Chemistry Reference
In-Depth Information
positive counterion. The degree of interaction between the macromolecular anion
and its counterion depends on the nature of the respective ions and the medium in
which the polymerization is proceeding.
Anions that are active enough to cause polymerization through reactions of
carbon
carbon double bonds are strong bases. However, if the carbonion end of the
growing chain is an extremely strong base, it will be converted to the corresponding
hydrocarbon by transfer of a proton from solvent or any other species in the reaction
mixture which is less basic. The kinetic chain would be terminated in such cases
since the new anions produced by these transfer reactions would not be basic enough
to reinitiate the propagation sequence. (Recall that a propagation reaction is one in
which monomers are consumed, the location of the active site is changed, but the
number of active sites remains the same. Monomer addition is the main propagation
reaction.) Practical anionic polymerizations therefore require the use of macromolec-
ular carbonions which are relatively stable under accessible reaction conditions.
Monomers that can be polymerized anionically include those in which some
anion stability is realized by delocalization of the negative charge over a larger
region of the molecule. Examples include styrene and butadiene in which the
C
Q
C double bonds are conjugated and cyclic heteroatom compounds in which
the negative charge can be delocalized onto atoms that are more electronegative
than carbon. The latter group includes ethylene oxide, propylene sulfide, caprolac-
tam, and siloxane ring compounds. Vinyl monomers with electron-withdrawing
substituents like cyano and nitro groups (Section 9.10.2) are also suitable for
anionic polymerizations, because attack of an anion is facilitated on the electron-
poor unsubstituted carbon of the double bond.
If the monomers contain reactive groups that could be attacked by carbonions
they will not be suitable for anionic polymerizations. Halogen-containing vinyl
monomers are difficult to polymerize in these systems because of the elimination
of alkyl halides. Interactions between many initiators and the carbonyl groups of
methacrylates or acrylates necessitate the use of special reaction conditions, like
very low temperatures, for the anionic polymerization of these monomers.
Protection of some reactive functional groups is possible during polymerization,
using known organic chemical methods, such as silanation of alcohol or phenol
groups. This expedient and the subsequent deprotection step will, of course, com-
plicate the overall polymerization.
Solvents with high dielectric constants are generally not employed in anionic
polymerizations. This is because protic liquids like alcohols or amines are acidic
enough to destroy carbanionic active centers, and highly polar aprotic solvents
may form strong complexes with the anions and hinder addition of monomers.
Anionic polymerizations are performed as a consequence in fairly nonpolar sol-
vents with dielectric constants in the range of 2 to 10. (At room temperature the
dielectric constant of benzene is 3 while that of water is 78.5. Recall that the
attractive force between opposite charges is inversely proportional to the dielec-
tric constant of the medium in which they are immersed. Dissociation of ion pairs
is facilitated in solvents with higher dielectric constants.)
