytic sensitivity of (NPCl2) n high polymer is a consequence of its phosphorus-chlorine bonds rather than phosphorusnitrogen bonds (by analogy with the chemistry of PCI3, PCI5, POCI3, and similar compounds). Thus, it seemed possible that if the chlorine atoms could be replaced by organic groups, the derivative polymers would be stable. However, chemical reactions cannot be carried out easily on an insoluble material. Hence, an essential first step was to find a method for the polymerization of (NPCl2)3 without generating crosslinks. The answer came from a carefully conducted study of the polymerization reaction. We found that the polymerization was a two-stage process. In the first stage, molten trimer polymerizes to an uncrosslinked, veryhigh-molecular-weight open-chain type polymer. In the second stage (after about 70% of the trimer has been consumed), the chains crosslink spontaneously. Thus, time and temperature control of the polymerization is critical. Once prepared, the uncrosslinked polymer was dissolved in an organic solvent such as benzene, toluene, or tetrahydrofuran. It proved to be amenable to halogen replacement by a wide variety of organic reagents. The stability of these organic-substituted derivatives exceeded all expectations. Thus, this pivotal discovery came from a fundamental investigation of a polymerization mechanism, coupled with exploratory synthesis. Even though this pathway has proved to be remark-
ably successful as a general synthesis route, it was clear from the beginning that an alternative route was possible, at least in principle. This alternative involves the initial introduction of organic side groups at the cyclic trimer or tetramer stage, followed by polymerization of the cyclic organophosphazene. This possibility is especially appealing because substitution reactions carried out on cyclic trimers or tetramers, and the purification and characterization of the products are much easier than the corresponding processes for the high polymers. This polymerization route corresponds to the approach used to prepare most poly(organosiloxanes). So far, all attempts to polymerize cyclic trimers substituted with six organic groups or cyclic tetramers substituted with eight organic groups have failed to yield high polymers. Species with alkyl-, phenyl-, or fluoroalkoxy side groups undergo ring-ring equilibration to other cyclic phosphazenes (cyclic, trimer, pentamer, and so on), but they do not yield high polymers. A possible reason for this is the greater intramolecular steric hindrance expected in a linear high polymer when side groups larger than chlorine or fluorine are present, but this explanation is not entirely satisfactory. However, if both halogen and organic side groups are present as side groups on the same phosphazene ring, polymerization does occur at elevated temperatures, even if the phosphazene ring bears a bulky substituent such as a carboranyl or metallocenyl group. Interestingly, as shown recently by Geoffrey H. Riding and Karen D. Lavin in my research group, if a ferrocenyl or ruthenocenyl group is linked to the phosphazene ring in a transannular fashion (a so-called "ansa" structure), ring-opening polymerization of the cyclophosphazene takes place more readily than in the absence of the bridge, presumably a consequence of the release of steric strain. Equally interesting is the fact that organocyclophosphazenes that will not polymerize on their own sometimes copolymerize with (NPCl2)3 or have their polymerization initiated by traces of (NPC12)3. These results raise questions about the polymerization mechanism. The best explanation at the present time is that the initiation step involves ionization of a halogen atom from phosphorus. Work with trimers that contain both organic and halogen substituents suggest that the three repeating units from each trimeric ring enter the polymer as a unit—that is, depolymerization to the monomer does not precede polymerization. However, monomers could be involved in what is ostensibly a condensation polymerization route to polyphosphazenes, a method developed by Patty Wisian-Neilson and Robert H. Neilson at Texas Christian University. Methyl-, ethyl-, and methyl-phenyl-phosphazene high polymers can be prepared by elimination of a trimethyl alkoxysilane from a silylmonophosphazene. In theory, monomer molecules could be generated as transient intermediates in this reaction. Although simple phosphazene "monomers," R 2 P = N , have not been isolated as stable species, strong evidence for the existence of one (highly hindered) derivative has been reported by Guy Bertrand, Jean-Pierre Majoral, and coworkers in Toulouse, France. March 18, 1985 C&EN
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Special Report Characteristics of organo-substituted polyphosphazenes vary with their side groups
Water-soluble, film-forming polymer. Excellent matrix for salts in solid battery applications. Antifoaming agent
Hydrophilic. Hydrolytically degradable at slow rate to nontoxic small molecules. Prospective biomedical material
Noncrystalline, low-temperature elastomers. Prone to side-group rearrangement at elevated temperatures
Microcrystalline, film- and fiber-forming polymer. Hydrophobic. High stability. Soluble in ketenes
