J. MATER. CHEM., 1994, 4(9), 1387-1392 Polymer-mediated Crystallisation of Inorganic Solids: Calcite Nucleation on the Surfaces of Inorganic Polymers Kim K. W. Wong," Brian J. Brisdon,*a Brigid R. Heywood,bAnnabelle G. W. Hodson"and Stephen Manna a School of Chemistry, University of Bath, Bath, UK BA2 7AY Department of Chemistry, University of Salford, Salford, UK M5 4WT Department of Chemical and Physical Sciences, University of the West of England, Bristol, UK BS16 IQY Nucleation of calcite from supersaturated calcium hydrogencarbonate solution has been achieved on the surfaces of poly(dimethylsi1oxane)films formed by cross-linking in the presence of catalytic quantities of dibutyltindilaurate or zinc octanoate. Films containing high concentrations of the diorganotin catalyst produced, in addition to calcite, amorphous calcium silicate and calcium silanolate under similar conditions.Polar functional groups incorporated into the polysilox-ane network served to promote calcite growth, but prolonged heating of all films at 90°C rendered them inactive. The controlled deposition of inorganic crystalline solids upon polymeric surfaces is of considerable interest because of the potential technological' and biological applications2 of such processes, particularly in the area of nanoscale synthesis, crystal engineering, and microstructural fabrication., Mineral nucleation and growth upon the surfaces of organic polymers has been widely but to date we are not aware of analogous studies conducted on the controlled growth of inorganic materials upon polymers with an inorganic back-bone, with the exception of a single preliminary publication concerning poly(organosi1oxane) films.' Both the polarity of the Si-0 link, and the ease with which a range of organofunc- tional substituents can be introduced in a controlled and reproducible manner into polysiloxane membranes9-'' pro-vides a combination of highly desirable properties which may be useful in the study of crystal engineering and fabrication of hybrid composites.Polysiloxane membrane films, produced by cross-linking Me,SiO [MeSi(H)O],SiMe, (xz40) with a,co-dihydroxypoly-(dimethylsiloxane), in the presence of traces of tetraethoxysil-ane, using a dibutyltindilaurate catalyst, are formed as elastic, transparent, permeable membranes with a random three-dimensional network of polysiloxane chains.A variety of organofunctional groups can be introduced into these films by reactions leading in effect to partial replacement of the Me substituents on silicon, and as reported previously,' both functionalised and unfunctionalised membranes promote the growth of inorganic solids from a supersaturated calcium hydrogencarbonate solution. The solid deposits were shown to contain crystalline calcite and amorphous calcium silicate, but the role of the substituent groups on Si in promoting these effects was unclear. In this paper we define in more detail the nature of the inorganic solids nucleated from aqueous calcium hydrogen-carbonate on poly(organosi1oxane) surfaces, and elucidate possible mechanisms by which they are formed.Experimental Preparation of SupersaturatedCalcium Hydrogencarbonate Solution Kitano Method12 Scrubbed carbon dioxide gas was bubbled into a stirred, aqueous suspension of calcium carbonate (10 g calcite in 4 dm3) at a rate of 0.18 m3 h-l for ca. 70 min. The suspension was filtered and the filtrate purged with C02 gas for ca. 35 min to dissolve any residual crystal nuclei. The resultant solution had a pH of 5.8-6.2. The total dissolved calcium (ca. 9 mmol drnp3)was determined by EDTA titration. Slow loss of C02 gas from the solution resulted in CaCO, crystallihation. CaCO,(s) +C02(g)+H20(1)SCa2+(aq)+2HCO,( hq) Metastable Method13 Equal volumes of sodium hydrogencarbonate and calcium chloride dihydrate, solutions (8 mmol dmP3 in 0.1 mmol dm-3 NaCl) were mixed at ambient temperature (ca.25 "C) and the pH raised to 8.3 with NaOH (as) (0.1mol drnp3), whilst stirring, to give a metastable solution of calcium hydrogen-carbonate. 