Ionic Polymerization

Anionic Polymerization

General Formula

B- Anionic active center; Counter-ion

What monomer? Electron withdrawing groups (ester, cyano) or groups with double bonds (phenyl, vinyl) are needed as the R groups because these can stabilize the propagating species by resonance.

Question:

What are the characteristic Q-e values for such monomers? (q1)

Anionic Initiation

For initiation to be successful, the free energy of the initiation step must be favorable. Therefore, it is necessary to match the monomer with the appropriate strength of initiator so that the first addition is "downhill."

Bottomline: Nucleophile agents

Concept of Living Polymerization

There are only• Initiation: Chains are initiated all at once

(fast initiation) • Propagation: All chains grow under

identical conditions; fast; generally

needs low temperature (why? q2).

BUT• Little or no termination (except purposeful)• Little or no chain transfer

Living Polymerization

• Consequently,

Reaction rate

[M-]: concentration of all anions in the system

[M]: concentration of the monomer

[C]: concentration of the initiator

n: the number of anions that an initiator molecule produces

Degree of polymerization

MW in Living Anionic Polymerization

Poisson Distribution

Narrow distribution

Typical Products

CASE 1

CASE 2

One or more blocks of flexible, low Tg polymer, with other blocks of rigid, high Tg polymer. The polymer actually exhibits two glass transitions, one at high temperature and a second one below room temperature.

unlike conventional rubbers that are covalently crosslinked (vulcanized), these block copolymers can be processed thermally and molded.

Mono-disperse Polystyrene: Gel Permeation Chromatography (GPC) standard

Answer q2: ionic polymerizations necessarily carry along a counteri

on, and their rates are much more sensitive to reaction conditions (e.g., solvent polarity, temperature).

q1. monomers for anionic polymerization generally have High Q or high e values

Cationic Polymerization

The mechanism of cationic polymerization is a kind of repetitive alkylation reaction

Initiation and Propagation

• Propagation is usually very fast must often be run at low

temperatures cooling large reactors is difficult and expensive.

• Trace of water kills reaction Careful drying of ingredients is

necessary

Cationic Polymerization: MonomerElectron donating groups are needed as the R groups because these can stabilize the propagating species by resonance

Cationic Polymerization: Initiators

Proton acids with unreactive counterions

Lewis acid + other reactive compound

Cationic Polymerization: side reactions

Cationic vinyl polymerization is plagued by numerous side reactions, most of which lead to chain transfer.

It is difficult to achieve high MW because each initiator can give rise to many separate chains because of chain transfer.

These side reactions can be minimized but not eliminated by running the reaction at low temperature.

Review: Vinyl Polymer TacticityVinyl polymers that have single substituents (e.g., propylene, the example below) or two unsymmetrical substituents (e.g., methyl methacrylate) have pseudoasymmetric carbon atoms in the backbone.

• atactic polypropylene is a useless, gummy solid,

• the isotactic version is a highly crystalline, tough plastic that can even be

made into fibers.

Coordination (Ziegler-Natta) Polymerization

Important discovery: R3Al + Lewis acids:

Another important discovery: tacticity control:

Results:1. Nobel Prize in Chemistry for Zeigler and Natta (1963) 2. Multibillion $ industry

Overall Scheme of Coordination Polymerization

• Limited to ethylene and other a-olefins like propylene. (Actually, it is the only good way to polymerize these monomers.)

• Produces linear polymer, with very few branches (e.g., high density polyethylene, HDPE).

• Capable of producing homo-tactic polymers.

• Most commercial initiators are insoluble complexes or supported on insoluble carriers. .

The mechanism is poorly understood because it takes place on the surface of an insoluble particle, a difficult situation to probe experimentally.

An approximation

Concept Check Questions

1. Styrene is almost a unique monomer, in that it can be polymerized by practically all methods of chain polymerization.

A. Free radicalB. AnionicC. CationicD. Co-ordination (i.e., with a catalyst)

Which of these methods would you use to make isotactic polystyrene?

2. If you needed to synthesize a set of narrow molecular weight standards (i.e., with poly-dispersities close to 1), which of the above methods would you use?

3. Commercial atactic polystyrene is synthesized by which of the

above methods?

Concept Check Questions

4. What method would you use to synthesize a triblock copolymer?A) Free radical polymerizationB) Anionic polymerizationC) Using a Ziegler Natta catalystD) By putting it into a bloody great pot and spitting on it to initiate polymerizationE) Condensation polymerization

5. Suspension free radical polymerization of styrene would be preferred over bulk polymerization to overcome the problem ofA) BranchingB) Cross-linkingC) Stereo-isomerismD) Polymeric impuritiesE) Temperature control during polymerization

Concept Check Questions

6. In emulsion polymerization, the principal place where the monomer polymerizes isA) Monomer dropletsB) Aqueous phaseC) Surfactant micellesD) Surface of reactorE) Air-liquid interface

7. Polypropylene produced commercially using a Ziegler-Natta catalyst is predominantlyA) AtacticB) IsotacticC) Syndiotactic