2-Vinylpyridine oligomerization

PDMS nanocomposites with layered mica-type silicates were also reported.374 A two-step sol-gel process of the in situ precipitation of silica led to the development of siloxane-based nanocomposites with particularly high transparencies.3 5 Some unusual nanocomposites prepared by threading polymer chains through zeolites, mesoporous silica, or silica nanotubes were reviewed.3 6 Poly(4-vinylpyridine) nanocross-linked by octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane was reported.377... 

The results for the potassium system are likewise closely similar to the distribution of stereoisomers calculated by equating the stereoselectivity of 2-vinylpyridine addition with that of methylation. With Li as counterion, the oligomerization of 2-vinylpyridine continues to be highly stereoselective, at least up to ten monomer units and most likely considerably higher ( ). 

The applicability of the proposed mechanism to the alpha methyl derivative of 2-vinylpyridine, 2-(2-pyridyl)propene is of considerable interest because of its similarity to the analogous methylmethacrylate. Thus the stereochemistry of oligomerization of this monomer was investigated (Eqn. 2)., CH,... 

Alkoxides, Amides, and other Bases. The use of alkoxides as initiators has recently received some attention because of their ease of handling relative to metal alkyls. In principle such species might be expected to be active only with monomers capable of yielding a more stable anion. Potassium methoxide in dimethylsulphoxide/methanol solution will oligomerize 4-vinylpyridine and here initiation seems to proceed via an acid/base equilibrium rather than by addition to the vinyl double bond (Scheme 15). With methyl methacrylate lithium t-butoxide is an initiator although an inefficient one. Furthermore, unused... 

Vinylpyridine Oligomers. Oligomerizations of 2-vinylpyridine are closely related to polymerizations of methacrylates. Products formed by initiation using lithium and sodium salts of 2-ethylpyridine in THF at —78 °C are predominantly (>95%) isotactic. Methylation of 1,3-di(2-pyridyl)butane anion with... 

When methyl iodide is replaced by 2-vinylpyridine in these reactions we have in effect the propagation step of polymerization or oligomerization, and indeed reaction proceeds in the same selective fashion. In the case of lithium counterion >95 % of isotactic trimer is formed by the next addition. Epimerization of the selectivity methylated product using K+OBu " in dimethylsulphoxide at 25 °C for 2 weeks yields a final mixture of approximately 1 2 1 of isotactic, heterotactic, and syndiotactic trimers. Thus the trimerization process must be kinetically controlled. 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate (86). Thus, using oligomeric (o -methylstyryl)lithium as initiator in THF at —78°C, the molecular weight distribution (M /Mn) of the polymer was 3.61 in the absence of lithium chloride but 1.2 in the presence of lithium chloride ([LiCl]/[RLi] = 5). In the presence of 10 equiv of LiCl, f-butyl acrylate was polymerized with 100% conversion and 95% initiator efficiency to provide a polymer with a quite narrow molecular weight distribution (My,/Mn = 1.05). More controlled anionic polymerizations of alkyl methacrylates are also obtained in the presence of lithium chloride. Other additives, which promote controlled pol5unerization of acylates and methacrylates, include lithium f-butoxide, lithium (2-methoxy)ethoxide, and crown ethers (47,48). The addition of lithium chloride also promotes the controlled anionic polymerization of 2-vinylpyridine. 

Many cereal endosperm storage proteins—especially glutelins— are, in their native forms, disulfide-bonded oligomers or polymers. Even if soluble, such proteins may resolve poorly upon RP-HPLC and elute as broad peaks [22], in part due to their many possible polymeric forms. Reduction is necessary to release disulfide-bonded subunits from cereal glutelins or oligomeric prolamins for analysis [19,32,42,76]. Thus, protein resolution, extractability, and stability are often improved by extraction under reducing conditions. To prevent reoxidation and further enhance resolution, resulting cysteine residues may be stabilized by alkylation, usually with 4-vinylpyridine [19,21,29,70,76]. 

We have also examined the oligomerization of alpha-methyl-2-vinylpyridine according to equation (4). Oligomers [18] are crystalline products with sharp melting points and the tetramer... 

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