8 Inorganic and Organometallic Polymers

By I. MANNERS

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto M5S IA 1, Ontario, Canada

The resurgent interest in macromolecules which contain inorganic elements is primarily a consequence of the increased technological demand for new speciality materials with unusual or unique high performance properties. Thus, studies of the relatively few well-characterized inorganic and organometallic polymer systems prepared to date have clearly indicated that the presence of inorganic elements can often provide access to characteristics which are difficult or impossible to achieve with conventional organic polymers.'-4 At present, the commercial significance of this area is best illustrated by the polysiloxanes or silicones (1) which now represent a billion dollar industry worldwide, with applications as diverse as low temperature elastomers, adhesives, and biomedical polymer^.'.^ The other two major classes of inorganic polymers, the polyphosphazenes (2) and polysilanes (3), are still at a relatively early stage of technological development although commercial applications for several examples of these materials have already been r e a l i ~ e d . ' . ~ , ~ However, in many ways it has been the unusual and useful properties of these two classes of polymers which have helped restimulate an area which after a period of intense activity starting in the 1950s faded during the 1960s. Indeed, the tremendous current interest in polymeric materials derived from inorganic elements has led to the recent birth of a new Journal' and the first introductory textbook on the subject designed for students and research worker^.^

This review concentrates on developments in inorganic and organometallic poly- mer chemistry during 1991. In addition, because this is the first review of a series, other recent developments are mentioned where they help provide perspective.

Inorganic and Organometallic Polymers; ed. M. Zeldin, K. Wynne, and H. R. Allcock ACS Symp. Ser. 360, Washington D.C., 1988. Silicon-Based Polymer Science' ed. J. M. Zeigler and F. W. G. Fearon, .Advances in Chemistry 224, American Chemical Society, Washington D.C., 1990. 'Siloxane Polymers, ed. J. A. Semlyen and S. J. Clarson, Prentice Hall. Englewood Clips, N.J. 1991. J. E. Mark, H. R. Allcock and R. West, 'Inorganic Polymers', Prentice Hall, 1992. Journal of Inorganic and Organornefallic fol~vrners, Plenum, New York, 1991.

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78 I. Manners

Because of the enormous body of journal and patent literature, particularly in the area of the silicones, the survey does not aim to be exhaustive but instead aims to highlight some of the most interesting and significant developments in the past year. The first part of the review sequentially covers the well-established inorganic polymer systems; polysiloxanes, polyphosphazenes, and polysilanes. The second part consists of separate sections that consider other polymers based on main-group elements and transition metals.

1 Polysiloxanes (Silicones)

Polysiloxanes currently represent the most commercially significant class of inorganic polymers. The wide range of applications for these materials is mainly a consequence of the unusual flexibility and high thermo-oxidative and radiation stability of the backbone of alternating silicon and oxygen atoms. Since the initial development of silicones, such as poly(dimethylsi1oxane) (PDMS) [Me,SiO],, in the 1930's and 1940's the chemistry of these polymers has been extremely well-explored and many excellent reviews are a ~ a i l a b l e . ~ - ~ Nevertheless, virtually all aspects of the chemistry and technology of these polymers continue to attract enormous interest.

An important area of current research involves the incorporation of siloxane segments into copolymer structures to give new materials with unusual combinations of properties. For example, the synthesis of triblock PDMS-PS-PDMS copolymers with polystyrene (PS) has been reported by using difunctional living polystyryl dianions to initiate the ring-opening polymerization of [ Me2Si0]3.6 These materials exhibit microphase separation in the solid-state to give a silicone phase with a very low glass transition temperature ( T'.) of ca. - 110 "C and a glassy polystyrene phase with a Tg of 65-85 "C. McGrath and co-workers, on the other hand, have reported the synthesis of polysulfones which possess grafted siloxane chains.' The synthesis of these materials was accomplished via the free radical terpolymerization of hexenyl-functionalized polysiloxane macromonomers with 1-butene and sulfur dioxide (Equation 1). The

+ so1 Et

resulting materials (4) possess a backbone of

RO-OR - I SiMezBu

- PBS - PDMS

poly(butenesu1fone) (PBS) and side groups of poly(dimethylsi1oxane) and are of interest as electron beam resists and as polymers with resistance to oxygen reactive ion etching.

In addition, the development of new elastomeric PDMS-derived silicone materials with unusual network chain length distributions, and the generation of inorganic-

' A. Dems and G. Strobin Makromol. Chem., 1991, 192, 2521. ' J. M. DeSimone, G. A. York, J . E. McGrath, A. S. Gozdz, and M. J. Bowden Macromolecules, 1991, 24,

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Inorganic and Organometallic Polymers 79

organic composites by carrying out sol-gel reactions in the presence of functionalized polysiloxane chains has been reviewed by Mark.' The network structure of silicone release coatings which are used in the pressure-sensitive adhesive industry has also been investigated by Desorcie and Stein.'