Elastomers. Low glass-transition temperature. High chemical stability. Soluble only in fluorocarbons. Fire resistant
Microcrystalline, film-forming polymer. Soluble in hot aromatic solvents
Extremely sensitive to decomposition by water Water soluble
Prototype for anesthetic high polymer Water soluble. Water stable
Water insoluble. Water stable, film former
The phosphorus-halogen bonds in poly(dihalophosphazenes) are inherently unstable to moisture, having roughly the same reactivity as PCI3, PCI5, or organic acid chlorides. This precludes the use of halogenophosphazenes themselves for most practical purposes, although in the laboratory they are valuable polymers for structural and other physicochemical studies. The key to the utilization of polyphosphazenes as stable and useful materials lies in the replacement of the halogen atoms by organic groups. In this sense, the principal drawback of poly(dihalophosphazenes)—their high reactivity—is also their main attribute for macromolecular substitution processes. Substitution reactions carried out on organic polymers are usually slow and 32 March 18, 1985 C&EN
Glass-type polymer at room temperature
incomplete. However, the high reactivity of phosphorus-chlorine bonds provides an extremely facile pathway for substitution by organic nucleophiles. For example, the reaction of (NPCl2) n with CF3CH2ONa to yield [NP(OCH2CF3)2]n is essentially complete within a few minutes at moderate temperatures, especially when phase-transfer catalysts such as tetraalkyl ammonium halides are present. Because many inorganic element-halide covalent bonds have a similarly high reactivity, the same general approach should be feasible for other inorganic polymers with halogen side groups. The first fully substituted, uncrosslinked poly(organophosphazenes) were prepared by us in the mid-1960s.
Since then, the organic substitution chemistry derived from (NPCl2)n and (NPF 2 ) n has grown to be an extremely diverse field. Only the outlines of it will be sketched below. First, as mentioned earlier, the replacement of chlorine atoms in (NPCl2)n by alkoxy- or aryloxy side groups is the oldest and most deeply studied aspect of the field. Side groups such as CH3O, C 2 H 5 0 , CF 3 CH 2 0, HCF2(CF 2 )nCH 2 0, C 6 H 5 0, and RC 6 H 4 0 form the basis of most of the technological phosphazenes available today. An interesting characteristic of these polymers is their tendency to depolymerize to small-molecule cyclic phosphazene analogs at elevated temperatures—for example, above 300 °C—known as the ceiling temperatures. This tendency seems to be exacerbated when all the side groups are bulky units, such as aryloxy groups, presumably because the steric crowding in the polymer can be relieved by depolymerization and cyclization. A great deal needs to be learned about the depolymerization mechanisms, thermodynamics, and ceiling-temperature phenomena. Only when these fundamentals are understood can the depolymerization reaction be controlled. Recently, new types of alkoxy or aryloxy side groups have been attached to phosphazene chains. These include steroidoxide units (with a view to the preparation of bioactive phosphazenes) and CH 3 OCH 2 CH 2 6 groups that confer water-solubility properties. (Polymers of the latter type have recently been shown by Duward F. Shriver and Peter Blonsky at Northwestern University to be excellent polymeric salt electrolytes for solid batteries.) Glucose residues also have been incorporated as side groups and these generate water-soluble behavior. The second class of polyphosphazenes that have been studied in detail are the poly(aminophosphazenes), prepared by the reaction of (NPCl 2 ) n with primary or secondary amines. A wide variety of amines can be used in this reaction, ranging from methylamine, dimethylamine, aniline, bioactive amines such as procaine or benzocaine, and amino acid esters such as ethyl glycinate or the ethyl ester of phenylalanine. The methylamino derivative is soluble in water to yield stable, basic solutions and has been used as a coordination carrier for transition metals and metalloporphyrins. Polymers with amino acid ester side groups are slowly hydrolyzed in water to yield ethanol, amino acid, phosphate, and ammonia. Hence, they are attractive matrices for the controlled delivery of chemotherapeutic agents. One question that arose early in the development of polyphosphazene chemistry was whether the inorganic backbone was sufficiently stable after the chlorine atoms had been replaced by organic units to allow organic chemistry to be carried out on those side groups without damage to the polymer backbone. We now know that, in general, such reactions are possible without the backbone's being affected. Here are a few examples from our laboratory. Bromoaryloxy groups attached to the polymer chain can be lithiated and treated with diphenylchlorophosphine to
Substituent groups give polyphosphazenes varied properties Polyphosphazene
For water solubilization, ft = CH3NH, Glucose residues, CH3OCH2CH20 For water destabilizatfon t R CH3CH202CCH2NH, ImldazoyI
For water stabilization, R = CF 3 CH,0, C6HgO, C e H s , CH 3
For biological activity, R « tf&pmn, RCI 2 (as W-coordirration adduct), X
(Steroid)
(Procaine)
(Dopamine)
(Sulfadiazine)
