Polymers difunctional growing ends

An especially simple way to prepare triblock polymers having identical end blocks involves the employment of difunctional initiators, such as l,4-dilithio-l,l,4,4-tetraphenylbutane. The growing chain has the structure... 

Thus the first step in the initiation reaction [Eq. (64)] involves a reversible electron transfer reaction from the alkali metal to the styrene monomer to form the styryl radical anion in a rapid subsequent reaction, two radical anions couple to form a di-anion which can grow a polymer chain at both ends. In the case of the soluble alkah metal aromatic complexes, the overall initiation reaction is extremely fast, due to the high concentrations of radical anion ( 10 M) and monomer (-1M), and so is the subsequent propagation reaction. However, in the case of the alkah metal initiators, the electron transfer step [Eq. (64)] is very much slower, due to the heterogeneous nature of the reaction, so that the buildup of radical anions is much slower. In fact, there is evidence [153] that, in such cases, a second electron transfer step can occur between the metal and the radical anion to form a di-anion, rather than coupling of the radical anions. In either case, the final result is a di-anion, i.e., a difunctional growing chain. 

In order to participate in polymer formation, a molecule must be able to react at both ends (difunctional) so that the polymer chain can grow in length. 

These are the most important. The two double bonds mutually activate each other conjugation is essentially not destroyed by addition to the growing chain end. Therefore the conjugated dienes are difunctional monomers. They are polymerized by a relatively simple mechanism. Of all the polymers generated in living tissues, we have so far been able to imitate most closely natural rubber, poIy-cis-l,4-isoprene. Butadiene, isoprene and chloroprene are the dienes most often employed in macro-molecular chemistry. 

The resulting product molecule is still difunctional Its left end can react with another diacid molecule and its right end with another molecule of diol, and after each subsequent reaction, the growing molecule is still difunctional and capable of undergoing further growth, leading to a true polymer molectde. 

When the initiator is based on the reaction product of sodium and naphthalene (Figure 4.8), the growing chain is difunctional. It is possible to put various functional groups on both ends of a single chain in a single step. In fact, if done correctly, this initiator can be used to produce ABA polymers directly by polymerization of one monomer followed by addition of a second type. 

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