# 29.8: Block, Graft, and Ladder Polymers

A variation on the usual variety of copolymerization is the preparation of polymer chains made of rather long blocks of different kinds of monomers. A number of ingenious systems have been devised for making such polymers, including the Szwarc method described in Section 29-6D. Another scheme, which will work with monomers that polymerize well by radical chains but not with anion chains, is to irradiate a stream of a particular monomer, flowing through a glass tube, with sufficient light to get polymerization well underway. The stream then is run into a dark flask containing a large excess of a second monomer. The growing chains started in the light-induced polymerization then add the second monomer to give a two-block polymer if termination is by disproportionation, or a three-block polymer if by combination. Thus, with $$\ce{A}$$ and $$\ce{B}$$ being the two different monomers,

Block polymers also can be made easily by condensation reactions:

The very widely used polyurethane foams can be considered to be either block polymers or copolymers. The essential ingredients are a diisocyanate and a diol. The diisocyanate most used is 2,4-diisocyano-1-methylbenzene, and the diol can be a polyether or a polyester with hydroxyl end groups. The isocyano groups react with the hydroxyl end groups to form initially an addition polymer, which has polycarbamate (polyurethane) links, and isocyano end groups:

A foam is formed by addition of the proper amount of water. The water reacts with the isocyanate end groups to form carbamic acids which decarboxylate to give amine groups:

The carbon dioxide evolved is the foaming agent, and the amino groups formed at the same time extend the polymer chains by reacting with the residual isocyano end groups to form urea linkages:

$\ce{R'N=C=O} + \ce{RNH_2} \rightarrow \ce{R'NHCONHR}$

Graft polymers can be made in great profusion by attaching chains of one kind of polymer to the middle of another. A particularly simple but uncontrollable way of doing this is to knock groups off a polymer chain with x-ray or $$\gamma$$ radiation in the presence of a monomer. The polymer radicals so produced then can grow side chains made of the new monomer.

A more elegant procedure is to use a photochemical reaction to dissociate groups from the polymer chains and form radicals capable of polymerization with an added monomer.

Modern technology has many uses for very strong and very heat-resistant polymers. The logical approach to preparing such polymers is to increase the rigidity of the chains, the strengths of the bonds in the chains, and the intermolecular forces. All of these should be possible if one were to make the polymer molecules in the form of a rigid ribbon rather than a more or less flexible chain. Many so-called ladder polymers with basic structures of the following type have been prepared for this purpose:

With the proper structures, such polymers can be very rigid and have strong intermolecular interactions. Appropriate syntheses of true ladder polymers in high yield usually employ difficultly obtainable starting materials. An example is

Although there seem to be no true ladder polymers in large-scale commercial production, several semi-ladder polymers that have rather rigid structures are employed where high-temperature strength is important. Among these are