Another area of silicone chemistry which is attracting considerable attention aims at expanding the range of properties accessible via the incorporation of different side groups. For example, in the past year Zeldin and co-workers reported the synthesis of polysiloxanes with aminopyridine functions." These materials were prepared via polycondensation reactions of difunctional silane monomers and are of interest as macromolecular catalysts for a range of reactions including phosphory- lation and ester rearrangements. A particularly fertile method for the introduction of different side groups to the siloxane chain involves the hydrosilylation reaction of silicone polymers that contain Si - H bonds with functionalized terminal olefins (Equation 2).11 A selection of the side groups which have been successfully incorpor- ated in the past year include glucose and galactose units,I2 photoactive cinnamic acid units,13 metha~rylate, '~ epoxy," amino groups,16 and mesogenic groups which give rise to liquid crystallinity."

+ 7 R

Pt catalyst - In recent years, liquid crystalline polysiloxanes with either rod-like or disc-like

groups in the polymer main-chain or side-group structure have received considerable attention. Intriguing materials have been recently developed such as polysiloxanes ( 5 ) with chiral smectic C* phases which possess a tilted, layered structure and have potential applications in electro-optical devices.'' This area has been excellently reviewed in the past year."

( 5 )

' J . E. Mark, J. Inorg. Organomet. Polym., 1991, 1, 431. J . L. Desorcie and J . Stein, J. Znorg. Organomet. Polym., 1991, 1, 591. S. Rubinsztajn, M. Zeldin, and W. J . Fife, Macromolecules, 1991, 24, 2682.

" J. Stein, L. N. Lewis, K. A. Smith, and K. X. Lettko, J. Znorg. Organomet. Polym., 1991, 1, 325. l 2 G. Jonas and R. Stadler, Makromol. Chem. Rapid Commun., 1991, 12, 625. l 3 S. H. Barley, A. Gilbert, and G. R. Mitchell, Makromol. Chem., 1991, 192, 2801.

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S. K . Duplock, J . G. Matisons, A. G. Swincer, and R. F. 0. Warren, J. Inorg. Organomet. Polym., 1991, 1, 361.

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l 5 J . V. Crivello and M. Fan, Polymer Prepr. (Am. Chem. SOC. Div. Polym. Chem.), 1991, 32(1), 340. l 6 J . R. Babu, G. Sinai-Zingde, and J . S. Rifle. Polymer Prepr. (Am. Chem. SOC. Din Polym. Chem.), 1991,

32(1), 152. G. Staufer and G. Latterman, Makromol. Chem., 1991, 192, 2421.

Mater., 1990, 2, 539. S. Boileau and D. Teyssie, J. Inorg. Organomet. Pol-vm., 1991, 1 , 247.

I' H . Kapitza, R. Zentel, R. J. Twieg, C. Nguyenm, S. U. Vallerien, F. Kremer, and C. G . Willson, Adu.

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80 I. Manners

Another method for achieving control over polysiloxane microstructure involves the use of living polymerizations. The living polymerization of strained cyclo- trisiloxanes to provide well-defined polymers is of increasing importance and has been reviewed.,'

The unusual conformational properties of polysiloxanes also continue to attract considerable attention. For example, solid state 13C and 29Si NMR studies of PDMS indicate that the ribbon-like, 2-fold, helical conformational model, for crystalline PDMS proposed by Damaschun is not correct and a modified helical structure has been suggested.21 The conformational properties of poly( methylphenylsiloxane) have also been examined theoretically.22

2 Polyphosphazenes

Since the discovery of soluble poly( dichlorophosphazene) by Allcock and Kugel in the early 1960s, polyphosphazenes have attracted considerable attention as an unusual class of inorganic macromolecules. 1,4*23 Polyphosphazenes are used indus- trially as high performance elastomers and are under development for a variety of other applications including biomedical uses. One of the features of polyphos- phazenes is their remarkable tunability which allows access to a wide range or properties. This is a consequence of the main method of synthesis which involves ring-opening polymerization of [ NPC1,I3 to yield a polymer, polydichlorophos- phazene [ NPCl,], , that functions as a macromolecular intermediate with respect to substitution reactions. Thus, polydichlorophosphazene can be reacted with oxygen and nitrogen-based nucleophiles which are designed to impart specific properties to the p~ lymer . ' , ~**~ Significant recent developments reported by Allcock et al. include the synthesis of photochromic polymers, via the attachment of spiropyran units,24 and polymers with azoxybenzene side groups which X-ray diffraction studies show to possess a tilted layer m o r p h o l ~ g y . ~ ~ Polyphosphazenes have also been prepared with pendant arenechromium tricarbonyl groups,26 oligopeptide side g r o ~ p s , ~ ' and substituents designed to give rise to second-order non-linear optical properties.28 The latter provide valuable information on the relationship between polymer side group structure and the measured x(,) values. The photochemical behaviour of several polyphosphazenes which contain aryloxy and naphthoxy side groups and the synthesis of polymers for singlet oxygen generation have also been reported by Gleria and c o - w o r k e r ~ . ~ ~

