MacromolecularMacromolecular ChemistryChemistry
MichaelMichael SzwarcSzwarc LewisLewis J.J. FettersFetters
Lecture 15 Chemistry 367L/392N Anionic polymerization Some History
1914, Schlenk reacts Na with butadiene and styrene 1929, Ziegler proposes a mechanism
1952 Higginson, styrene, KNH 2, kinetic study 1956 Szwarc, sodium naphthalene, Styrene, living polymerization conception 60's, commercial products were available 90‘s, study on the living polymerization of polar monomers 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 Radical 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 Alkali metal 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 polymer 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 monomer
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 chain transfer
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!!!