9 Inorganic and Organometallic Polymers

By I. MANNERS Department of Chemistry, University of Toronto, 80 St. George St., Toronto, M5S 1A1,

Ontario, Canada

Macromolecules containing main group elements or transition metals as part of the main chain structure continue to attract considerable attention because of their interesting and unusual properties and also their potential applications as speciality materials. l P 1 This review focuses on developments in inorganic and organometallic polymer chemistry during the year 1993. The review has the same format and follows on from previous articles7y8 in the series which cover the years 1991 and 1992. The first sections of the review cover new developments concerning the well-established inorganic polymer systems namely, the polysiloxanes, polyphosphazenes, and poly- ~ i l a n e s . ~ A brief introduction to each of these classes of inorganic polymer systems was included in the appropriate sections of the first article of this ~ e r i e s . ~ Following these sections, recent developments concerning other polymers based on main-group elements and transition metals are discussed. As with previous articles in this series, the main emphasis is placed on polymers with inorganic elements within the main chain rather than in the side-group structure.

1 Polysiloxanes (Silicones)

Polysiloxanes continue to be the focus of considerable attention, particularly liquid crystalline materials.’ ’-’

Phthalocyaninantopolysiloxanes such as (1 ) represent a very interesting example of a rigid rod macromolecular system with flexible alkoxy side chains which dramatically

‘Silicon-Based Polymer Science’, ed. J M. Zeigler and F .W.G. Fearon, Advances in Chemistry 224, American Chemical Society, Washington D.C., 1990. ‘Inorganic and Organometallic Oligomers and Polymers’, ed. by R. M. Laine and J. F. Harrod, Kluwer Publishers, Amsterdam, 1991. ‘Siloxane Polymers’, ed. J . A. Semlyen and S. J. Clarson, Prentice Hall, Engiewood Cliffs, N.J., 1991. J. E. Mark and H. R. Allcock, and R. West, ‘Inorganic Polymers’, Prentice Hall, 1992.

’ Journal of Inorganic and Organometallic Polymers, Plenum, New York, 1991 onwards. ‘ I . Manners, Polymer News, 1993, 18, 133.

I. Manners, Ann. Rep. Prog. Chem., Sect. A Inorg. Chem., 1991, 88, 77 . I. Manners, Ann. Rep. Prog. Chem., Sect. A Inorg. Chem., 1992, 89, 93. I. Manners, Ado. Mater . , 1994, 6, 68.

l o H. R . Allcock, Ado. Mater . , 1994, 6 , 106. I ’ T. Sauer, Macromolecules, 1993, 26, 2057. ’* E. Chiellini, G. Galli, E. Dossi, F. Cioni, and B. Gallot, Macromolecules, 1993, 26, 849. l 3 I.G. Shenouda and L.C. Chien, Macromolecules, 1993, 26, 5020. l 4 C.S. Hsu, L. J . Shih, and G. H. Hsiue, .Macromolecules, 1993, 26, 3161. I ’ R. Fu, P. Jing, J . Gu, Z . Huang, and Y. Chen, Anal. Chem., 1993, 65, 2141.

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104 I . Manners

enhance solubility in organic solvents. Sauer has studied the phase behaviour of a series of these polymers and has found three different types of behaviour depending on the side-chain length.’ ’ Short side-chain derivatives do not show any phase transition up to the decomposition temperature. Medium length side-chains lead to polymers which show a transition to a highly viscous liquid crystalline phase. On the other hand, polymers with long-chain alkoxy substituents have an additional mesophase at higher temperatures. The rod-like polymer molecules were found to pack in a two- dimensional hexagonal lattice. This hexagonal columnar phase has high thermal stability as a consequence of the cylindrical symmetry and the very high chain stiffness of the polymer structure. The thermal stability of the polymers was found to be limited by side-chain cleavage which takes place in the 250-300”C range. The cleavage mechanism was believed to involve 8-elimination.

OR’ OR’ OR’ 0 R’

Chiellini et al. have reported the synthesis and properties of new chiral smectic polysiloxanes from mesogenic olefin or vinyl ether monomers.’ Such materials can display optimum values of spontaneous polarization and response time which are comparable to those of small molecule liquid crystals with which they can compete effectively for high information screen applications.

Fu and co-workers have reported side-chain liquid crystalline polysiloxanes (2) containing crown ether moieties in the side-group structure which are useful as stationary phases for capillary gas chromatography.”