A particularly interesting development in polyphosphazene chemistry was also reported by Allcock and co-workers and involves the synthesis of polymers which contain controlled, short-chain, bran~hing.~' Thus, the bis(trichlorophos-

J. Chojnowski, J. Inorg. Organomet. Polym. 1991, 1, 299. F. C. Schilling, M. A. Gomez, and A. E. Tonelli, Macromolecules, 1991, 24, 6552. A. Horta, I. Pierola, A. Rubio, and J. J. Freire, Macromolecules, 1991, 24, 3121. H. R. Allcock, Chem. Eng. News, 1985, 63(11), 22. H. R. Allcock and C. Kim, Macromolecules, 1991, 24, 2846. H. R. Allcock and C. Kim, Macromolecules, 1991, 24, 2841. H. R. Allcock, A. A. Dembek, and E. H. Klingenberg, Macromolecules, 1991, 24, 5208. H. R. Allcock and J. Y. Chang, Macromolecules, 1991, 24, 993. H. R. Allcock, A. A. Dembek, C. Kim, R. L. S. Devine, Y. Shi, W. H. Steier, and C. W. Spangler, Macromolecules, 1991, 24, 1000. G. Facchin, F. Minto, M. Gleria, R. Bertani, and P. Bortolus, J. Inorg. Organomet. Polym., 1991, 1, 389. D. C. Ngo, J. S. Rutt, and H. R. Allcock, J. Am. Chem. Soc,, 1991, 113, 5075.

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Inorganic and Organometallic Polymers 81

phazo)cyclophosphazene ( 6 ) undergoes thermal ring opening polymerization at 150 "C to yield the poly(phosphazophosphazene) (7). The resulting polymer can be reacted with alkoxides or aryloxides to yield hydrolytically stable derivatives. Inter- estingly, substitution of the chlorines present in the pendant phosphazo groups occurs first and this allows different groups to be attached to the pendant and the skeletal phosphorus atoms (Scheme 1).

NaOR, 25 "C J r P(OR)3 1 b c1 c1

I I -j-- T = N - ~ = N - P = N - I

L

I I

I I N c1 c1 II

I I I 1 OR OR

n L

Scheme 1

Another interesting development in polyphosphazene chemistry involves the syn- thesis of polymers with thiourethane and thiourea side groups. High molecular weight macromolecules with these substituents have been prepared via the derivatiz- ation of poly{bis(isothiocyanato)phosphazene} (8) with alcohols [to give (9)] and amines [to give ( High molecular weight poly {bis(isothiocyanato)phos- phazene} was prepared via the reaction of polydichlorophosphazene with K[ NCS]. By contrast, according to viscosity measurements the ring-opening polymerization of [ NP( NCS),], appears to yield relatively low molecular weight polymers.

1 s HN-C--OR'

HN-C--OR'

HN-C-N-R'

P=N

HN-C-N-R' 1. (8) (9) (10)

3' H. R. Allcock and J . S. Rutt, Macromolecules, 1991, 24, 2852.

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82 I. Manners

Another area of polyphosphazene #chemistry to experience significant recent advances involves polymers with ferrocenyl substituents. The use of transannular ferrocenyl groups to impart strain to cyclic halogenophosphazenes to facilitate their thermal polymerization was reported several years ago. An important recent develop- ment has been the use of this methodology to permit the polymerization of cyclic organophosphazenes which would not otherwise be expected to p ~ l y m e r i z e . ~ ~ , ~ ~ For example, in general, cyclotriphosphazenes with less than three halogen substituents do not undergo thermal ring-opening polymerization. However, a range of cyclic phosphazenes without any halogen side groups (1 1) have been found to polymerize, provided transannular ferrocenyl substituents are present (Equation 3) . In most cases, the presence of a small amount of an initiator such as [NPCI,], is necessary for polymerization to occur. X-ray crystallographic studies of several of these strained ferrocenylorganocyclophosphazenes have shown that the phosphazene ring is forced into a high energy non-planar c ~ n f o r m a t i o n . ~ ~ By contrast, in most cyclo- triphosphazenes the phosphorus-nitrogen ring is virtually planar.