yield phosphino-aryloxy phosphazenes. This work was done by Evans, Lavin, and Norris M. Tollefson. These polymers have been tested as carriers for organometallic catalysts in hydroformylation reactions by Philip E. Garrou and Robert A. Dubois at Dow Chemical's New England Laboratory. The polymer backbone is stable, but the phosphine units become separated from the aryloxy side groups during the catalyzed reaction. Polyphosphazenes with nitro-aryloxy side groups can be catalytically reduced to the amino derivative and coupled with aldehydes to form Schiff base species. Similarly, formyl or ketonic aryloxyphosphazenes form Schiff bases with primary amines, including bioactive amines. Aminoaryloxy groups attached to a polyphosphazene chain can be diazotized and coupled to phenols to yield polymeric azo dyes or coupled to catecholamines to yield, for example, dopamine-bound polymers. As shown by Paul E. Austin in my research group and Wesley C. Hymer in the biochemistry department at Penn State, these latter systems retain the biological March 18, 1985 C&EN
33
Special Report activity of the bound dopamine at the surface of the solid polymer. Amide coupling reactions also can be performed on amino-aryloxy side groups, a preliminary step to the attachment or growth of oligopeptide chains. Sulfa drugs and N-acetylpenicillamine have been linked via similar chemistry. Other examples include the quaternarization of halomethylene-aryloxy side groups and coupling to heparin, and the linkage of porphyrins via coupling reactions. These reactions were developed by Austin and Thomas X. Neenan in my laboratory. In all these reactions, the phosphazene backbone remains intact. Clearly, side group and even skeletal atom modification of these types markedly increase the scope of polymers for fundamental and other studies.
Organometallic problem The reactions of alkoxides, aryloxides, and amines with (NPCl 2 ) n or (NPF 2 ) n have been developed to a high degree of sophistication. Yet all of these reactions yield polymers in which the organic side group is linked to the skeleton through an oxygen or nitrogen atom. In some cases, this linkage atom is the source of thermal or chemical instability. Thus, the synthesis of polyphosphazenes with alkyl or aryl groups connected directly to the skeleton through carbon-phosphorus bonds is of considerable interest. Such polymers would be structural analogs of alkyl and aryl polysiloxanes. As mentioned earlier, a few polymers of this type are available via a condensation polymerization route. Also, a number of macromolecules with both alkyl or aryl and alkoxy or amino side groups have been prepared by the ring-opening polymerization method followed by halogen replacement. But an appealing route to structures of this kind is through the reactions of organometallic reagents with (NPCl2)n or (NPF2)n. A great deal of effort has gone into exploring this route, and some interesting conclusions have emerged, especially from studies using small-molecule model compounds. First, conventional organometallic reagents, such as Grignard or organolithium reagents, react with (NPCl2)n by a complex pathway. The pathway involves metalhalogen exchange, replacement of chlorine by alkyl or aryl groups, skeletal cleavage and—in some cases— metallation of the alkyl groups already introduced, coupled with crosslinking probably through the formation of intermolecular phosphorus-phosphorus bonds. Use of (NPF2)n as a substrate simplifies the synthesis somewhat, since metal-halogen exchange and attack on skeletal nitrogen are both reduced, but (NPF2)n is soluble only in fluorocarbon solvents, which constitutes a severe experimental problem. At the small-molecule level, alkyl copper reagents react with (NPCl2)3 with the introduction of alkyl groups and a hydride unit at the geminal position. The hydride function can then be metallated and a second alkyl group introduced. Alternatively, Grignard reagents induce phosphorus-phosphorus coupling reactions to give products that also provide a route to dialkyl or alkyl-aryl-substituted trimers. Much of this chemistry has been worked out in my laboratory by Paul J. Harris, Mark S. Connolly, and James L. Desorcie. 34
March 18, 1985 C&EN
At the present time we do not know how much of this chemistry really applies to the high polymers, although we are attempting to perform the same reaction sequences at the macromolecular level. At the very least, this pathway is expected to allow the introduction of one or two alkyl or aryl groups every three repeating units along the chain. However, a more difficult challenge is to develop reactions that allow a wide range of transition metalcontaining side groups to be attached to the skeleton via organometallic substitution processes. Solving this problem has been one of tne main objectives of my research group during the past three years. As indicated earlier, lithiometallocenes and lithiocarboranes react cleanly with halogenophosphazenes, with metallation of the carborane function being carried out after linkage to the skeleton. The lithiometallocene reactions are especially interesting, with metal-halogen exchange reactions predominating when chlorophosphazenes are employed but not with fluorophosphazenes, where substitution is the main pathway.
