Oligomers, acetylene terminated polymerization

A family of acetylene-terminated phenyl quinoxalines have been synthesized by the Polymer Branch of the Materials Laboratory. ( 1) These phenyl quinoxalines are remarkable for their thermooxidative stability and resistance to moisture. These materials have potential for structural applications as adhesives or composite matrix resins.(2) The feature of moisture resistance makes the materials especially attractive for bonding aluminum. However, problems arise from the fact that aircraft aluminum alloys (and their surface oxiges) are altered by exposures to temperatures above 177 C (350 F) and this is much lower than the polymerization temperatures of the acetylene-terminated oligomers. 

The polyimide matrix resin employed in all composites was THERMID 600, a thermosetting resin polymerized from acetylene-terminated oligomers. Its chemical structure is shown below (9). 

A substantial effort in this laboratory has been directed toward the synthesis and characterization of acetylene terminated oligomers for use as addition-curable, moisture-resistant, thermoset systems. Early work on the acetylene-terminated phenylquinoxalines demonstrated the moisture insensitivity of the product generated from the thermal cure. Studies of various difunctional acetylene-terminated monomers has demonstrated that the polymerization is a free radical propogation of the acetylene moiety to a linear conjugated polyene. 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

Fig. 10.3 Reversed-phase high-performance liquid chromatography (HPLC) of Me3C(CH=CH) jCMe3 prepared by the polymerization of acetylene (5 equivalents) in toluene, initiated by W(=CHCMe3)(=NAr)(OCMe3)2 (1 equivalent) in the presence of quinuclidine (5 equivalents) and terminated by Me3CCHO. The peak marked 3t3 is for the oligomer with jc = 3 and 3 trans double bonds likewise for the other marked peaks. The unmarked peaks refer to oligomers containing at least one cis double bond. The peak marked with an asterisk is the internal standard (guaiazulene) (Schlund 1989).

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