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Chemistry of Plastics
Posted Date: 12 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: How things work
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Posted By: Olufemi Member Level: Diamond
Rating:  Points: 5
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All plastics are polymers, but not all polymers are plastics. Some familiar nonplastic polymers include starches (polymers of sugars), proteins (polymers of amino acids) and DNA (polymers of nucleotides -- see How DNA Works). The simplified diagram below shows the relationship between monomers and polymers. Identical monomers can combine with each other to form homopolymers, which can be straight or branched chains. Different monomers may combine together to form copolymers, which also may be branched or straight.
The chemical properties of a polymer depend on:
The type of monomer or monomers that make up the polymer. The chemical properties of homopolymer 1 are different from those of homopolymer 2 or the copolymers.
The arrangement of monomers within the polymer. The chemical properties of the straight polymers are different from those of the branched polymers.
The monomers that are found in many plastics include organic compounds like ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol, vinyl chloride and acetonitrile (we'll examine many of these as we discuss various plastics). Because there are so many different monomers that can combine in many different ways, we can make many kinds of plastics.
The Chemistry of Carbon
All plastic is made of carbon. Man-made plastic uses carbon derived from oil, while biopolymers or bioplastics use carbon derived from natural materials. Carbon is essential because it has a fairly unique ability to combine with itself in many ways. Carbon can make single, double or triple covalent bonds with itself (electrons are shared between two atoms). Carbon atoms in compounds have four bonds around them. Carbon atoms can combine in linear chains, branched chains, or rings (single- or multiple-ring structures). Often, carbon combines with hydrogen and oxygen atoms, but it can also form bonds with other atoms -- like nitrogen, phosphorus and chlorine. Carbon compounds can be small, simple molecules like methane, or large, complex molecules like proteins and plastics. The carbon atoms in the carbon-containing monomers make bonds with other carbon atoms in other monomers in a variety of ways to form plastics. The types of monomers and the ways that they are arranged give different chemical properties to various plastics.
All biological compounds are organic compounds (like sugars, fats and proteins). Benzene, alcohols, graphite and gasoline are nonbiological organic compounds. There are thousands upon thousands of organic compounds, both natural and man-made. Organic chemistry is devoted to the study of carbon compounds.
Condensation and Addition Reactions
There are a few ways that monomers combine to form the polymers of plastics. One method is a type of chemical reaction called a condensation reaction. In a condensation reaction, two molecules combine with the loss of a smaller molecule, usually water, an alcohol or an acid. To understand condensation reactions, let's look at another hypothetical polymer reaction
Monomers 1 and 2 both have hydrogen (H) and hydroxyl groups (OH) attached to them. When they come together with an appropriate catalyst (an atom or a molecule that speeds up the chemical reaction without being used up in it), one monomer loses a hydrogen while the other loses a hydroxyl group. The hydrogen and hydroxyl groups combine to form water (H2O), and the remaining electrons form a covalent chemical bond between the monomers. The resulting compound is the basic subunit of copolymers 1 and 2. This reaction occurs over and over again until you get a long chain of copolymers 1 and 2.
Another way that monomers can combine to form polymers is through addition reactions. Addition reactions involve rearranging electrons of the double bonds within a monomer to form single bonds with other molecules. Imagine that two people (each a monomer) stand close together and each person has his/her arms folded (double bond). Then they unfold their arms and hold hands (single bond). The two people now make a polymer, and the process can be repeated.
Various polymer chains can interact and cross-link by forming strong or weak bonds between monomers on different polymer chains. This interaction between polymer chains contributes to the properties of specific plastics (soft/hard, stretchy/rigid, clear/opaque, chemically inert).
Olufemi
Nigeria
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