Chapter 3. Continued

Examples of addition polymers

Free radical polymerization was widely used in industry

The advantages of this polymerization include:

• Robust, mature technique
• Wide range of monomers (any double bond containing)
• Operating conditions friendly
• Aqueous media: Emulsion polymerization

The disadvantages are:

• Non-selective reaction
• The distribution of molecular weight is broad

Other initiators: 

"Nitrogen type" initiators are also possible, such as azobisisobutylnitride (AIBN):

           CH3            CH3

            |                   |

CH3 - C - N = N - C - CH3

            |                   |

           CN              CN

Molecular weight control
In a polymerization process, the aim is to control the molecular weight. The following variables influence the molecular weight:

1. Temperature
2. Solvent
3. Concentration of Initiator
4. Concentration of Monomer
5. Type of Initiator

Chapter 4  Ionic and Coordination Chain Polymerization

cationic polymerization, anionic polymerization, coordination polymerization.

In contract to radical polymerization, termination in ionic polymerization never involves the reaction between growing chains, this makes the polymer molecular weight very high..

The advantages of this polymerization are:

• Normally produces _______________________. For instance, the polystyrene thus made from anionic polymerization can be used as a gel permeation chromatography (GPC) standard.
• leads to ______________________ polymer and extreme high rate and low (often below 0 ^oC) reaction temperature.

The disadvantages are:

• Strict operating conditions. Any impurities that neutralize the catalyst can provent the reaction, and many aromatic, heterocyclic, olefinic, and acetylenic compounds could retard or terminate the polymerization.
• Not all the vinyl monomers can undergo these reactions.

A. General description

Formation of Initiator -- By ionization of the initiator, such as H⁺ from BF₃-H₂O

Initiation -- transfer of the electron from initiator to a monomer, three cases can be differentiated as the following

a. A cation adds to a monomer to form a monomer cation, for example:

      H⁺  +  CH₂=CHCH₃ --> CH₃CH⁺CH₃

b. An anion adds to a monomer to form a monomer anion, for example

     C5H11^-   +   CH₂=CHCH₃ --> C₅H₁₁CH₂CH^-CH₃

c. A neutral molecule add to a monomer to produce a zwitterion, for example

Propagation and Termination. Further addition of monomers to the growing chain or destruction of the growing chain.

Homework:

1. Unlike free-radical polymerization, both cationic and anionic polymerizations show a marked dependence on the type of solvent used. Discuss the causes and nature of these effects.