Polymethylene, homologation

It was an interesting idea to create the giant tube-like structures 46 capped with a Lewis acid center by the homologation reaction of 42 with dimethylsulfoxonium methylide and their deboronation to the three-armed star polymethylene polymers 47 incorporating a m, r-l,3,5-trisubstituted cyclohexane core (Scheme 12) <2003JA12179, 20010L3063>. 

The solvolysis of 7,7-dibromo-bicyclo[4.1.0]heptane in methanol in the presence of silver perchlorate leads to ( -2-bromo-3-methoxycyeloheptene [172, 173]. A more detailed study of this reaction — having been performed by Ito in 1986 [174] — has revealed that the stereochemical result of the solvolysis depends on the length of the polymethylene chain in these bicyclic dihalides. For the lower homologs (n = 2 to 4) it is the ( )-isomer XIII that is produced, whereas for the more extended systems (n = 5 to 8) the Z-bromoethers XIV are produced. 

The equation for the diffusion coefficient in a gaseous state, Dc, is the starting point for the derivation of an equation for diffusion coefficients in an amorphous solid state (S). The homologous series of n-alkanes is used as a reference class of chemical compounds which asymptotically reach an unlimited molecular chain in the form of poly methylene. The diffusion coefficient, DSji, is derived theoretically for a member of the series with j carbon atoms at infinite dilution in a matrix of a n-alkane with i carbon atoms in the molecule. In addition, the diffusion in polymethylene, which is considered as the reference structure for all polymers is also derived theoretically. 

Most of the subject polymers that have been studied to date have contained flexible spacers composed of polymethylene chains. The monomers or precursors for these spacers are readily available as either diols or diacids or as dihalides, and this availability has led to the preparation of several homologous series of thermotropic polymers with different mesogenic units. Indeed, the first reported main chain, thermotropic LC polymer contained a polymethylene flexible spacer... 

The most frequently encountered homologous series in medicinal chemistry are monoalkylated derivatives, cyclo-polymethylenic compounds, straight chain difunctional systems, polymethylenic compounds and substituted cationic heads. 

The method used most in the present work to estimate amorphous polyethylene properties consisted in using oligomer (ra-alkane) properties to calculate limiting properties (Pcx>, properties of an infinite-length homolog) sueh limiting properties resemble the corresponding properties of amorphous, linear polyethylene (or amorphous polymethylene) (Table 11). 

A proposed mechanism for the formation of polymethylene is shown in Scheme 11.5 (Shea, 2000). The reaction is initiated by attack of the nucleophilic ylide 1 on the Lewis-acidic trialkylborane to form a zwitterionic complex 7, which subsequently undergoes a 1,2-migration of one of the three alkyl substituents on boron to the methylene group. This results in a homologated borane 8 and a molecule of DMSO. Since the homologated product 8 is a trialkylborane, after each migration the Lewis-acidic boron is regenerated so it can react with... 

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