Triphenylmethyl methacrylate

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

A bulky methacrylate, triphenylmethyl methacrylate (TrMA), is a unique monomer which gives an almost 100% isotactic polymer in anionic polymerization with n-butyllithium both in nonpolar and polar solvents. Moreover, even free-radical polymerization affords a highly isotactic polymer from this monomer.23 The isotactic specificity of TrMA polymerization is ascribed to the helical formation of the main chain. When TrMA is polymerized in toluene at —78°C... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

The main limitation of these CSPs is their limited pressure stability, which makes them not very suitable for HPLC application. However, they have proved to be an excellent tool for the preparative separation of drugs by low-pressure HPLC. To make these CSPs accessible to HPLC, silica gel-based phases were developed. " This type of phase is available from Merck (Darmstadt, Germany) under the name Chiraspher. Polymer phases of different types have been developed by Okamoto s group. > They are prepared by the asymmetric polymerization of triphenylmethyl-methacrylate monomers. The original character of these polymers is that they do not possess any chiral centre and therefore their chirality is only due to their helicity. However, clear mechanisms have not been proposed... 

Another result of great importance—the conformational asymmetric polymerization of triphenylmethyl methacrylate realized in Osaka (223, 364, 365)— has already been discussed in Sect. IV-C. The polymerization was carried out in the presence of the complex butyllithium-sparteine or butyllithium-6-ben-zylsparteine. The use of benzylsparteine as cocatalyst leads to a completely soluble low molecular weight polymer with optical activity [a]o around 340° its structure was ascertained by conversion into (optically inactive) isotactic poly(methyl methacrylate). To the best of my knowledge this is the first example of an asymmetric synthesis in which the chirality of the product derives finom hindered rotation around carbon-carbon single bonds. 

The only exception to this statement (outside of the triphenylmethyl methacrylate case) in the literature up to now concerns the polychloral obtained in the presence of tetraethylammonium 0-methyl- or O-acetylmandelate (366-368). The monomer was treated so as to give directly a polymeric film that is endowed with extremely high rotatory power ([a]o exceeding —1800°, but preliminary data obtained with other initiators led the authors to suppose that values > 3000° could be attained). 

The hydrocarbon 25 has been partially resolved by asymmetric complexation with Newman s reagent [TAPA ( )-a(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)prop-ionic acid] thereby establishing its chiral Z)2-structure 53). Similarity, the naphthaleno-phane 27b could be resolved by chromatography on silicagel coated with (—)-TAPA 49) and recently also by HPLC on optically active poly(triphenylmethyl methacrylate)49a) which also proved to be very useful for the optical resolution of many other axial and planarchiral aromatic compounds 49b>. 

Stereospecific polymerization of various methacrylates has been extensively studied (1). Among many, triphenylmethyl methacrylate (TrMA) is one of the most interesting monomers and brings about some unusual results as follows This monomer forms only... 

A similar improvement in sensitivity via copolymerization was observed in the copolymers of methyl methacrylate with a,a-dimethylbenzyl methacrylate (Table VI) and triphenylmethyl methacrylate (Table VII). In the former case the mechanism of sensitivity enhancement should be the same as that for the poly(a,a-diphenylethyl methacrylate-co-methyl methacrylate), although the enhancement of sensitivity with copolymerization is rather low. 

The homopolymer of triphenylmethyl methacrylate is not soluble in usual organic solvents but it is solubilized by the incorporation of methyl methacrylate units. The copolymers containing over 90 mol% methyl methacrylate are soluble in the organic solvents such as toluene and xylene. In... 

Table VII the electron-beam exposure characteristics are given for the soluble poly (triphenylmethyl methacrylate-co-methyl methacrylate)s. The sensitivity on alkaline development was strongly influenced by the copolymer composition. The highest sensitivity was obtained on the copolymer containing 93.7 mol% methyl methacrylate. The copolymer of highest sensitivity showed the 7-value of 6.3, which was nearly twice as large as that for poly(methyl methacrylate). Formation of methacrylic acid units on exposure is obvious from the infrared spectrum. However, the mechanism of the occurrence should be different from the case of the a,a-dimethylbenzyl methacrylate polymer since there are no /3-hydrogen atoms in the triphenylmethyl group, and may be similar to the case of poly (methyl methacrylate). This will be explored in the near future.

FIGURE 9 Structures of (a) poly(triphenylmethyl methacrylate) [poly(TrMA)] and (b) diphenyl-2-pyridyl methacrylate [poly(D2pyMA)] chiral selectors used for the preparation... 

In spite of the development of more successful and reliable CSPs (Chaps. 2-8), these miscellaneous types of CSP have their role in the field of the chiral resolution also. The importance of these CSPs ties in the fact that they are readily available, inexpensive, and economic. Moreover, these CSPs can be used for some specific chiral resolution purpose. For example, the CSP based on the poly(triphenylmethyl methacrylate) polymer can be used for the chiral resolution of the racemic compounds which do not have any functional group. The CSPs based on the synthetic polymers are, generally, inert and, therefore, can be used with a variety of mobile phases. The development of CSPs based on the molecularly imprinted technique has resulted in various successful chiral resolutions. The importance and application of these imprinted CSPs lies in the fact that the chiral resolution can be predicted on these CSPs and, hence, the experimental conditions can be designed easily without greater efforts. Because of the ease of preparation and the inexpensive nature of these CSPs, they may be useful and effective CSPs for chiral resolution. Briefly, the future of these types of CSP, especially synthetic polymers and polymers prepared by the molecularly imprinted technique, is very bright and will increase in importance in the near future. 

Okamoto, Y., Suzuki, K., Ohta, K., Hatada, K., and Yuki, H. (1979) Optically active poly(triphenylmethyl methacrylate) with one-handed helical conformation, J. Am. Chem. Soc. 101, 4763-4765. 

Okamoto, Y. and Hatada, K. (1986), Resolution of enantiomers by HPLC optically active poly(triphenylmethyl methacrylate), J. Liq. Chromatogr. 9, 369-384. 

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