Homopolymer

Polythene covers are manufactured with polyethylene which is a polymer molecule. Polymer molecules are macromolecules composed of a large number of monomer units. It is composed of ethene molecules; an unsaturated hydrocarbon with two carbon atoms which are bonded with each other through covalent bond and form a long polymeric chain. Today we have large number polymers which are commonly called as plastics or resin. Chemically they are compounds where molecules or monomer units are bonded together in long repeating chains. The chemical and physical properties of polymers depend on the structure and bonding of monomer units. They can be synthesized with different chemical compounds and some of them are also found in nature. The polymers with natural sources are called as natural polymers whereas polymers formed in laboratory are called as synthetic polymers. Rubber, starch, DNA, RNA, cellulose and proteins are some common examples of natural polymers. 

Polymers have a range of applications such as for the manufacturing of adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices and optical devices etc. Polymers are widely used in agriculture and agribusiness improves aeration in soil and to promote plant growth. Many biomaterials such as heart valve replacements and blood vessels are manufactured of polymers. Dacron, Teflon and polyurethane are common polymer used in medical field. Polymers are also used in manufacturing in plastic containers, garbage disposal bags, clothing manufacturing and floor coverings. Some common properties of polymers are impact resistant, toughness and brittle nature, translucent, malleable, soft and elastic. The reaction of formation of polymer by monomer units is called as polymerization. So we can say that polymerization is the method of creating a polymeric chain by the combination of small monomer molecules through covalent bonds. On the basis of polymerization, polymers can be classified as step growth polymers and chain growth polymers. In chain growth polymerization, monomer molecules are bonded directly with one another whereas step growth polymerization involves certain steps which results the formation of polymeric chain. Cross linked polymers cannot re-bond after the cross linked bond between monomer units is broken. They are rigid, hard polymers with good thermal properties.

Some common examples of polymers and their uses are listed below.

1. Polypropylene (PP) – Used for manufacturing of carpet, upholstery
2. Polyethylene low density (LDPE) – Mainly used for making of grocery bags
3. Polyethylene high density (HDPE) – Used to manufacture detergent bottles and toys
4. Poly(vinyl chloride) (PVC) – Used in piping and decking
5. Polystyrene (PS) – Used in making of toys, foam
6. Polytetrafluoroethylene (Teflon) – Used in manufacturing of  non-stick pans and also for electrical insulation
7. Poly(methyl methacrylate) (PMMA) – Used for face shields and skylights
8. Poly(vinyl acetate) (PVAc) – Added in paints and as adhesives
9. Polychloroprene  - Used in making of wetsuits

Homopolymer Definition

On the basis of origin, polymers can be classified as natural and synthetic polymers. On the basis of chemical nature, they can be classified as organic and inorganic polymers. Organic polymers have carbon atoms in the polymer backbone of chain. For example; polypropylene, poly (vinyl chloride) are organic polymers because the polymeric backbone is composed of carbon atoms. Inorganic polymers do not have carbon atoms in their polymeric chain such as Silicone rubbers. 

On the basis of monomer composition, polymers can be classified as homo-polymers and co-polymers. Homopolymers can be defined as the polymers which have one type of constituent monomer units in polymeric chain. They can be represented as;

-A-A-A-A-A-A-A-A-A-A-A-A-

Copolymers are composed of two or more different types of monomer units and generally represented as; 

-A-B-A-B-A-B-A-B-A-B-

Homopolymers can be linear, branched or cross-linked which depends on the linkage between monomer units. 

Copolymers are also known as heteropolymers which can be further classified as graft, random, alternating and block polymers. Random copolymers are composed of two different monomer units which are bonded in random manner. They can be represented as;

-A-B-B-A-A-B-A-A-A-B-A-A-B-

Alternating polymers have two different monomer units which are arranged in alternate manner. 

-A-B-A-B-A-B-A-B-A-B-A-B-A-B-

Homopolymer Examples

Polyethylene: It is one of the most common examples of Homopolymer which is composed of ethylene monomer units. 

Ethene is an alkene with one double bond between two carbon atoms. It involves in addition polymerization which is also known as chain growth polymerization and form long polymeric chain which can be linear or branched. The density of polymer depends on the linear and branched structure of polymer.

High density polyethylene (HDPE) is a linear polymer whereas presence of short or long branches on parent polymeric chain form low densitypolyethylene (LDPE). 

Both high density polyethylene (HDPE) and low density polyethylene (LDPE) have very different physical properties.  The melting point of LDPE is 115º C whereas HDPE melts at 135º C. Compare to HDPE, LDPE is more flexible has less strength. No doubt both of these polymer chains are composed of ethylene monomer units only.

