Experimental Evidence for the Quasi-“Living” Nature of the GrignardMetathesis Method for the Synthesis of RegioregularPoly(3-alkylthiophenes)Mihaela Corina Iovu, Elena E. Sheina, Roberto R. Gil, andRichard D. McCullough*Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213Received May 31, 2005; Revised Manuscript Received August 15, 2005ABSTRACT: The Grignard metathesis (GRIM) polymerization of 3-alkylthiophenes proceeds by a quasi-“living” chain growth mechanism, not by a step growth process. Kinetic studies of the Grignard metathesispolymeriza tion of 2,5-dibrom o-3-alkylth iophenes showed th at the molecular weight o f poly(3-alkyl-thiophenes) is a function of the molar ratio of the monomer to nickel initiator, and conducting polymerswith predetermined molecular weights and relatively narrow molecular weight distributions (PDIs )1.2-1.5) can be made. Sequential monomer addition resulted in new block copolymers containing differentpoly(3-alkylthiophene) segments.IntroductionConventional design of advanced organic materialsthat display a variety of desirable properties in acontrollable way continues to be one of the greatchallenges of the contemporary polymer research. Oneclass of these advanced materials is organic conductingpolymers. Since the initial discovery of these polymersin the late 1970s, various applications of these materialshave been explored due to their exceptional electronicand photonic properties.1-3The straightforward synthesis of polythiophene de-rivatives (PT) generates soluble and processable poly-mers with a wide range of practical and potentialapplications, including rechargeable batteries,1electro-chromic devices,1chemical and optical sensors,1light-emitting diodes,4-6and field-effect transistors.7Whiletraditional approaches to synthesizing PTs derivativesvia electrochemical or oxidative chemical polymerizationmethods yield polymers with various degrees of regio-regularity,3the regioselective synthesis of poly(3-alkyl-thiophenes) (PATs) results in almost exclusively head-to-tail (HT) couplings.The synthesis of regioregular PATs, first discoveredby McCullough et al.8,9and shortly followed by Rieke,10results in the formation of defect-free, structurallyhomogeneous HT-PATs that have greatly improvedelectronic and photonic properties compared to regio-random analogues,11,12including significantly enhancedconductivity. One drawback with the McCullough andRiecke methods lies in their use of cryogenic tempera-tures.8The discovery of the Grignard metathesis (GRIM)method allows the polymerization to occur at roomtemperature or at reflux, hence leading to a quick andcost-effective technique for the large-scale synthesis ofhigh molecular weight, regioregular PATs.13,14All of the aforementioned polymerizations are transi-tion-metal-catalyzed cross-coupling reactions.15The gen-erally acce pted mechanism for these Ni(II) catalyzedcross-coupling reactions involves a catalytic cycle ofthree consecutive steps: oxidative addition, transmeta-lation, and reductive elimination.16-26The course of thecatalytic reaction has been extensively studied andproven to be affected by both the ligand structure andthe choice of the metal.15It has been recently reportedthat the nickel-initiated cross-coupling polymerizationproceeds via a chain-growth mechanism.27,28Further-more, the addition of various Grignard reagents (R′MgX)at the end of polymerization results in the end-cappingof regioregular PATs with an R′ end group.29Here we show that the Grignard metathesis polym-erization generates regioregular poly(3-alkylthiophenes)with precise molecular weights and very narrow poly-dispersities. Furthermore, by the sequential addition ofdifferent 3-alkylthiophene monomers, this method leadsto the synthesis of block copolymers. The near “living”nature of the polymerization also presented and allowsfor the development of new architectures and the abilityto create regioregular poly(3-alkylthiophenes) with spe-cific functionalities.Experimental PartMaterials. Synthesis of 2,5-dibromo-3-hexylthiophene and2,5-dibromo-3-dodecylthiophene were performed according tothe literature.13,14Tetrahydrofuran (THF) was dried overK/benzophenone under nitrogen and freshly distilled prior touse. [1,3-Bis(diphenylphosphino)propane]dichloronickel(II) (98%)(Ni(dppp)Cl2), tert-butylmagnesium chloride (2 M in diethylether) and p-dimethoxybenzene (98%) were purchased fromAldrich Chemical Co., Inc., and used without further purifica-tion.Polymerization Kinetic Experiments. In a typical ex-periment, a dry 100 mL three-neck round-bottom flask wasflushed with N2and charged with 2,5-dibromo-3-hexylt hio-phene (1.6 g, 5 mmol), p-dimethoxybenzene (internal standard)(0.2 g), and anhydrous THF (50 mL). A 2 M solution of tert-butylmagnesium chloride (2.5 mL, 5 mmol) in diethyl ether(Et2O) was added via a deoxygenated syringe, and the reactionmixture was gently refluxed for 2 h. At this time an aliquot(0.5 mL) was taken out and quenched with water. The organicphase was extracted in Et2O analyzed by GC-MS to determine* Corresponding author. E-mail: [email protected] 2005, 38, 8649-865610.1021/ma051122k CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 09/24/2005the composition of the reaction mixture. The main componentsof the reaction mixture were 2-bromo-5-chloromagnesium-3-hexylthiophene and 5-bromo-2-chloromagnesium-3-hexylthio-phene regioisomers. Usually less than 5% of unreacted 2,5-dibromo-3-hexylthiophene was detected by GC-MS analysis.The concentration of 2-bromo-5-chloromagnesium-3-hexylthio-phene isomer was considered as the initial monomer concen-tration. The oil bath was then removed, and the reactionmixture was allowed to cool to 23-25 °C, at which timeNi(dppp)Cl2(0.04 g, 0.075 mmol) was added as a suspensionin 1 mL of anhydrous THF. After addition of Ni(dppp)Cl2,aliquots (1 mL) were taken at different time intervals, andeach was precipitated in methanol (5 mL). For each aliquot asample was prepared in Et2O (2 mL) and analyzed by GC-MSfor the determination of concentration of unreacted monomer.After filtration through PTFE filters (0.45 µm), the molecularweight of the pristine polymer samples was measured by GPC.Chain Extensi on Experiment for th e Synthesis ofPoly(3-hexylthiophene)-b-poly(3-dodecylthiophene). Adry 250 mL three-neck round-bottom flask (A) was chargedwith 2,5-dibromo-3-hexylthiophene (1.6 g, 5 mmol), p-dimethoxy-benzene