5.16 Other Polymers

Polyurethanes are made from two monomers that react together, forming an alternating pattern. One monomer has an isocyanate -N=C=O group at both ends, and the other is a polyol such as ethylene glycol. The result is a polymer of -CO-NH-R-NH-CO-O-R'-O- units, where the exact values of R and R' determine much of the material's properties.

Epoxy resins are those based on an epoxide group, that is, a three-membered ring of two carbon atoms and one oxygen. Such a ring is unstable due to bond strain, and can easily react with phenols, amines, acids, alcohols, or thiols. By selecting molecules with two or more functional groups, the epoxy reaction can be made to produce various polymers.

Bakelite is formed by the reaction of phenol with formaldehyde. Formaldehyde in water converts reversibly to methylene glycol, and the two compounds constantly interconvert. Methylene glycol can react with phenol at the ortho- and para- positions, releasing water and bridging the aromatic rings with methylene groups. The result is a haphazard arrangement of monomers stuck together any which way, that cannot be melted or dissolved in solvents because the entire piece is one huge molecule. Its strength comes from the fact that it is held together by covalent bonds, not hydrogen bonds or van der Waals forces like most polymers.

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