250°C

R L R ( 3 )

R = non-halogen substituent

Further advances in the condensation route to phosphazene polymers, first repor- ted by Neilson and Wisian-Neilson at the beginning of the 1980s, have also recently taken place.35 In particular, this method provides access to polyphosphazenes with substituents bound by P-C bonds which are difficult to prepare uia either the ring-opening or the macromolecular substitution routes. Interestingly, the facile low-temperature synthesis of polymers with fluoroalkoxy side groups has recently been reported.36 The past year saw the first preliminary report of polymers (12) which possess CF3 substituents directly bound to the polymer backbone.37 These polymers are reportedly prepared, along with cyclic products, via the elimination of trimethylbromosilane from phosphoranimine precursors (Equation 4).

CF3 I heat .+F+ - Me,SiBr I

Br

R-P=N-SiMe3

R = alkyl or aryl (12)

I . Manners, G. H. Riding, J . A. Dodge, and H. R. Allcock, J. Am. Chem. SOC., 1989, 111, 3067. H. R. Allcock, J . A. Dodge, I . Manners, and G . H. Riding, J. Am. Chem. SOC., 1991, 113, 9596. H. R. Allcock, J . A. Dodge, I. Manners, M. Parvez, G. H . Riding, and K. B. Visscher, Organometallics, 1991, 10, 3098. R. H. Neilson and P. Wisian-Neilson Chem. Reu. 1988, 88, 541. R. A. Montague and K. Matyjaszewski, J. Am. Chem. SOC., 1990, 112, 6721. R. H. Neilson, D. L. Jinkerson, S. Karthikeyan, R. Samuel, and C. E. Wood, Polymer Prepr. (Am. Chem. SOC. Div. Polym. Chern.), 1991, 32(3), 483.

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Inorganic and Organometallic Polymers 83

Because of the useful properties they are likely to impart, the incorporation of cyclic phosphazene moieties into the side-group structure of both organic and inorganic polymers is also receiving attention. Advances in these areas in the last year have been reported by Allen38 and van de G r a m ~ e l . ~ ~

3 Polysilanes

Since the first reports of soluble polysilanes in the early 1980s, these macromolecules have attracted tremendous interest from both fundamental and applied perspec- t i v e ~ . ~ " ~ ~ The backbone of silicon atoms gives rise to unique electronic and optical properties as a result of the delocalization of u-electrons. In addition, polysilanes are of interest as preceramic materials and are used industrially as precursors to silicon carbide.40

The main method of preparing polysilanes involves the thermally induced Wurtz coupling reaction of dichlorosilanes with sodium metal. The harsh conditions required for this reaction tends to limit the side groups that can be successfully incorporated to non-functionalized alkyl and aryl units. In addition, the procedure generally affords very low to moderate yields of high molecular weight polymer. Because of these limitations, considerable effort has been focused on the development of new synthetic routes to polysilanes and recent developments in this area have been reviewed by Matyjas~ewski.~~ The transition metal catalyzed dehydrogenative route to oligosilanes discovered'by Harrod during the mid 1980s is now attracting intense attention from several research groups (Equation 5).43 The search for an understanding of the mechanism of this reaction and the development of new catalysts which may yield higher molecular weight products or stereoregular poly- mers are areas under intense current investigation.

RSiH3 catalyst H 1; Si in H + H z

For example, an elegant study by Marks and co-workers of the dehydrogenative oligomerization of phenylsilane catalyzed by a range of organolanthanide complexes has provided important mechan(ktic insight into this type of reaction.44 Thus, the observation that complexes such as CpTLaR(Cp* = q-C5Me5, R = H or CH( SiMe,),) function as active catalysts excludes the possibility of metal-centred redox processes in the catalytic cycle (i.e. oxidative additionlreductive elimination sequences). Instead, the kinetic data obtained provided clear support for a mechan- ism that involves four-centre heterolytic bond-scission and bond-formation (Scheme 2).

D. E. Brown and C . W. Allen, J. Inorg. Organornet. Polyrn., 1991, 1 , 189. R. hyenbroek , A. P. Jeklel, and J. C., van de Grampel, J. Inorg. Organornef. Polyrn., 1991, 1 , 105. R. West, J. Organornet. Chern. 1986, 300, 327. R. D. Miller and J. Michl, Chern. Reo., 1989, 89, 1359. K. Matyjaszewski, J. Inorg. Organornet. Polyrn., 1991, 1 , 463. F. Gauvin and J . F. Harrod, Polymer Prep. ( A m . Chern. SOC. Diu. Polyrn. Chern.), 1991, 32(1), 439. C . M. Forsyth, S. P. Nolan, and T. J . Marks, Organornetallics, 1991, 10, 2543.

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84 I.

