Paul W. Morgan
Textile Fibers Department E. I.d u Pont de Nemours & CO., Inc. Wilmington, Delaware
II
Models for h e m P0ly.m
O n e of the steps in teaching the general character of polymers and polymer formation is portraying the concept of long chain molecules. Polymer chains may be represented by linked paper clips or plastic "Pop-It" heads in various colors. B1ackweIl1 has described the latter system in detail. These models are simple and most helpful, but somewhat static and lifeless. A group of more active models, which has been used successfully before grade and high school students, is based on the use of small bar magnets as the connecting links for a lighter material representing the body of the polymer intermediate. (The use of magnets as connecting links in models for coordination compounds has been des~rihed.~) Models for Addition or Vinyl Polymers
Many well-known synthetic polymers are made by the catalyzed addition of unsaturated monomeric substances to themselves to form chains without any byproduct. Some of these are polyethylene, polystyrene, poly(methy1methacrylate), polyacrylonitrile, and poly(vinyl chloride). R
h~ 'atalyst C H 2 =R
r1
R
R
I RI '
-cHpLcH,-c-cH3-+
k,
Models for this group of polymers can be made by inserting small bar magnets in sections of a light body material so that a north and a south pole is exposed in each Lcmolecule"of intermediate (monomer). Alternatively, one end may contain a magnet with constant polarity and the other a nail head. (Figs. 1A and IB.) Appropriate coloring or structural appendages can he added to indicate the R groups and their position. A single bar magnet can be used to represent an intermediate molecule, if the center is covered with a ball or cylinder of inert material (Fig. 1C). For a demonstration a dozen or so monomer models are placed in a shallow paper bag and interacted by moderate shaking or tumbling. The operator then pulls from the bag slowly a chain of linked units (Fig. 2). The actual end-to-end contact requires a little manipulation in the bag as the chain grows, but this does not detract from the effect. The single magnet type (1C) will form chains when they are rolled together on a table surface.
71
'1
R' n
R and R' may be hydrogen or some other subst.itnent, group.
Figure 2.
Models for Condensation Polymers Figure
I.
Side views
d
modelr far polymer intermedioter.
BLACKWELL, R. Q., J. CEEM.EDUC., 34,500 (1957). WENDLANDT, W. W., J. CEEM.EDUC.,34, 223 (1957).
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Journol of Chemicol Education
Many condensation volvmers are formed by the interaction bf two complemeitary intermediate; to form a chain in which the two components alternate, as in many nylons, polyesters, and polyurethanes. These
are often spoken of as AA-BB structures. Condensation polymers are also formed from intermediates which contain the two interacting groups at opposite ends of a single intermediate molecule. Such polymers are proteins, cellulose, and the nylon, polycaprolactam. These are designated AB-AB condensation polymers. (AA could he hexamethylenediamine; BB, adipic acid, and AB, r-aminocaproic acid.) The models for addition polymers can serve as well for AB-AB condensation polymers. A model for the AA-BB type is prepared by making two groups of monomers, one with both outer poles being north and the other with the outer poles being south. (Fig. ID.) To save magnets one group could have iron screws or nails as the inserts. The difference between the two intermediates should be indicated by color, shape, or material of construction. The demonstration is carried out as before, using an equal number of each type of monomer. A chain having an alternating sequence should form (Fig. 2). This is a good companion experiment for the demonstration of condensation polymeri~ation.~
The analogy between these models and polymers or polymer behavior is schematic as well as inaccurate in many ways. For instance, they do not show the elimination of a small by-product molecule which accompanies a condensation reaction. The purpose is not exactness, however, but the dramatization of the general concept of linear polymers. The scheme can be elaborated to show copolymerization, branching, and chain termination in condensation polymerization by monofunctional materials, or failure to reach high polymer because of nonequivalence of complementary reactants. Excellent magnets (I/*" X 3/16" X 4 / g ' ' ) of Alnico V, alloy may be obtained inexpensively from Sunset House 100 Sunset Building, Beverly Hills, California. Balsa wood is readily purchased in small pieces from hobby and model shops. Other possible body materials are sponge rubber, foamed polystyrene, urethane foams, and papier-mbch4. a
MORGAN, P. W.,
AND
KWOLEK, S. L.,J. CHEW.EDUC.,36,
182, 530 (1959).
Volume 37, Number 5, April 1 9 6 0
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