Electron Accepting Oligomers and Polymers: Chain Scission Under Quaternization Conditions
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Electron Accepting Oligomers and Polymers: Chain Scission Under Quaternization Conditions Douglas S. Dudis and B. Robert McKellar Materials and Manufacturing Directorate, Polymer Core Technology Area Air Force Research Laboratory 2941 P Street AFRL/MLBP Wright-Patterson Air Force Base, Ohio 45433-7750, USA ABSTRACT The methylation of polybenzobisthiazole in dimethylsulfate produces an ndopable polymer which has previously been shown to have high electrical conductivity. The reaction conditions have been shown to chemically degrade the polymer chains, decreasing the molecular weight to about 20% of the original value. A methylationdemethylation scheme has been used to clarify the degree of chain scission. INTRODUCTION Poly(benzobisazoles) [1,2,3] are heterocyclic rigid-rod polymers which have been developed in large part for their high thermooxidative stability. Included in this class are the poly(benzobisthiazoles), poly(benzobisoxazoles) and poly(benzobisimidazoles). These polymers have been synthesized in high molecular weights, and have been processed into films and fibers with high molecular ordering (liquid crystallinity) affording very high tensile strength and high modulus. When film or fibers of poly(pphenyl)benzobisthiazole) (PBZT) are exposed to sulfuric or hydrochloric acid and contacted with metals or reduced electrochemically, electrical conductivities on the order of 24 S/cm have been measured [4]. This conducting state of PBZT has been attributed to reduction (either by a metal or electrical potential) of the protonated form of the polymer. This may be considered as an n-doped conducting polymer, since the conducting state of the polymer is reduced relative to the non-conducting state. Similar to protonation, quaternization (alkylation) of the nitrogen of the benzothiazole moiety of PBZT may be expected to produce an n-dopable polymer with a positively charged polymer backbone isoelectronic with the protonated form of the polymer. We have recently reported [5,6] that high molecular weight (intrinsic viscosities = 13 dL/g) polyphenylene(benzobisthiazole) (PBZT) will dissolve in hot dimethyl sulfate (170° C, 6d). Free-standing films were cast from dimethylsulfate reaction mixtures and D.C. conductivity measurements [7] on order of 1 S/cm while were made on these undoped films using the standard 4 probe method. The dissolution of PBZT in dimethyl sulfate is interesting since previously, PBZT had been known only to dissolve in strong mineral acids, such as sulfuric or methanesulfonic acid, or in anhydrous nitrobenzene or nitromethane in the presence of a Lewis acid (aluminum chloride) [8]. The dissolution of PBZT in dimethyl sulfate is presumed to be due in part to methylation (quaternization) of the thiazolium nitrogens by the solvent. The proposed methylated poly-quaternary salt structure (as shown in figure 2) is supported by characterization (IR, NMR, elemental analysis) of model compounds [5,6] synthesized in good yield under similar conditions as used in the polymer reaction. C8.1.1
Quaternization
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