Polyacetylene is a best known conductivity polymer, whose doped form exhibited metallic conductivity. Its poor intractability and instability significantly limited the scope of its applications. Synthesis of substituted polyacetylene has attracted much interest among polymer scientists because polyacetylene with suitable substituent not only will have better processibility and stability but also may possess novel properties that are not found in its parent form. Introduction of mesogen pendants into the polyacetylene structure will further advance polyacetylene science and open up new avenues in developing polyacetylene-based specialty materials....[ Read more ]

Polyacetylene is a best known conductivity polymer, whose doped form exhibited metallic conductivity. Its poor intractability and instability significantly limited the scope of its applications. Synthesis of substituted polyacetylene has attracted much interest among polymer scientists because polyacetylene with suitable substituent not only will have better processibility and stability but also may possess novel properties that are not found in its parent form. Introduction of mesogen pendants into the polyacetylene structure will further advance polyacetylene science and open up new avenues in developing polyacetylene-based specialty materials.

New acetylene-based monomers were synthesized and polymerized. The polymerization behaviors and thermal properties of the polymers were influenced by many factors such as spacer length, tail length, types of mesogen, rigidity of the main chain and so forth.

First, the effect of spacer length was studied. We synthesized poly[n- {[(4'-heptoxybiphenyl-4-yl)carbonyl]oxy}-I-alkynes] with different spacer length. Results show that long spacer length favors better ordering of the mesogens and enhances the formation of typical focal-conic Smectic A texture. But it was not true in poly[n-({[({[{(4'-[(nonyl)oxy]biphenyl-4- l)carbonyl)oxy]hexyl}oxy}carbonyl)-1-alkynes] and PolYL[n-[({[(4'- [(hexyl)oxy]biphenyl-4-l)oxy]hexyl} oxy)carbonyl]-1-alkynes]. Polymers with long spacer length in these two systems not only forms ill-defined Smectic A texture but also displays crystal to crystal transition.

Second, orientation of ester functional group in 5-{ [(4'-heptoxybiphenyl- 4-yl)carbonyl]oxy}-1-pentyne (A3EO7) gave 5{[(4'-heptoxybiphenyl- 4-yl)oxy]carbonyl}-1-pentyne (PA3E'O7). Results show that under the same conditions, polymerization of A3E'O7 gave polymer in low yield and low molecular weight. Even both polymers form Smectic A mesophases, only PA3E'O7 displays typical focal-conic texture.

Third, the effect of tail length on the mesomorphic property of the polymer was investigated. It was found that ordering of the mesogens seems to improve for initial increase in tail length but reduce for further increase. The effect, however, is generally small compared with the increase in spacer length.

Poly {5-[((4'-undecyl)carbonyl)oxy)-biphenyl-4-lyl)oxy]-l-pentyne} (PA3OEll) and Poly{5-[(((4'-undecyl)caonyl)oxy)-biphenyl-4-lyl)carbonyl) oxy]-1-pentyne} (PA3EEll) are two polymers with different chemical bonds connecting the mesogens to the main chain. PA3EEll displayed typical focal-conic Smectic A texture. Even result from X-ray powder diffraction suggests the formation of Smectic A mesophase in PA3OE11, texture developed by the polymer is fan-like and the observed crystal size is very small. Low dipole moment along the mesogen may result in poor ordering between mesogens, therefore, formation of typical focal-conic texture is forbiddened.

Forth, monomer with biphenyl benzoate (A3E6) was synthesized. Unlike monomers bearing bipheny rings as mesogens, it exhibits nematic mesophase. However, upon polymerization, it forms interdigitated Smectic A mesophase with weak side chain interactions.. Even the flexible spacer is three methylene groups, typical focal-conic texture was observed.

Last, substituted phenyacetylene with biphenyl ring as mesogen was prepared. "Classical" metathesis catalysts are effective to polymerize alkyne monomers but inactive for the phenylacetylene monomer. Rh-based catalysts such as [Rh(nbd)Cl]₂ initiated the polymerization of the non- mesomorphic monomer, producing polymer in high yield and high molecular weight. POM showed that the polymer was non-mesomorphic. DSC, GPC and NMR found that the polymer displayed cis-trans thermal isomerization and thermal degradation. The polymer is stable below 120℃ and no molecular weight and conformation change were observed. The isomerization and intramolecular cyclization took place above 150℃ and 1,3,5-triphenylbenzene derivative was obtained as a intramolecular cyclization product. No 1,2,4-triphenylbenzene derivative was observed by NMR.