1

Anionic Polymerization

RNu- + R

Nu

ConceptsGeneral Mechanism

Initiation

R+R

NuR

R

NuR

RnR

Nu

Propagation

Reaction is facilitated by electron withdrawing groups, e.g. :

NO2, CN, CO2R, CH=CH2, Aryl

In Nu-E+

Anionic Polymerization

R

NuR

Rn E+

R

NuR

Rn

E

Concepts

General Mechanism

Termination

R

ER

Rn

HE+

R

ER

Rn

β-elim

Anionic Polymerization

R

R- Li+ + Li+

Chemistry

Initiators

1. Nucleophilic: Strength of nucleophilic initiater depends on monomer

n-C4H9Li, ArylMgBr, RO-, CN-, NaNH2, LiNR2, RNH2

R

RR

+

R

Li+Li+

2

Anionic Polymerization

In + M In.+ + M

.+

Chemistry

Initiators

2. Electron Transfer

+ Na Na+= Na+-

Anionic Polymerization

Na+-+ +

- -

--

greenish-blue reddimerize

Chemistry

Anionic Polymerization

ROMe OH

O O

Chemistry

PropagationMonomer Structure: anion stabilized by electron withdrawing group

CNOMe

O

CN CN

Rates

Rate determined by both sterics and polar factors. Trend is not the same as free radical

3

Anionic Polymerization

B A

Chemistry

B- A+

B- A+

covalent contaction pair

solvent separated ion pair

Ion Structure: Species

B-A+

B-

A+

B-

-B

AggregateIons

B- A+

free ions

Anionic PolymerizationChemistry

Reactivity is that with the weakest bond (releases more highly active free ions)

Solvation not important in dioxane, therefore, the ion pair with the highest

24.5Cs+

21.5Rb+

19.8K+

3.4Na+

0.94Li+

Rate in Dioxane(less polar)

Counterion

Counterions

Anionic PolymerizationChemistrySolvent Effects

38005.51,2 Dimethoxyethane

5507.6THF

52.2Dioxane

22.2Benzene

kpappDielectric

ConstantSolvent

4

Anionic Polymerization

Na+ or K+

but not Li+

H H

+ NaH

Chemistry

Termination

1. Hydride Transfer

2. Chain Transfer or Protonating Impurities

H

Li++ H+Z-

(e.g. H2O)

H

H

Anionic PolymerizationChemistry

Termination

3. Intramolecular

OMeO

O OMe O OMe

COOMeCOOMe

O

Anionic Polymerization

Rd = kd [Nu- E+]Nu- E+ Nu- + E+

K =[Nu-] [E+][Nu- E+]

Kinetics

[Nu-] =[E+]

K [Nu- E+]

Ri = ki [M] [Nu-]

ki K [Nu- E+] [M]

[E+]Ri =

Initiation

Dissociation: e.g. free ions

5

Anionic Polymerization

Rp = kp [M] [NuM-]

Rt = kt [H2O] [NuM-]

ki K [Nu- E+] [M]

[E+]Ri = = Rt = kt [H2O] [NuM-]

Kinetics

ki K [Nu- E+] [M]

kt [H2O] [E+][NuM-] =

Steady state approx:

[NuM-] [M]+ [NuMM-]

[NuM-] [H2O]+ [NuMH] + OH-

Propagation

Termination:

Anionic Polymerization

Rp =kp ki K [Nu- E+] [M]2

kt [H2O] [E+]

Rp = kp [M] [NuM-]

DP = Rp/Rt = Rp/Ri

Kinetics

kp ki K [Nu- E+] [M]2

kt [H2O] [E+]

ki K [Nu- E+] [M]

[E+]=

=kp [M]

ki [H2O]

Note: = [M] / [I] or 2[M] / [I]

Degree of Polymerization

Anionic Polymerization

AAA- BAAABBB- AAABBBCCC-C

Telechelic Transformations

Block Copolymers

AAA- AAA Cl

O

AAA+SbF6-

AgSbF6

COCl2

BAAABBB+SbF6

-

Reverse Transformation

6

Anionic Polymerization

AAA-O

AAA OH1.

2. H2O

Br

AAA

1. CO22. H2O

AAA OH

O

Telechelic Transformations

Synthetic Transformations

AAA- + SiCl4 SiAn

AnAn

An

Coupling Reactions

Anionic Polymerization

Nu-E+ +H

O

H Nu

O-

E+

Nu

O-

E+

H

O

H+ NuO O- E+ O n O- E+O

Nu

O n O- E+ONu

+ H2O O n OH E+OH-ONu

+

Carbonyl Polymerization

O n OHONu

O n OONu

O

O

O

O

Initiation

Propagation

Termination

End Capping

Anionic Polymerization

CH

R H

H

R, I, C

R+n n

R

N C ORA N

R

On

C CHRR, I, C

R

n

Other Anionic Polymerizations

C NRI, R

CR

N n

7

Anionic Polymerization

+ + ++ nbuLi

1,4 cis 1,4 trans 1,2 3,4

Stereochemical ConsiderationsDiene Polymerization: e.g. Polyisoprene (synthetic rubber)

38755smallpentaneK

53740smallpentaneNa

45550---Et2OLi

7525------THFLi

6510250Pentane/THFLi

6------94PentaneLi

3,41,21,4 trans1,4 cisSolventCounterion

Anionic Polymerization

Li+Li+

Li+

Stereochemical Considerations

Li+

+Li+

Li+

Note: Na+ or K+ not effective—too large to promote cyclic transition state