Polymer International Polym Int 48 :157–158 (1999)

Rapid Report

Tricarbonylchromium promoted tacticityvariations in theWurtz synthesis ofpoly(methylphenylsilane)Andrew J Wis eman, Simon J Holder, Michael J Went* and Richard G JonesCentre for Materials Res earch, School of Phys ical Sciences , Univers ity of Kent , Canterbury, Kent CT2 7NR,UK

Polysilanes have a range of potential applicationsstemming from the unusual properties exhibited bythe catenated silicon backbone arising from delocali-zation of the p-electrons.1 Despite several detractivefeatures, the Wurtz reductive-coupling of dichloro-silanes remains the most practical method of synthe-sis of polysilanes. However, the harsh reactionconditions limit the use of monomers containingfunctional groups. Metal functionalization could be apromising approach to tuning the properties of poly-silanes, but to date only the robust ferrocenyl grouphas been exploited in a Wurtz synthesis which wasused to prepare a copolymer of methylphenylsilaneand methylferrocenylsilane.2 Recently we havereported methods of metal functionalizing poly-silanes.3 Of particular relevance here is a methodinvolving the co-ordination of metal–ligand frag-ments, such as tricarbonylmolybdenum, to thearene rings of preformed poly(methylphenylsilane)(PMPS).4,5 A recent report of the synthesis ofoctamethyltetrasila[2.2]paracyclophane via a Wurtzcoupling using a tricarbonylchromium template ledus to re-examine the possibility of using metal-containing monomers.6

Reaction of dichloromethylphenylsilane (I) withdichloromethyl(g6-phenyltricarbonylchromium)silane(II) in ratios 3 : 1 and 9 : 1 under Wurtz conditionsusing sodium in reýuxing toluene (Scheme 1)resulted in low molecular weight polymers (as deter-mined by size exclusion chromatography calibratedwith polystyrene standards) in poor yields (Table 1).

Scheme 1.

1H NMR spectroscopy showed no co-ordination oftricarbonylchromium fragments to the phenyl ringsof PMPS. Moreover, analysis of the products byatomic absorption spectroscopy revealed that nochromium had been incorporated in the polymer.The low yields are not atypical in many Wurtz typepolysilane syntheses.

The tacticity of a sample of PMPS can be conven-iently and accurately established by deconvolutionof its 29Si NMR spectrum using a programme suchas PeaksolveTM.7 PMPS displays three main reso-nances at [39.2, [39.9 and [41.2ppm that havebeen assigned by Fossum and Matyjaszewski to het-erotactic, syndiotactic and isotactic triads. Analysisof the products of runs 1 and 2 (Table 2) shows thatthe tacticities of the polymers are the same as for

Table 1. Polymerization reactions

Run Solvent Ratio I : II Yield (%) Mw

Metallated

arene

rings (%)

1 Toluene 3 : 1 8 6 600 0

2 Toluene 9 : 1 16 9 700 0

3 THF 1 : 1 5 6 900 3

4 THF 3 : 1 8 27 200 3

5 THF 9 : 1 60 24 800 2

Table 2. Tacticities determined by deconvolution of 29Si

NMR s pectra

Run Heterotactic Syndiotactic Is otactic

1 0.46 0.15 0.39

2 0.49 0.14 0.37

3 0.32 0.28 0.40

4 0.36 0.18 0.46

5 0.47 0.12 0.41

* Corres pondence to : J Centre for MaterialsMichael Went,

Res earch, School of Phys ical Sciences , Univers ity of Kent, Can-

terbury, Kent CT2 7NR, UK.

