EFFECT OF MOLECULAR-WEIGHT ON SPECTROSCOPIC AND SPECTROELECTROCHEMICAL PROPERTIES OF REGIOREGULAR POLY(3-HEXYLTHIOPHENE)

Using experimentally established sequence of extractions, we were able to fractionate regioregular poly(3-hexylthiophene) (R-P3HT) into four fractions differing significantly in their molecular weight (M-n) and exhibiting low polydispersity coefficients. H-1 NMR analysis of low m olecular fractions enabled us to propose the dominant chain terminatio n mechanisms and identify the sources of regioregularity defects. In p articular, it turned out that the presence of small amounts of undesir ed isomer of 2-bromo-3-hexylthiophene, namely, 2-bromo-4-hexylthiophen e, strongly influences the molecular weight and to a much lesser exten t the regioregularity of the polymer obtained via Grignard type polyco ndensation. In the reaction with the growing chain, 2-bromo-4-hexylthi ophene either causes its termination, lowering in this manner the mole cular weight of the resulting polymer, or introduces regioregularity d efects of TT-HH (TT = tail to tail; HH = head to head) type. The avera ge conjugation length in R-P3HT increases with the increase of M-n as manifested by a bathochromic shift of the lambda(max) and the appearan ce of the vibrational structure in the W-vis-near-IR spectra of the fr actions of higher molecular weight. This conclusion is supported by FT IR data. The onset of the oxidative doping of R-P3HT shifts to lower p otentials with the increase of the molecular weight as evidenced by cy clic voltammetry and UV-vis-near-IR spectroelectrochemistry.