MacromolecularMacromolecular ChemistryChemistry

MichaelMichael SzwarcSzwarc LewisLewis J.J. FettersFetters

Lecture 15 Chemistry 367L/392N Anionic Some History

1914, Schlenk reacts Na with butadiene and 1929, Ziegler proposes a mechanism

1952 Higginson, styrene, KNH 2, kinetic study 1956 Szwarc, sodium , Styrene, living polymerization conception 60's, commercial products were available 90‘s, study on the living polymerization of polar Alkenes with electron Withdrawing Groups

examples

CH3 H C C H 2 C C H 2 H 2 C C H C O CN N O O OCH3 Acrylonitrile Nitroethene Methyl methacrylate Reactivity of monomers

H C C(CN) > H C C COOC H > H C CH Group A: 2 2 2 2 5 2 CN NO 2

H C CHCN Group B: 2 > H 2C C CH 3 > H 2C CH C CH 3 CN O

H 2C CH > H C C CH Group C: 2 3 COOCH 3 COOCH 3

CH3

H2C CH H2C C Group D: H2C CH CH CH2 Conjugated dienes are classical

CH3

H2C C H2C CH

styrene ααα-Me-styrene

H2C CH C CH2 H2C CH CH CH2 CH3 butadiene isoprene Initiators and initiation

(1) alkali metals ….one electron reductions

Lithium (Li) Sodium (Na) Potassium (K) as mirrors or fine dispersions.

e electron - + Na H C CH . CH CH Na + 2 transfer 2 anion

dimerize + - - + Na CH CH2 CH2 CH Na radical couple Szwarc’s favorite Solubility of alkali metals homogeneous vs heterogeneous

Li, K and Na dispersion: insoluble in hydrocarbons

Reaction occurs on the surface

adsorption reaction desorption complexs

Aromatic radical-anions: Sodium naphthalene Lithium naphthalene One electron reduction

- . + Na + [ ] Na

e complex (greenish blue color)

Alkali metal Aromatic compound initiator - . Na + Initiation: [ ] + H2C CH

electron . CH CH- Na + transfer 2 +

radical couple - - Na + + CH CH2 CH2 CH Na dimerize

soluble in polar solvents only ( such as: ethers) Szwarc’s Experiment

Living test Other Initiators Organometallic compounds Alkyl metal compound: RLi

Metal amides: NaNH 2, KNH 2 Complexity!!  Grignard reagent: RMgX

- + Initiation: CH3CH2CH2CH2 Li + H2C CH

(n-BuLi)

initiation - + n-BuCH2 CH Li

Soluble in nonpolar solvents!! Active center State of ion pair - B + A positive counterion

- + - - + A B AB A B+ A + B

Loose ion pair Covalent species Solvent-separated Tight ion pair ion pair Contact ion pair Free ion pair Intimate ion pair Polymerization rate

- + - + - A B AB A B A + B+ + + + M M M

POLYMERIZATION

reactivity

capacity of controlling stereo structure of chain Factors influencing the state of ion pair (solvent, temperature, counterion)

- + - + - A B AB A B A + B +

polarity of solvent temperature (in nonpolar solvent)

size of counterion For example:

n-BuLi n H C CH CH CH 2 2 n

In cyclohexane: several hours in THF: 10 min.

H H + + CH Li 2 C CH2 C Li Nucleophilic initiators

......

R 3 PR 3 N ROHHOH An unshared pair of electrons on the heteroatoms Generally requires strong EWG on the

initiator example: CN H C C C OC H 2 2 5 + H2O POLYMER O Matching initiators to monomers

Reactivity of initiator

Li, Na, NaR, LiR Strongest RMgX, ROLi, stronger bases

NR 3, weaker bases, H 2O weakest

Reactivity of monomer… see groups in earlier slide

Success requires matching initiators and monomers Reactivity Type of polymerization

Group A R only >>

Group B and group C R , R >>

+ Group D R , R , R

Initiator must strong base Propagation

- + Insertion propagation L i CH 2 Active species exist are ion pairs

H 2 C CH δδδ + X Both Single carbanions and dicarbanions - C Na + M + M M M - M C Na + M Na + - C

Low Activation energy Fast initiation! Termination and

Termination

+ + C Li + Li C

No coupling between chain ends!! No disproportionation!!!

sec -BuLi

H 2C CH

There is no termination Only Chain Transfer to impurity or Inhibitor H + H O CH C Li + 2 + LiOH

H + C Li + ROH CH + ROLi

H + C Li + RCOOH CH + RCOOLi

No Impurity No termination!!! Special Feature of anionic polymerization

Condition: nonpolar monomers, nonpolar solvent

Fast initiation Slow propagation No termination No chain transfer

“““living”livingliving ”” polymerization!!!polymerization!!! polymerization!!!