Variety of polyphosphazenes are made by organic reactions on preformed polymer
Carrier polymer for transition metal catalysts
Schiff s base, hydrolyzable to release bioactive amine
Heparin
Amide-coupled bioactive species
Nonthrombogenic biomedical polymer
Dopamine
Polymer-bound dopamine, elicits response similar to free dopamine in rat pituitary cells
Carrier molecule In solution and solid states for metaltoporphyrins
Organometallic anions, such as [Fe(CO)2Cp]~, [Fe 2 (CO) 8 ] 2 -, and [Cr(CO) 3 Cp]- (Cp is cyclopentyldienyl), react with (NPCl2)3 or (NPF2)4 to yield a range of unusual small-molecule organometallic phosphazenes, several of which have been prepared at Penn State. In many cases, these compounds are stable in the atmosphere and are intriguing prototypes for their macromolecular analogs. Although not yet achieved in practice, reactions of this type suggest a way toward the synthesis of hybrid phosphazene-transition metal "outrigger" polymers, which would be interesting molecules for electrical conductivity studies. These organometallic reactions of phosphazenes are more complex than the reactions with organic alkoxides, aryloxides, or amines, but they illustrate the exciting opportunities that exist in this system for molecular design and exploratory synthesis.
Molecular structure and physical properties
A particularly puzzling feature of these molecules is the nature of the bonding along the backbone. Unlike organic 7r-conjugated polymers such as polyacetylene, most polyphosphazenes are colorless, do not conduct electricity, and have an inherent skeletal flexibility. These differences can be ascribed to the nature of the 7r-bonds formed between phosphorus and nitrogen. The structure can be viewed as a stabilized excited state, with the phosphorus atoms using their 3d-orbitals to form 7r-bonds with the p-orbitals of the flanking nitrogen atoms. Unlike organic 7r-systems, d^-pT bonds do not impose severe conformational restrictions on the molecule. Any one of the five 3d orbitals at each phosphorus can 7r-bond to the nitrogen p-orbitals. Thus, each skeletal bond can rotate through 360° without incurring a serious energy penalty. It is as if torsion of an inorganic bond of this type resembles a rotary rheostat, whereas, in the same terms, torsion of an organic p^-p* bond would correspond to torsion of a two-position rotary switch. This explains the low glass-transition temperature values found for some polyphosphazenes and the high deformability of some derivatives, but it does not explain the lack of color or electrical conductivity. The presence of color and conductivity is usually associated with lowenergy electronic transitions that involve the TT or TT* orbitals. Such transitions normally require electron de-
In its most sophisticated role, synthesis is a powerful technique for the exploration of why specific molecular structures give rise to unusual properties. Polyphosphazene chemistry offers a rare opportunity to study the properties of a wide variety of macromolecules with the same main chain structure but with different side groups attached to that chain. At Penn State, we have investigated the role played by the side group in altering both the ease of backbone torsional freedom and the conformation assumed by the polymer in its microcrystalline state. The tools Phosphazenes containing transition metals are employed for this have ranged from challenging synthetic targets nuclear magnetic resonance analysis through conformational energy calculations to x-ray diffraction studies. The picture that is beginning to emerge is one in which the polyphosphazene backbone is a highly flexible structure with considerable torsional and angular freedom in the —P—N— skeletal system. This, in turn, permits facile conformational changes that, in the presence of small side groups and in the absence of microcrystalline order, give rise to elastomeric behavior even at very low temperatures—in one case as low as —90 °C. In this aspect, the phosphazene backbone resembles the siloxane skeleton in silicones. This provides an insight into the prospective behavior of other related inorganic macromolecular systems. Thus, the glasstransition temperature, which is a measure of the reorientational freedom of the polymer chains, varies widely according to the diThese compounds, prepared at Pennsylvania State University by reaction mensions, stiffness, and polarity or between organometallic reagents and cyclophosphazenes, are prototypes hydrogen bonding capability of the for linear polymers with similar side-group structures side groups.