Corriu and co-workers have prepared organic-inorganic hybrid networks via the hydrolysis of alkoxysilylferrocene derivatives such as (3). This leads to silsesquioxane

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Inorganic and Organometallic Polymers 105

materials (4) which were characterized by IR and solid state I3C and 29Si NMR spectroscopy. Co-hydrolysis of (3) with Si(OMe), leads to hybrid silica gels.16

In other developments concerning polysiloxane chemistry, Zeldin and co-workers have described new polymers with pendant pyridyl groups which are of interest as nucleophilic catalysts for acyl transfer reactions. Crivello has reported the synthesis and polymerization of monomers containing epoxy and alkoxysilane groups.' * Riffle has reported the preparation and properties of silicone-oxazoline diblock copolymers. '

A particularly novel development involves the synthesis of silsesquioxane-siloxane copolymers ( 5 ) from polyhedral dihydroxysilsesquioxanes by Lichtenhan and co- workers.20 This was achieved via the reaction of the latter species with difunctional halogeno- or amino-silanes or oligosiloxanes. The resulting polymers had molecular weights (M,) of 15 oo(r200000. These materials have potential as processable, preceramic polymers. Preliminary experiments showed that these polymers yield ceramics containing SiO, and SiOC when pyrolyzed.

2 Polyphosphazenes

Polyphosphazenes are a remarkably diverse class of inorganic macromolecules that continue to attract considerable attention.

Allcock and Turner have reported2' interesting studies of the polymerization behaviour of a series of transannular bridged and spirocyclic cyclotriphosphazenes l 6 G. Cerveau, R.J.P. Corriu, and N. Costa, J . Non-Cryst. Sol., 1993, 163, 226. " M. Zeldin, E. Granger, and W.K. Fife, J. Inorg. Organomet. Polym., 1993, 3, 141.

J.V. Crivello and D. Bi, J. Polym. Sci. A , Polym. Chem., 1993, 31, 3121. l 9 Q. Liu, G.R. Wilson, R. M. Davis, and J.S. Riffle, Polymer, 1993, 34, 3030. 2o J.D. Lichtenhan, N.Q. Vu, J.A. Carter, J. W. Gilman, and F. J. Feher, Macromolecules, 1993, 26, 2141.

H. R. Allcock and M. L. Turner, Macromolecules, 1993, 26, 3.

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106 I . Manners

[e.g. (6) and (7)]. These provide a comparison with transannular ferrocenylcyclot- riphosphazenes which undergo ring-opening polymerization to yield high molecular weight polymers. The transannular and spirocyclic species underwent ring-ring equilibration to yield higher cyclics or moderate molecular weight polymers. In some cases trace amounts of N,P,Cl, were required for polymerization to occur.

The Allcock group has also reported studies of the arene sulfonation of cyclic phosphazenes and the corresponding high polymeric derivatives, and full details of work on polyphosphazene interpenetrating polymer network^.^^.^^

Matyjaszewski and co-workers have reported further advances in the use of the anionic copolymerization of phosphoranimines to prepare random polyphosphazene copolymers with alkoxyethoxy and trifluoroethoxy functional groups.24 The phos- phoranimine monomers (8) were prepared via the Staudinger reaction of the appropriate phosphites P(OR), with Me,SiN,. Simultaneous copolymerization of the phosphoranimines using [Bu4N]F as initiator at 133 “C for 13 h yielded a series of random copolymers (9) which were characterized by 31P NMR, ‘H NMR, gel permeation chromatography, and differential scanning calorimetry. The resulting copolymers possessed monomodal molecular weight distributions with M , = 10000-150000 and M , = 8000-100000. In addition, the solubilities and thermal and mechanical properties were found to be dependent on the repeat unit ratios.

Allcock and co-workers have reported the synthesis of polyphosphazenes (10) with Fe(CO),Cp side groups via the reaction of lithiated poly(ary1oxyphosphazenes) with F~I (CO) ,CP .~ Similar surface modification reactions were carried out on cross-linked films of [NP(p-OC,H,Br),],, by treatment initially with BunLi and then with the same iodo-organoiron species. The surface metallated materials were characterized by scanning electron microscopy and X-ray microanalysis along with other techniques.

” H. R . Allcock, E. H. Klingenberg, and M . F. Welker, Macromolecules, 1993, 26, 5512. 2 3 H. R. Allcock, K . B. Visscher, and I . Manners, Chem. Muter., 1992, 4, 1188. 24 K . Matyjaszewski, M. S. Lindenberg, M . K. Moore, M . L. White, and M . Kojima, J . Inorg. Organornet.

