Although water promotes Suzuki-Miyaura coupling reaction, it also induces side reactions such as deboronation and dehalogenation. Therefore, Suzuki-Miyaura polymerization of triolborate halothiophene monomer 1 with (t) Bu3 PPd(o-tolyl)Br (2) in dry tetrahydrofuran (THF) is investigated. However, the resultant poly(3-hexylthiophene) (P3HT) shows a broad molecular weight distribution and uncontrolled polymer ends. Model reactions of a number of boron reagents 3 with 2,5-dibromothiophene (4) in the presence or absence of water indicate that intramolecular transfer of the catalyst is hardly affected by the boron moiety of 3, whereas it is hindered in the absence of water. Indeed, polymerization of 1 with 2 in H2 O/THF affords P3HT with a narrower molecular weight distribution and controlled tolyl/H ends, as compared to the reaction in dry THF.