Ionic Chain-Growth Polymerization - Principles of Polymer Chemistry - page 192

Table 4.3 Effect of counterion and solvent on the microstructure of polyisoprene a approximate% of adduct
Initiator

Solvent a Cis -1,4 Trans -1,4 1,2 3,4

Lithium dispersion Alkane 93 1 - 5-6

Ethyllithium Alkane 94 - - 6

Butyllithium Alkane 93 - - 7

Sodium dispersion Alkane - 43 6 51

Ethylsodium Alkane 6 42 7 45

Butylsodium Alkane 4 35 7 54

Potassium dispersion Alkane - 52 8 40

Ethylpotassium Alkane 24 39 6 31

Butylpotassium Alkane 20 41 6 33

Rubidium dispersion Alkene 5 47 9 39

Cesium dispersion Ether 4 51 8 37

Ethyllithium Ether 6 30 5 59

Ethylsodium Ether - 14 10 76

Lithium dispersion 3 27 6 64

a From various sources in the literature. The alkane is a low boiling aliphatic hydrocarbon

Polymerizations of polar monomers, like acrylic and methacrylic esters with alkyllithium
initiators yield the greatest amount of steric control [ 151 ]. An almost all isotactic poly(methyl
methacrylate) forms at low temperatures. Addition of Lewis bases such as ethers or amines reduces
the degree of isotactic placement. Depending upon the temperature, atactic or syndiotactic
polymers form [ 151 ]. Also, butyllithium in heptane yields an isotactic poly( N 0 -dibutyla-

N ,

crylamide) at room temperature [ 201 ].
The propagation rates for methyl methacrylate polymerization in polar solvents like tetrahydrofu-
ran or dimethylformamide are lower than the rates of initiation [ 203 ]. There is no evidence, however,
that more than one kind of ion pairs exist [ 204 - 206 ]. The ion pairs that form are apparently contact-
ion pairs [ 203 ]. Furthermore, based on the evidence, the counterions are more coordinated with the
enolate oxygen atoms of the carbonyl groups than with the -carbons. As a result, they exert less

influence on the reactivity of the carbanions [ 203 ]. The amount of solvation by the solvents affects the
reaction rates. In addition, “intramolecular solvation” from neighboring ester groups on the polymer
chains also affects the rates. In solvents like dimethylformamide, tetrahydrofuran, or similar ones
[ 203 ], the propagating chain ends-ion pairs are pictured as hybrid intermediates between two extreme
structures. This depends upon the counterion, the solvent, and the temperature [ 203 ]:

a

O

O

S

S O

Me

O

Me

S

S S

where S means a solvent molecule; Me represents a metal.
Several mechanisms were offered to explain steric control in polymerizations of polar monomers.
Furukawa and coworkers [ 157 ] based their mechanism on infra-red spectroscopy data of interactions
between the cations and the growing polymeric chains in polymerizations of methyl methacrylate and
methacrylonitrile. They observed a correlation between the tacticities of the growing molecules and
the carbonyl stretching frequencies. The higher the frequency, the higher is the amount of isotactic