Helix-Sense-Selective Radical Polymerizations

Figure 4. Helix-sense-selective radical polymerization using optically active thiol as a chain-transfer agent or initiator.

Helix-sense-selective radical polymerization of PDBSMA was also performed using a chiral Co(II) complex, Co(II)—Li (21).74 Complex Co(II)—Lj can possibly interact with the growing radical in the... 

Helix-sense-selective radical polymerization of PDBSMA was also performed using a chiral Co(II) complex, Co(II)-Li (41). Complex Co(II)-Li can possibly interact with the growing radical in the polymerization system because Co (II)-Li is a d species. Regarding the interaction of Co(II) species with a growing radical, several examples of catalytic chain transfer in methacrylate polymerization by the use of Co(II) have been reported. The polymerization was carried out in the presence of Co(II)-Li in a CHCla-pyridine mixture at 60 ° C. Although the polymer yield and the molecular weight of... 

Optically active polymers can be prepared by free-radical additions that give polymers whose chirality is the result of an excess of one single-screw sense. Most polymers will not maintain a helix screw conformation in solution unless the chain backbone is rigid or the polymer side-chains are very large and prevent conformational relaxation. Polymers derived from trityl and related methacrylates have this apparent capacity, i.e. they display excess helical content in solution. Comprehensive reviews of helix-sense-selective anionic polymerizations have appeared [12], and in this section, we highlight some of the recent developments in this field related to radical polymerizations of these highly hindered methacrylates. 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

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... 

As discussed so far in this section, the helical polymethacrylates are synthesized predominantly using anionic polymerization techniques. However, recently, more versatile, inexpensive, and experimentally simple free-radical polymerization has been proved to be an alternative, effective way to prepare helical polymethacrylates from some monomers. Although the stereochemical control of radical polymerization is generally more difficult compared with that in other types of polymerization,69 an efficient method would make it possible to synthesize helical, optically active polymers having functional side chains by direct radical polymerization without using protective groups. In the radical polymerization of bulky methacrylates, helix-sense selection is governed by the chirality of a monomer itself or an additive. 

Helix-sense selection was also realized during the radical polymerization of DBPAMA at 0 °C using... 

Phenyl-2-pyridyl-otolylmethyl methacrylate (PPyo-TMA, 27) having a chiral ester group is known to lead to highly enantiomer-selective and helix-sense-selec-tive polymerization by anionic catalysis.84-86 The selection was also found in the radical polymerization of optically active PPyoTMA having various ee s,... 

Triphenylmethyl methacrylate (TrMA) is a unique monomer, which affords a highly isotactic polymer (PTrMA) even by a radical process, and the polymer with more than a 95% triad isotacticity can be obtained by the anionic polymerization with butyllithium (BuLi) [10]. In 1979, we found that optically active PTrMA is formed during the anionic polymerization of TrMA with the complex of ( )-sparteine-n-BuLi at —78°C (Fig. 5) [11, 12]. This is the first example of helix-sense-selective polymerization preferentially producing a stable one-handed helical polymer through the polymerization process. The results clearly indicated that the existence of a stable helical polymer even in solution is possible on a vinyl polymer without chiral side groups. The helical structure is maintained due to the steric hindrance of the bulky triphenylmethyl groups. Therefore, when the ester groups are hydrolyzed with an acid, the optical activity of the polymer disappears. 

We have been studying the helix-sense-selective polymerization of bulky methacrylates such as triphenylmethyl methacrylate (TrMA), diphenyl-2-pyridylmethyl methacrylate (D2PyMA)phenyl-2-pyridyl-o- and m-tolylmethyl methacrylates (PPyoTMA and PPymTMA), and (S)-(-)-diphenyl(1-methylpyrrolidin-2-yl)methyl methacrylate (DMPMA) with chiral anionic initiators. In these polymerizations, optically active polymers with one-handed helical structure have been obtained and some of the polymers exhibited characteristic conformational transition in solution. In the present paper, we describe helix-sense-selective polymerization of 1 -phenyidibenzosuberyl methacrylate (PDBSMA), diphenyl-3-pyridylmethyl methacrylate (D3PyMA) and phenyl[bis(2-pyridyl)]methyl methacrylate (PB2PyMA) by means of chiral anionic initiators and conformational transition of the obtained polymers. Asymmetric radical polymerization of PDBSMA is also reported because this monomer can also afford a helical, highly isotactic polymer by radical mechanism. 

On the basis of above results, we performed the helix-sense-selective polymerization of PDBSMA by radical mechanism. i When the radical polymerization of PDBSMA was carried out with (/-PrOCOO)2 in (+)- and (-)-menthol containing a small amount of toluene at 50°C, optically active (-)- and (+)-poly(PDBSMA)s ([a]36S ca. 200°) with high Isotacticity (mm >97%), respectively, were obtained. CD spectral pattern of these polymers was similar to that of the anionically obtained (+)-poly(PDBSMA). Therefore, one of right- and left-handed helical structures must be produced preferentially over the... 

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