Chemistry Reference
In-Depth Information
observed, these were living polymerizations. The polymers that formed had the dispersity of 1.1 and
were high in syndiotactic sequences.
Novak and Boffa, in studying lanthanide complexes, observed an unusual facile organometallic
electron transfer process takes place that generates in situ bimetallic lanthanide(III) initiators for
polymerizations of methacrylates [
239
]. They concluded that methyl methacrylate polymerizations
initiated by the Cp*
2
Sm complexes occur through reductive dimerizations of methyl methacrylate
molecules to form “bisinitiators” that consists of two samarium(III) enolates joined through their
double bond terminally [
239
]. Their conclusion is based on the tendency of Cp*
2
Sm complexes to
reductively couple unsaturated molecules:
O
CO
2
Me
2
OCH
3
2
Cp*
2
Sm(III)
Sm
O
Sm
CH
3
O
OCH
3
Sm
O
CO
2
CH
3
CO
2
CH
3
CH
3
O
H
excess
n
n
CH
3
O
2
C
N or P),
designed for the polymerization of ethylene, are effective for home- and copolymerization of butyl
acrylate, methyl methacrylate, and styrene. Their role as radical initiators was demonstrated from the
calculation of the copolymerization reactivity ratios. It was shown that the efficiency of the radical
initiation is improved by the addition of PPh
3
to the nickel complexes as well as by increasing the
temperature. The dual role of nickel complex as radical initiators and catalysts was exploited to
succeed in the copolymerization of ethylene with butyl acrylate and methyl methacrylate.
Multiblock copolymers containing sequences of both ethylene and polar monomers were thus
prepared.
Montei and coworkers [
240
] reported that Nickel complexes [(X,O)NiR(PPh
3
)] (X
¼
6.14.2 Acrylic Elastomers
Polymers of lower
-alkyl acrylates are used commercially to only a limited extent. Ethyl and butyl
acrylates are, however, major components of
acrylic elastomers
. The polymers are usually formed by
free-radical emulsion polymerization. Because acrylate esters are sensitive to hydrolysis under basic
conditions, the polymerizations are usually conducted at neutral or acidic pH. The acrylic rubbers,
like other elastomers must be cross-linked or
vulcanized
to obtain optimum properties. Cross-linking
n
