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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18
Polymers, Synthesis 763
SCHEME 12 Generation of some typical free radicals.
that do have small but significant quantities of head-to-
head and tail-to-tail addition steps. This has most often
19
been demonstrated through the use of F NMR spec-
troscopy. Even in this case, the abnormal addition is rela-
tive minor and is of the order of ∼10% of the overall chain
structure.
The tacticity or stereochemistry of vinyl chains is a very
important parameter. Following the definitions of Natta
(Scheme 14) it is common to depict chains as isotac-
tic, syndiotactic, or atactic. The isotactic chains are ca-
pable of undergoing crystallization and, indeed, isotac-
tic polypropylene made via coordination polymerization
is a polymer of major commercial importance. Further
comments on this process will be made later. Syndiotactic
polymershaveplacementsoftheRgrouponoppositesides
ofanextendedchain.Itisalsoquitepopulartocharacterize
the stereostructure by NMR in terms of meso or racemic
types of placements, diads, triads, pentads, and so on. In
general, one should not that free-radical polymerization
tends to produce mostly atactic or heterotactic placements.
There is some tendency to produce moderately high syn-
diotactic structures in certain polar monomers, such as
vinyl chloride or acrylonitrile. Moreover, the syndiotac-
tic structure is favored as one reduces the polymerization
SCHEME 13 Free radical polymerization; X provides resonance
stabilization to the growing radical. temperature.