’’ H. R . Allcock, E . N . Silverberg, C. J . Nelson, and W . D. Coggio, Chem. Mater . , 1993, 5, 1307. Polym., 1993, 3, 317.

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Inorganic and Organometallic Polymers 107

L

Majoral, Bertrand, et al. have reported a route to poly(ally1phosphazenes) from poly(dichlorophosphazene).26 This new method for introducing organometallic side-groups to phosphazenes involves the photochemical reaction of allyltri(n- buty1)stannane with [NPCl,], (Equation 1 ). Intrinsic viscosity measurements showed that the phosphorus-nitrogen skeleton had not been cleaved to a significant extent. This is important as the reaction of most organometallic reagents with poly(dich1orophosphazene) leads to skeletal cleavage as well as substit~tion.~’

Crumbliss, Wisian-Neilson, and co-workers have reported studies of the redox properties of a series of polyphosphazenes with pendant ferrocenyl groups. The polymers were synthesized by side-group modification of poly(phenylmethy1phos- phazene) (Equation 2).28 The latter was prepared by the condensation polymerization of a phosphoranimine precursor. The electrochemical studies of the polymers in solution or as films on an electrode indicated increasing charge-transfer efficiency with the number of ferrocene groups present. This is consistent with an increasing contribution from electron-hopping between the metallocene groups.

In another development concerning polyphosphazenes, detailed analyses of the physical properties of aryloxy-substituted copolymers, including studies of stress-strain isotherms and thermoelasticity, have been carried out by Mark et a/.29930

26 H. Rolland, P . Potin, J . P. Majoral, and G. Bertrand, Inorg. Chem., 1993, 32, 4679. ’’ H. R . Allcock, J . L. Desorcie, and G . H. Riding, Polyhedron, 1987, 6, 119. 2 8 A. L. Crumbliss, D. Cooke, J . Castillp, and P. Wisian-Neilson, Inorg . Chem., 1993, 32, 6088. 29 G . B. Sohoni and J. E. Mark, J . Inorg. Organomet. Polym., 1993, 3, 331. 30 J. E. Mark and G. B. Sohoni, J . Inorg. Organomet. Polym., 1993, 3, 347.

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108 1. Manners

Ph

tt=Nk Me

(1) 6u"LI

3 Polysilanes

Polysilanes continue to attract intense interest from both fundamental and applied perspectives.

The remarkable properties of polysilanes has led to their use in electroluminescent devices. For example, light emitting diodes (LEDs) which utilize poly(methylpheny1- silane) as the hole-transporting material have been reported by Haarer and co- w o r k e r ~ . ~ The LED devices comprised three functional polymer layers which were all fabricated by wet spin-coating processes (hole-transporting, emissive, and elec- tron-transporting) and two electrodes (Figure 1). The high hole-mobility in poly(methylphenylsi1ane) compared to other organic photoconductors is a conse- quence of the a-delocalized structure.

polymethylphenylsilane

aluminium electrode 30 wt% PBD in polystyrene 1.5 wt% DCM in polymethylphenylsilane

indium tin oxide electrode strate

Figure 1

The LEDs fabricated were found to electroluminesce at a threshold voltage of ca. 40V in a continuous D C mode under forward bias. The light from the LED devices was yellow and was easily observed in a dark room. The emissive layer consisted of poly(methylphenylsi1ane) doped with 4-dicyanomethylene-2-methyl-6-(p- dimethylaminostyryl)-4H-pyran (DCM). The electroluminescence spectra for the device and the photoluminescence spectrum of DCM were extremely similar. This supported the idea that the electroluminescence of the device arises from the decay of the singlet excited state of the DCM molecules in the emissive layer which is generated by the recombination of holes and electrons injected from the two electrodes. The high operating voltages for the devices (ca. SCrlOOV) led to lifetimes of less than 1 h. However, the authors are optimistic that the device performance can be considerably improved by optimization of the structure, especially the layer thickness and the choice of electrode materials. The authors also note that poly(methylphenylsi1ane) shows a

3 1 H. Suzuki, H. Mayer, J . Simmerer, J . Yang, and D. Haarer, Adu. Mater . , 1993, 5, 743.

