The ability of chemists to design and synthesize :m-conjugated organic polymers with precise control remains the key to technological breakthroughs for using polymer materials in electronic and photonic devices. In this communication, the controlled chain-growth polymerization of regioregular poly (3-hexylthiophene) (P3HT) from an external initiator using 1,3-bis(diphenylphosphino)propane (dppp) as a catalyst ligand is reported. The complexes cis-chloro(phenyl)(dppp)nickel(II) and cis-chloro(o-tolyl)(dppp)nickel(II) were synthesized and characterized by P-31 NMR spectroscopy. These complexes served as initiators in the polymerization of 2-bromo-5chloromagnesio-3-hexylthiophene in THF at room temperature, affording fully regioregular P3HT with controlled molecular weights and narrow molecular weight distributions, as demonstrated by gel-permeation chromatography and H-1 NMR spectroscopy. MALDI-TOF mass spectrometry revealed that the polymers had almost complete incorporation of the initiating aryl group, and when the aryl group was o-tolyl, only Tol/H end groups were observed. Although external initiators have been used previously with a PPh3 ligand, that methodology led to polymers with broad molecular weight distributions. This is the first example in which complete control over the externally initiated P3HT polymerization has been achieved.