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An Overview of Inorganic and Organometallic Polymers Martel Zeldin * INTRODUCTION The chemistry and technology of polymeric materials have had a major impact on all facets of our life and the economy of the world. Competition among nations for leadership positions in the development of materials science and technology is economically driven. In fact, it can be predicted that the counties with the most advanced and sophisticated materials technology will offer their citizens the highest standards of living and the greatest hope for national security in the decades to come. In spite of the importance and widespread use of organic polymers, increasing attention has been given to macromolecules that contain metals and metalloids, that is, inorganic and organometallic polymers. The reasons for the growth in these "hybnd" species are linked to need. For example: Organic polymers are often susceptible to reaction with oxygen and ozone. The presence of metals or metalloids reduces that propensity. Organic polymers thermally decompose to volatile materials that burn; the presence of inorganics increases thermal stability, reduces inflammability of the volatiles, or increases nonvolatile residues. Organic oolvmers degrade when exposed to ultraviolet and higher energy radiation; norgan~cs are less susceptible to radiative degradation. Organic polymers are somewhat more soluble or swellable in solvents. Inorganics generally form stronger bonds and offer greater resistance to free radical cleavage reactions than carbon compounds. Longer bond lengths and higher valencies give rise to greater torsional mobility and thus greater flexibility and other dynamic-mechanical properties. ~ A- ~ · · . MAJOR CLASSIFICATIONS OF INORGANIC AND ORGANOMETALLIC POLYMERS Although many elements (metals, semimetals, and nonmetals) have been incorporated in one manner or another into an inorganic and/or organometallic polymer unit, certain elements lend themselves better to products with commercial potential. Thus, for example, silicon- containing polymers are used extensively for industrial and consumer products, whereas boron- containing polymers (i.e., as prepolymer for boron nitride (BN) ceramics or as carborane *Science and Technology, The College of Staten Island-City University of New York. i51
152 Improved Fire- aM Smoke-Resistant Materials siloxane ultra-high-temperature elastomers) are produced in limited quantity for very special applications. The following is a brief discussion of some of the more important and interesting inorganic and organometallic polymers. Although commercialization has been attained only in a few examples, the potential for design of materials with unusual and significant properties is unlimited. Silicon-Based Polymeric Materials Polysilaxanes R ski - - R H. hydrocarbon Polysiloxanes (PSs), compounds containing Si-O bonds in the polymer backbone, are the oldest and largest class of inorganic-organometallic polymers of commercial significance (Rochow, 1987~. Worldwide production amounts to about 0.5 million metric tons, with commercial value exceeding $3.5 billion. Their physical properties (see Table I), which are sufficiency different from those of organic polymers, continue to catch the attention of academic scientists and industrial research and development laboratories. Thus, the growth and development of this class of polymer have remained unabated. In fact, a review of the current literature reveals an increase in the number of publications about and patents for PSs throughout the world. Some of the reasons for the remarkable and sustained growth in this area are: The raw materials, principally silicon dioxide, methy~chioride and other organic compounds, for the production of PSs are readily available. Consumer and industrial applications of PSs are found in commodity and other industries, for example, the plastics, rubber, paper, coatings, automotive, construction, food, biomedical, cosmetics, and specialty chemicals inclustries. PSs' copolymers can be prepared for tailoring physical, dynamic-mechanical, and chemical properties of materials. What is on the horizon for PSs? Some of the significant developments in the field are summarized below: . Recent progress in understanding the mechanism(s) of PS polycondensation and ring-opening polymerization will permit the synthesis of linear polymers with regio- and stereoselectivity. Synthetic control of these materials will inevitably lead to dominion over polymer microstructure and hence macroscopic properties (Chojnowski, 1993~.
