Oxireductases in the Enzymatic Synthesis of Water-Soluble Conducting Polymers - Enzymatic Polymerisation - page 3

Scheme 1 Catalytic cycle of peroxidase. L ligand, R reduced compound. (Reprinted with
permission from Gross et al. [ 9 ]. c 2001, American Chemical Society)

out of the reaction solution, which limits the applicability of this method [ 11 ] .
Although several strategies have been used to avoid this precipitation, such as mod-
ification of monomer structures [ 12 , 13 ] , synthesis in micellar media [ 14 ], reverse
micelles [ 15 , 16 ], and interfacial polymerizations [ 17 ] , the final conductivity of the
obtained PANI is poor in most cases. This poor solubility is said to be due to the
presence of different structures in the polymer, giving a highly branched instead of
a predominantly head-to-tail linear structure, as is formed in the traditional chem-
ical polymerization of aniline. Liu and coworkers [ 18 , 19 ] successfully improved
the conductivity of enzymatically synthesized PANI by using a polymer electrolyte
such as poly(sodium p -styrenesulfonate) (PSSNa) in the reaction media. This water-
soluble polyelectrolyte serves as a linear template. Since PSSNa and aniline have a
pKa of 0.70 and 4.63, respectively, at pH close to 4, PSSNa is anionically charged
while aniline is cationically charged. Therefore, aniline and PSSNa form an electro-
static complex, which leads to a pre-orientation of the aniline monomers and favors
a para -directed reaction. This results in a more linear, highly conjugated structure.
Consequently, the PANI conductivity is improved by this method. The same effect
is obtained with dodecylbenzene sulfonic acid (DBSA), hexadecyltrimethylammo-
nium bromide (HDTMAB) and polyoxyethylene isooctylphenyl ether (PEOPE) as
templates. These molecules aggregate to form micelles above a critical concentra-
tion (critical micellar concentration, CMC), creating another template configuration
for a guided polymerization. However, the use of small molecules such as sodium
benzene sulfonic acid (SBS), without the ability to form these templates, does not
result in a highly conducting PANI by enzymatic synthesis. Therefore, it seems
clear that the pre-orientation of anionic species (via aggregation of small molecules
or polymeric electrolytes) is essential to create an adequate environment by (1)
providing the necessary counteranions for the doping of the conducting polymer,