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Inorganic and Organometallic Polymers 109

high effective mobility of holes compared to poly(phenyleneviny1ene) (PPV) which has been used previously for polymer-based LED fabrication. Thus, this particular polysilane might be useful for fast switching applications. In addition, the lack of a visible absorption for poly(methylphenylsi1ane) makes this material attractive for the fabrication of LEDs in the whole range of the visible spectrum, including the blue region as reabsorption of electroluminescent light does not occur.

In a separate Hadziioannou and co-workers have shown that the incorporation of well-defined polysilane segments in poly (thiophenes) allows the tuning of the photo- and electro-luminescence of the resulting multiblock copolymers [e .g . (11): M , = 14280, M , = 68001. The band gap of the oligothiophene blocks strongly depends on delocalization length as inferred from the blue shift of the absorption maxima when the number of thiophene rings is decreased. In contrast, a change in the length of the larger band gap oligosilane block had only a minor effect on the absorption maximum. This indicated that only weak coupling occurred between the o-conjugated oligosilane blocks and the 7t-conjugated oligothiophene blocks along the main chain. Studies of the photoluminescence of the copolymers confirmed that the oligothiophene block was the active unit. Electroluminescent devices were made from the copolymers which emit light with colours ranging from blue to orange-red depending on the length of the oligothiophene block. Significantly, some of the fabricated devices showed no apparent degradation after seven months.

Studies by Wang and West33 have shown that poly(methylphenylsi1ane) doped with fullerenes greatly increases the photoinduced charge-generation efficiency. The combination of high charge-generation efficiency and high carrier mobility makes such materials one of the best polymeric photoconductors known. Pure poly(methylpheny1- silane) was found to have very low charge-generation efficiency and no surface discharge was detected when a film was irradiated with a tungsten lamp. However, for a film of the same polysilane doped with fullerenes (1.6%) the photoinduced discharge rate was enhanced by orders of magnitude. The low field-charge-generation efficiency was found to be about an order of magnitude better than for fullerene-doped poly(vinylcarbazo1e) but less than that of thiapyrylium dye aggregate doped amine/polycarbonate photoconductor, which is generally regarded as the best commercial polymeric photoconductor. However, at higher field, the charge-gener- ation efficiency of the fullerene/polysilane photoconductor is greater than the latter. Attempts to extend this approach to other polysilanes such as poly(cyclohexylmethy1- silane) and poly(dimethylsi1ane) were unsuccessful and no significant photoinduced discharge was observed for these polysilanes even after doping with fullerenes. This result is surprising in view of the previously demonstrated mobility of photoinduced

32 G.G. Malliaras, J . K . Herrema, J . Wildernan, R . H . Wieringa, R.E. Gill, S . S . Lampoura, and G.

3 3 Y. Wang, R. West, and C. H . Yuan, J . .4m. Chem. SOC., 1993, 115, 3844. Hadziioannou, Adv. Mater., 1993, 5, 721.

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110 I . Manners

holes in these polymers. These results indicate that a specific interaction exists between fullerene and the phenyl group of poly(methylphenylsi1ane) which leads to the formation of a weakly bonded complex which enhances electron-transfer from the polysilane to the fullerene.

Other areas of polysilane research are also attracting continued attention. The main method of preparing polysilanes, the thermally induced Wurtz coupling of di- chlorosilanes with alkali metals, has been the subject of further study. Jones and co-workers have shown34 that the blue colour which develops during this reaction arises from the presence of colloidal alkali metal particles and that these play a role in determining the proportion of the very high molecular weight polymer that is formed. Oligomeric materials that are always generated were shown to be formed uia alkali-metal-induced polymer degradation reactions.

The transition metal catalyzed dehydrogenative route to oligosilanes discovered by H a r r ~ d ~ ~ , , ~ continues to attract intense attention (Equation 3). Tilley has reviewed research carried out in his laboratories on the coordination polymerization of silanes to polysilanes involving a a-bond metathesis mechanism.,’ Using certain Zr catalysts, poly(phenylhydrosi1ane) with molecular weights M , = 5000 have been produced from PhSiH,.

of up to ca. M , = 100oO and

+ H2 (3)

Waymouth and co-workers have shown that trifluoromethyl-substituted phenyl- silanes undergo dehydrogenative coupling in the presence of zirconocene catalysts to yield poly{ (trifluoromethy1)phenylhydrosilanes) with M , = 1000-3000.38 Copoly- merization reactions with phenylsilane allowed the relative rates of polymerization of the trifluoromethyl-substituted phenylsilanes to be deduced. Harrod and co-workers have found that, in contrast to the Ti and Zr analogues, dimethylhafnocene does not react with PhSiH,.39 This was attributed to the low reactivity ofthe M-C bonds in this species to a-bond metathesis which inhibits the formation of the true catalyst which is believed to be a silyl hydride complex. Tilley has also reported the use of dehyd- rogenative coupling reactions to synthesize disilanylenearylene oligomers and polymers (Equation 4).40 Heating of the reaction mixtures leads to increases in molecular weight which are believed to arise from cross-linking via dehydrocoupling of the ‘backbone’ Si-H groups. Eventually, insoluble polymers are formed. These materials gave much higher ceramic yields (ca. 67%) on pyrolysis compared to the low molecular weight linear oligomers formed at room temperature.