Marte! cretin Polysilazar~es 153 Advances in methods for copolymer synthesis, both siloxane-siloxane (Kennan, 1993) and siloxane-organic (Smith et al., 1992) polymers, offer greater flexibility in modifying and improving properties for specific applications. New functionalized polysiloxanes enable the formation of miscible polymer mixtures and polymer blends for improved properties (Lu et al., 1993~. Controlled synthesis of cross-linked network PSs, for example, silsesquioxanes and related copolymers (Lichtenhan et al., 1993), interpenetrating networks (U.K. Patent 804,199; U.S. Patent 3,971,705; Lestel et al., 1990), and hyperbranched hybrid PSs (Rubinsztajn, 1994; Rubinsztajn et al., in press), has led to materials with significantly improved thermal, mechanical, and optical properties. Development of the colloid chemistry of silica (sol-gel processes and technology) has had a significant impact on the preceramic, ceramic, and surface coatings industry (Blinker et al., 1988; Huang et al., 1988; Saklca et al., 1988; Schmidt, 1988; Bergna, 1994~. , . . TABLE 1 Some Properties and Major Uses of Polysiloxanes Properties Uses Low glass transition temperature Low thermal coefficient of viscosity High thermal stability Oxidative stability Hydrophobicity-lipophilicity Elasticity Low surface energy Biological inertness Photochemical stability Low dielectric constant Heat exchange fluids Dielectric fluids Antifoams Lubricants Polishes RTV and HTV rubbers Chromatographic resins Refractory materials Damping fluids Biomedical applications R Isis 1 1 ~ nil n n R H. hydrocarbon Polysilazanes, compounds containing Si-N bonds in the polymer backbone are an example of organometallic precursors to advanced non-oxide ceramic materials, for example, for reinforcement of ceramic, plastic, and metal matrix composites (Zeigler and Fearon, 19871. Such ceramic materials offer a wide variety of unique physical properties that include extreme hardness, high structural and thermal stability under extremes of environment conditions, and
154 Improved Fire- arm Smoke-Resistant Materials potential for electronic and optical properties. The major challenges facing further commercialization of polysilazanes and ceramic progeny are the following: synthesis of tractable prepolymers, that is, precursors with solubility in common organic solvents; precursors with sufficiently high molecular weight or optimum degree of cross- linking for shape retention in the pyrolytic process of ceramic formation; ability to attain high ceramic conversion; control of the pyrolysis process to minimize ceramic porosity; and reduction of carbon content in the ceramic product. Polycarbosilanes R ~S'-Y ~ Y = CH2, unsaturated hydrocarbon Like polysilazanes, polycarbosilanes, compounds containing Si-C bonds in the polymer backbone are an example of organome~lic precursors for advanced silicon carbide (SIC) ceramic materials. Their route through polysilanes has been commercialized by the Nippon Carbon Co. into silicon carbide fibers ~CALON_) (see scheme below). (CH 3) 2sia2 [(CH3~2~ ] / Na 1 1. air, 280 °C ogle ~ ii CH2)D. i N29~300 0C SiC \<~/ CHe t(CH3~2Sit SiC-based ceramic materials offer a wide variety of physical properties that include extreme hardness, high structural and thermal stability, and future potential as coating materials for electronic and optical devices. Recently, novel carbosilane dendritic macromolecules have been prepared (Zhou and Roovers, 1993~. These starburst-hyperbranched polymers are three- dimensional symmetric species with well-defined 32- and 64-arm structures of intermediate molecular weight. These materials may have interesting physical properties. Additionally, stable comb-like polymers with carbon un saturation in the polymer backbone and pendant
Marte! Ze~in 155 oligofoxyethylene3 units have been synthesized (Wang and Weber, 1993; Chen et al., 1993~. Significantly, ceramic materials from their pyrolysis have high char yield. The combination of solubility and hydrolytic and thermal stability of these polycarbosilanes offers opportunities to tailor properties by using appropriate preceramic polymer blends. Similarly, new branched polyhydridocarbosilanes are precursors to high-residue ~ ~ 70 percent) SiC ceramics (Froehling, 1993~. Patents for these materials are in progress. Polysilar~es R HI ~ R Organic Although polysilanes, compounds with Si-Si bonds in the polymer backbone, have been known for decades, only recently has synthetic methodology been sufficiently perfected to afford soluble oligomers and high molecular weight polymers that can be fully characterized. These substances are stable to 300 °C, mildly susceptible to hydrolysis, and, contrary to early predictions, inert to oxygen at ordinary temperatures. Perhaps the most significant and surprising discovery is that these materials possess interesting and commercially important photochemical and electronic properties. For example, since poly~diorganosilane~s