Triphenylmethyl methacrylate polymerization

Nakano T, Matsuda A, Okamoto Y, Okamoto Y. Pronounced effects of temperamre and monomer concentration on isotactic specificity of triphenylmethyl methacrylate polymerization through free radical mechanism. Thermodynamic versus kinetic control of propagation stereochemistry. Polym J. 1996 28 556-558. 

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). 

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... 

Type II sorbents are based on an inclusion mechanism. Chiral recognition by optically active polymers is based solely on the helicity of that polymer. Optically active polymers can be prepared by the asymmetric polymerization of triphenylmethyl methacrylate using a chiral anionic initiator [264]. Helical polymers are unique from the previously discussed chromatographic approaches because polar functional groups are not required for resolution [265]. These commercially available sorbents have been used to resolve enantiomers of a-tocopherol [266]. The distinction between this group (lib) and the sorbents containing cavities is vague (Ila). 

Triphenylmethyl methacrylate (TrMA) and azobenzene-modified methacrylates were randomly copolymerized in toluene at — 78°C with chiral catalysts to give optically active helical copolymers (19 in Fig. 6) [65]. The optical activity (optical rotation) of the copolymers decreased with the increasing content of the azobenzene-modified methacrylates in the copolymers. The single helical conformation of PTrMA is quite stable in solution, but the copolymers of TrMA with less bulky methacrylates cannot keep their helical structure and lose their optical activity during the polymerization or after the polymerization in solution, which is highly dependent on the bulkiness of the comonomers [22]. The copolymer (19 x = 2) containing 26 mol% azobenzene units, also lost its optical activity upon irradiation within 20 min. This change is due to the helix-to-coil transition of the copolymer and can occur in the dark. 

Vinyl polymers with a stable helical conformation are obtained from methacrylates with a bulky side group by isotactic specific anionic or radical polymerization.13,34 This type of polymer was first synthesized by asymmetric anionic polymerization (helix-sense-selective polymerization) of triphenylmethyl methacrylate (TrMA, 5) using a complex of n-BuLi with (—( sparteine (Sp, 6).13 Although, as discussed... 

Poly(triphenylmethyl methacrylate) Analogues Anionic Polymerization... 

Derivation of polymer also serves as a useful method for tacticity determination. Poly(triphenylmethyl methacrylate) is easily converted to PMMA by hydrolysis and the subsequent methylation with diazomethene. The polymers obtained by anionic polymerization not only in toluene but also in tetrahydrofuran are highly isotactic.189 Even the radical polymerization of the monomer gives an isotactic polymer. 

Optically active oligomers of methyl methacrylate were obtained by the asymmetric polymerization of triphenylmethyl methacrylate, followed by the substitution of methyl for triphenylmethyl, and subsequent GPC separation275 and optical resolution.276 Detailed study on the stereostructure of the oligomer made it possible to discuss precisely the mechanism of polymerization leading to the formation of polymer with one-handed helical conformation. 

Some isotactic polymers such as polychloral and poly(triphenylmethyl methacrylate)289 are known to exist only in purely helical conformation. The helical structure of the polymers is rigid even in solution, owing to the bulkiness of the side-groups. This has been demonstrated by the measurement of high optical activity of the polymers prepared by asymmetric polymerizations the optical activity is based on a one-handed helical conformation of the polymer chain. 

One of the most studied polymerization systems employs alkyllithium initiators that are modified by chiral amine ligands for the polymerization of sterically bulky methacrylates [8,38,39,40,41], acrylates [42],crotonates [43], and acrylamides [44]. A primary example is the reaction of triphenylmethyl methacrylate with an initiator derived from 9-fluorenyllithium and (-)-sparteine (3) at -78 °C (Scheme 4). The resultant isotactic polymer is optically active, and is postulated to adopt a right-handed helix as it departs from the polymerization site. This polymer has been particularly successful as a chiral stationary phase for the chromatographic resolution of atropisomers [8]. Many modifications of the or-ganolithium initiator/chiral ligand system have been explored. Recently, Okamo-to has applied enantiopure radical initiators for the enantioselective polymerization of bulky methacrylate monomers [45]. 

Poly(triphenylmethyl methacrylate) analogues Anionic polymerization... 

Free radical propagation is poorly stereocontrolled, with nearly equal proportion of meso and racemic dyads in polymerization of monosubstituted alkenes and a preference for syndiotactic placement for disubstituted monomers such as methacrylates (rr = 0.62, mm = 0.04). The sequence distrihution follows a first-order Markov model with a slight deviation from Bernoulian statistics. However, for very bulky substituents, as in polymerization of triphenylmethyl methacrylate, the preference for isotacticity was observed (mm = 0.64, rr = 0.12). Recently, complexation with Lewis acids and acidic solvents enabled to enhance stereocontrol in polymerization of vinyl esters and acrylamides, and to a smaller degree in polymerization of methacrylates (127-129). 

In a study on the radical polymerization of various methacrylates in 2-methyltetra-hydrofuran (MTHF) rigid glass, Kamachi et al. observed even at room temperature the ESR spectrum of the poly(triphenylmethyl methacrylate) (polyTPMA) radical, which has a bulky side group. They investigated the temperature dependence of the... 

Concerning the growing radicals in polymerization reactions, they can be studied directly by ESR spectroscopy as in the case of triphenylmethyl methacrylate and MMA [95]. In the latter case it was concluded that there are two stable conformations of the propagating radicals. The steric effect of the a-methyl group of MMA is not only responsible for the comparatively low heat of the polymerization reaction, but also for a certain control of the propagation steps. Therefore, in radical solution polymerization the polymethacrylates exhibit in most cases a favored syndiotacticity. 

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