| Abstract: | Radical polymerization of 4-vinylpyridine (4-VP), 2-vinylpyridine (2-VP), and 2-methyl-5-vinylpyridine (MVP) was studied in concentrated DMF solutions of ZnCl2, ZnBr2, ZnI2, Zn(CH3COO)2, and Cd(CH2COO)2 at 50°C. Polymerization of 4-VP and MVP was accelerated by the addition of the metal salts, while the polymerization of 2-VP was greatly retarded. The sequence of the accelerating effect of metal salts for 4-VP was in the following order: Cd(CH3COO)2 > ZnCl2 > Zn(CH3COO)2 > ZnBr2 > ZnI2. This sequence is almost the same as that reported in a previous report for MVP. However, the order was reversed for the retarding effect on the polymerization of 2-VP. At the intermediate concentration of metal salts, polymerization of 4-VP proceeded heterogeneously, which was explained by considering crosslinking of poly-4-VP by the metal ion. Since a linear correlation between the rate Rp and the degree of polymerization was observed for the 4-VP–Zn(CH3COO)2 system, the accelerating effect was postulated to be due to the enhancement in kp. Results of copolymerization of VP with styrene as M2 in a concentrated solution of Zn(CH3COO)2 indicated the strong activation of 4-VP by complex formation (r1 = 2.7 ± 0.5, r2 = 0.08 ± 0.03), whereas the change in the monomer reactivity of MVP is smaller (r1 = 2.0 ± 0.2, r2 = 0.35 ± 0.05). The behavior of 2-VP was abnormal (r1 = 3.35 ± 0.3, r2 = 0.55 ± 0.15, then r1r2 > 1), which was attributed to the steric effect by complex formation. Solid complexes formed between pyridine, 4-VP, 2-VP, or MVP and zinc salts were prepared as samples for infrared spectroscopy. The shifts in infrared absorption bands of these amines were studied by comparing the infrared spectra of the amines before and after the complex formation, and the results were interpreted in terms of electronic as well as steric interactions of metal salts with ligands. Conjugation of the metal salt with the ligand π-orbitals was necessary to explain both infrared spectra and polymerization results. |