2NaHC03(aq)+CaCl,(aq) g2NaCl (as)+Ca(HCO,),(aq)I1 CaCO,(s)+ CO,(g)+H,O(l) Preparation of Polysiloxane Membrane Films Unfunctionalised poly(dimethylsi1oxane) membrane films were prepared by the addition of dibutyltindilaurate (0.016-0.36 g; 0.025-0.57 mmol drnp3) to a thoroughly stirred mixture of poly(methylhydrosi1oxane) Me,SiO [MeSi(H)O],SiMe, (xz 40; 0.60 g; 0.23 mmol dm-,), a,o-dihydroxypoly (dimethylsiloxane) (Mwz 74 000; 6.0 g; 0.08 mmol drn-,) and tetraethoxysilane (0.60g; 2.9 rnmol drn-,).The resulting gel was held between Cellophane hheets and pressed under ca. 12.5kg cm-2 to produce large trans- parent films with a thickness of ca. 0.1 mm. Unfunctionalised membrane films cross-linked with dibutyltindiacetate (0.001-0.60 g; 0.003-1.7 mmol drn-,), and dibutyltindiineth- oxide (0.15-0.92 g; 0.56-3.5 mmol dm-,) or zinc octanoate (0.18 g; 0.51 mmol drnp3) dispersed in 1.97 g polydimethyl-siloxane, were prepared similarly. Unfunctionalised poly(dimethylsi1oxane) membrane films containing only two silicon-containing components were also prepared in a similar manner by the addition of dibutyl- tindilaurate (ca. 0.2g; 0.3 mmol dm-,) to a thoroughly stirred mixture of a, o-dihydroxypoly(dimethy1silouane) 6.57 g; 0.014 mmol dm-,) with either pol!;(me-(M, ~74000; thylhydrosiloxane) Me3Si0[MeSi( H)O],SiMe, (x z40; 0.66 g; 0.26 mmol drn-,), or tetraethoxysilane (0.66g; 3.2 mmol drn-,).Synthesis of Zinc Octanoate Zinc octanoate was synthesized by dissolving sodium octano- ate (24.40 g, 0.15 mol drn-,) in warm EtOH (250 cm3), and adding anhydrous ZnC1, (10 g, 0.073 mol drn-,). The reaction mixture was heated under reflux for 24 h, then allowed to cool to room temperature and treated with water (100cm3). The insoluble solid was filtered, washed thoroughly with water, and dried over P,05 in uucuo to afford a white crystalline product (18.8 g, 73%). mp 134-135 "C; v,,,(Nujol)/cm-l; 1533 (C=O str.).'H NMR (DMSO; 400MHz; 120°C) SH: 0.88 (6 H, q, 2 x -CH,, J 7.0), 1.29 [16 H, s, 2 x CH,fCH,),CH,], 1.52 (4 H, d of t, 2x -O-CH2-CH2-, J 7.0), 2.14 (4 H, t, 2 x -O-CCH,-CH,-, J 7.0). Found: C, 54.30; H, 8.86%; C16H3,Zn0, requires: C, 54.63; H, 8.60%. Preparation of Functionalised Polysiloxane Films9-11 A series of linear polymers of general formula Me,SiO [MeSi( H)0], { MeSi[( CH,),L] 0},%Me, containing side-chain functionalities -(CH,),L (L =-C,H,, -C02Me and -C6F5) were prepared by the platinum-catalysed addition of the appropriate l-alkenyl derivative to the poly (methylhydrosiloxane) Me,SiO [MeSi( H)O],,SiMe, . The molar concentrations of reagents were adjusted to produce materials with a high degree of functionalisation.The remaining Si-H groups in the linear fluids Me,SiO [Me%( H)O], { MeSi [(CH,),L]O),SiMe, were cross- linked with the same a,o-disilanol in the presence of minimum tetraethoxysilane and dibutyltindilaurate catalyst, to produce membranes containing up to 60 mol%t of -C3H7, -CO,Me or -C,F, groups (Fig. 1). Alternatively, functional groups were introduced into the polysiloxane membrane films by the reaction of commercially available alkoxysilanes (RO),Si (CH,),L (R =Me, Et), which t Mol% functionality expressed as a percentage of the total moles of Si atoms bearing functionalities. Me3SiO[MeSi( H)O],+iMe3 + CH2=CH(CH2),-2L n = ca.40 L = functional group Me3SiO[MeSi(H)O]fleSi(CH2),&(0)],,SiMe3 HO(Me$iO),HI cross-linked membrane film Me Me Me3Si-0 Si-0 I I L L Fig.1 Idealised structure of a functionalised polysiloxane membrane film prepared by method A J. MATER. CHEM., 1994, VOL. 4 contain a side-arm functionality L [L= -NMe,, -C5H4N, -CN, CH,=C(CH,)CO,-and -NHCO,Et] with an a,o-dihydroxypoly(dimethylsi1oxane)of molecular weight 16000, to yield pressed membranes containing up to 30mol% of functionalised Si atoms (Fig. 2). Crystallisation Experiments All experiments were conducted at ambient temperature (ca. 20-25 "C). Membrane film samples (ca. 18 mm x 18 mm) were cut from the polysiloxane membrane film, mounted onto glass slips and immersed at the bottom of clean crystallisation dishes containing supersaturated calcium hydrogencarbonate solu- tions.A glass slip was used as control surface. Mineral deposition was allowed to occur for ca. 24 h under still conditions. The calcium hydrogencarbonate solution was removed and the samples removed for optical microscopy, X-ray diffraction, transmission infrared and FTIR spec-troscopy, and scanning and transmission electron microscopy. Treated Films Crystallization experiments were also undertaken with mem- brane films cured at 80-90°C