Linear polymers complexes

Nickel halide complexes with amines give mixtures of linear polymer and cychc trimers (30). Nickel chelates give up to 40% of linear polymer (31). When heated with ammonia over cadmium calcium phosphate catalysts, propargyl alcohol gives a mixture of pyridines (32). 

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Amylopectin accounts for the remaining 80% of starch and is more complex in structure than amylo.se. Unlike cellulose and amvlose, which are linear polymers, amylopectin contains 1- 6-cr-glycoside branches approximately every 25 glucose units. 

What determines the propensity of cyclization of the different diethynylated complexes Looking carefully at the monomers, there are three variables which could influence the formation of cycles as opposed to linear polymers. One is the angle a between the two alkyne arms. The second is the parameter /, which indicates how far the two alkyne groups are apart at their origin, while the third is the parameter the bulk of the monomer. 

Moreover, since the repeating unit in a-type linear polymers is about 7.5 A, according to a molecular model evaluation, the stacks in the starting crystals prefer to have repeating units of a similar length (L). However, L (5.932 A) in 2 OPr-a-dimer crystals is apparently shorter than the favourable distance (—7.5 A). On the other hand, in the 2 OPr-a-dimer-PrOH complex, reactant dimer molecules are separated from each other by a more suitable distance (L = 6.736 A). 

In this work, the kinetics of these reactions are closely examined by monitoring photopolymerizations initiated by a two-component system consisting of a conventional photoinitiator, such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and TED. By examining the polymerization kinetics in detail, further understanding of the complex initiation and termination reactions can be achieved. The monomers discussed in this manuscript are 2-hydroxyethyl methacrylate (HEMA), which forms a linear polymer upon polymerization, and diethylene glycol dimethacrylate (DEGDMA), which forms a crosslinked network upon polymerization. 

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