Synthesis and characterization of phenylene-thiophene all-conjugated diblock copolymers

Grignard metathesis (GRIM) polymerization for all-conjugated diblock copolymers comprising poly(2,5-dihexyloxy-1,4-phenylene) (PPP) and poly(3-hexylthiophene) (P3HT) blocks were systematically studied with LiCl as additive and 1,2-bis (diphenylphosphino) ethane nickel dichloride (Ni(dppe)Cl₂) or 1,3-bis(diphenylphosphino) propane nickel dichloride (Ni(dppp)Cl₂) as catalyst. It was found that the addition order of the monomers was crucial for the success of copolymerization. With the monomer addition in the order of phenyl and then thienyl Grignard reagents, all-conjugated PPP-b-P3HT diblock copolymers with different block ratios were successfully synthesized. In contrast, the inverted addition order only afforded a mixture containing both block copolymers and deactivated or end-capped homopolymers. Mass spectroscopic analysis indicates that the effect of the addition order of the monomers on copolymerization is attributed to the low efficiency of intramolecular Ni transfer from thiophene to phenylene units. The resulting PPP-b-P3HT diblock copolymers were characterized by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). It was found that both PPP and P3HT blocks in the copolymers were crystalline, and microphase separation between them took place, as indicated by two endothermal transitions corresponding to the melting of PPP and P3HT blocks, respectively. These unique properties may render PPP-b-P3HT diblock copolymers potential applications in optoelectronics.