H f r i H I

c p 2 L I; /; 1 1

H-

Hz

Manners

R

H

dimer

Scheme 2

Another interesting development in this area is the discovery by Waymouth and c o - ~ o r k e r s ~ ~ of stereoselectivity in the catalytic oligomerization of phenylsilane by chiral zirconocene complexes. In addition, Corey and co-workers have reported detailed studies of the dehydrogenative coupling of secondary silanes using catalysts derived from Group 4 metallocene dichlorides and two equivalents of b ~ t y l l i t h i u m . ~ ~

An interesting, new route to polysilanes has been reported by Matyjaszewski which involves the ring-opening polymerization of strained cyclic ~ i l a n e s . ~ ~ The majority of cyclic silanes are thermodynamically stable and cannot be converted into linear polymers. Even cyclotetrasilanes wth substituents such as phenyl groups are resistant to polymerization. Indeed, [Ph,Si], is formed in high yield by the reductive coupling of Ph2SiC12, which indicates that steric interactions between the relatively bulky phenyl substituents are more important than angular strain in this particular species. However, cyclotetrasilanes such as [ PhMeSi], , which possess a mixture of methyl and phenyl substituents, have now been shown to p ~ l y m e r i z e . ~ ~ Species of this type can be prepared in two steps. The first involves the reaction of [Ph,Si], with triflic acid which leads to the replacement of up to four phenyl groups per silane ring without skeletal cleavage. The second step involves treatment of the intermediate [Ph(OTf)Si], , (OTf = OSO,CF,), with alkylating agents such as methylmagnesium iodide or methyl lithium to give the desired mixed-substituent

J . P. Banovetz, K. M. Stein, and R. M. Waymouth, Organornetallics, 1991, 10, 3430. J . Y. Corey, X . H. Zhu, T. C. Bedard, and L. D. Lange, Organornetallics, 1991, 10, 924. M. Cypryk, Y. Gupta, and K. Matyjaszewski, J. Am. Chern. Soc., 1991, 113, 1046.

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Inorganic and Organometallic Polymers 85

product [ PhMeSi], . Ring-opening polymerization of the latter can be achieved by using anionic initiators such as BuLi in THF/benzene (60: 40). This yields polymers with molecular weights ranging from 10 000 to 100 000. Interestingly, if the same reaction is carried out in 100% benzene then cyclopentasilanes are formed exclus- ively.

The physical properties of polysilanes continue to be the subject of intense investigation. Whereas the remarkable optical and electronic characteristics of these materials continue to attract considerable attention, now other properties are also attracting interest. For example, West et al. have shown that polysilanes with a wide variety of surface tensions are accessible and that the values depend on the specific nature of the alkyl or aryl side groups present.,' In addition, the same research group have reported characterization of the liquid crystalline properties of poly( n- butyl n-hexylsilane) ( PBHS).49 This polymer, which possesses quite different proper- ties from those of either poly(di-n-butylsilane) or poly(di-n-hexylsilane), was studied by differential scanning calorimetry, dynamic mechanical analysis, and X-ray diffraction. The studies showed that the polymer exists in a liquid crystalline mesophase over a very wide temperature range of -20 "C to 220 "C. Interestingly, the X-ray measurements on films of PBHS indicated that the hexagonal lattice of partially crystalline PBHS is maintained in the mesophase. Lovinger et a1 have also reported studies of the solid state structure and phase transitions in po lys i l ane~ .~~ These researchers studied poly( dimethylsilane) and found that this polymer possesses a trans-planar conformation which is in agreement with the findings for the diethyl and dipropyl homologues. Furthermore, the methylated polymer was found to exhibit no thermochromism in contrast to the widely studied poly(di-n- hexylsilane).

4 Other Polymer Systems Based on Main Group Elements

In recent years there has been considerable interest in the synthesis of new inorganic and organometallic polymer systems as this may provide access to materials with a range of new and interesting physical and chemical properties. In the past year several new classes of polymers have been reported and further details of other recently prepared systems have also been published. Several of these are constructed from main group elements.

A recent area of active interest is the synthesis and properties of polyheterophos- phazenes. These macromolecules possess backbones which contain phosphorus and nitrogen together with atoms of another element. Thus, they can be formally regarded as macromolecules derived from 'classical' polyphosphazenes via the replacement of skeletal phosphorus atoms by atoms of a hetero-element. The chemistry of cyclic heterophosphazenes has been developed in several research groups over the past 20-30 years. However, the first developments in polyheterophosphazene chemistry were provided5' by the initial preliminary report in 1989 of poly(metal1aphos- phazenes) which contain skeletal molybdenum and tungsten atoms. This was rapidly

R. Menescal and R. West, Macromolecules, 1991, 24, 329. T. Asuke and R. West, Macromolecules, 1991, 24, 343. A. J. Lovinger, D. D. Davis, F. C. Schilling, F. J. Padden, Jr., and F. A. Bovey, Macromolecules, 1991, 24, 132.

