Helical Poly methacrylates

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

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. 

To date, a number of optically active polymers, particularly helical polymers with an excess one-handedness, have been prepared with much interest. However, helical poly (triaryl methacrylates) are the only examples of successful application to HPLC stationary phases [22,103]. Helical polymers combined with photoresponsive molecules will offer promising functions and applications as realized by natural macromolecules. 

Sterically Restricted Poly (methacrylate ester)s. It was recognized by Okamoto and coworkers150 that the anionic polymerization of tri-phenylmethyl methacrylate (TrMA, 41) (Chart 8) at low temperature in the presence of an optically active initiator results in the formation of an isotactic, optically active polymer. The helical conformation of the backbone in these macromolecules is the result of steric interactions between the bulky trityl groups, as was shown by the loss of optical activity upon their conversion to methyl ester groups. This class of bulky... 

Helical ( )-)-poly(triphenylmethyl-methacrylate) has been applied successfully as a chromatographic material for the separation of racemates An overview of the helix types exhibited by crystalline macromolecules and some examples are given in Table 3. 

The latter benchmark reaction for the preparation of 84 was also used recently by Reggelin to study a completely different kind of chiral polymeric bases (Scheme 10.15). In their work, the authors used pyridyl N-oxide substituted helically chiral poly(methacrylate)s 82 prepared by helix sense selective anionic polymerization of... 

Because of their inability to obtain fiber patterns. X-ray studies have not provided any conclusive evidence as to the nature of the conformations of syndiotactic poly(methacrylic acid) (PMMA). IR dichroism measurements cannot be applied because the oriented films cannot be prepared however, it was possible to obtain Raman depolarization data in aqueous solutions (i 7). Figure 5 shows Raman spectra in the solid state and in aqueous solution. Because of striking similarity of the spectra, it was concluded that the local conformations in both phases are similar. The frequency shift of two bands at 1452 and 1685 cm" was attributed to the differences in the extent of hydrogen bonding. On the basis of the IR and Raman activity of polarized and depolarized spectra, it was proposed that both solid and aqueous PMMA exist as helices that contain more than six monomer repeating units per two... 

The helical polytriphenylmethyl methacrylate was the first synthetic chiral polymer able to separate a very limited number of enantiomers [28]. Recently a fully synthetic chiral stationary phase based on polymerized diacryloyl derivative of fra s-l,2-diaminocyclohexane [either (R, R) or (S, S)] bonded to silica gel in the form of a very thin layer was proposed as a new LC CSP [29]. This CSP could not resolve many enantiomeric pairs. However, when it could resolve a racemate, it was shown that the amount that could be loaded was much larger than that on most other CSPs. It means that the number of active sites is large. Hydrogen bonds were found to be pivotal in the chiral recognition mechanism of this CSP. The enan-tioselectivity was adjusted by the methanol content in the organic mobile phase. Polysodium A-undecanoyl-L-leucyl-leucinate (poly-SULL) and —L-leucyl-valinate... 

In the same manner, when methylmethacrylate is polymerized with a photosensitive agent and /-camphor, the syndiotactic polymer obtained at low temperature shows optical rotatory power ([a] d°= -1.84°), but atactic polymers obtained at high temperature are inactive (Table XI). This result suggests that the optical rotatory power of the above stated poly-(methacrylate), is due to molecular asymmetry of the helical configuration. 

Poly(l-proline)/poly(ethylene oxide)/poly(l-proline) water-soluble ABA block copolymers have been prepared from amine-terminated poly(ethylene oxide) and 1-proline-N-carboxyanhydride (169). In this polymer, the poly(amino acid) is in a helical conformation and the poly(ethylene oxide) is in a random coil. The polymer forms what appears to be 1 1 complexes with poly(methacrylic acid). 