absorb light in the visible and ultraviolet region of the spectrum as a function of substituent and chain length, and photodecay to volatile products, they have great potential as positive photoresists in the microelectronics industry and photoconductors in the electrophotography industry. In addition, they serve as free radical photoinitiators in organic reactions and display nonlinear optical properties for potential use in lasers and other optical devices (Mark et al., 1992~. Phosphorus-Based Polymeric Materials R MAP N- - R = Just about any type of substituent : Polyphosphazenes, compounds with P-N bonds in the polymer backbone is rivaled only by polysiloxanes with respect to diversity of species and potential for commercialization. The potential for growth and development of this unusual class of materials is reflected in the synthetic ease with which framing substituents, R. can be changed and modified, and in the
156 Improved Fire- arm Smoke-Resistant Materials consequential variations in physical and chemical properties that occur from the compositional versatility (AlIcock, 1988; AlIcock et al., 1992~. Thus, different pendant groups lead to dramatically different materials, that is, elastomers or glasses, hydrophobic or hydrophilic -I-' inert or highs reactive materials i ' - - - po~ymers, inert or highly reactive materials, insulators or conductors, photoactive or photoinert materials, and bioerodible or bioinert material. Table 2 summarizes some of the current and projected applications of polyphosphazenes. Some unusual polyphosphazenes are noted below. ~-R' ~ Pe En IN = CAN = I my l' = N -IF = N I P = No OR OR ~oR oR TABLE 2 Applications of Polyphosph~enes Elastomers Ionic electrical conductors Membranes Water repellents Nonburning textile fibers Foam heat and sound insulators Hydrocarbon solvent-resistant O-r~gs and gaskets Immobilized enzymes Controlled drug-release agents Hydrogels for prostheses and soft-tissue applications Liquid crystalline polymers for nonlinear optical properties Ceramics Although other inorganic and semi-inorganic polymers containing phosphorus (e.g., phory} resins, poly~alkylene phosphates), polycarbophosphazenes) are known and, in some instances, possess interesting properties (transparency, hardness, flame retardancy, adhesion, biocompatibility, and elasticity), they appear to suffer from three major disadvantages high cost, oxidative instability, and hydrolytic sensitivity. Perhaps further research on related compositions will produce new classes of materials without the problems.
Marte! Ze~in OTHER TYPES OF INORGANIC AND ORGANOMETALLIC POLYMERS Boron- and Almnmu~n-Contammg Polymers Polyborylamines and Polyborazines R R kB``N'B``N' R' R' R = Hydrocarbon 1 157 Hi H `,,N B\,N: N B x H H 2a /<N ~ x H- Be/ AN-H ,N=B , ~ h' -H 2b Polyborylamines (1) and polyborazines (2a, 2b), compounds with the B-N bond in the backbone as linear chain, or connected I,2 or 1,6 six-member rings, are precursors to non-oxide BN ceramics. Like the SiC, silicon nitride (SiN), and boron carbide (BC) materials, BN ceramics are of great interest due to their exceptional hardness and stability to oxygen and water under extremes of conditions. The use of BN ceramics as coatings, reinforcing fibers, and new- generation composite materials depends on the ability to produce readily processible polymers and prepolymers that can be shaped or spun into filaments before pyrolytic transformation into products. A considerable amount of effort has been expended in several laboratories, both here and abroad, in the search for soluble, processible polymers. Some success has been reported over the past decade (Narula et al., 1988; Paciorek et al., 1988; Shaw et al., 19881; however, the field needs considerably more research and development support. Hybrid Boron-Nitrogen-Carbon Polymers HsC3` \B~ \B/ OR H5C3/ C3H5 'C3H~ R = C6H4, (CH2)n
158 Improved Fire- and Smoke-Resistant Materials A new class of soluble boron-nitrogen-carbon (BNC)-containing polymers, prepared by ally~boration polymerization, has recently been reporter! (Chujo et al., 1992~. Relatively low- molecular-weight Ad,, 5,000-14,000), organic soluble oligomers in pure form are obtained and have been fully characterized. Although pyrolysis of the polymer at 900 °C gives a BNC ceramic-like material, the properties of the product have not been published. Carborane Cage Polymers . o Me- Si- Me Me FIR l l ~7 I~ si-o-s'~ X Me Me Synthesis of m-carborane cage polymers based on the decaborane cage structure, CB~oH~oC-, was achieved in the 1960s (see Alicock and Lampe, 19901. Incorporation of poly(dimethylsiloxyI) units between the cage functions results in elastomeric copolymers that retain their thermal stability ~` > 500 °C), low Tg (-30 °C), and chemical-oxidative stability. These properties lead to high-performance applications as specially (Dexil) resins for chromatography. Related materials, which contain diamine groups, have resulted in BN-like fibers. Although these sub spaces were studied extensively in the 1960s and 1970s, further development of them has essentially ceased due to the difficulty and cost of producing the parent carboranes. Oligo (alurninoxane)s MAIM SAC OR R = alkyl, Neal cy' b R' /~/ ,0-~' R', R" = alky', ary'