(Received 18 January 1999; accepted 20 January 1999)

( 1999 Society of Chemical Industry. Polym Int 0959-8103/99/$17.50 157

AJ Wiseman, MJ Went, RG Jones

PMPS prepared by a conventional Wurtz reaction(0.48 H, 0.14 S and 0.37 I). Furthermore, theyaccord with a model based on Bernoullian statisticswhich signiües that a single probability factor deter-mines the conüguration of successive stereogeniccentres.7

Homopolymerization of II in THF at room tem-perature also produced only very small amounts ofpolymer.9 However, copolymerization of I and II inTHF at room temperature produced higher molecu-lar weight material, but with a low incorporation ofchromium. Irrespective of the composition of themonomer mixture, only ca 2–3% of the arene ringswere metallated. This was established by atomicabsorption spectroscopy and by 1H NMR spectro-scopy which displayed broad resonances in theregion 5.5–4.5ppm corresponding to co-ordinatedphenyl rings. In addition, substantial amounts ofdimethyloxymethyl(g6 - phenyltricarbonylchromium)silane (III) were found in the product mixture, pre-sumably resulting from the methanolysis of unre-acted II during work-up. Signiücantly however, thepresence of II in the copolymerization greatlyaþected the tacticity of the resulting copolymer.Accepting the Matyjaszewski and Fossum assign-ments, which seems reasonable in view of the lowchromium content of the polymers, increasing theamount of II in the feed mixture dramaticallyincreases the proportion of syndiotactic triads at theexpense of the heterotactic triads whilst leaving theisotactic content relatively unchanged (Table 2).These variations are large compared with the eþectsof using as an alternative reducing agent (0.44C8KH, 0.12 S and 0.44 I).7 Most signiücantly, the tac-ticity ratios no longer conform to Bernoullian sta-tistics as might be expected for a copolymerization.The fragment is known to reduce n-Cr(CO)3electron density on one face of an arene ring and indoing so may facilitate alignment of arene rings inthe growing polymer.6 However, the low incorpor-ation of chromium in the products, notwithstandingthe fact that (g6-oligosilylarene)tricarbonylchromiumcomplexes have been prepared previously by post-polymerization thermal reactions,10 and the forma-tion of large quantities of III upon quenching thepolymerizations with methanol suggest that the main

function of II could be to modify the sodium surfacein such a way as to exercise some controlling inýu-ence on the incorporation of I into the polymer.

Although the PMPS syntheses that are of interestin the context of stereogularity do not have highyields they are the ürst to be produced by a Wurtz-type synthesis that have signiücant shifts in tacticitywhich have been accurately evaluated. Tacticityvariations have also recently been observed in thesynthesis of PMPS via alkali metal supramolecularcomplexes,11 but the 29Si NMR spectra have notbeen deconvoluted to establish sufficiently accurateintegrals for the proper determination of the tacti-cities.

The rationale for the shifts in tacticity is notentirely clear, but it does seem to be caused, in thiscase, by the presence of a metal-containing monomer,thus presenting considerable scope for further devel-opment. Future work will investigate the eþect ofother metal-containing monomers on the tacticity ofPMPS and probe the mechanisms behind theseeþects as well as studying the properties of the newpolysilanes produced.

We gratefully acknowledge an EPSRC studentshipfor AJW.

REFERENCES1 Miller RD and Michl J , Chem Rev 89 :1359 (1989).2 Pannell KH, Rozell JM and Ziegler JM, Macromolecules,

21 :276 (1988).3 Jones RG, Swain AC, Holder SJ, Benüeld RE, Went MJ and

Wiseman AJ, Metal Containing Polymeric Materials, PlenumPublishing Corporation, New York, 161 (1996).

4 Wiseman AJ, Jones RG, Swain AC and Went MJ, Polymer,37 :5727 (1996).

5 Wiseman AJ, Jones RG and Went MJ, J Organomet Chem

544 :129 (1997).6 Dyson PJ, Hulkes AG and Suman P, J Chem Soc, Chem

Commun, 2223 (1996).7 Jones RG, Benüeld RE, Evans PJ, Holder SJ and Locke JAM,

J Organomet Chem 521 :171 (1996).8 Fossum E and Matyjaszewski K, Macromolecules, 28 :1618

(1995).9 Jones RG, Budnick U, Holder SJ and Wong WKC, Macro-

molecules, 29 :8036 (1996).10 Ratel P and Pannell KH, Abstr Am Chem Soc, 205 :INOR35

(1993).11 Jedlinski ZJ, Kurcok P and Nozirow F, Macromol Rapid

Commun, 18 :483 (1997).

158 Polym Int 48 :157–158 (1999)