for 7 days. Some of these cured films were immersed in dilute acid (HC1,2 mol dm-,, 50 cm3) or base (NaOH, 2mol drn-,, 50 cm3) for 24 h, then thor- oughly rinsed in distilled water and immersed in water for 24 h before use. For comparison uncured samples of film were also immersed in acid (1mol dmP3 H,SO,; 2 mol dmP3 HNO,) or base [2mol dmP3 NH, (as)] for either 1 h or for 24 h at 40°C, washed thoroughly in water and then soaked in water for a minimum of 1h.The water was changed 0.5 h before the membranes were immersed in a calcium hydrogen- carbonate solution. Individual Components and Functionalised Glass Surfaces Separate samples of a,o-dihydroxypoly(dimethylsi1oxane) (a,co-disilanol; M, z 74OOO), poly(methylhydridosi1oxane) Me,SiO [MeSi( H)O],SiMe, (xG40) and tetraethoxysilane (ca. 0.25 cm3 each) were spread onto the surface of a solution of calcium hydrogencarbonate solution and a glass slip allowed to fall vertically through the surface film to the bottom of the crystallisation dish. Additionally, samples of a,o-disilanol (6.0 g) mixed with dibutyltindilaurate (ca.0.1 g), and a,o-disilanol (10g) mixed with lauric acid (0.05 g), which is liberated during the cross-linking process, were separately smeared onto glass slips before immersion in a supersaturated calcium hydrogencarbonate solution. Glass (RO)3Si(CH2),,L + HO(Me2SiO)J-l cross-linked membrane I I0 0 0 0 I I Fig. 2 Idealised structure of a functionalised polysiloxane membrane film prepared by method B J. MATER. CHEM., 1994, VOL. 4 slips (10 mm diameter) were silanised in a refluxing toluene- trialkoxysilane (RO),Si( CH,),L’ (R =Me, Et;) [L’ = -CN, -CH,NMe,, -(CH,),CH,, -C02CZH5, -CH,C,H,N,, -CH,NHC0,C2H, or -CH,CO,C(CH,)=CH,] mixture for 24 h. The glass slips were recovered, washed thoroughly in toluene and air dried before immersion in supersaturated calcium hydrogencarbonate solution.Results and Discussion Unfunctionalised Poly (dimethylsiloxane) Membrane Films Efect oj Cutulyst Concentration As noted previously,8 uncured polysiloxane films, prepared as above, promote the growth of inorganic solids on immersion in a supersaturated calcium hydrogencarbonate solution. The cross-linking catalyst, dibutyltindilaurate, used in the fabri- cation of unfunctionalised polysiloxane membrane films, had a profound effect on the rate of formation and composition of the mineral growth. At low concentration (0.2 wt.%) only calcite growth was observed [Fig. 3(a)], with the crystals being discrete and partially embedded in the membrane. Crystals were clustered, with sizes ranging from ca.0.5 to 3 pm, the larger crystals having well defined rhombohedra1 calcite morphology, and with no preferred crystal orientation. This habit is characteristic of the equilibrium form of calcite which was often observed in the control experiments, suggest- ing that the crystals, once nucleated on the membrane, grow unperturbed in the supersaturated solution. In contrast, at higher catalyst concentrations (up to 4 wt.%) the calcite growth was overlaid with a filamentous amorphous ‘coral’- like growth [Fig. 3(b)],this growth becoming more profuse and elaborate as the cross-linking catalyst concentration was increased [Fig. 3(c)].Changing the degree of supersaturation of the calcium hydrogencarbonate solution over the range 8.5-1.1 mmol dm-, had no significant effect on the growth of the amorphous solid.SEM and XRD analyses confirmed that calcite was present at a level of 5-10wt.% in the amorphous solid, which was shown by FTIR, TEM and energy dispersive X-rays (EDXA), to be amorphous calcium silicate together with a calcium salt or salts of one or more organosiloxane species. Attempts to separate the individual components have not been successful to date, but analytical and IR data on the amorphous material are fully consistent with it being a calcium salt of one or more oligomeric dimethylsilanolates of the type known for various other metal ions. Thus the solid exhibited strong IR absorption in the C-H, Si-Me (sym.and asym.) and Si-0-Si stretch regions, and on thermal decomposition, cyclo-Me6Si,0, was identified in the products from its mass spectrum. These products are presumably formed by a diffusion-limited process at the polymer/water interface from silaceous materials eman- ating from the