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5' H. W. Roesky and M. Lucke, Angew, Chem., Int. Ed. Engl., 1989, 28, 493.

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86 I. Manners

followed by reports of the synthesis and properties of poly(carboph~sphazenes)~~ and poly(thiophospha~enes),~~ which contain skeletal carbon and sulfur( IV) atoms, respectively. The first paper giving full details on the synthesis and characterization of poly(carbophosphazenes) appeared during 1991 (Scheme 3).54 Investigations of the thermal transition behaviour of a range of aryloxy substituted polymers (13) indicated that the presence of skeletal carbon atoms lowers the flexibility of the polymer backbone compared to 'classical' polyphosphazenes. One possible reason for this difference involves the inherent differences in the .rr-bond character of the C=N and P=N bonds. Thus, the C=N bond is of p ~ - p . r r type where the maximum wbond overlap occurs at only two points of the 360" bond twisting profile. This leads to a significant energy barrier which must be summounted before the bond can undergo significant torsion. By contrast, the P=N bond is believed to be of the p.rr-d.rr type where significant n-overlap can occur at any point of the 360" torsional profile. This would lead to a low energy barrier for torsional motions. Another possible reason for the lower skeletal flexibility of poly(carbophosphazenes) is the smaller size of the main-chain carbon atoms which would tend to lead to more severe steric interactions between side groups.

Cl I

c1'

r c1 c1 1

NaOAr / r OAr OAr 1

1 OAr OAr OAr 1 ( 1 3 )

n

Scheme 3

During 1991 the synthesis of the first members of a new class of polyheterophos- phazenes termed poly(thiony1phosphazenes) was reported by Manners' In contrast to poly( thiophosphazenes), these polymers possess skeletal four-coor- dinate sulfur(v1) atoms in addition to phosphorus and nitrogen. These polymers were synthesized via the thermal ring-opening polymerization of the cyclic

1. Manners, G. Renner, H. R. Allcock, and 0. Nuyken, J . Am. Chem. SOC., 1989, 111, 5478. J . A. Dodge, I . Manners, G. Renner, H . R. Allcock, and 0. Nuyken, J. Am. Chem. SOC., 1990, 112, 1268. H. R. Allcock, S. M. Coley, I . Manners, 0. Nuyken, and G. Renner, Mucromolecules, 1991, 24, 2024.

M. Liang, C. Waddling, and I. Manners, Polymer Prepr. ( A m . Chem. SOC. Div. Polym. Chem.) 1991,32(3), 487.

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5 5 M. Liang and I . Manners, J. Am. Chem. SOC., 1991, 113, 4044. 56

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Inorganic and Organometallic Polymers 87

thionylphosphazene NSOCl[ NPC1J2 (14) which was first synthesized independently by van de Grampel and coworkers and Klingebiel and co-workers in 1972. The resulting polymer ( 15) undergoes regiospecific substitution of the chlorine atoms at phosphorus with sodium phenoxide to yield a polymer (16) in which sulfur-chlorine bonds remain intact (Scheme 4). Remarkably, the regiospecificity detected is exactly the opposite to that observed for the analogous sulfur(rv) polymers which undergo preferential chlorine atom replacement at sulfur when reacted with aryloxide nu~ leoph i l e s .~~

I 1 (!I OPh OPh L J n

(16)

Scheme 4

The synthesis of fluorinated poly(thiony1phosphazenes) via the ring-opening polymerization of the cyclic thionylphosphazene NSOF[ NPClJz ( 17) with a fluorine substituent at sulfur has also been reported by the same re sea rche r~ .~~ The ring- opening process occurs at a slightly higher temperature (180 "C). Reaction of the polymer product with sodium phenoxide also leads to regiospecific substitution of the chlorine atoms at phosphorus to yield a very unusual example of an hydrolytically stable, elastomeric, inorganic polymer with fluorine atoms bound directly to skeletal sulfur atoms in the polymer main chain. The poly( thionylphosphazenes) possess much improved hydrolytic stability over their sulfur ( IV) analogues, the poly( thiophosphazenes), which have been reported to date.53 For example, polymer (18) showed no sign of hydrolysis when a solution of the polymer in 10% water in dioxane was heated at 65 "C for one month.57

M. b a n g and I . Manners, Makromol. Chem., Rapid Commun. 1991, 12, 613. 57

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88 I. Manners

Also during 1991 the synthesis of the first member of another new class of phosphorus-nitrogen-sulphur polymers was reported by Chivers In contrast to the sulfur-phosphorus-nitrogen polymers described above, these materials for- mally contain sulfur atoms in the +2 oxidation state. Polymer (19), which was synthesized by the route shown in Equation 6, was found to be insoluble in most solvents, but elemental analysis, IR spectroscopy, and solution 3'P NMR measure- ments in DMF were consistent with the proposed structure.