34 R.G. Jones, R . E. Benfield, R. H. Cragg, A.C. Swain, and S. J . Webb, Macromolecules, 1993, 26, 4878. 35 C. Aitken, J . F. Harrod, and E. Samuel, J . Organomet. Chem., 1985, 279, C11. 36 C. Aitken, J . F. Harrod, and E. Samuel, J . Am. Chem. SOC., 1986, 108, 4059. 3 7 T. D. Tilley, Acc. Chem. Res. , 1993, 26, 22. 38 J . P. Banovetz, H. Suzuki, and R. M. Waymouth, Organometallics, 1993, 12, 4700. 39 H. Li, F. Gauvin, and J. F. Harrod, Organornetallics, 1993, 12, 575. 40 T. Imori, H . G . Woo, J . F. Walzer, and T . D . Tilley, Chem. Muter., 1993, 5, 1487.

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Inorganic and Organometallic Polymers 111

In other developments in polysilane chemistry, poly(methylphenylsi1ane)-poly- styrene block copolymers have been prepared via the reaction of a,o-dich- loropolysilanes with p~lystyryllithium.~~ The resulting copolymers were characterized by UV/VIS and 29Si NMR spectroscopy and gel permeation chromatography. In addition, Schilling and co-workers have investigated the solid state structures, phase transitions, and thermochromism of polysilane copolymer^.^^ Matsuda et al. have synthesized and investigated the photochemical properties of network and branched polysilanes formed by the copolymerization of MePhSiC1, and PhSiC13.43

4 Other Polymer Systems Based on Main Group Elements

The design, synthesis, and development of new polymer systems containing main group elements in the polymer main-chain continues to attract growing attention.

Further studies of poly(thionylphosphazenes), an unusual class of sulphur-nitro- gen-phosphorus polymers which were first reported in 1991, have also been described in the past year6,44*45 and key developments in this area have been reviewed.6 These polymers possess skeletal, four-coordinate sulphur(v1) atoms in addition to phos- phorus and nitrogen and are synthesized via the thermal ring-opening polymerization of the cyclic thionylphosphazeiie NSOCl[NPCl,], or the fluorinated analogue NSOF[NPCl,],. The first polyniers with alkoxy side-groups were reported in 1993 (Scheme l).44 In addition, a6 initio calculations on mimics of poly(thiony1phos- phazenes) suggest that the polynier chain has a cis-trans helical c o n f ~ r m a t i o n . ~ ~

Full details of the synthesis, characterization, and properties of a series of representatives of a new class of sulphur-nitrogen polymers, the poly(oxothiazenes), which were first reported in 1992, were published by Roy.46 These polymers were produced via the condensation polymerization of N-silylsulfonimidates (Equation 5). The polymerization was found to be catalyzed with Lewis acids and bases and the polymers possess significantly higher glass-transition temperatures than the analogous polyphosphazenes. Theoretical studies indicate that a cis-trans helical conformation is the most stable for these polymers.

Further, significant advances were also reported in 1993 in the areas of poly(carbophosphazenes) and poly(thioph0sphazenes). These polymers possess back-

" s. Demouster-Champagne, A. F. de Mathieu, J. Devaux, R. Fayt, and Ph. Teyssie, J. Polym. Sci. A , Polym.

42 F.C. Schilling, A. J. Lovinger, D. D. Davis, and F. A. Bovey, Macromolecules, 1993, 26, 2716. 4 3 A. Watanabe, H. Miike, Y. Tsutsumi, and M. Matsuda, Macromolecules, 1993, 26, 2111. 44 M. Edwards, Y. Ni, M. Liang, A. Stammer, J. Massey, G. J. Vancso, and 1. Manners, Polymer Prepr. (Am.

