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 Degree of polymerization [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

MW in Living Anionic Polymerization Poisson Distribution Narrow distribution

Typical Products CASE 1 CASE 2 Mono-disperse Polystyrene: Gel Permeation Chromatography (GPC) standard CASE 2 unlike conventional rubbers that are covalently crosslinked (vulcanized), these block copolymers can be processed thermally and molded. 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.

Answer q2: ionic polymerizations necessarily carry along a counterion, 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 Initiation and Propagation The mechanism of cationic polymerization is a kind of repetitive alkylation reaction 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: Monomer Electron 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 Tacticity Vinyl 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: Nobel Prize in Chemistry for Zeigler and Natta (1963) 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 Styrene is almost a unique monomer, in that it can be polymerized by practically all methods of chain polymerization. A. Free radical B. Anionic C. Cationic D. Co-ordination (i.e., with a catalyst) Which of these methods would you use to make isotactic polystyrene? 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? 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 polymerization B) Anionic polymerization C) Using a Ziegler Natta catalyst D) By putting it into a bloody great pot and spitting on it to initiate polymerization E) Condensation polymerization 5. Suspension free radical polymerization of styrene would be preferred over bulk polymerization to overcome the problem of A) Branching B) Cross-linking C) Stereo-isomerism D) Polymeric impurities E) Temperature control during polymerization

Concept Check Questions 6. In emulsion polymerization, the principal place where the monomer polymerizes is A) Monomer droplets B) Aqueous phase C) Surfactant micelles D) Surface of reactor E) Air-liquid interface 7. Polypropylene produced commercially using a Ziegler-Natta catalyst is predominantly A) Atactic B) Isotactic C) Syndiotactic