The synthesis of new silicon-based polymers is also attracting considerable atten- tion. For example, West and co-workers have reported the synthesis of u / r conju- gated ethynylene-disilanylene copolymers (20) uia both condensation and ring- opening routes.59 Anionic catalysts such as BuLi were found to be effective for the polymerization of many of the cyclic compounds used in the ring-opening pathway. However, for cyclic species where R = Bu and Ph, electron transfer agents such as Na/K alloy and sodium naphthalide were found to be preferable. The polymers (20) formed by the ring-opening process possessed weight-average molecular weights (M,) of between ca. 3 000 and 14 000 (Scheme 5). The polymers display a UV absorption near 240 nm which is indicative of u/ r conjugation between the Si-Si and CEC moieties. Moreover, several of the polymers prepared were found to undergo solid-state transitions to form liquid crystalline mesophases.

R R R R I I I I

LiC EC-Si-Si-C CLi + Cl-Si-Si-Cl I I R R

R R

R-Si-C C-Si-R

R-Si-C ZZ C-Si-R

I I

I I

I I

\ catalyst ____,

I I R R

R R I I 1

Si-Si-C C

R R

R R (20)

Scheme 5

T. Chivers and M. N. S. Rao, Unpublished results. Reported at the 6th International Symposium on Inorganic Ring Systems (IRIS) held at Berlin, August, 1991. R. West, S. Hayase, and T. Iwahara, J. Inorg. Organornet. Polvm., 1991, 1, 545.

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Inorganic and Organometallic Polymers 89

An important discovery by Bianconi and co-workers involves the successful functionalization of polysilynes, a recently reported class of soluble random-network silicon polymers. Treatment of the phenylated derivative with triflic acid yields a product in which some of the phenyl groups have been replaced by triflate. Sub- sequent reaction with, for example, alkyl, aryl, alkoxy, or amine reagents yields new polysilynes (Equation 7).60 Interrante and co-workers reported the synthesis and

OTf = OSO,CF, R = alkyl, aryl etc.

characterization of highly branched hydridopolycarbosilanes from the reaction of C13SiCH2C1 with magnesium followed by treatment of the polymer product with Li[AIH,].61 These materials are of interest as silicon carbide precursors. Other interesting new routes to organosilicon polymers have been reported by Wagener62 and by Gibson.63

Continuing progress in the use of boron-nitrogen polymers as precursors to ceramic materials has been reported by S n e d d ~ n ~ ~ and by P a i r ~ e . ~ ~ In addition, the synthesis and properties of polyboranes has been reported by Chung et ~ 1 . ~ ~

5 Polymers Containing Skeletal Transition Metal Atoms

One of the major areas of advance in 1991 involved the synthesis and characterization of macromolecules containing transition metal atoms in the polymer main chain. The development of polymers of this type would be expected to provide access to a range of new materials with interesting and potentially useful electrical, optical, magnetic, or catalytic properties. Particular attention has been focused on a series of remarkable macromolecules with backbones which possess conjugated C z C and transition metal atoms, termed poly( metallaynes). The first examples of macromolecules of this type were reported in the 1970’s and these materials are receiving continued attention, in part, because of their unusual liquid crystalline behaviour in solution. Thus, the orientational characteristics of these polymers in the lyotropic nematic mesophase formed in trichloroethylene solvent have been studied in relation to their phase beha~iour.~’ In the past year, Lewis et al. have reported key synthetic developments in the area of rigid-rod poly(metal1aynes). They reported a new route to the polymers of this type via the reaction of

D. A. Smith, P. A. Bianconi, C. A. Freed, and D. M. Goncalves, Polymer frepr. (Am. Chem. Soc. Diu. folym. Chem.) , 1991, 32(3), 495. C. K. Whitmarsh and L. V. Interrante, Organomefallics, 1991, 10, 1336. K. B. Wagener and D. W. Smith, Macromolecules, 1991, 24, 6073. J. T. Anhaus, W. Clegg, S. P. Collingwood, and V. C. Gibson, J. Chem. SOC., Chem. Commun. 1991, 1720. P. J. Fazan, E. E. Remsen, and L. G. Sneddon, Polymer frepr. ( A m . Chem. SOC. Div. Polym. Chem.), 1991, 32(3), 544. R. T. Paine, J. F. Janik, T. T. Borek, D. A, Lindquist, and E. N. Duesler, Polymer Prepr. ( A m . Chem. Soc. Diu. folym. Chem.) , 1991, 32(3), 546. T. C. Chung, J. Inorg. Organomet. folym., 1991, 1, 37. A. Abe, N. Kimura, and S. Tabata, Macromolecules, 1991, 24, 6238.

