10 Inorganic and Organornetallic Polymers
By IAN MANNERS Department of Chemistry, University of Toronto, 80 St. George St., Torunto M5S 1A 7,
Ontario, Canada
1 Introduction
Inorganic polymeric materials continue to attract attention as a result of their interest- ing and unusuaI properties and applications as speciality This review focusses on developments in inorganic and organometalIic polymer science published in 1995 and has a similar format to, and follows on from, the five previous articles in the series which cover the years 1991-1995.5-9 The first section of review covers new developments concerning well-established inorganic polymer systems, namely the polysiloxanes, polyphosphazenes and polysilanes. 1-3 A brief introduction to each of these classes of inorganic polymer systems was included in the appropriate sections of the first article of this series.' Recent developments concerning other polymers based o n main-group elements and transition metals are d i~cussed .~ As with previous articles in this series,5L9 the main emphasis is pIaced on polymers with inorganic elements within the main chain rather than in the side group structure. A review of inorganic polymer science, which focusses mainly on new polymer systems prepared recently, was published in 1996 and may also be of interest to readers."
2 Pol ysiloxanes (silicones), pol yphosphazenes and pol ysilanes
Vancso and co-workers have established a route to well-defined liquid crystalline polysiIoxanes with narrow polydispersities (PDI == 1.2) via the anionic ring-opening polymerization of pen tamet h y lvin y lcyclo trisiloxaoe folIowed by the at tachmen t of mesogens using a hydrosilylation strategy (Scheme l)-" The resulting polymers showed very fast electrooptic switching times, which is a vital consideration for many device applications.
Other developments in the siloxane area include studies of block copolymers of poly(diethylsi1oxane) with polystyrene, which build regular, phase separated structures in the soiid state where the siloxane block exists as a mesophase.12 The controlled synthesis of polysiIoxanes with organosulfur side groups has also been described in detail.'
Attention in the past year has focussed particularly on the new condensation route
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I . Manners
I
?
7
? (YHdfl7
?
Me- Si- Me I
Me-Si-Me I
c) CN
C N
Scheme 1
to polyphosphazenes which was reported by Manners and Allcock in 1995. The cationic polymerization of the phosphoranimine CI,P=NSiMe, has been shown to be a 'living' process and provides routes to polyphosphazenes with narrow polydispersi- ties (Scheme 2).14 Samples of poly[bis(trifluoroethoxy)phosphazene] with molecular weights of M, 58W66400 with PDIs of < 1.25 were prepared. Species such as PCl, and SbCl, and Ph,C[PF,] were found to function as initiators.
Scheme 2
In other developments, studies of the molecular motion of phosphazene- Jound non h e a r optical chromophores have been described by Allcock and Haw and studies of calcium-deficient hydrox yapatite-polyphosphazene composites have been described by Brown and Allcock. ' 5 7 1 6 Further studies of polyphosphazene-polysiloxane hybrid materials and pdyphosphazene solid electrolyte materials have also been re- p ~ r t e d . ' ~ . ' ~ In addition, gas permeation studies of poly(methylpheny1phosphazene)
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Inorganic and Organometaltic Polymers 131
have been reported by Wisian-Neilson.” The same group has reported a route to polyfphospholenazene), a new type of phosphazene poIymer.20
In the area of polysilanes, studies of the energy and electron transfer distances in Langrnuir-Blodgett films have been described and further studies of the synthesis and properties of dendritic polysilanes have been reported.21-22
3 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 continue to attract significant attention.
PoIy(thionylphosphazenes), which possess backbones of suIfur(vI), nitrogen and phosphorus atoms, continue to be of interest. The synthesis of amino derivatives were reported in the past year (Scheme 3).23 These materials were shown to possess an atactic structure by 31P NMR spectroscopy.
Scheme 3
The reactions of poly(thiony1phosphazene) with aryloxides has previously been shown to give rise to materials in which substitution of halogens only occurs at phosphorus. Studies have been reported which show that the reactions of c y c k thionyIphosphazenes display the same regi~selectivity.~~
Applications of poly(thiony1phosphazenes) as matrices for phosphorescent sensors were also reported in 1996.25 Phosphorescence quenching has been used to provide measurements of oxygen diffusion in pol y(n-butylaminothionylphosphazene) 1.26 This material was found to possess a very high permeability for oxygen making it ideal for use as a matrix in gas sensors.
1
Further significant developments have been reported in the last year in the area of polystannanes. Highly branched materials have been prepared via a dehydropolymer- ization/rearrangement proces~.~’ Whereas Ti and Zr metallocene complexes have previously been shown to afford linear poIystannanes via dehydropoIymerization of Bun2SnH2, the Rh’ complex [RhH(CO)(PPh,),] was found to afford either cyclic oligomers of polymers depending on the reaction conditions (Scheme 4).
