Catalytic hydrogenation of heteroaromatic polymers
Amine containing polymers are regarded as some of the most versatile and useful polymers. Preparation of amine containing polymers is usually not straightforward due to the high reactivity of amino functionality. A feasible route to synthesize a simple amine polymer through hydrogenation of pyridine rings was demonstrated. Heterogeneous catalytic hydrogenation of poly(vinylpyridine)s was carried out using various platinum metal catalysts to obtain saturated polymers, poly(vinylpiperidine)s. The hydrogenated polymers retain their well defined structures inherited from the precursor polymers, while they obtain unique characters such as water solubility, high basicity, and improved thermal stability which the precursor polymers do not possess. The stereoregular polymer, isotactic poly(2-vinylpiperidine) exhibits some helical conformation order in the solid state and in the solution state.
Heterogeneous hydrogenation of block copolymers containing vinylpyridine could yield polymers with fully hydrogenated for both blocks or selectively hydrogenated for one block. When the block copolymer has an olefinic unsaturated structure, hydrogenation reaction occurred in both olefinic and pyridine blocks. Hydrolysis and hydrogenation on poly(t-butylacrylate-b-2-vinylpyridine) gave us two kinds of polymers, poly(acrylic acid-b-2-vinylpyridine) and poly(acrylic acid-b-2-vinylpiperidine). The former was an amphiphilic polymer and the latter was a water soluble, amphoteric polymer. Poly(styrene-b-2-vinylpyridine) was hydrogenated selectively by tweaking of the reaction conditions. In stoichiometrically acidic condition, pyridine block was exclusively hydrogenated over styrene block. When hydrogenation was carried out in the selective solvent with strong acidic condition, the styrene block was selectively reduced. The morphology of well defined poly(styrene-b-2-vinylpiperidine) was investigated using TEM, STXM, and RSXS which shows a unique phase separation where piperidine blocks aggregate in sphere forms dispersed in polystyrene matrix. This morphology and thermal behavior of the hydrogenated block copolymer were found to be similar to those of ionomeric block copolymers.
Various vinyl polymers and diallyl amino cyclopolymers having heteroaromatic rings were also synthesized and hydrogenated. Poly(2-vinylpyrazine) was readily hydrogenated to yield poly(2-vinylpiperazine). Poly(2-vinylpyrimidine), poly(2-amino-5-vinylpyrimidine), poly(N,N-diallylamino-2-pyridine), and poly(N,N-diallylamino-2-pyrimidine) were partially hydrogenated to yield tetrahydro compounds. The resulting guanidine or amidine structures endowed these polymers with highly basic characters in aqueous solution. Tetrahydro poly(N,N-diallylamino-2-pyridine) was then hydrolyzed in the existence of water to generate poly(N,N -diallylamine) and δ-valerolactam.