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Photoinduced racemization of an optically active helical polymer formed by the asymmetric polymerization of 2,7-bis(4-tert-butylphenyl)fluoren-9-yl acrylate.

The helix is a fundamental macromolecular structure. The construction and mutation of helical structures have been studied for a variety of synthetic polymer systems. Macromolecular stereomutation is triggered by chemical or thermal stimuli in most cases, and fewer examples are known of stereomutation triggered by light. Existing examples of photoinduced stereomutation involve photochromic groups incorporated into the side chains or main chain of polymers. These groups cause the isomerization of a double bond or bond cleavage and formation. Herein we report the synthesis of optically active preferred-handed helical poly(2,7-bis(4-tert-butylphenyl)fluoren-9-yl acrylate) (poly(BBPFA)), a novel polyacrylate, and its effective racemization induced by photoirradiation. In contrast to the previous examples, the stereomutation observed for this polymer is governed by simple rotation about the single bonds between the side-chain fluorenyl moieties and the tert-butylphenyl substituents without any changes in chemical bonding. Polymerization was carried out with the complex formed between 9-fluorenyllithium (FlLi) and (+)-1-(2-pyrrolidinylmethyl)pyrrolidine (PMP) as the initiator in toluene at 78 8C (Scheme 1, Table 1). This initiator system has been used to synthesize preferred-handed helical polymers from bulky (meth)acrylates. The polymerization of BBPFA occurred with almost quantitative monomer conversion. The observed increase in Mn with an increase in the [BBPFA]/[FlLi] ratio (from 10:1 to 40:1) implies that the polymerization has