Chem. Soc., Div. Polym. Chem.), 1993, 34, 324. J. B. Lagowski, R. Jaeger, I. Manners, and G. J. Vancso, Polymer Prepr. (Am. Chem. SOC., Div . Polym. Chem.), 1993, 34, 326.

Chem., 1993, 31, 2009.

4 5

46 A. K. Roy, G.T. Burns, G. C. Lie, and S. Grigoras, J. Am. Chem. Soc., 1993, 115, 2604.

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112 I . Manners

X = C I W F X = C I W F

X =CI Or F, R = OR w OAr

Scheme 1

R = Me or Ph

bones of phosphorus, nitrogen, and either three-coordinate carbon or sulphur(1v)

Exciting new developments have been reported concerning the macromolecular chemistry of the group 14 elements. For example, Bianconi et al. have reported the synthesis of polygermynes, which are random network polymers analogous to the previously reported poly~ilynes.~’ The polygermynes were synthesized via the treatment of trichloroorganogermanes with an emulsion of Na/K alloy in pentane in the presence of ultrasound (Equation 6).

* [RGeI, Na/K

pentme; ultrasound

The resulting coffee-coloured materials possessed NMR spectroscopic properties similar to those of their silicon analogues. In addition, X-ray diffraction showed no peaks, and no melting points were detected. The UV/VIS absorptions tail into the

*’ H. R. Allcock, S. M. Coley, I. Manners, K. B. Visscher, M. Parvez, and 0. Nuyken, G. Renner, lnorg. Chem.,

48 H. R. Allcock, J . A. Dodge, and I. Manners, Macromolecules, 1993, 26, 1. 49 W. J. Szymanski, G. T. Visscher, and P. B. Bianconi, Macromolecules, 1993, 26, 869.

1993,32, 5088.

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Inorganic and Organometallic Polymers 113

visible up to ca. 800nm and account for the deep brown colour. By contrast, polysilynes absorb up to 450 nm. In a similar way to polysilynes, polygermynes under- go photooxidation to give cross-linked networks of polygermoxanes [RGeO ,Jn . The latter show a characteristic IR band at 846cm-' assigned to Ge-O-Ge stretching. Copolymers containing d y n e and germyne units were obtained by copolymerization (Equation 7). These materials were found to possess electronic properties intermediate between those of the two parent homopolymers. For example, the UV/VIS absorption of [Bu"Si),~,(PhGe),,,], has an onset at ca. 650nm. The molecular weights of the monopolymers and copolymers. estimated by GPC, were in the range of M , = 300&15000.

A particularly exciting development, reported by Tilley and co-workers, involves the use of dehydrogenative coupling as a route to p~lystannanes.~' Treatment of neat Bu",nH, with CpCp*ZrMe(Si(SiMe,),) yields the polystannane (12) in 93% yield as a yellow solid (Equation 8). Analysis by GPC showed the presence of (presumably) cyclic oligomers and a higher molecular weight fraction with M , = 17 500 and M , = 7800. The UV/VIS spectrum of the polystannane with M , = 7800 possessed a A,,, at 382 nm, significantly red-shifted compared to the corresponding polygermane. The polystannanes are photosensitive and photobleaching typical of polysilanes and polygermanes was shown to occur.

5 Polymers Containing Skeletal Transition Metal Atoms

Polymers containing transition metal atoms in the main-chain structure represent a rapidly expanding area of research with many interesting developments reported in the past year.

Poly(metallaynes), which are novel macromolecules with backbones possessing conjugated C E C units and transition metal atoms (M), have been studied in some detail in the past few years. Frapper and Kertesz have reported extended Hiickel calculations to study the electronic structure of polymers of structure (13).'

The electronic structures and properties of these materials appear to depend on the size and nature of the oligo-yne bridging ligand, the coordination number of the metal,

T. Imori and T. D. Tilley, J . Chem. SOC., Chem. Commun., 1993, 1607. '' G. Frapper and M. Kertesz, Inorg. Chern., 1993, 32, 732.

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114 I . Manners

(13)

and the metal itself. The authors concluded that the best opportunity for high electrical conductivities is provided by a d8 square planar configuration for the transition metal (e.g. M = Pt, x = 2) which leads to a larger bandwidth for the conduction band compared to a situation when a d6 octahedral transition metal (e.g. M = Fe, x = 4) is present.

Advances continue to be made in the area of metallocene-based polymers. Unusual face-to-face metallocene polymers (14) have been reported by Rosenblum and co-workers.”

