The first chemical synthesis of regioregular poly(3-alkylthiophenes) (P3AT), consisting predominantly of HT couplings, was reported by McCullough and coworkers [1]. In 1999, McCullough discovered a simple and convenient route for the synthesis of highly regioregular P3ATs at room temperatures by the use of Grignard metathesis (GRIM) to overcome the cryogenic temperatures adopted in his former method, which made it not so ideal for large scale synthesis [2]. Early attempts at the synthesis of P3ATs by GRIM produced however polymers with large PDIs and uncontrolled molecular weights. In 2004, Yokozawa showed that it is possible to synthesize rr-P3HT by GRIM with a controlled molecular weight and narrow PDI (about 1.15) [3]. The Kiriy group was the first to explore in 2007 the synthesis of rr-P3HT through a novel external initiation polymerization by the preparation of a suitable nickel initiator, Ar-Ni(PPh3)2-Br [4]. The Luscombe group further extended the work on external initiation polymerization of P3ATs, obtaining fully regioregular P3HTs with PDIs of 1.1-1.2, by using Ph-Ni(dppp)-Cl and o-tolyl-Ni(dppp)-Cl as initiators [5]. In this communication we perform a Raman spectroscopic study of some rr-P3ATs with narrow PDI and controlled molecular weights, obtained through external initiation polymerization, and compare the results with those collected for regiorandom and regioregular P3HTs obtained through other synthetic procedures, to derive structural information about the degree of molecular order in the different types of polymers. We use different laser excitation wavelengths (resonant, nonresonant and preresonant conditions), with particular attention to the C=C stretching spectral region. We also analyze the Raman spectra of the polymers after thermal annealing.
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