POLYMER END-GROUP ANALYSIS: THE DETERMINATION OF AVERAGE
Background reading: This exercise uses a strong acid titration by a strong base with a visual
indicator. See; Harris, 7
ed., Chap. 11.
Skoog, West, Holler and Crouch, 7
ed., Chap. 14.
Polymers are a special form of macromolecules They are compounds of high molecular
weight formed by combining a large number of small molecules. The small molecules, called
monomers, may all be of one type, as in the compound used in this experiment, or may be of different
Polymers are very important in biological systems. For example, proteins are composed of intricate
sequences of amino acids, and polysaccharides contain repeating units of simple sugar molecules.
Our everyday lives are also greatly influenced by polymers that are not obtained from natural
sources. Synthetic polymers are known to us as various fibers (e.g., Dacron), plastics (e.g., polyvinyl
chloride), abbreviated PVC, and polystyrene), and elastomers. See Fig. 1.
Like PVC and polystyrene, the compound involved in this experiment is synthesized from only one
kind of monomer molecule, ethylene glycol. The monomers are joined by elimination of a molecule of
water to form a series of ether linkages.
-OH + n H
The polymer is called either poly(ethylene glycol) (PEG for short), poly(ethylene oxide), or, more
properly, poly(oxyethylene). PEG is commonly found in the list of ingredients in hair preparations and
cosmetics. It is a good lubricant and also has the desirable property of being soluble in water.
Therefore, PEG is heavily used as a base for therapeutic ointments, and it is used industrially as a
lubricant in the formation of textile fibers and metal products.
depend heavily on their molecular weights, which vary according
to the number of repeating monomer units (the n’s in the
structures in Fig. 1) per polymer molecule. For example, the
PEG molecules to be studied in this experiment have molecular
weights less than 1000 and are viscous liquids. As the molecular
weight increases, the compound is more grease-like, and, if the
MW is greater than about 6000, PEG is a white solid.
The use of the term “molecular weight” is somewhat misleading,
because it implies that a sample of a polymer has a uniform
formula weight (i.e., that n is the same for all the polymer
molecules in a sample). For many biopolymers, most notably
proteins, the sequence of the monomers is dictated by the
specific biological role of the compound, and the structure and
molecular weight is the same for all molecules of that type.