Sometimes C chains become extremely long containing thousands of C atoms. When the number of C atoms in a molecule reaches these large numbers, a giant molecule is created, which we call a polymer.
Polymer means many parts, because usually these molecules are formed by linking many smaller molecules (called monomers) together, like an elephant train. • There are 2 sources of polymers. • There are many natural polymers found in Nature, some of which we will study in later chapters. • The first synthetic polymer was derived from a natural polymer, cellulose, and was called celluloid and was first used as a replacement for ivory in billiard balls.
Then beginning in 1909, the first truly synthetic polymer was produced in a laboratory and this has spawned a multibillion dollar industry. Everything that consumers call plastics are synthetic polymers. We will now look briefly at some of these.
2 Types: 1) Addition 2) Condensation
Addition polymers – Large molecule composed of one or sometimes 2 smaller molecules (called monomers), linked together end to end, basically (sort of like a train of elephants). Requirements of monomers For addition, at least one double bond – Linking takes place by breaking the double bonds and adding each monomer at that point.
A few common addition polymers are described in your book on pages 268 – 271 and 272 - 273. You should read these. • Note that in each case, the monomer has at least one double bond. • Table 10.1 on page 275 has a more detailed list of common addition polymer plastics and their uses.
Generally, plastics are insulators (don’t conduct heat or electricity. • But there is even a plastic called polyacetylenewhich does conduct electricity and is sometimes used as a metal substitute. • The key idea to remember is that polymers can be designed and made to meet almost any need.
By adjusting the reaction conditions, chemists can adjust the exact structure of these compounds. The same general polymer, such as polyethylene can be made elastic or rigid, soft or hard, clear or opaque. We won’t go into these specifics, except in the following situation:
Diene Polymerization - C=C-C=C. Usually only one C=C is utilized, leaving ...-C-C=C-C... , leaving one C=C in the middle. Natural rubber is a polymer of a diene: The repeating unit is found to be:
Natural rubber was not a very useful product until Charles Goodyear in 1844, discovered that if you heat natural rubber in the presence of S, cross links are made between different strands of the polymer utilizing S atoms as the bridge. • This process is called vulcanization. It produced a much tougher & less gummy product.
When this was done for the first time in a laboratory, the product had some rubber-like properties, but not all & it was basically not usable (sticky & tacky) We need to digress for a few minutes here. .
Further study showed that natural rubber was almost all cis around C=C, while the synthetic was a mixture of cis & trans. Using special catalysts the synthetic polymer with almost all cis was made & it was just as good as natural rubber. Thus after World War II, synthetic rubber compounds became a major industry. Various variations on monomers have yielded various different synthetic rubbers.
The second type of polymer : Condensation Polymers – Polymer formed when 2 or more monomer molecules linked together via a condensation reaction, each reaction eliminating a small molecule, usually H2O or HCl.
The monomer requirement for condensation polymerization is that it must contain 2 functional groups • One functional group on the first monomer reacts with the opposite functional group on the second monomer producing a larger molecule, that still has 2 functional groups. • This process can continue on and on.
Condensation Polymers: 1. polyamide - Nylon (different types) & protein.Fibers of nylon are formed by the following: Melt spinning - Heat to just above M.P., force at high pressure through tiny holes into stream of N2 gas to cool. Solidifies quickly. Then drawn into 4 times original length, which orients polymer chains into regular side by side position --> increases MP - harder, denser, tougher. (extensive H bonding)
2. Polyesters – polymer formed when a molecule containing 2 COOH groups (a di-acid) and a molecule containing 2 OH groups, a di-alcohol or glycol
Another way we classify plastics is whether they can be remolded or not. • A thermoplastic polymer can be softened by heat and pressure and then reshaped. This can be done repeated times. • These can be recycled with some effort. Some are currently being recycled by many communities. • A thermosetting plastic cannot be softened by heat and remolded. Heating causes discoloration and decomposition.