solid matrix into a CaZf-rich solution. A similar growth pattern was also observed when membrane films were immersed in calcium hydrogencarbonate solutions prepared by mixing chemical solutions rather than CO, gassing. As precipitation in the latter is driven by the evolution of C02 gas, it seems unlikely that promotion of crystal growth in the presence of the membranes is due to the dissolution of carbon dioxide in the films with concomitant crystal forma- tion.It seems probable therefore that calcite growth results from calcium-ion clustering around polar sites on or within the surface of the membrane film, resulting in a lowering of the activation energy to calcite nucleation. These polar sites may be unreacted silanol groups, which IR measurements reveal to be present in uncured films and which arise from hydrolysis of residual Si- H and/or Si-OR groups. Moreover, these polar residues may be segregated into Fig. 3 Scanning electron micrographs of (a)calcite, (b)and (c ) amor-phous overgrowth, deposited on the surface of unfunctic malised polysiloxane membranes. Scale bar = 10 pm. ‘domains’ of exposed Si-0-H linkages within the three- dimensional network of cross-linked polysiloxane chains such that they act as regiospecific nucleation sites.EfSects of Membrane Composition A study of the individual components from which the film was prepared was performed in order to determine R hether the activity of the membrane film might be related dirrctly to only one of them. None of the individual components pro- moted ‘coral’ growth or calcite formation while the m:iterials were immersed in supersaturated calcium hydrogencar bonate solution. Membrane films were also fabricated from the u,m- disilanol plus either tetraethoxysilane or the polyhydride, and were cross-linked using a dibutyltindilaurate catalyst, in order to ascertain whether a combination of only two components was responsible for the calcium silicate overgrowth.Growth occurred on all such membrane films on immersion in calcium hydrogencarbonate solution. However, those fabricated with (EtO),Si appeared to be the more active, probably due to a higher final concentration of silanol groups after immersion in the aqueous phase. Membranes containing DifSerent Catalysts The effect of two other tin-containing cross-linking catalysts, dibutyltindiacetate and dibutyltindimethoxide, were investi- gated in order to determine the effect of the catalyst composi- tion on mineral growth. Catalyst concentrations were varied from 0.01 to 7.6% (ca. 0.001-0.60 g) and 1.2-11.3% (0.15-0.92 g), respectively, of the total film weight. Unlike membranes cross-linked with dibutyltindilaurate, films cross-linked with dibutyltindiacetate and dibutyltindime- thoxide did not promote the growth of amorphous material, but crystalline calcite was deposited.The dibutyltindiacetate catalyst [Fig. 4(a)] was the more effective, producing discrete crystals, with sizes ranging from ca. 1 to 3 pm, which exhibited well defined rhombohedra1 morphology, and with the majority of crystals possessing secondary crystal growth. Crystals grown on the membrane films cross-linked with dibutyltindi- methoxide were of similar size, discrete but less well defined, and with a nucleation density ca. 20% of that found on membranes containing the former catalyst at a comparable -Fig. 4 Scanning electron micrographs of calcite deposited on the surface of unfunctionalised polysiloxane membranes cross-linked with (a)dibutyltindiacetate and (b)zinc octanoate.Scale bar = 10 pm. J. MATER. CHFM., 1994, VOL. 4 Sn concentration. The crystals had the appearance of being embedded in the surface of the membrane film in both instances. Varying the catalyst concentration did not greatly enhance the activity of the films. Membranes were also prepared using zinc octanoate? as a 9% dispersion in poly(dimethy1siloxane) as catalyst in order to ascertain whether mineral growth promotion was a prop- erty innate only to tin catalysts. Such films promoted calcite growth only, and it was found that zinc octanoate was more active in this respect than dibutyltindimethoxide. Crystals were discrete, with evidence of some secondary growth, and nucleation densities were similar to those on films containing an equivalent concentration of dibutyltindiacetate