POLYMERIZATION

POLYMERIZATION

In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups present in reacting compounds  and their inherent steric effects explained by VSEPR Theory. In more straightforward polymerization, alkenes, which are relatively stable due to σ bonding between carbon atoms form polymers through relatively simple radical reactions; in contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis due to the way in which reacting molecules polymerize.

As alkenes can be formed in somewhat straightforward reaction mechanisms, they form useful compounds such as polyethylene and polyvinyl chloride (PVC) when undergoing radical reactions, which are produced in high tonnages each year due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. Polymers such as PVC are generally referred to as "homopolymers" as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one molecule are referred to as copolymers (or co-polymers).

STEP^-GROWTH

Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers. Most step-growth polymers are also classified as condensation polymers, but not all step-growth polymers (like polyurethanes formed from isocyanate and alcohol bifunctional monomers) release condensates, in this case we talk about addition polymers. Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e. >95%).

To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves loss of small molecules during its synthesis, or contains functional groups as part of its backbone chain, or its repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded. 

ADDITION POLYMER

An addition polymer is a polymer which is formed by an addition reaction, where many monomers bond together via rearrangement of bonds without the loss of any atom or molecule. This is in contrast to a condensation polymer which is formed by a condensation reaction where a molecule, usually water, is lost during the formation.

With exception of combustion, the backbones of addition polymers are generally chemically inert. This is due to the very strong C-C and C-H bonds and lack of polarization within many addition polymers. For this reason they are non-biodegradable and hard to recycle. This is, again, in contrast to condensation polymers which are bio-degradable and can be recycled.

Many exceptions to this rule are products of ring-opening polymerization, which tends to produce condensation-like polymers even though it is an additive process. For example, poly[ethylene oxide] is chemically identical to polyethylene glycol except that it is formed by opening ethylene oxide rings rather than eliminating water from ethylene glycol. Nylon 6was developed to thwart the patent on nylon 6,6, and while it does have a slightly different structure, its mechanical properties are remarkably similar to its condensation counterpart.

Chain-growth polymerization (or addition polymerization) involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form the repeating chain. Chain-growth polymerization is involved in the manufacture of polymers such as polyethylene, polypropylene, and polyvinyl chloride (PVC). A special case of chain-growth polymerization leads to living polymerization.

In the radical polymerization of ethylene, its pi bond is broken, and the two electrons rearrange to create a new propagating center like the one that attacked it. The form this propagating center takes depends on the specific type of addition mechanism. There are several mechanisms through which this can be initiated. The free radical mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided in to three stages: chain initiation, chain propagation, and chain termination.

The formation of a polymer by addition polymerization is an example of a chain reaction. Once a chain reaction gets started, it is able to keep itself going. The three steps of this reaction to focus on are
 how the reaction gets started (INITIATION)
 how the reaction keeps going (PROPAGATION)
 how the reaction stops (TERMINATION) 

CONDENSATION POLYMER

Condensation polymers are any kind of polymers formed through a condensation reaction, releasing small molecules as by-products such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymers include polyamides, polyacetals and polyesters.

Condensation polymerization, a form of step-growth polymerization, is a process by which two molecules join together, resulting loss of small molecules which is often water. The type of end product resulting from a condensation polymerization is dependent on the number of functional end groups of the monomer which can react.

Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular weight. Linear polymers are created using monomers with two reactive end groups and monomers with more than two end groups give three dimensional polymers which are cross linked.

The monomers that are involved in condensation polymerization are not the same as those in addition polymerization. The monomers for condensation polymerization have two main characteristics:.

 Instead of double bonds, these monomers have functional groups (like alcohol, amine, or carboxylic acid groups).

 Each monomer has at least two reactive sites, which usually means two functional groups.

The production of monomers and intermediates is clearly tied to the market penetration and sales of particular polymers. Since the distribution of hydrocarbon structures in the feedstock does not coincide closely with the repeat structures of tonnage polymers, there are clear problems of balancing supply with demand. Since vinyl polymers are in a mature stage of development, the demand for ethylene exceeds that for propylene with the result that polypropylene prices are much lower than they would be otherwise. Moreover, there are many unused co-products (meta-xylene is a good example) which cannot be used in quantity to make polymers. Even if new and interesting polymers based on these intermediates were developed, it would be many years before market penetration would mop up available supplies of this chemical

CHEMICAL REDUCTION

Reduction reactions

Reduction is the loss of oxygen from a molecule or the gaining of one or more electrons. Reduction is a process where a substance

• Gains one or more electrons
• Loss  an oxygen atom or atoms
• Gain  a hydrogen atom or atoms 

Reduction is gain of electrons and thus gaining of negative charge. The atom that acquired electrons is said to be reduced.

Methods of reduction

In chemical reduction process, the choice of reducing agent depends upon the chemical reactivity of the metal.

Sub Topics

1.     Smelting  (Carbon reduction method)

2.     Hydrogen reduction method

3.     Carbon monoxide reduction method

4.     Magnesium reduction method

5.     Aluminium  reduction method  (Alumino Thermic Process)

6.     Self-reduction method

7.     Reduction by more electropositive metals (precipitation or hydrometallurgy)

Smelting (Carbon reduction method

This method is used for the extraction of lead, zinc, iron, copper, manganese and tin. In this method, the roasted oxide ore is mixed with carbon (charcoal, coal or coke) and a flux, and is heated to a very high temperature in a suitable furnace. Carbon reduces the oxide to metal.

Hydrogen reduction method

Hydrogen can reduce certain oxides to metals e.g.,

Carbon monoxide reduction method

In certain cases CO gas produced in the furnace itself can be used as a reducing agent. For example,

Magnesium reduction method

Oxides of certain metals are reduced by Mg e.g.,

Aluminium reduction method (Alumino Thermic Process)

Certain metal oxides cannot be reduced by carbon. Such metallic oxides can be reduced by aluminium powder. This process has been widely used to reduce TiO₂, Cr₂O₃ and Mn₃O₄ to get the corresponding metal.

Self-reduction method

When the sulphide ores of less electropositive metals like Hg, Cu, Pb, Sb etc., are heated in air, a part of the ore gets oxidized to oxide or sulphate, which then reacts with the remaining sulphide ore to give the metal and SO₂. This process is also known as self-reduction method.

Reduction by more electropositive metals (precipitation or hydrometallurgy)

This method is employed when leaching method had been used to concentrate the ore. The metals are obtained by reducing their ions in the solution as precipitates by a more electropositive metal. This method is also called as the hydrometallurgy method of reduction.

For example, when a heap of copper glance (Cu₂S) is exposed to air and water, it gets converted to copper sulphate. Copper is recovered from copper sulphate solution by adding some iron scrap to its solution.