MadSci Network: Chemistry

Re: explain process of polymerisation of vinyl chloride to polyvinyl chloride

Date: Mon Oct 11 02:03:35 1999
Posted By: Werner Sieber, Research Scientist, Pigment Division, Ciba Specialty Chemicals Corp.
Area of science: Chemistry
ID: 939388524.Ch

Dear Cynthia,
The key to understanding polymerization is understanding INITIATION. To start 
any polymerization, the molecule with the double bond must add a RADICAL, so 
that it becomes a radical itself ( I think you know that a radical, in 
chemistry, is a molecule with an "unpaired" electron). The unpaired electron 
(symbolized by a period) then sits on that carbon of the ex-double bond, to 
which the radical has NOT added.

R. + C=C  ->  R-C-C.

 The first radical, which starts the chain reaction, is generated by 
decomposition of a special molecule called an initiator. Initiator molecules are 
unstable towards heat or light, causing some SINGLE bond within themselves to 
break into two radicals. Those are very shortlived, but if a large excess of 
monomers surrounds them, each radical adds to one of them, and so becomes the 
beginning of a chain.

 Eventually, chains are terminated e.g. by adding another radical (eg. a 
partially grown chain: this is called RECOMBINATION).

 Since initiators are added in very small amounts, they are often not mentioned 
in popular discussions of polymerization. Under some conditions (e.g UV light or 
very high temperatures), ordinary molecules, solvents, impurities or even 
monomers can act as initiators, leading to "spontaneous" polymerization.

Best regards
Werner Sieber

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