If R = H, the dark purple polymer possessed a bimodal molecular weight distribution (with M , = ca. 18 000, M , = ca. 14 OOO). If R = Me, a partially soluble polymer product resulted with a bimodal molecular weight distribution with a component with M , = ca. 139000. Mixed metal polymers containing nickel in addition to iron were also synthesized, however, these tended to be rather insoluble and the molecular weights were lower. Bulk magnetic susceptibility measurements provided evidence for the presence of nickelocene units in the polymer backbone.

Advances have also been reported concerning high molecular weight poly(ferro- cenylsilanes) (16), which were first synthesizeds3 in 1992 by a ring-opening polymeriz- ation route (Equation 9). 54-62 The monomers, silicon-bridged ferrocenophanes (1 5),

5 2 H.M. Nugent, M. Rosenblum, and P. Klemarczyk, J . Am. Chem. Soc., 1993, 115, 3848. 53 D.A. Foucher, B.Z. Tang, and I. Manners, J. Am. Chem. Soc., 1992, 114,6246. 54 W. Finckh, R. Ziembinski, B.Z. Tang, D.A. Foucher, D.B. Zamble, A. Lough, and I. Manners,

5 5 D. A. Foucher, R. Ziembinski, B. Z. Tang, P. M. Macdonald, J. Massey, R. Jaeger, G. J. Vancso, and I.

5 6 I. Manners, J. Inorg. Organomet. Polym., 1993, 3, 185. 5 7 B.Z. Tang, R. Petersen, D.A. Foucher, A. Lough, N. Coombs, R. Sodhi, and I. Manners, J. Chem. SOC. ,

5 8 D.A. Foucher and I. Manners, Makromol. Chem., Rapid Commun., 1993, 14, 63. 5 9 R. Ziembinski, C. Honeyman, 0. Mourad, D. A. Foucher, R. Rulkens, M. Liang, Y. Ni, and I. Manners,

6 o J. M. Nelson, H. Rengel, and I. Manners, J. Am. Chem. SOC., 1993, 115, 7035. 6 1 M.T. Nguyen, A. F. Diaz, V. V. Dement’ev, and K. H. Pannell, Chem. Muter., 1993, 5, 1389. 62 D. A. Foucher,C. Honeyman, J. M. Nelson, B. Z. Tang, and I. Manners, Angew. Chem., Int. Ed. Engl., 1993,

Organometallics, 1993, 12, 823.

Manners, Macromolecules, 1993, 26, 2878.

Chem. Commun., 1993, 523.

Phosphorus, Sulfur, Silicon Rel. Elem., 1993.76, 219.

32, 1709.

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Inorganic and Organometallic Polymers 115

have strained ring-tilted structures. For example, (15) (R = Me) has a tilt-angle between the planes of the cyclopentadienyl ligands of 20.8(5)".54 In contrast, the corresponding C2lferrocenophane containing an (SiMe,), bridge has a tilt angle of only ca. 4" and appears to be resistant to thermal ring-opening polymerization.

heat ___)

The synthesis, characterization, and properties of a series of symmetrically- substituted poly(ferrocenylsi1anes) (16) (R = Me, Et, Bu", and n-hex) was reported in the first half of 1993.55 UV/VIS studies showed that these materials possess structures which are essentially localized, whereas electrochemical studies clearly show that the iron atoms interact with one another. Mossbauer spectroscopic studies of doped samples of (16) (R = Me) are also consistent with an essentially localized electronic structure.56 Poly(ferrocenylsi1anes) have also been shown to function as pyrolytic precursors to magnetic FeSiC ceramics at ca. 500 "C and unusual depolymerization products have also been detected, isolated, and characterized." In late 1993, studies of more crystalline samples of the previously reported poly(ferrocenylsi1anes) (1 6 ) (R = Me, Et, Bun, and n-hex) were also described.61 In 1993, the synthesis of the first examples of poly(ferroceny1germane.s) (1 7)'* and poly(ferrocenylethy1enes) (1 8)60 via ring-opening polymerization routes were also reported. Strained, ring-tilted german- ium- and hydrocarbon-bridged ferrocenophanes function as suitable precursors to these materials (Equations 10 and 11).