March 18, 1985 C&EN
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Special Report
"Outrigger" polymers might show electrical conductivity
Polyphosphazene chain
localization or conjugation over many atoms to lower the energy of the 7r* state. In polyphosphazenes it appears that 7r-delocalization does not occur over more than three skeletal atoms, being interrupted by the orbital node at each phosphorus. This situation was predicted by Michael J. S. Dewar, then at the University of London, and his coworkers 25 years ago, long before the first stable polyphosphazenes had been prepared. It is an important concept for understanding and predicting the properties of other polymers that contain second-row elements in the skeleton. What factors may affect the exploration and development of new inorganic macromolecules in the years ahead? What needs to be done to stimulate advances in this area? The principal impediment in the past has been a scarcity of investigators who understand inorganic reaction chemistry and are willing to learn the tech-
niques of polymer chemistry. Interfacial areas of chemical research are often neglected until they have gathered considerable momentum, and this has certainly been true for inorganic macromolecular research. However, it is clear from recent work that dramatic advances in both fundamental science and technology would be possible if the high-polymer chemistry of the representative elements were to be studied in detail. The key problem in inorganic macromolecular research is the polymerization process itself. Thus, it seems clear that the immediate focus of new research should be on the study of inorganic ring-opening and condensation polymerizations. In turn, the development of new ring-opening polymerizations will depend on a fundamental understanding of ring-ring and ring-chain equilibria in inorganic and organometallic systems, a subject that in the past has not received the attention it deserves. In addition, because polymer substitution reactions probably will play an increasingly important role in the development of this field, much more fundamental information is needed about the interaction of chemical reagents with very large molecules, both inorganic and organic. All this has to be done as part of a revitalized exploration of the chemistry of the main-group elements by both inorganic and organic chemists. It must also be recognized that metal-metal bonding between maingroup or transition elements offers the promise of metallo-backbone polymers that could further bridge the gap among metals, ceramics, and macromolecules. •
Reprints of this C&EN special report will be available at $3.00 per copy. For 10 or more copies, $1.75 per copy. Send requests to: Distribution, Room 210, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036. On orders of $20 or less, please send check or money order with request.
Polyphosphazene structure can be viewed as an inorganic 7r system
bond Electron-dot description of the electrons left over when electron pairs from the skeletal elements and side groups are assigned to conventional sigma bonds is shown above. A working description of the remaining electron structure is that two electrons from nitrogen form a lone-pair orbital (responsible for the marked basicity of alkyl or amino phosphazenes). The remaining two electrons per repeating unit then form a 3d7r-2p7r bond, which can be visualized as a stabilized excited state (below).
36
March 18, 1985 C&EN
Harry R. Allcock is professor of chemistry at Pennsylvania State University, University Park, Pa. He received his Ph.D. from the University of London in 1956 and has been a member of the Penn State faculty since 1966. Allcock was trained as a physicalorganic chemist working with organosilicon compounds, but he developed an interest in inorganic synthesis and polymer chemistry as a result of postdoctoral and industrial experience in those fields. He received the 1984 American Chemical Society Award in Polymer Chemistry, sponsored by Mobil Chemical, for his pioneering research on the synthesis of polyphosphazenes. His research interests include small-molecule and macromolecular synthesis; structure I mechanism studies with main-group, transition metal, and organic systems; and the synthesis and function of bioactive macromolecules.
