Conducting Polymers with Chemically Sensitive Traps And Barriers: New Molecule-Based Sensors
- PDF / 241,057 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 68 Downloads / 180 Views
X = 0 or -0H2 -0planar
z = 1, 2
twisted
Scheme 1
263 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
The synthesis of the monomers will be reported elsewhere. 5 Polythiophenes have been synthesized by both electrochemical and chemical oxidation as well as through chemical coupling reactions. 1 We chose to produce polymers by chemical coupling reactions (Scheme 2 and 3) since these methods are better suited for producing materials which can be characterized by standard solution chemical methods. In our initial studies we were not concerned with production of high molecular weight polymers since it is well know that the electronic structure of polythiophenes is determined at low degrees of polymerization. In the case of polymer 3 we introduced a new polymerization method whereby the dilithiated monomer 2 is formed and then reacted with Fe(acac)3 to produce the homopolymer 3. In this Fe(III) polymerization, insoluble Fe(acac)2 is separated by filtration to produce materials free of the iron salts which typically plague FeC13 oxidative polymerizations. Additionally, 1H NMR showed no evidence of the deleterious ax-P coupling which may be observed in oxidative polymerizations of thiophenes.1 After precipitating 3 in methanol, the degree of polymerization of the soluble (low molecular weight) portion was determined to be 10 thiophene units long by 1H NMR integration of the two doublets resulting from the disubstituted thiophene end groups. We have also made use of Stille coupling methodology in the synthesis of copolymers. 6 In this case, the dilithio species of 2 and 7 are reacted in situ with trimethyltin chloride and then subjected to palladium catalyzed cross-coupling with 5,5'-dibromo-2,2'-bithiophene. The molecular weights produced in this procedure were most likely solubility limited since these copolymers precipitated during the course of the reaction. The molecular weights as determined by GPC relative to polystyrene standards were determined to be 2800, 3700, 2000, and 1900 for polymers 4, 5, 8, and 9 respectively. However, the absolute molecular weights may be higher since the absolute molecular weight determined by NMR end group analysis for 3 shows the true molecular weight to be almost double that determined by GPC (1800 vs. 960).
00 So
x
S
kO X=Li 2 00
a
i~ /~
b,c
2eq. BuLi[
S
x
S
O0Z
63
n
£o 0] S
e2 X=Li
4 n=l 5 n=2 Scheme 2
264
/1
S
z=1,2 0
x S 2eq. BuLi
0
S X 6 X=Br -77X=Li
8 n=l
9 n=2
1/
n n
a. Fe(acac) 3, THF, reflux. b. Me 3SnCI, THF. c. 5,5'-dibromo-2,2'-bithiophene, PdCI 2(AsPh 3)2, THF, reflux. Scheme 3 The sensitivity of effective conjugation length to different side chains is apparent from comparison of the Xmax of the pure polymers (Table 1). Polymers 3, 4, and 5 show a higher kmax than polymers 8 and 9 indicating that the conformational restrictions are greater when the methylene spacer is incorporated between the thiophene ring and the polyether tether. Comparison of the Xmax of 4 with that of its homologue 3, suggests that there is som
Data Loading...