Polypropylene: Like polyethylene, polypropylene (PP) is also composed by an alkene with three carbon atoms; propylene. Due to presence of double bond in the molecule, it involves in addition polymerization and form long polymeric chain. 
Some common properties of PP are semi-rigid nature, translucent, tough, good chemical resistance, heat resistance and fatigue resistance with integral hinge property.

Poly (vinyl chloride): It is commonly known as PVC. It is a thermoplastic which can be mold in different shapes. It is supplied in powder form which shows resistance against oxidation and degradation. It is a chain growth polymer of vinyl chloride (CH2=CHCl). 

Polystyrene: Polystyrene (PS) is a Homopolymer because it is composed of same kind of monomer units; styrene. Unlike PE, PVC and PP, it is an aromatic compound. 

Polystyrene is mainly used in the manufacturing of drinking cups which are formed by polystyrene foam and known as Styrofoam. The arrangement of aromatic ring along with polymeric chain determines the properties of polymer. On the basis of presence of aromatic ring polystyrene can be two types; atactic and syndiotactic. 

Polypropylene Homopolymer

Polypropylene is a Homopolymer as it is composed of same kind of monomer units. Monomer unit for PP is propylene which is an unsaturated hydrocarbon with three carbon atoms. 

Polypropylene is also abbreviated as PP which is a thermoplastic and form by addition polymerization of propylene monomer units. The main application of PP is for packaging of consumer products, plastic parts such as automotive industry, living hinges, and textiles. It was first polymerized by Paul Hogan and Robert Banks in 1951. Some common properties of PP are listed below.

• Chemical resistance: No reaction with diluted bases and acids
• Elasticity and toughness- it has ability to deform without breaking.
• Fatigue resistance: It retains its shape after bending or flexing. 
• Insulation: It exhibits high resistance to electricity.
• Transmissivity: It can be made transparent or opaque in color.  

Acetal Homopolymer

Homopolymers are composed of same type of monomer units. On the basis of arrangement of monomer units they can be classified as linear, branched, cross linked and network polymers. Linear polymers have single continuous chain of monomer units whereas branched chains have side chains of monomer units on the main chain of polymer. Cross linked polymers have interconnections between polymeric chains whereas network homopolymers have numerous interconnections between parent chains which make them more complicated than cross linked. 

Acetal polymers are also known as polyacetals such as polyformaldehyde and polyoxymethylene (POM). Polyformaldehyde has formaldehyde (HCHO) as the starting material for the polymerization. Polyoxymethylene (POM) is an acetal polymer. It is a thermoplastic polyacetal and polyformaldehyde. Some common properties of polyacetal are high strength, high stiffness, low friction, opaque white, density of 1.410–1.420 g/cm3 and excellent dimensional stability.  They are mainly marketed as Delrin,  Duracon, Celcon, Ramtal,  Kepital and Hostaform. Polyoxymethylene is used in high-performance engineering components like small gear wheels, ski bindings, fasteners, eyeglass frames, ball bearings, guns, knife handles etc.

Homopolymer vs Copolymer

Polymers are made of same or different types of monomer units which are bonded through addition or condensation polymerization reaction. On the basis of types of monomer units, polymers can be classified as Homopolymer and copolymers. 

Homopolymers are composed of same types of monomers whereas copolymers are composed of different monomers. Some common examples of homopolymers are nylon 6, nylon 11, polyethylene, polypropylene, polyvinyl chloride, and poly-acrylonitrile. Polyester, Nylon-6, 6 are examples of copolymers. Copolymers are polymerized from two or more different monomers. They can be classified as alternating copolymer, block copolymer, graft copolymer and random copolymer. Some other differences are listed below. 

Homopolymers	Copolymers
These polymers have same type of monomer units.	This is also known as heteropolymer as they are composed of two different kinds of monomers.
They can be classified as linear, branched, cross linked and network homopolymers.	They can be classified as alternating copolymer, block copolymer, graft copolymer and random copolymers.
Linear homopolymers have linear long polymeric chain of same types of monomer units. For example; HDPE	In an alternating copolymer, the two monomers are arranged in an alternative way and can be represented as ABABABABAB.
Branched homopolymers have short or long branches bonded on parent polymeric chain such as LDPE.	In random copolymer the monomers are arranged in any order such as AABAAABBBBAB.
Cross linked and network homopolymers have braches on parent chain which are interconnected with each other to form cross linked and network polymer.	In block copolymers, two blocks of homopolymers are joined together.It can be represented as AAAAAAABBBBBBB.
Short term stiffness	Long term stiffness
Short term impact strength	Long term impact strength
Example: Polyethylene, Poly(vinyl chloride), Polypropylene, Polystyrene	Example: Nylon-6,6, polyester, protein, Buna-S