Despite their high conductivity and good solubility, regioregular poly(3-alkylthiophenes) do not posses optimal mechanical and processing properties. This issue could be addressed by integrating poly(3-alkylthiophene) in copolymer structures with various polymer blocks that display desirable mechanical properties, leading to a variety ofpolymeric materials with improved properties (e.g. flexible plastics, rigid plastics, elastomers).

The synthesis of diblock copolymers containing head-to-tail-coupled poly(3-alkylthiophenes) (HT-PATs) can be accomplished by executing post polymerization reactions on the end groups of the polymer.  The target molecule contains a functional group that intiates atom transfer radical polymerization (ATRP).  ATRP is a “living” free-radical polymerization method developed by Matyjaszewski and co-workers to produce a wide variety of conventional polymers¹. Two methods of synthesizing diblock copolymers containg regioregular poly(3-alkylthiophene) will be presented.^2,3

The first method of synthesis of diblock copolymer described in 2002 is based upon the formation of a HT-PAT with H/Br end groups2 with subsequent end group functionalization reactions⁴ (Figure 1).  Since PATs are chemically stable polymers, the ability to modify the end groups on one side of the polymer is similar to polymer- or bead-supported organic synthesis.  Between each reaction, the polymer is isolated, purified by simple precipitation and filtration and characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and NMR. 

Figure 1. Synthesis of diblock copolymers.

The HT-PHT (2) is functionalized by reaction with thiophene derivative (5) and [Ni(dppp)Cl2] (dppp=propane-1,3-diylbis(diphenylphosphane)) to yield polymer 6.  Deprotection of 6 leads to HT-PHT with one hydroxyl end group (7).  The hydroxy-terminated PHT (7) was further modified by reaction with 2-bromopropionyl bromide to generate an atom transfer radical polymerization (ATRP) macroinitiator (8). The following diblock copolymers has been synthesized:  polyhexylthiophene-b- polystyrene (PHT-PS; 9a), and polyhexylthiophene-b- polymethylacrylate (PHT-PMA; 9b) by ATRP using 8 as the initiator, styrene or methyl acrylate as the monomer, and CuBr/N,N,N’,N’,N” -pentamethyldiethylenetriamine (PMDTA) as the catalyst.

Recently our group has developed another method to produce diblock copolymers containing regioregular poly(3-alkylthiophene).³  This method is similar to the above procedure by using post polymerization reactions to create a macroinitiator for ATRP, however the new method involves fewer synthetic steps.  A vinyl terminated PHT was used as the diblock copolymer precursor (Figure 2).

The first synthetic step involves the formation of a vinyl terminated PHT via in situ chain end functionalization3, a method developed in our group.  The vinyl end group was easily converted to hydroxyethyl end group, that reacts with 2-bromopropionyl bromide to give a bromoester terminated PHT. The latter was used as macroinitiator for ATRP of acrylates. 

Figure 2. Synthesis of poly(3-hexylthiophene-b-polyacrylates

References

1. a) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1997, 119, 5614; b) Patten, T. E.; Xia, J.; Abernathy, T.; Matyjaszewski, K. Science 1996,272, 866; c) Matyjaszewski, K.; Patten, T. E.; Xia, J.J. Am. Chem. Soc.1997, 119, 674; d) Matyjaszewski, K.; Gobelt, B.; Paik, H. –J.; Horwitz, C. P. Macromolecules 2001, 34, 430.
2. Liu, J.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Agew. Chem. Int. Ed. 2002, 41,
3. Iovu, M. C.; Jeffries-El, M.; Sheina, E. E.; Cooper, J. R.; McCullough, R. D. Polymer, 2005, 46, 8582.
4. Liu, J.; McCullough, R. D. Macromolecules 2002, 35, 9882-9889