Part 1 L4-6 Flashcards
Initiators for cationic polymerization
(a) Classical protonic acids or acid surfaces–HCI, H2SO4,HCIO4;
(b) Lewis acids or Friedel-Crafts catalysts– BF3, AICI3 , TiCI4 ,SnCI4; + ionization agent (protogen or cationogen)
(c) Carbenium ion salts.
What is a protogen?
Protogen: proton donor such as water, hydrogen halide, alcohol, and carboxylic acid, etc
What is a cationogen?
Cationogen: carbocation donor such as an alkyl halide (e.g., t-butyl chloride and triphenylmethyl chloride), ester, ether, or anhydride, etc
Give an example of termination with the counter ion in cationic polymerization
S10 L4
What does a polar solvent do to cationic polymerization?
Polar solvents which favour ion-pair separation (e.g. dichloromethane) and large counter-ions (e.g. SbCl6
-), which associate less strongly with cations, give rise to
higher values of kp.
What does a large Counter ion do in cationic polymerization?
larger and less tightly bound anions give a higher rate of reaction. Small and tightly bound = no space for monomer.
Cationic Ring Opening Polymerization. Draw mechanism with reaction with R+ A-
Slide 17 L4
Mechanism for living cationic polymerization
Slide 20 L4
Initiators in anionic polymerization
A variety of basic (nucleophilic) initiators have been used:
Covalent or ionic metal amides such as NaNH2 and LiN(C2H5)2, alkoxides, hydroxides, cyanides, phosphines, amines, and organometallic compounds such as n-C4H9Li and PhMgBr. Initiation involves the addition to monomer of a nucleophile (base), either a neutral (B:) or negative (B:-) species.
Termination of anionic polymerization to form either carboxylic acid, alcohol or alkene
Reaction with CO2, ethylene oxide (followed by H+) and alcohol respectively.
Termination of anionic polymerization to form either acid chloride or star polymer
Reaction with COCl2 or SiCl4
Anionic polymerization: Effects of solvent
- The reactivity of ion pairs is greatly enhanced by the use of a more polar solvent which increases the ion-pair separation.
- When the solvation is absent or weak, the rate coefficient for ion-pair propagation increase as the size of the counter-ion increases (i.e. K+ > Na+ > Li+) due to the consequent increase in the separation of the ions. However, in polar solvating solvents, the opposite trend often is observed because the smaller counter-ions are
more strongly solvated.
What is the formula for average degree of polymerization final
Xn(final) = [M]0/ [I]0 = amount of monomer consumed/number of chains
[I]0 is the number of initiators
Monomer conversion formula
x = ([M0]-[M])/[M0]
Anionic Ring Opening mechanism (reaction with alkoxides)
Slide 32 L4