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90 I. Manners

bis(trimethylstanny1)diynes with readily accessible trans-dichlorobis(phosphine) Ptrl complexes. This afforded rigid rod polymers (2 1) with weight-average molecular weights up to 100000 (Equation 8).68

-CISnMe, C G C - S n M e 3 -

+

fruns-[PtCl,(PBu,),] 4 C G C G 1 \ C 3 C - P t r t (8)

P B u ~ n

(21)

Lewis and co-workers also showed that this procedure could be extended to allow the incorporation of other transition elements into the polymer main chain such as iron (to give (22)) and rhodium by using FeC12(Et2PCH,CHzPEt,)2 or [ Rh( PMe,),]CI, respectively, as the transition metal containing reactant.69

In another important development in this area, Marder et al. showed that a range of rhodium containing poly(ynes) (23) are also accessible via the reaction of the unsubstituted diynes with Rh( PR,),Me in a reaction that involves reductive elirnina- tion of methane and the loss of a phosphine ligand (Equation 9).70 In the case where R = Me the polymers are insoluble but if R = Bu soluble polymers are obtained which yield free-standing films from solvents such as THF.

S. J . Davies, B. F. G. Johnson, M. S. Khan, and J. Lewis, J. Chem. SOC., Chem. Commun., 1991, 187. B. F. G. Johnson, A. K. Kakkar, M. S. Khan, and J . Lewis, J. Orgonomet Chem., 1991, 409, C12. H. B. Fyfe, M. Mlekuz, D . Zargarian, N. J . Taylor, and T. B. Marder, J. Chem. SOC., Chem. Commun., 1991, 188.

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Inorganic and Organometallic Polymers 91

Another interesting class of materials currently receiving attention is the polymeric metal nitrides exemplified by compound (24). These materials have the formal structure [X,M=N], which indicates that they can be regarded as transition metal analogues of polyphosphazenes. However, in all well-characterized cases to date, the polymeric chains consist of alternating triple and single bonds where the single-bond is rather weak and does not survive in solution. Hopkins and co-workers have carried out studies of the non-linear optical (NLO) and excited state properties of the molybdenum and tungsten material^.'^ The second harmonic generation (SHG) capability for powdered samples was measured to be only 30% of that of urea. However, interestingly the polymers were found to possess electronic excited states which are highly emissive at low temperatures (77 K) and which possess very long lifetimes. Indeed, the excited-state lifetime for the molybdenum polymer was amongst the longest known for any transition metal species, which indicates that emission is a spin forbidden process. The emissive energy of the [(RO),M=N], polymers was found to be only moderately dependent on the nature of the alkoxide ligand, which indicates that the orbital character of the emissive state arises mainly from within the metal-nitrogen backbone. Analysis of the vibronic structure in the emission spectra of the molybdenum polymer indicated that a significant 0.2 A distortion along the M-N coordinate occurs for the band-gap excitation from the least [MN], bonding orbital of the filled band to the least [MN], antibonding orbital of the empty band. This indicates that the electron is not extensively delocal- ized along the polymer chain which is consistent with the rather unexpectedly low value of x ( 2 ) measured for these materials. Nevertheless, the optical transparency of the polymers suggests that structural modification of the backbone to improve SHG efficiency may lead to promising NLO materials.

L (C0)Fe- Fe(C0) \ / 1. C

L (CO),Mo - Mo(C0)3

( 2 5 ) 0

( 2 6 )

Other areas of transition metal polymer chemistry are also attracting

J"

attention. For example, Tyler et al. have developed condensation routes to polyurethanes which contain Mo-Mo bonds (25) (R = urethane linkage) or Fe(p-CO),Fe units (26) in the polymer backbone.'* These polymers are of interest as photoreactive

" T. P. Polagi, T. C. Stoner, R. F. Dallinger, T. M. Gilbert, and M. D. Hopkins, J. Am. Chem. Soc., 1991, 113. 703. S. C. Tenhaeff and D. R. Tyler, Organornerallics, 1991, 10, 473. 1 2

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92 I. Manners

materials as metal-metal bonds can often be cleaved photochemically. Also in the past year Chisholm and co-workers have reported some remarkable compounds which contain M-M quadruple bonds which are molecular models for subunits of prospective stiff-chain polymers.73 In addition, Fox and co-workers have reported the synthesis of a range of interesting nickel-phosphine coordination polymers (27) which possess semiconducting proper tie^.'^

r 1

(27)

R. H . Cayton, M . H . Chisholm, J . C. Huffman, and E. B. Lobkovsky, J. Am. Chem. Soc., 1991, 113,8709. M . A. Fox and D. A. Chandler, Ado. Muter., 1991, 3, 381.

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