Polymerization

Monomers

These are simple molecules of the same chemical compound that undergo polymerization. Such molecules must meet certain criteria:

1. They need to contain multiple bonds, usually double bonds, which can be broken and, as a result, release two electrons allowing the formation of new bonds.
2. They must contain two reactive functional groups in their structure.

A mer is the smallest, repetitive part of the polymer chain. The general equation of the polymerization reaction has the following notation:

_{MONOMER                           POLYMER                          MER}

Types of polymerization

The reaction that enables the formation of polymers can be classified according to its mechanism. So, on the basis of the mechanism, one may distinguish:

• chain polymerization,
• polyaddition,
• condensation polymerization.

Chain polymerization

Most often, it is a radical reaction that occurs only for monomers containing multiple bonds capable of generating radicals during breaking. These radicals have the ability to be combined into long chains. The reaction consists in repeated attachment of monomers to the same active site. This reaction must always be initiated by chemicals having the ability to produce a light quantum as a result of thermal decay or redox reactions of high-energy radicals with a short lifetime and poor resonance stabilization. A large part of polymers (>70% of vinyl polymers) produced on an industrial scale is obtained using this method. Its advantages are: high efficiency, simple technology, high resistance to pollution, possibility to use a commonly available solvent – water, as well as the possibility to easily predict kinetics of the process. The chain polymerization reaction is divided into several stages: initiation, propagation – chain elongation, chain transfer and termination. In addition to radical polymerization, the chain mechanism can also be anionic or cationic. In the case of anionic polymerization, e.g. in vinyl monomers, the inductive effect of the substituents is utilized.A group capable to attract electrons is able to induce a positive charge present on adjacent atoms due to the stronger bonding of the electron pair of the double bond. In the case of cationic polymerization, it is necessary to use monomers including a donor moiety, e.g. vinyl ethers.

Polyaddition

Also known as gradual polymerization, the process is based on the rearrangement of atoms between monomers with the formation of by-products, just like in the case of chain polymerization. However, it is gradual in nature.

Condensation polymerization

It is a type of reaction occurring only for monomers with at least two functional groups able to react with the release of a by-product, most often in the form of water. There are two possible types of such condensation. Heteropolycondensation – if the monomer contains two different functional groups that react with each other, or homopolycondensation – if the monomer is made up of two identical functional groups. They can only react with a comonomer, i.e. the second monomer present in the reaction, containing other functional groups.

Classification of polymers based on physical and chemical properties

1. Elastomers – highly flexible, rubber-like polymers with the ability to stretch multiple times and return to their original dimensions, e.g. cross-linked polybutadiene.
2. Duromers – excellent construction materials. Their main properties are: hardness, lack of flexibility and a very high mechanical strength. Heavy melting polymers from this group are called duroplasts. They include e.g. Bakelite and epoxy resins.
3. Plastomers – also known as thermoplasts, are slightly less rigid than duromers. After melting, it is possible to process them, but repeated thermal treatment has an adverse effect on their mechanical and functional properties. This group of polymers includes polyethylene, polypropylene, methyl polymethacrylate, etc.

Classification of polymers based on origin

1. Natural polymers are polycompounds that occur in nature. They can be used directly or after modification. The most commonly used ones are:
   a) natural rubber – polyisoprene, which, when subjected to a sulphur vulcanization process, allows the production of rubber (approx. 3% of sulphur) used in various gaskets, tires, toys, flexible fabrics and household items, and Ebonite (approx. 25-30% of sulphur) used in battery boxes, chemical devices and insulating materials.
   b) polysaccharides – such as ribose, glucose, fructose, starch and cellulose. These are readily used materials made of monosaccharides binded by a glycosidic bond. Cellulose is used in the production of paper, adhesives or rayon, and starch is used virtually in every industry, including textiles, pharmaceuticals and cosmetics.
   c) proteins – their monomers are known as α-amino acids, such as glycine, cysteine and phenylalanine. They are the fundamental constituents of all living organisms and have various biological functions. Amino acid residues are linked by peptide bonds. They are mainly used in the food industry and in medicine.

2. Artificial polymers are man-made compounds obtained through chemical synthesis. Polymerization can occur by a chain reaction, polyaddition, and polycondensation mechanisms. Examples include:
   a) chain polymers, such as: polyethylene used in the production of films, packaging or toys, polypropylene used in insulation materials, water pipes and car body parts, as well as polyvinyl chloride used in the production of floor coverings, electrical insulation materials, doors and windows.
   b) synthetic rubbers, including polybutadiene used in gaskets, latex paints and adhesives, and polychloroprene used in life rafts, diving suits and bands used for physical therapy purposes.
   c) polyaddition polymers, e.g. polyurethanes used in the furniture, automotive and footwear industries, as well as epoxy resins found in laminates, adhesives and various types of composites used in aviation, the automotive industry and boatbuilding.
   d) condensation polymers, e.g. polyesters most often used as PET, i.e. poly(ethyl terephthalate) for the production of dishes, bottles, packaging and fibres, polyamides, mainly Nylon, widely used as a component of stockings, tights, ropes, toothbrushes and Kevlar, polycarbonates being transparent thermoplasts with very good mechanical properties, often used for layers that make glass break-resistant, helmets and CD/DVDs, phenoplasts used mainly as Bakelite, aminoplasts and silicones.

Read also about emulsion polymerisation.