Fractional Precipitation

Experimental and Theoretical Evaluation of the Reactions Leading to Formation of Internal Double Bonds in Suspension PVC

“…Fractionation of PVC samples was performed via a modified fractional precipitation method. 13,14 The lowest molecular weight fractions, which could not be obtained by fractionation, were isolated by stirring the PVC powder overnight in a mixture of hexane and acetone (75:25). The material insoluble in this mixture of solvents was removed via filtration.…”

“…The resulting fractions were characterized by GPC and 1 H and 13 C (performed on reductively dehalogenated material) NMR spectroscopy using the same procedure described in detail in refs 12 and 14. The tacticity was determined by analysis of 13 C spectra of the unmodified resin.…”

“…A method for discriminating between these alternative mechanisms for the formation of internal double bonds is the analysis of the 13 C spectra of reductively dehalogenated samples. Unsaturated structures undergo 1-5 free radical cyclizations upon reaction with Bu 3 SnH, wherein a carbon-centered radical, formed from a C-Cl moiety, adds to a double bond to form a five-membered ring.…”

“…23 This addition results in various types of cyclopentane moieties, depending upon the location of the double bond in the chain (see also Scheme 2). The cyclic structures possess very similar chemical shifts, [23][24][25][26] but the monoalkylcyclopentane (MCP) structure that is formed by backbiting on a double bond in positions 5-6 (Scheme 2) can nevertheless be distinguished unambiguously in 13 C spectra. The chemical shifts of carbons R, 3 and 4 or 1 in the MCP structure (Scheme 2) have been determined (by comparison with 13 C spectra of model compounds) 23 not to overlap with those of other structures in reductively dehalogenated PVC.…”

3. Chemical Composition and Analysis of Wood

“…The distribution of the DP values in a cellulose sample can be determined by fractional precipitation from solution (Kotera 1967;Philipp, Linow 1965) or by fractional dissolution (Elliott 1967). The resulting fractions contain portions of a certain DP range.…”