Moreover, the dark yellow gum-like polymer formed, 2 ( M , = 50240, PDI = 1-43), was found to possess a branched structure by ‘”Sn NMR spectroscopy and by analysis of the cleavage products after treatment with I2 followed by MgPhBr which included SnBuPh, as well as SnBu,Ph,. As a result of the branching, the i.,,, in the
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132 I . Manners
Bun2SnH2 IAhH(CW'Ph3)d : +{nu+
Bun
2
Scheme 4
UV/VIS spectrum of the material (in pentane) was 394nm, slightly red-shifted from that of linear poly(di-n-butylstannane) (&,ax = 378-380 nm in the same solvent). The branched material was reported to possess greater stability to air-oxidation and light in the solid state than the linear polymer.
The first successful and detailed report of polystannanes uia Wurtz coupling in a refereed journal was reported in 1996.28 Poly(di-n-butylstannane) was prepared from the reaction of SnBu,CI, with a sodium dispersion in toluene in the presence of 15- crown-5 at 60 "C in the dark (Scheme 5). The maximum yield of poIyrner was reached after 4 h when very high moIecular weight materials was formed (M, > lo6). Pro- longed reaction gave rise to chain scission. The spectroscopic data were consistent with the earlier reports for the same polymer by Tilley and co-workers using the dehydrocoupling route.
Scheme 5
TilIey and co-workers have now reported high molecular weight poly(diary1stan- nanes) by dehydrocouphng react i~ns.~ ' These materials were found to possess even smaller band gaps than their alkyl analogous. For example, poIy[bis(tert-butyl- pheny1)stannanel was isoIated as an orange solid.
Another exciting development in Main Group polymer chemistry involved the discovery of a demethanative coupling route to poIygermane~.~* The reaction reported by Berry et al. involved the coupling of GeHMe, with the ruthenium catalyst [Ru(PMe,),(GeMe,),](Scheme 6).
' he ' n
Scheme 6
The polygermane formed was found to have a highly branched structure. The pro- posed mechanism for the formation of linear and branched chains in this reaction is shown in Schemes 7 and 8 below.
In other areas, further developments concerning the use of poIy(borazy1enes) as precursors to boron nitride ceramic fibres have been d e ~ c r i b e d . ~ ~ In addition, further studies of poIy(carbosi1anes) with a variety of side groups have been reported by I n t e ~ r a n t e . ~ ~ Studies of materials with alkyl side groups with between four and six carbon atoms have been published by F r e ~ . ~ ,
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Inorganic and Organometallic Polymers 133
R = Me, (GeMe&Me I? = PMQ
GeMe3
t
! 1 I
I" Me
Scheme 7
linear ,GeMe3 i t
t , = : 1.3-Me migration
I '
t : GeMezGeMenR
Ru\
,GeMe3 Ge,- GeMe2R
rngralion - RU/ Me \ \ 'GeMe2R Me
Me branched
Scheme 8
4 Polymers containing skeletal transition-metal atoms
Macromolecules containing skeletal transition-metal atoms represent a continually' growing area of r e~ea rch .~
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134 I . Manners
An interesting class of self-oscillating gels have been reported by Yoshida et al.34 These materials 3, which possess [Ru(bipy)f complexes in the side group structure, undergo the BeIousov-Zhabotinsky (BZ) reaction and exhj bit spontaneous mechan- ical oscillations.
C=O 1 NH I
,cy CHj CH3
I I NH
CH2 1 NH
I C=O I
-CH2-CH-
3
Further advances have also been reported concerning high molecular weight poIy(ferrocenes) which are accessible via a ring-opening polymerization route. Full details of the ‘living’ anionic ring-opening polymerization of silicon-bridged [llferro- cenophanes were published in 1996.35 This methodology allows access to materials with controiled moIecuIar weight and architectures such as di-, tri-, and penta-bIock materials (see Scheme 9).
‘Living’ anionic polymerization of phosphorus-bridged [ llferrocenophanes was also reported for the first This allows similar contro1 of molecular weight and polymer architecture; block copolymers with other poly(ferrocenes) and also poly(si1oxanes) were described.
Manners and Geiger have reported the synthesis of linear o~igo(ferrocenyIsilanes) with between two and nine ferrocene units by anionic oligomerization methods.37 Derailed electrochemical studies of the resulting species have provided convincing evidence for the concept of initial oxidation at alternating iron atoms originally postulated for these materiats. The structure of the pentarner in the solid state was obtained by singIe-crystal X-ray diffraction and provided insight into the IikeIy conformations of the analogous high polymer in the solid state. Interesting studies of n-hexyl-substituted oligo(ferroceny1enes) and low molecular weight polymers have also been reported by Nishihara et
O’Hare and co-workers have reported a moIecular mechanics study of oligomeric models of high molecular weight poly(ferrocenylsilanes).39 The calculations utilized the new generalized ESFF force field and showed that the neutral molecules are conformationally flexible with the lowest energy configurations having close contacts between the electropositive iron atoms and the electronegative cyclopentadienyl ligands of their neighbors. For the isolated molecules and conformations are governed by intramolecular interactions whereas in the solid state intermolecular interactions are more important.
