Freight Trains-A
Useful Analogy for Polymers
Students in my chemistry for the health sciences class have found freight trains a useful analogy for polymers and polymerization. Many freight cars (monomers) are coupled (bonded together) to form a single entity, the train (polymer). Freight cars are difunetional-they have acoupler oneach end. Thestepwise addition of cars to the growing train is aclose parallel to stepwise condensation polymerization. When a car is added to the train, one coupler (functional group) is used up, but the second one on the other end is available for coupling (reaction), so the train (polymer) can be made very long without running out of couplers (reactive sites). Reaction continues until the train is assembled (the polymer made). Many students have commented that this really clarifies the concept of stepwise addition of monomers. Locomotives have aeab a t one end, the "tail". In multiengined trains, thelocomotives can be attached to eachother in any sequence. If the head of each is coupled to the tail of the next, the locomotives are isotactic: if each head is attached t o a head, and each tail to a tail, the locomotives are syndiotactic, and random attaching is atactie. The individual cars remain separated until the switch engine assembles them. Similarly, the alkene monomers for an addition polymer remain unreaeted until the peroxide or light initiates the reaction. Thus, alkenes do not spontaneously react in thestorage tank by themselves.
William C. Hoyt Saint J o s e p h College Windham. ME 04062
718
Journal of Chemical Education
