The role of oligomers in the synthesis of polysilanes by the Wurtz reductive coupling reaction Daniel Bratton, Simon J. Holder, Richard G. Jones *, William K.C. Wong Centre for Materials Research, School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NR, UK Received 14 January 2003; accepted 6 March 2003 Abstract Size exclusion chromatography and 29 Si-NMR spectroscopy have been used to investigate the evolution of low-molecular weight molecules during the synthesis of poly(methylphenylsilane) by alkali metal-mediated reductive coupling in diethyl ether, tetrahydrofuran and toluene, revealing clear evidence for the important role played by a dimer. In addition, the distinct origins of the stable cyclic oligomers that are ubiquitous side products of these reactions have been established, cyclopentamers being formed through endbiting during chain growth and cyclohexamers by a degradative backbiting reaction. # 2003 Elsevier B.V. All rights reserved. Keywords: Polysilanes; Oligosilanes; Wurtz reductive coupling; Backbiting; Endbiting 1. Introduction After the first tractable polysilanes were synthesised in the late 1970s and early 1980s [1  /4] by the alkali metal- mediated coupling of diorganosilanes (see Scheme 1), there was a significant drive to understand the origins of the polymodal, usually trimodal, molecular weight distributions that invariably resulted. They were origin- ally thought to arise from non-interacting polymerisa- tion mechanisms but as evidence accumulated it was increasingly clear that this is not the case. A number of researchers have made significant contributions to the development of what is now an acceptable understand- ing validated by a mathematical model [5] (vide infra) but for a full historical description the reader is referred to a recent review [6]. A typical product molecular weight distribution from a Wurtz-type synthesis of poly(methylphenylsilane) (PMPS) as usually carried out in boiling toluene or xylene consists of three fractions: (i) an oligomeric fraction corresponding mainly to cyclic pentamers and hexamers, (ii) a broad and dominant fraction of inter- mediate molecular weight corresponding to a degree of polymerisation of about 35  /40 and (iii) a lesser amount of a narrow high-molecular weight fraction correspond- ing to a degree of polymerisation in the region of 50,000. The first of these is well understood to arise from endbiting and backbiting reactions that arise during the course of the polymerisation. However, until recently, why there should be two distinct polymeric fractions was an enigma. It is now understood that the polymer grows from the alkali metal surface in a random-coil conformation consisting of a mixture of irregular P- and M-helical segments, each separated from the next by reversal of the helix. These reversals diffuse rapidly within the chain, occasionally reaching the alkali metal surface, at which juncture the chain end can backbite with the elimination of a cyclic oligomer and concomi- tant disengagement of the growing chain from the surface. This is a polymerisation termination process, which is most probable when there is only one helix reversal present in the growing chain, and it is respon- sible for the formation of the intermediate-molecular weight fraction. Those chains that survive this possibi- lity continue to grow and as their length increases, so does the number of helix reversals. The probability of adjacent reversals colliding and undergoing an inevita- ble mutual annihilation increases correspondingly. This increases the range of diffusion of the remaining * Corresponding author. Tel.:  /44-1227-823321; fax:  /44-1227- 827558. E-mail address: r.g.jones@ukc.ac.uk (R.G. Jones). Journal of Organometallic Chemistry 685 (2003) 60  /64 www.elsevier.com/locate/jorganchem 0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-328X(03)00641-7