In another development, spirocyclic ferrocenophanes have been used as novel crosslinking agents for poly(ferrocenes) prepared via ring-opening polymer i~a t ion .~~ Full details of the properties of poly(ferrocenylsi1anes) with methylated Cp ligands and
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Inorganic and Organornetallic Polymers 135
Li-
(2) MeJSiCl
r
6" t
'Y
Me 1
-'Y
6-cH21H26.- x X
x = 43, y= 35
Me1 r
--ti
Y
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136 I . Manners
their charge transfer complexes with electron acceptors have also been de~cribed.~ ' Detailed studies of a series of poly(ferrocenylgermanes) were described in 1996.42
Further developments concerning the transition-metal-cataIyzed ROP of silicon- bridged [llferrocenophanes, which was discovered in 1995, have been reported in the past year. Manners and Foucher reported that whereas [llsilaferrocenophane con- taining Si-C1 bonds undergoes thermal ring-opening polymerization at more elevated temperatures, transi tion-met aka talyzed ring-opening polymerization occurs at ruom temperature in the presence of ca. 1 mol% of PtCI, (Scheme The resulting poly(ferroceny1silanes) were found to be tunable and halide replacement yieIded polymers with different side groups.
trace PtCI2
ME?
-"i Scheme 10
Transi tion-metal-catalyzed copolymerization of silicon-bridged [ llferro- cenophanes with silacyclobutanes was a k o reported in 1996 (Scheme 1 l).44
The first step in the transition-metal-catalyzed ring-opening polymerization reac- tions of silicon-bridged Cl-Jferrocenophanes is believed to involve insertion into the strained ipso-carbon-bridging-atom bond. The first example of this type of reaction has been reported and involves the reaction of [Pt(PEt3)J with a silicon-bridged f 1 jferrocenophane (Scheme 12).45
+ - M e
@i-Me
Me
Scheme 11
A review concerning the mechanisms for the thermal and transition-metal-catalyzed ring-opening polymerization reactions of [I] ferrocenophanes was published in the past year.46
A series of novel dendritic systems has been reported in the past year. Cuadrado and co-workers have recently reported the synthesis of a series of novel ferrocenyl-func- tionalized poly(propy1enimine) dendrimers 4 which undergo oxidative precipitation
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inorganic and Orgunornetallic Polymers 137
Me Scheme 12
onto electrode surfaces.47 Moss has described large organoruthenium dendrimers with 48 ruthenium atoms from 'dendritic wedge' precursors such as 5.48
4
Constable and co-workers have described a convergent approach to metalIocentric metallodendrimers 6 containing up to seven metal atoms with poIy(pyridy1) ligand
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138 I. Manners
5
environment^.^' The same research group has also described convergent routes to organoruthenium dendrimers with organic centre^.^'
Peng and Yu have reported the synthesis of conjugated polymers 7 containing ionic ruthenium bipyridyl complexes in the main chain using the Heck r ea~ t ion .~ ' The resulting polymers possess a delocalized backbone and exhibit photoconductive prop- erties.
Rigid-rod poIymers containing metals in the backbone continue to attract attention.
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Inorganic and Organometallic Polymers 139
6 (M = Fe or Co)
RO
Potyrner I: x = 0, y = 1 [q] = 0.48 dl g-'
Polymer II: x = 0.1, y = 0.9 [q] = 0.58 dl g-I Potyrner Ilk: x = 1, y = 0 [q] = 0.60 dl g-'
7
Butlers2 has reported a vibrational spectroscopic investigation of platinum acetylide polymers and Puddephatts3 has reported the synthesis of oligomers and polymers with Pt-Pt bonds in the main chain. Endo and co-workers have described the
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140 I . Manners
synthesis and characterization of liquid crystalline po€yesters with cyc- lobutadienecobalt moieties in the backbone.54 Further interesting work has been reported concerning poIymers with metaIlacyclopen tadiene units in the main chain. Nishihara and co-workers described full details of the synthesis and properties of a range of poly(coba1tacyclopentadienes) which were prepared uia the reactions of conjugated diacetylenes with CCO(PP~,)~('I-C,RH,}].~~ In the cases where R = H the polymers were insoluble but soluble materials were prepared where R = n-hexyl with values of M , up to lo4. The UV/VIS band edge was found to shift on moving from the monomer to the dirner and on to the polymer; the opticaI band gap values for the pofyrners were 2.1-2.3 eV, which are similar to the value for poly(thiophene) (2.0 eV). Photoconductivity was detected for the polymer (q-C,RH,)CoC,Me,C,H.1, where the photocurrent was four times greater than in the dark.
Further significant developments in the area of well characterized lanthanide-based polymers have also been reported by Archer and c o - w o r k e r ~ . ~ ~ The first linear luminescent polymers with europiurn(II1) in the backbone have been synthesized and characterized. Mixed metal Y and Eu polymers were also prepared and possessed enhanced emission properties over the homopolymers.
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