Question

Explain the advantages and disadvantages of complex coordination polymerizations over other addition polymerizations

Answer 1

A) Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes.

The main advantages are:
1. Reduction in branching in the final
product.
2. Better stereo control over polymer
tacticity, producing either syndiotactic
or isotactic, thus producing polymers
with greater crystallinity, and hence
better physical properties.
3. The formation of higher MW polymers.
4. The ability to produce copolymers with
polar co-monomers.

Types of coordination polymerization of alkenes:

1) Heterogeneous Ziegler–Natta polymerization:

Coordination polymerization started in the 1950s with heterogeneous Ziegler–Natta catalysts based on titanium tetrachloride and organoaluminium co-catalyst. The mixing of TiCl₄ with trialkylaluminium complexes produces Ti(III)-containing solids that catalyze the polymerization of ethene and propene.

2)Homogeneous Ziegler–Natta polymerization:

In some applications heterogeneous Ziegler–Natta polymerization has been superseded by homogeneous catalysts such as the Kaminsky catalyst discovered in the 1970s. The 1990s brought forward a new range of post-metallocene catalysts.

Principles :Coordination polymerization has a great impact on the physical properties of vinyl polymers such as polyethylene and polypropylene compared to the same polymers prepared by other techniques such as free-radical polymerization. The polymers tend to be linear and not branched and have much higher molar mass. Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinityin otherwise amorphous polymers. From these differences in polymerization type the distinction originates between low-density polyethylene (LDPE), high-density polyethylene (HDPE) or even ultra-high-molecular-weight polyethylene.

B) An addition polymer is a polymer that forms by simple linking of monomers without the co-generation of other products. Addition polymerization differs from condensation polymerization, which does co-generate a product, usually water.

the three stages of addition polymerization :

This process has three stages :  initiation, propagation, and termination. In the first stage, a substance is split into two identical parts, each with an unpaired electron. (Peroxides, which contain an O-O bond, are often used in this role.) A molecule with an unpaired electron is called a free radical.

Addition polymerization occurs by a chain reaction in which one carbon-carbon double bond adds to another. Monomers continue to react with the end of the growing polymer chain in an addition polymerization reaction until the reactive intermediate is destroyed in a termination reaction.

Disproportionation and dimerization are two possible termination reactions. In disproportionation, a hydrogen atom at a carbon atom α to the radical center is abstracted by a radical in another chain. This produces a double bond in one polymer molecule, and the other polymer molecule becomes saturated. Because no new radical intermediates are formed, the next propagation step cannot occur.