Published: September 13, 2011 r2011 American Chemical Society 7610 dx.doi.org/10.1021/ma201453s | Macromolecules 2011, 44, 7610–7616 ARTICLE pubs.acs.org/Macromolecules Discovery of an Anionic Polymerization Mechanism for High Molecular Weight PPV Derivatives via the Sulfinyl Precursor Route Inge Cosemans, † Lieve Hontis, † David Van Den Berghe, † Arne Palmaerts, † Jimmy Wouters, † Thomas J. Cleij, † Laurence Lutsen, ‡ Wouter Maes, † Thomas Junkers, † and Dirk J. M. Vanderzande* ,†,‡ † Institute for Materials Research (IMO), Hasselt University, Universitaire Campus, Building D, B-3590 Diepenbeek, Belgium ‡ Division IMOMEC, IMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium ’ INTRODUCTION During the past decades, conjugated polymers have gained a lot of interest because of their excellent characteristics for applications in electronic devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and generally organic photovoltaics (OPVs). 1 Poly(p-pheny- lenevinylene) (PPV) and its derivatives in particular are con- jugated polymers with interesting electrical and optoelectronic properties. To introduce these materials in all kinds of devices, it is of utmost importance that the solubility and processability of these polymers are guaranteed. 2 PPVs with suitable physical properties are accessible via so-called precursor approaches based on in situ formation of the active monomer that yields nonconjugated polymers that can, however, easily be trans- formed into the final PPV via elimination reactions. There are many known precursor routes toward PPV materials, notably the Gilch, 3 Wessling, 4,5 xanthate, 6 sulfinyl, 7 and the dithiocarba- mate 8,9 route. All these routes have in common that they proceed via the in situ formation of a p-quinodimethane system, which is formed through a base-induced elimination reaction (Scheme 1). For all these routes, except for the xanthate route, a self-initiating radical mechanism has been proposed. 10À16 The sulfinyl route should be distinguished from all other precursor routes since it starts from a nonsymmetrical monomer, which allows for highly efficient formation of the p-quinodimethane system which was confirmed by in situ UVÀvis experiments and theoretical calculations. 17 At the same time the sulfinyl functional group prevents the formation of head-to-head and tail-to-tail additions. In this way, polymers with a low intrinsic chemical defect level can hence be synthesized. 18 Some years ago the sulfinyl precursor route was studied in our group in N-methylpyrrolidone (NMP) as the solvent and sodium tert-butoxide (NatBuO) was applied as the required base. Thereby bimodal polymer product distribu- tions were observed, and indication was given that a competition between a radical (resulting in high molecular weight polymer) and an anionic (resulting in low molecular weight material) poly- merization mechanism was responsible for the bimodality. 19À22 In this paper, we now report on a polymerization of a mono- substituted p-quinodimethane system that purely proceeds via an anionic polymerization mechanism, which is achieved via careful selection of reaction conditions and type of base employed to form the monomer. ’ EXPERIMENTAL SECTION Materials. All solvents and reagents were purchased from Acros or Aldrich and were used without further purification. Tetrahydrofuran (THF) was dried by distillation from Na/benzophenone. Analytical size exclusion chromatography (SEC) was performed using a Spectra Series P100 (Spectra Physics) pump equipped with two mixed-B columns (10 μm, 2 cm  30 cm, Polymer Laboratories) and a refractive index detector (Shodex) at 70 °C. THF was used as the eluent at a flow rate of 1.0 mL/min. Molecular weight distributions were determined relative to polystyrene standards. Synthesis of Monomer 4 (1-(Chloromethyl)-4[(octylsul- finyl)methyl]benzene). Monomer 4 was synthesized according to a known procedure. 23À25 In the final step a few drops of concentrated Received: June 26, 2011 Revised: August 21, 2011 ABSTRACT: The polymerization of PPV via the sulfinyl precursor route has been investigated with respect to its mechanism. When polymerized in sec-butanol, a purely radical polymerization mechanism is observed as in most precursor polymerization routes. Accordingly, an increase in the reaction temperature induced an increase in the overall yield alongside with a reduction of the average molecular weight of the polymer. Upon changing the monomer concentration in solution before addition of the base NatBuO, an increase in molecular weight is observed, signifying that the polymerization is faster than the mixing of the two reaction components. When changing the solvent to NMP, a competition of anionic and radical polymerization has been established while in THF an anionic polymerization mechanism occurs exclusively. To prevent termination reactions, LDA and LHMDS were introduced as base whereby LHMDS shows less propensity to initiate anionic chain growth due to higher steric hindrance. With polymerizations in presence of the radical quencher TEMPO, the anionic polymerization mechanism could unambiguously be proven.