Polymerization

Monomers are small, individual units of a larger molecule that can chemically link together to form polymers. For example, nucleotides are monomer units that comprise the building blocks of DNA and RNA, that are genetic macromolecules.

Polymer [Greek, poly (many) + meros (parts)] is a large molecule whose structure repeats units of simpler chemical compounds. Thus, polymers contain repeating units of monomers.

Here are some examples:

• Polyethylene – CH2=CH2 (monomer) -> n (polymer) = CH2-CH2-CH2-CH2-CH2…
• Polypropylene – CH(CH3)-CH2 (monomer) -> n (polymer) = CH(CH3)-CH-CH(CH3)-CH-…
• Polyvinyl chloride – CH2=CH-Cl (monomer) -> n (polymer) = CH2-CH-Cl-(…)-Cl

Different forms of Polymers 

• Homopolymer

If the monomers are identical, the polymer is called a “homopolymer”, such as polyethylene or polypropylene.

• Copolymers

If the polymer contains more than one type of monomer, it is called a “copolymer”. For example, ethylene-propylene copolymer. 

Polymerization

Polymerization combines monomer molecules in a chemical reaction to form three-dimensional networks, polymer chains, and plastic.

Types of Polymerization

• Addition Polymerization

Addition polymers are formed by adding monomers together via covalent bonds. Examples include polyethylene (polythene) and polystyrene.

• Condensation Polymerization

Condensation polymers are formed by condensing monomers together via covalent bonds that result in the loss of small molecules such as water or methanol. E.g., nylon.

• Chain Reaction Polymerization

Chain reaction polymers result from chain reactions that use free radicals to propagate polymerisation. Examples of chain reaction polymers include styrene-butadiene rubber and polyvinyl chloride.

Classification of Polymers 

There are three classes of polymers.

(A) Based on Structure

• Linear Polymer

A polymer in which the monomers join to form long straight chains is linear. The simplest example of a linear polymer is polyethylene (polythene). 

• Branched Polymer

A polymer in which the main chain has branches emanating from it is called a branched polymer. The simplest example of a branched polymer is propylene oxide.

• Cross-Linked Polymer

A polymer in which one or more monomers are joined by multiple bonds instead of single bonds is called a cross-linked polymer. 

(B) Based on Polymerisation

Addition Polymerisation occurs when monomer units combine to form long chains without releasing other molecules like water, alcohol etc. 

Condensation Polymerisation is when many monomers combine to form a long chain to eliminate a small molecule such as an alcohol or water molecule from the polymer chain.

(C) Based on Molecular Forces

1. Elastomers are polymers that can be stretched to great extents and return to their original length upon stress release. 
2. Fibres are used for textiles and papermaking because of their strength and rigidity.
3. Thermoplastic polymers are also known as engineering plastics. 
4. Thermosetting polymers are extensively used as adhesives as they have excellent binding properties.

Commercially Important Polymers

Here are some economically-valued polymers:

• Addition Polymers

• Polyethylene: Polyethylene has a linear structure. But it has variants with branched structures consisting of side chains of varying lengths.

Preparation of LDPE: Manufacturers prepare low-density polythene by free radical polymerisation in a slurry process at a temperature of about 170 °C and pressure of about 1000 atm. The monomer ethylene does not fully convert into the polymer during this process.

The catalyst used for preparing LDPE include peroxides such as dibenzoyl peroxide or benzoyl peroxide and sodium persulphate in the presence of an alcohol such as ethanol or methanol.

Preparation of HDPE: Commercial manufacture of high-density polythene employs additional polymerisation of ethylene at a temperature of about 250 °C and pressure of about 2000 atm. The monomer ethylene fully converts into the polymer at this condition.

The catalyst used for preparing HDPE includes chromium oxide, silica-alumina, and titanium silicate.

• Condensation Polymers

• Polyamides

Polyamides are polymers in which amide groups link the monomers. Examples include Nylon, Kevlar, Nomex, and Brominated Polystyrene.

Preparation of Nylon: Industries manufacture Nylon 6,6 from adipic acid and hexamethylenediamine (1,6-diamino hexane). They mix the acid and the diamine with water in a reactor at high temperatures and pressure.  

• Polyesters

Polyesters are a class of polymers that contain the ester functional group in every repeat unit of their main chain. It most commonly refers to a polymer used to make clothing and plastic bottles as a specific material.

Alkyd resins: Alkyd resins are polyesters derived from a dicarboxylic acid or an ester-forming derivative (anhydride, acid chloride) and polyhydric alcohol. 

Melamine: Melamine resin is a synthetic polymer often combined with formaldehyde to produce melamine-formaldehyde, durable thermosetting plastic with fire-retardant properties.

• Rubber

Natural Rubber is a polymer of isoprene (2-methyl -1, 3- butadiene) and has the empirical formula (C5H8)n. 

Tapping latex sap from rubber trees is the natural process to produce rubber. It is similar to maple syrup production. In contrast, human-made processes create synthetic rubbers.  

Vulcanisation of Rubber: Vulcanisation refers to adding sulphur to the rubber polymer chain. This reaction occurs at temperatures of 140-220 °C and results in a change in the material that no longer melts but rather “glasses over” around 130 °C.

Conclusion

The most common polymers are synthetic polymers made from petroleum products through a process known as polymerisation. Some polymers occur in nature, such as proteins and DNA. Today thousands of synthetic polymers are used for several applications, including electronics, packaging and construction.  

Fibres are polymers as well. One can spin them from melts or solutions; we can also produce synthetic fibres from gases (e.g., rayon).  

Biodegradable polymers are a form of plastic that decomposes via natural processes (e.g., by the action of microorganisms) into compounds that are not harmful to the environment.