Effect of Molecular Weight on Spectroscopic and Spectroelectrochemical Properties of Regioregular Poly(3-hexylthiophene)

• Published in: Macromolecules Vol. 31; no. 15; pp. 5051 - 5058
• Main Authors: Trznadel, Miroslaw, Pron, Adam, Zagorska, Malgorzata, Chrzaszcz, Ryszard, Pielichowski, Jan
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 28.07.1998
• Subjects: Applied sciences; Electrical, magnetic and optical properties; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Catalytic reaction; Solution polycondensation; Head to tail polymer; Termination reaction; Experimental study; Molecular weight property relation; Conducting polymers; Electrochemical properties; Grignard reaction; Thiophene derivative polymer; Optical properties; Reaction mechanism; Optical absorption
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma970627a

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. 1H NMR analysis of low molecular fractions enabled us to propose the dominant chain termination mechanisms and identify the sources of regioregularity defects. In particular, it turned out that the presence of small amounts of undesired isomer of 2-bromo-3-hexylthiophene, namely, 2-bromo-4-hexylthiophene, strongly influences the molecular weight and to a much lesser extent the regioregularity of the polymer obtained via Grignard type polycondensation. In the reaction with the growing chain, 2-bromo-4-hexylthiophene either causes its termination, lowering in this manner the molecular weight of the resulting polymer, or introduces regioregularity defects of TT−HH (TT = tail to tail; HH = head to head) type. The average conjugation length in R-P3HT increases with the increase of M n as manifested by a bathochromic shift of the λmax and the appearance of the vibrational structure in the UV−vis−near-IR spectra of the fractions of higher molecular weight. This conclusion is supported by FTIR data. The onset of the oxidative doping of R-P3HT shifts to lower potentials with the increase of the molecular weight as evidenced by cyclic voltammetry and UV−vis−near-IR spectroelectrochemistry.