R

90 -1 50 "C

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116 I . Manners

Interestingly, electrochemical studies have shown that the iron atoms in poly(ferroceny1germanes) interact with one another in a similar way to those in poly(ferrocenylsilanes).62 In contrast, poly(ferrocenylethylenes), which possess a longer, two-atom bridge between the ferrocene units, possess virtually non-interacting transition metal centres .60* 62

New developments concerning polyferrocenylene persulfides, which were first reported6, in 1992, have also been described by Rauchfuss and c o - w o r k e r ~ . ~ ~ These polymers were synthesized by a novel atom-abstraction induced ring-opening polymerization route. The atom abstraction route using PBu, as the desulfurization agent has been extended to the preparation of very high molecular weight ( M , = 5000&1 OOOOOO) network polymers (19) by using C3lferrocenophanes with two trisulfido bridges as monomers.

Poly(ferroceny1enes) are continuing to attract attention with respect to sensor applications. The first examples of the use of poly(ferrocenes) as mediators in amperometric biosensors have recently been r ep~r t ed .~ ’ Specifically, the poly(ferro- cenylmethylenes) (20) (R = H or 2-methoxyphenyl, n = ca. 12-14), which were synthesized via the condensation of the aldehydes RCHO with ferrocene in the presence of a Lewis Acid, are capable of acting as mediators which shuttle electrons between glucose oxidase and a carbon electrode.

6 3 P.F. Brandt and T.B. Rauchfuss, J . Am. Chem. SOC., 1992, 114, 1926. 64 C.P. Galloway and T. B. Rauchfuss, Angew. Chem., Int. Ed. Engl., 1993, 32, 1319. 65 S. P. Hendry, M. F. Cardosi, A. P. F. Turner, E. W. Neuse, Anal. Chim. Acta, 1993, 281, 453.

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Inorganic and Organometallic Polymers 117

r

L

Other areas of transition metal polymer chemistry are also attracting attention. Polymers consisting of metallacarborane units, ‘polymetallacarborane staircase oligomers’, are of particular i n t e r e ~ t . ~ ~ ’ ~ ’ Studies of oligomers with between 5 and 17 metal atoms [e.g. (21)] consisting of multidecker sandwiches linked by a phenylene group have shown that there is good evidence for electron delocalization within the individual multidecker stacks, but that the electronic communication between the stacks is small. The lack of interstack delocalization was attributed to the tilt (ca. 4&60”) of the connecting phenylene spacers relative to the planes of the (methyl- ated)cyclopentadienyl ligands which prevents n-conjugation. This tilt was attributed to the presence of steric repulsions between the methyl groups on the cyclopentadienyl rings and the phenylene moiety. m-0-f I

co

I co

(21) 0 = CR

Puddephatt and co-workers have reported full details of an interesting new method for the incorporation of metals into the side-group structure which involves the use of oxidative addition reactions.68 Metalled monomers were prepared uia the reaction of

66 X. Meng, M. Sabat, and R.N. Grimes, J . Am. Chem. SOC., 1993, 115,6143. 67 J. R. Pipal and R. N. Grimes, Organometallics, 1993, 12, 4458.

S. Achar, J . D. Scott, J. J. Vittal, and R. J . Puddephatt, Organornetallics, 1993, 12, 4492.

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118 I . Manners

bromoacetic acid or its derivatives with the platinum(I1) complex PtMe,(bipy). Free radical polymerization subsequently yielded poly(viny1acetate) with pendent organo- platinum groups. Similar polymers were obtained by the esterification of poly(viny1 alcohol) to give poly(viny1bromoacetate) followed by oxidative addition using an excess of PtMe,(bipy).

The use of transition metal activated nucleophilic substitution has provided an elegant route to soluble poly(phenylenesu1fide) and poly(pheny1ene oxide) derivatives (22) with pendent Ru(q-C,Me,) groups attached to arene rings in the polymer backbone.69 In addition, Cr(CO), groups have been used to solubilize poly(phenyleneterephtha1amide) (PPTA) and these led to a retention of lyotropic liquid ~rystallinity.~'

Ru' SO& F3-

(22) X = 0 or S

In the area of rigid rod polymers Puddephatt and c o - ~ o r k e r s ~ ~ have utilized isocyanide-arene-acetylide bidenate ligands to form monomeric and oligomeric gold(1) complexes (23). The latter materials were insoluble in common organic solvents and are probably of low molecular weight.

69 A . A . Dembek, P.J. Fagan, and M. Marsi, Macromolecules, 1993, 26, 2992. A. A. Dembek, R. R. Burch, and A.E. Feiring, J . Am. Chem. SOC., 1993,115, 2087. G. Jia, R . J . Puddephatt, J. J . Vittal, N. C. Payne, Organometallics, 1993, 12, 263.

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