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Poly(triarylmethyl methacrylate)s

PTrMA (1) is the first vinyl polymer prepared by the helix-sense selective polymerization of an achiral (prochiral) monomer, triphenylmethyl methacrylate (TrMA), using anionic initiators such as 9-fluorenyllithium (FlLi) complexed with chiral ligands such as (—)-sparteine (Sp), resulting in a single-handed, fully isotactic helical polymer with a large optical rotation ([a]D = ca + 380°) (Fig. 11.2) [9, 17]. The chiral ligand controls the main-chain configuration as well as the helical sense of the polymer. In addition, the bulky triphenylmethyl groups restrict unfolding of the helical conformation produced through the polymerization reaction, so that the optical activity of PTrMA disappears when the triphenylmethyl groups are removed for conversion to methyl esters. The helical conformation of PTrMA was further evidenced by the optical resolution of the optically inactive PTrMA prepared by achiral anionic initiators into fractions showing opposite optical rotations by chiral chromatography. The optically active helical PTrMA shows a remarkable chiral recognition for a variety of racemic compounds, giving a practically useful chiral packing material for high-performance liquid chromatogra-phy (HPLC), as described later in Section 11.5.

Fig. 11.2 Schematic representation of helix-sense selective anionic polymerization of TrMA and structures of methacrylates bearing a bulky ester group.

Okamoto et al. prepared a series of analogous helical poly(triarylmethyl metha-crylate)s by the helix-sense selective polymerization of bulky methacrylates (5-8) and investigated their helical structures and chiral recognition abilities as well as the mechanism of the helix-sense selective polymerization. Although anionic polymerization techniques have often been used for the synthesis of the helical polymethacrylates, more versatile, free-radical polymerization has been proved to be an alternative way to produce a helical polymethacrylate from 7 [18]. The free-radical polymerization of 7 with a, a'-azobisisobutyronitrile (AIBN) in the presence of chiral chain transfer agents or cobalt (II) complexes (9) interacting with the growing radical [19] or in chiral solvents gave an almost perfect isotactic polymer with an excess of the one-handed helical sense [9]. Radical polymerization of n-butyl-substituted N-triphenylmethyl methacrylamide (10) in menthol produced a highly isotactic, optically active helical polymer soluble in chloroform, although the helix-sense excess of the polymer may not be as high as that of PTrMA [20].

Scheme 11.1

A low molecular weight, one-handed helical poly(diphenyl-2-pyridyl methacry-late) prepared from 5 showed an unprecedented inversion of the macromolecular helicity in solution accompanied by a gradual decrease in the optical rotation with time. This change was ascribed to the helix-helix transition of the main chain, that is, the change from the one-handed helix to a mixture of right- and left-handed helices, as evidenced by the further chromatographic separation into fractions showing opposite optical rotations [21]. The activation energy (DGz) for the helix-sense inversion (racemization) was estimated to be 23 kcal mol_1.

11.2.2 Polychloral

Isotactic polychloral (poly(trichloroacetaldehyde) (2), prepared by the helix-sense selective polymerization of chloral with optically active lithium alkoxides and car-boxylates as initiators, possesses a 4/1 helical conformation and showed a high optical activity in films ([a]D + 4000°) [15]. Because the polymer is totally insolu-

Scheme 11.1

R1 R2

R10--C-SHR2

CCI3 Jn l

H H H H H f-C4H9O-C-0-C-0-C-0-C-0-C-0-C0CH3 11 {r? = 5)

CCI3 CCI3 CCI3 CC!3 CC!3

CSwi1

CSwi1

CH30-C-0-C-0"C-0-C-0-C-0-CH3 12 (n - 5) CCI3 CCI3 CCI3 CCI3 CCI3

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