Microspheres also have been prepared from poly(methacrylic acid-g-ethylene oxide) (1 la). The ability of the copolymer to form reversible hydrogen-bonded complexes suggests a potential for use as biosensors and/or carriers for controlled drug delivery. In a physiological environment, the copolymer was more useful in a microsphere form than in the usual helical or random-coil form. 

Crystalline samples of syndiotactic poly(methyl methacrylate) (st-PMMA) may be obtained from chloroacetone 178). This guest could be completely replaced by a variety of other guest molecules such as acetone, 1,3-dichloroacetone, bromoacetone, pinacolone, cyclohexanone, acetophenone and benzene. The X-ray diffraction patterns for these inclusion compounds were similar. These data indicate that the st-PMMA chains adopt a helical conformation of radius about 8 A and pitch 8.85 A. The guest molecules are located both inside the helical canals and in interhelix interstitial sites. 

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

Isotactic poly(methyl methacrylate), also, is an intricate case, resolved only after a 20-year debate. The repetition period along the chain axis is 10.40 A corresponding to S monomer units the entire cell contains 20 monomer units (four chains). At first, the stmcture was resolved as a 5/1 helix (183) with = 180° and 62 — 108° but no reasonable packing was found using this assumption. Further conformational calculations showed that helices like 10/1 or 12/1 should be more stable than the 5/1 helix. The structure was solved by Tadokoro and co-workers (153b) who proposed the presence of a double helix. Two chains, with the same helical sense and the same direction but displaced by 10.40 A one from the other are wound on each other, each chain having 10 monomer units per turn [i(10/l)] and a 20.80-A repeat period. As a result, the double helix has a 10.40-A translational identity period, identical to that found in the fiber spectmm. The conformational parameters are Of = 179° and 2 = -148°. Energy calculations indicate that the double helix is more stable by 4.4 kcal per-mole of monomer units than two isolated 10/1 helices, a result that is in line with the well-known capacity of this polymer to form complexes in solution (184). 

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Another type of synthetic polymer-based chiral stationary phase is formed when chiral catalyst are used to initiate the polymerization. In the case of poly(methyl methacrylate) polymers, introduced by Okamoto, the chirality of the polymer arises from the helicity of the polymer and not from any inherent chirality of the individual monomeric subunits (109). Columns of this type (eg, Chiralpak OT) are available from Chiral Technologies, Inc., or J. T. Baker Inc. 

The intramolecular interaction energy was calculated for five isotactic polymers, namely, isotactic polypropylene, poly(U-methyl-l-pentene), poly(3-methyl-1-butene), polyacetaldehyde, and poly(methyl methacrylate) (23). The molecular structures of the first four polymers have already been determined by x-ray analyses as (3/1) (2k), (7/2) (18,25.,26), (U/l) (21), and (U/l) helices (28), respectively. Here (7/2) means seven monomeric units turn twice in the fiber identity period. For isotactic poly(methyl methacrylate) (29), a (5/l) helix was considered reasonable at the time of the energy calculation in 1970, before the discovering of... 

The other three polymers have additional rotation angles in the side chains, x and/or x. For poly(3-methyl-1-butene), the minimum was found in the three-dimensional plot. For poly(U-methyl-l-pentene) and poly(methyl methacrylate), the stable conformation of the side chain was first calculated with the fixed main chain conformation corresponding to the (7/2) and (5/1) helices, respectively. The potential energy was calculated against the main chain rotation angles, x and t2, by fixing x and x of the side chain at the values thus obtained. ... 

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. 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

Benzoin acetate, methylben-zoin, Troger s base, tra/w-stilbene oxide and 1,1 -binaphthyl-2,2 -diol Aminopropyl silica gel (LiChrospher 1000, 5 pm) coated with helically chiral poly(diphenyl-2-pyridylmethyl methacrylate) (PDPM) Methanolic solution of ammonium acetate (2.5 mM, pH 4.5) 300 mm x 100 pm i.d. 200 mm effective length, chiral separation 163... 

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