Marte! Zeldir~ 159 Oligotaluminoxane~s, prepared by condensation polymerization of AlCI3 in the presence of aqueous organic acids or amides, are low molecular weight, highly branched, soluble materials with the Al-O bond as the basic repeat structural unit. These compounds have found application as ~drag-reducingU agents for fluid toil or water) flow, drying and gelling agents, coatings, fuel additives, catalysts, and lubricants. These oligomers and the amide analogs can be melt-spun into fibers that, upon pyrolysis, yield Al203 (alumina) and AIN (aluminum nitrides. Sulfur-Containing, Polythiazy' (S-N)X, Polymers Sulfur-containing polymers have been studied for decades. Elemental sulfur itself is a polyatomic, polymorphic material, and exists as chains and rings (i.e., S6, Ski. Although the element is low in cost and plentiful, there was only intermittent research and development activity until the 1970s. ,N So Nit x In 1975, the spotlight was turned onto S-N polymers because of their remarkable metal- like appearance and their properties, for example, malleability, electrical conductivity, oriented epitaxial structure, optical polar~zability, and superconductivity (at 0.3 °C) (Mikulski et al., 1975~. The polymer can be fabricated as thin films on glass or synthetic polymeric surface by epitaxial polymerization. They have a gold color by reflected light and blue color by transmitted light. They are "relatively" inert to air and moisture; however, they detonate on compression and depolymerize under ambient conditions on long standing. Polythiazyls are the forerunners of polyacetylenes. Tin-Containing Polymers R R Drum ~ ~ FIR R _ 914 R 0~1 R ladder /
60 Improved Fire- and Smoke-Resistant Materials Most notable of the tin-containing macromolecules are the ladder-like polymers. Interest in these materials is relatively recent and a result of their high temperature stability. Moreover, it has been found that these stannsequioxanes exist in several structural types ("ladder," "drum," and "butterfly") and contain hypervalent tin (Holmes et al., 1988~. The biological activity of tin species has discouraged their continued investigation as commercially useful materials. Ferroceny' Polymers Homoannular Heteroannutar it"""' (~CH-CH2= ~C~ - M (CO)3 Pendant Metallocenyl polymers, which contain a wide variety of metallic elements, have been known for a long time. Most of the studies on these materials occurred in the 1960s, principally with iron as the metal center (Neu se, 19821. The structural units depicted above (homoannular, heteroannular, and pendant) represent only a few of the types that have been synthesized. The major limitations in further development of home- and heteroannular materials results from low solubility, low-to-moderate molecular weights, and lack of mechanical properties for applications. Recently, pendant-type water-soluble polymers, such as polyaspartamide-bound ferrocene compounds, have been prepared and characterized. These materials were found to have salient biomedical applications (Neuse et al., 1990~. A strained-ring ferrocenyl-silane (below), which can be thermally polymerized to an amorphous magnetic iron-containing SiC ceramic, has been recently reported (Manners, 1993). Hi R -R' heat R Mn, 30000-800000 500°C, Fe-Si~
Marte! Ze~in ~0 it> 014 gN,$~ :~-I N - N 161 Phthalocyan~ne (shiskabab) Polymers Phthalocyanines are planar macrocyclic rings composer' of aromatic groups and nitrogen atoms capable of tetra- coordina~cion with a meW or Adenoid. Polycondensation of a monomeric Riot derivative, where M = Si, Ge, or Sn, leads to one-dimensional stacked me~Ioxy chain macromolecules with unusual physical and dynamic-mechanical properties. These polymers dissolve in strong acids and are spinnable into fibers. Since they are highly oriented, they can be co-spun with such materials as polyaramids (e.g., Kevia~) to impart strength and thermal stability. Moreover, upon doping with iodine, before or after spinning, the phthalocyanine polymers possess interesting magnetic and electronic (conductor and semi- conductor), as well as optical, properties (Marks et al., 1988). MetalloporphyFin Polymers `1 `~ I N ~ , \ 1 , I ,N-M N / Hi\ INS FAX M = Fe(III), Co(II), Ni(II), Zn(II) X = Organic function Metalloporphyrin polymers have been popular recently due to potential catalytic, electrochemical, and electrical-photonic properties. For example, work by Lindsey and colleagues has demonstrated that, with M = Zn and the appropriate in-chain polymer spacer groups, an oligomer can act as a 9-nary macromolecular photonic wire with 76 percent energy transfer efficiency (Wagner and Lindsey, 1994~.