catalyst. Crystal size ranged from 1 to 10 pm, with the smaller crystals (<3 pm) possessing well defined calcite morphology [Fig.4(b)].Attempts to use high zinc catalyst concentrations were unsuccessful due to the highly insoluble nature of the catalyst in suitable solvents of low volatility. We conclude that unfunctionalised membranes containing tin or zinc catalysts promote calcite growth on the membranes, and that tin does not therefore play a unique role in determin- ing the nucleation and growth of this mineral. although some catalysts are more effective in producing active films than others. Of the three tin catalysts investigated only dibutyltindi- laurate containing films generated calcium silicate/calcium silanolates.The effect of laurate appears to be very subtle in that its addition as the free acid to dibutyltindiacetate cross- linked membrane films resulted in increased sizes (ca.5-20 pm) of calcium carbonate crystals. Its role in the promotion of other amorphous calcium salts is not completely clear, but as siloxane linkages are acid-sensitive, both calcium silicate and calcium silanolates may arise from fragmentation of the polymer matrix at its SiO,,, and R,SiO,/, centres induced by free lauric acid from the catalyst. Alternatively, disruption of the polysiloxane framework, when immersed in the calcium hydrogencarbonate solution, may be initiated by the acid at heterosiloxane linkages involving the metal catalyst, which for Sn are known to be particularly hydrolytically unstable14 t The cross-linking process when zinc octanoate, as a 9% dispersion in poly(dimethylsiloxane), is used as a catalyst is very slow compared to other catalysts in this study.R2’Sn(OCOR2)2t H20 ,0COR2 R2’Sn, R,Si(OR), ROH f%’Sn,OSi(OR),&,,0COR2OH 1 OCOP R,’Sn’I + HOSi(RO),3R, OH Jca-Ca2[0Si(R0),&L Fig. 5 Catalytic cycle of the SiOH/SiOR condensation illustrating a possible pathway for the formation of silanolates J. MATER. CHEM., 1994, VOL. 4 and easily cleaved by protic medial5 (Fig. 5). We note that a sample of a di butyltindilaurate-containing membrane film on immersion in a calcium chloride solution (9mmol dm-3) yielded after 5 days maturation, a similar amorphous deposit to that described above, although the coverage on the mem- brane’s surface was not extensive.Treated Membranes Curing the dibutyltindilaurate membrane films by prolonged heating rendered them inactive. A timed study showed that very little effect was evident on the rate of mineral growth after 8 h at 80-90°C, but a marked decrease was observed after 24 h curing, with complete inhibition of mineral growth on membranes after 3-4 days at this temperature, by which time no silanol groups were detectable by IR measurements. Such conditions are also likely to lead to rearrangement of Fig. 6 Scanning electron micrographs of growth deposited on the surface of unfunctionalised polysiloxane membranes: (a) amorphousovergrowth, cross-linked with dibutyltindilaurate and treated with dilute HNO,; (b) calcite, cross-linked with dibutyltindiacetate and treated with dilute aqueous NH,; and (c) calcite, cross-linked with dibutyltindilaurate, cured and treated with dilute HC1.Scale bar = 10 pm. Si-0-Sn linkages to more stable Si -0-Si and Sn -0-Sn combinations. Acid treatment of uncured dibutyltindilaurate cross-linked films enhanced solid formation, compared to untreated membrane films. The crystal morphology of the calcite formed was poorly defined [Fig. 6(a)], and crystals were extensively overgrown by amorphous calcium salts. Base treatment resulted in the growth of calcite (ca.1 pm) with well defined crystal morphology and with a very low nucleation density. Uncured dibutyltindiacetate-containing membranes yielded significantly greater quantities of calcite after treatment with NH3 (as) [Fig. 6(b)].Crystals were discrete, possessing well defined calcite morphology, were large in size (ca. 10 pm) with a narrow size distribution, and showed no preferred crystal orientation. Treatment of such films with dilute H,S04 resulted in calcite only with a poorly defined morphology and low crystal nucleation density. Treatment with HNO, inhibited mineral growth completely. Treatment of cured, inactive dibutyltindilaurate-cont aining membrane films with 2mol dmd3 HC1 caused reactivation