Enantiomer-Selective Polymerization

Propylene sulfide (174) can also be polymerized enantiomer-selectively [252-262], In the polymerization with a ZnEt2-(-)-binaphthol initiator system at room temperature, the... 

Propylene sulfide (291) can also be polymerized enantiomer-selectively with optically active initiators. The enantiomer selectivity is generally higher than that in the polymerization of 286, and especially high electivity has been reported for polymerization with the ZnEt2/(S)-(-)-2,2 -binaphthol initiator system the ratio of (S)-monomer consumption rate to (P)-monomer consumption rate ( sAr) was 20 at room temperature. 

The first example of kinetic resolution catalyzed by an organometallic compound was the partially enantiomer-selective polymerization of racemic propylene oxide induced with a diethylzinc optically active alcohol system (50). 

Selective polymerization, enantiomers, 185 Semico rrin-copper complexes, 199 Sharpless epoxidation, racemic alcohols, 45 Side-chain units, prostaglandins, 310 Sigmatropic reactions, 222 Silanes, oxidative addition, 126 Silica gel, 285, 352... 

In the presence of chiral polymerization catalysts, enantiomeric monomers are consumed at different rates (Scheme 75). Enantiomer-selective polymerization of racemic propylene oxide catalyzed by a diethylzinc-(-f)-bomeol system is a classical example of such kinetic resolution H 176). The polymeric product has an [a]D of +7.4°. The mechanism... 

Optically active catalyst 1 can be obtained either by enantiomer-selective reaction of rac.-2 with optically active lithium (l,l -binaphthyl)-2,2 -diolate or by direct resolution by chiral HPLC. Optically active 21 and 22 in addition to 1 were successfully obtained by HPLC resolution and used for the polymerization of 1,5-hexadiene [60-62], Both catalysts gave an optically active polymer through cyclopolymerization. The optical activity and the content of tranj-structure in the main chain of the polymers obtained with 21 and 22 were comparable with those of the polymers synthesized with 1 [61,62],... 

Enantiomer selection is also found in vinyl ether polymerization [226,227], The polymerization of cis- and trans- 1-methylpropyl propenyl ethers using (-)-menthoxyaluminum dichloride [227] and the copolymerization of rac- 1-methylpropyl vinyl ether with optically active monomers [226] are enantiomer selective. 

The enantiomer selectivity and polymerization activity is greatly dependent on the stmcture of initiator complexes [232,240] and monomer conformation [236,241]. The complexes with 163,164, and 165 were nearly inactive in MBMA polymerization, whereas the complexes with 166 and 167 can polymerize MBMA with much lower enantiomer selectivity compared with the Sp-complex [4]. Sp and 163-167 have different shapes and sizes of the cavity of the ligand, which play an important role in chiral discrimination and influence the activity of the complex. 

Enantiomer-selective polymerization of MBMA has also been attained by using the reaction products of chiral amine compounds, 168 and 169, with cyclohexylmagnesium bromide as initiator [242,243] and by using the aluminum porphyrin complex 170 in the presence of optically active aluminum alkoxide compounds 171a-e [244], In the latter systems, the enantiomer selection is based on enantiomer-selective coordination of the chiral aluminum compounds to MBMA as revealed by NMR analysis. With 171e as a catalyst, the ee of the unreacted monomer is 40% at 75% monomer conversion ratio in the polymerization at -70°C. 

TABLE 11.6. Enantiomer-Selective Polymerization of (+)-55 Using Preformed Living Poly- or Oligo[(+)-55] Anion in Toluene at -78°C 2... 

Enantiomer-selection was also confirmed in the free-radical polymerizations of 55 [ 132] and 148 [185 having various enandoselectivities, though the selectivity was much lower than that observed in the anionic polymerization. 

The first enantiomer-selective polymerization was performed with propylene oxide (172) as a monomer [245], The polymerization was carried out with a ZnEt2/(+)-bor-neol or ZnEt2/(-)-menthol initiator system. The obtained polymer was optically active and the unreacted monomer was rich in (S)-isomer. Various examples are known concerning the polymerization and copolymerization of 172 [246-251 ]. A Schiff base complex 173 has been shown to be an effective catalyst In the polymerization at 60°C, the enantiopurity of the remaining monomer was 9% ee at 50% monomer conversion [250],... 

Enantiomer-selective polymerization of a-amino acid NCAs has been reported [270-274]. In the polymerization of y-benzylglutamate NCA using [(+)-C2H5(CH3)CHCOO]2NiPBu5 in dioxane at 30°C, the enantiopurity of monomeric units in the polymer chain was estimated to be (-) 34% ee at 28% polymer yield [272]. 

Similar enantiomer selection at the growing chain end is commonly observed in the cationic polymerizations of bicyclic acetals having bicyclo[3.2.1]octane skeletons. [24]... 

Valuable data on the properties of active centres are obtained from kinetic measurements. They reveal the simultaneous existence of several centre types. The stable centres are active during the whole course of polymerization in addition, some fraction of decaying centres is also present. Isotactic centres exhibit stereoregulating ability and are, moreover, extremely active. Centres may oscillate between active and inactive (dormant) forms and some centres selectively polymerize enantiomers from a racemate. External effects, caused by specific properties of centres, will be discussed in subsequent chapters. In addition, the centres on which dienes are polymerized will be treated in Chap. 5, Sect. 4. The structure of these centres is a function of the coordinated diene, and it is therefore better presented together with propagation. 

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,... 

In the polymerization of the (—)-monomers with various ee s, enantiomer selection was observed though the selectivity was lower compared with that of the polymerization of IDPDMA.83-87 In this experiment, a nonlinear relation was observed between the ee of the monomer in the feed and the optical activity of the obtained polymer (Figure 6). This indicates that the optical activity of the polymer is not based only on the side chain chirality. Furthermore, the chirality of a one-handed helical part induced by a successive sequence of the (—)-monomeric units (monomeric units derived from a (—)-monomer) can overcome the opposite chiral induction by the sporadic (+)-monomeric units. In other words, once a one-handed helical radical comes under the influence of the (—)-monomeric units, an entering (+)-monomer becomes a part of the one-handed helix whose direction may be unfavorable to the chiral nature of the (+)-monomer. 

Configurationally chiral, optically active polymers having stereogerric centers in the side chain or main chain can be obtained by enantiomer-selective pol5nnerization (lUPAC nomenclature asymmetric enantiomer-differentiating polymerization). In enantiomer-selective polymerization, one antipode of a racemic chiral monomer is preferentially polymerized to afibrd an optically active polymer. In this process, kinetic resolution of the racemic monomer takes place. The first clear polymerization of this typ>e was reported for propylene oxide. 

In the polymerization of the above racemic a-olefins with TiCl4/Zn[(S)-2-CH2CH(CH3)C2H5]2 catalyst, enantiomer selectivity has been observed, although it is rather low. The selectivity decreased as the distance between double bond and the asymmetric carbon increased, and no selectivity was observed for 5-methy-l-heptene (283), whose asymmetric center is at the y-position with respect to the double bond. Enantiomer selectivity has also been observed in a-olefin polymerization with MgCl2-supported catalysts modified with optically active Lewis bases and in the copolymerization... 

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