162 Improved Fire- aM Smoke-Resistant Materials SUMMARY Research on inorganic and organometallic polymers has grown enormously over the past 30 years. The above overview represents a relatively small perspective of the breadth and scope of the field that has taken numerous, interesting diversions based, on the one hand, on Intellectual curiosity and creativity, and on the other, on economic objectives. As long as special properties and function are desired, new organic, inorganic, organometallic, hybrid materials will be discovered. REFERENCES Allcock, He R., and FeWe Lampe. 1990. Contemporary Polymer Chemistry, 2nd ed. Englewood Cliffs, New jersey: Prentice-Hall. Allcock, H.R. 1988. Current status of polyphosphazence chemistry. Pp. 250-267 in Inorganic and Organometallic Polymers. M. Zeldin, K.~. Wynne, and H. R. Allcock, eds. ACS Symposium Series 360. Washington, D.C.: American Chemical Society. Allcock, H.R., I.E. Mark, R.E. West, eds. 1992. Inorganic Polymers. Englewood Cliffs, New Jersey: Prentice-Hall. Bergna, H.E. 1994. The colloid chemistry of silica. Pp. I-50 in ACS Advances in Chemistry Series 234. H.E. Bergna, ed. Washington, D.C.: American Chemical Society. Brinker, C.~., B.C. Bunker, D.R. Tallant, K.~. Ward, and R.~. Kirkpatrick. 1988. Structure of sol-gel derived inorganic polymers: Silicates and berates. Pp. 314-332 in Inorganic and Organometallic Polymers. M. Zeldin, Kit. Wynne, and H.R. Allcock, eds. ACS Symposium Series 360. Washington, D.C.: American Chemical Society. Chen. M.W.. C.X. Liao. and W.P. Weber. 1993. Synthesis. characterization. and platinum . ~ ~ ~ ~ ·~ . · ~ · ~ · ~- ~ ~ , ~ ~ ·~ ~ I ~ ~ 1 ~ camyzeo nyclrosllatlon crossing of COpOly (metnylsllylene /1,4-pnenylene /methylvinylsilylene3: Properties of aromatic carbosilane thermosets. journal of Inorganic and Organometallic Polymers 3:241-249. Chojnowski, I. 1993. Siloxane Polymers, pp. I-62. S.~. Clarson and I.A. Semlyen, ecis. Englewood Cliffs, New jersey: Prentice-Hall. Chujo, Y., I. Tomita, and T. Saegusa. 1992. Allylboration polymerization: Synthesis of boron- containing polymers by the reaction between triallylborane and dicyano compounds. Macromolecules 25: 3005-3006. Froehling, P.E. 1993. Synthesis and properties of a new, branched polyhydridocarbon as a precursor for silicon carbide. journal of Inorganic and Organometallic Polymers 3:251-258. Holmes, R.R., R.O. Day, V. Chandrasekhar, C.G. Schmid, K.C.K. Swamy, and J.M. Holmes. 1988. A new class of oligomeric organotin compounds. Pp. 469-482 in Inorganic and Organometallic Polymers. M. Zeldin , K.~. Wynne, and H. R. Allcock, eds. ACS Symposium Series 360. Washington, D.C.: American Chemical Society. Huang, H.-H., R.H. Glaser, and Gig. Wilkes. 1988. New hybrid materials incorporating poly(tetramethylene oxide) into tetraethyloxysilane-based sol-ye! glasses: Structure- property behavior. Pp. 354-376 in Inorganic and Organometallic Polymers. M. Zeldin,
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