towards calcite growth only [Fig. 6(c)].Crystals were discrete with a high nucleation density approaching that produced by analogous uncured films, and they were significantly larger (ca. 10 pm) than previously observed, with no preferred orien- tation. Curing and then treatment with dilute H2S04 or HN03 rendered the membrane films very much less active. Treatment of such films with 2mol dm-3 NaOH, dld not reactivate them, and this may be due to neutralisation of free lauric acid and Na’ ions capping the available nucleation sites required for mineral growth. Organo-functionalised Poly(dimethylsi1oxane) Membranes Uncured functionalised membrane films promoted, to varying degrees, mineral growth of a similar nature to that above, H Fig.7 Optical micrographs illustrating the effect of organo-functionalities on mineral deposition: (a) CH, =C(CH3)C ‘02-; (h) -C,F,; (c) -NHC02Et; and (d) -CN. Scale bar= 500 prn. (a ) H Fig. 8 Optical micrographs illustrating the effect of increasing mol% organo-functionality -(CH,),L (L= -OMe) on the amorphous overgrowth: (a) 15, (h)30, (c)40 and (d) 50 molyo. Scale bar =500 pm. with calcite as a minor component (Fig. 7). The activity of the functionalities increased in the order: -NMe, z -C,H,N zz -C7HI3 < CH,=C(CH,)CO,-< -OMe< < -C,F, < < -CN d -NHCO,Et. A series of uncured and cured/acid-treated membrane films with 15-50 mol% methyl ester loadings were also studied. The quantity of amorphous calcium salts deposited appeared to increase per unit time on uncured films with increasing methyl ester loading (Fig.8). SEM analysis confirmed that calcite growth had also occurred, with crystals often branching off from a central calcite crystal on the membrane film surface (cf unfunctionalised membrane), but these centres were surrounded by ‘islands’ of amorphous material. Curing the films rendered them completely inactive, but subsequent treatment with 2 mol dmP3 HC1 partially reactivated the membrane films towards calcite and calcium silicate/silanolate formation. Glass slips, treated with organo- functional alkoxysilanes, were not found to promote mineral growth. Thus the more polar functionalities serve only to enhance the inherent ability of uncured, cross-linked poly- (dimethylsiloxane) membranes to promote calcite growth.Conclusions We have shown that it is possible to promote and control the growth of the inorganic mineral calcite on unfunctionalised J. MATER. CHEM., 1994, VOL. 4 polysiloxane membrane film surfaces. This can be achieved by adjusting the concentration of the cross-linking catalyst, dibutyltindilaurate, used to fabricate the membrane films. In this way the growth of amorphous Ca/Si-containing materials can be inhibited, permitting the growth of calcium carbonate to predominate. The growth activity is not restricted to tin- containing films, since a zinc catalyst can also be used to form membranes which promote the growth of calcium carbonate. In addition we have shown that all salt deposits can be completely inhibited by curing procedures and that these films can be re-activated by acid treatment to induce calcium carbonate growth only.We have also shown that the presence of different organo- functional groups incorporated into the polysiloxane network may increase or inhibit the nucleation activity of the mem- brane films. Films with accelerated growth activity show growth patterns similar to those of the unfunctionalised membrane films. We conclude that clusters of polar functionalities at the surface of the film (silanol or organofunctional) provide the effective nucleation sites, and that the polar backbone of the polymer does not influence the crystal growth of the inorganic material calcite.The authors thank SERC for their support in this work. References 1 P. D. Calvert and S. Mann, J. Muter. Sci., 1988,23, 3801. 2 L. Addadi, J. Moradian, E. Shay, N. G. Maroudas and S. Weiner, Proc. Natl. Acad. Sci. 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Chem., 1991, 1,889. 14 V. Gouran, B. Joussume, M. Pereyre, J-B. Verlhac, J-M. Frances, in Chemistry and Technology of Silicon and Tin, ed. V. G. Kumar Das, N. S. Wang and M. Gielen, Oxford University Press, Oxford, 1992, p. 239. 15 F. W. van der Weij, Makromol. Chem., 1980,181,2541. Paper 4/00032C; Received 5th January, 1994