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
Main advantage with living anionic polymerization
PDI can be lower than in living radical polymerization
Main disadvantages with living anionic polymerization
- Extremely clean conditions
- Limited number of solvents
- Limited number of functional groups (no OH, COOH)
- Limited number of functional monomers (in principle)
How can you make a block copolymer using living anionic polymerization
Synthesis of block copolymers by sequential monomer addition
What is the properties and structure of an isotatic polymer?
Can crystallize L5 S3
What is the properties and structure of an syndiotatic polymer?
L5 S3
What is the properties and structure of an atatic polymer?
Amorphous cannot crystalize L5 S3
How can isotatic polymers be formed
Highly isotactic polymers can be prepared by ionic polymerization if there is strong coordination of the counter ion with the terminal units in the polymer chain and with the incoming molecule of monomer
Requirements
• Monomer with polar substituent groups (for strong coordination)
• Low temperature
• Non polar solvent
• Initiator which yields a small counter ion
For non polar monomers, the Isotactic polymers can be prepared by polymerizations involving coordination to transition metals
What is the properties of cis/trans isoprene.
Trans
1 4 polyisoprene is able to crystallize due to its regular structure —- hard, rigid materials
Cis
1 4 polyisoprene has a less symmetrical structure that does not allow easy crystallization under normal conditions amorphous rubbery material
Draw the Ziegler
Natta coordination polymerization Mechanism
Propagation: mechanistic overview there are certain mechanistic features which now are widely accepted on
the basis of experimental evidence
1. Monomer initially is coordinated at vacant d orbitals of transition metal atoms at the catalyst surface
2. The orientation of a coordinated molecule of monomer is determined by its steric and electronic interactions with the ligands around the transition metal atom One particular orientation is of lowest energy
3. The propagation step is completed by insertion of the coordinated molecule of monomer into a metal carbon bond
4. The orientation of the molecule of monomer as it inserts into a metal carbon bond determines the configuration of the asymmetric carbon atom in the newly formed terminal repeat unit
5 Isotactic polymer is formed when the preferred orientation for coordination of monomer is of much lower energy than other possible orientations each successive molecule of monomer then adopts the
same preferred orientation as it undergoes coordination and then insertion
6 The mechanism of monomer insertion always leads to the formation of linear polymer chains, irrespective of the detailed stereochemistry.
