Enantiomorphic Catalysts

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Audisio studied the microtacticity of the 1,4-rrans-polypentadiene [—CH2—CH=CH—CH(CH3)—] in connection with that of the poly-(methyltetramethylene) [—CH2—CH2—CH2—CHfCHs)—] obtained from the preceding compound by reduction (109, 110), and succeeded in evaluating the distribution of the triads mm, mr, and rr. He has proposed an interpretation according to a one-parameter model based on enantiomorphic catalyst sites (111) (see Table 4, column 4, 3/ = 1). 

The probabilistic aspect of error propagation in isotactic polypropylene was treated both as a second-order Markov chain (in terms of m and r dyads) (408) and, in terms of a model of enantiomorphic catalyst sites, as asymmetric Ber-... 

The 13C NMR analysis of polypropylene obtained with the Me2C(Cp)(Flu) ZrCh —[Al(Me)0]x catalyst showed that the polymer chains consist of long blocks of r diads separated by pairs of m diads (Figure 3.47b), which is indicative of polymerisation stereocontrol by the enantiomorphic catalyst site [23]. In this case, it is obvious that an occasional change in the configuration of the last inserted monomeric unit has no influence on the chirality and tends to remain isolated. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

The enantiomorphic catalyst sites control mechanism was found to operate in the stereospecific polymerisation of tiiranes. Sigwalt et al. [79,153] found that... 

In (53) PJ and Pg represent chains with active L and D ends in the case of an end-controlled polymerization such as the one we are considering (or, in the case of the enantiomorphic catalyst site model, catalyst sites preferring L- and D-monomers, respectively), P L, P D etc. are the... 

According to (55), constancy of R implies constancy of [P ]/[Pd ] This condition is always fulfilled for an enantiomorphic catalyst of which the different sites remain in constant concentration. In an end-controlled mechanism, a ratio [P ] /[Pg ] can be achieved by use of an optically... 

The asymmetric Bernoulli mechanism described above is characteristic for the polymerization of configurationally different monomers with enantio-morphous catalysts, but is not limited to these cases. With enantiomorphous catalysts, one kind of active position preferentially polymerizes D-monomers, ana the other kind of active position should preferentially polymerize l monomers. The probability of adding on a d monomer to a d unit at a d... 

Figure 15-3, Relationship between triad fractions and the syndiotactic diad fraction. The lines give the theoretical relationship for an enantiomorphic catalyst with placements independent of the previously occurring placement the circles give the experimental results for the polymerization of methyl vinyl ether with Ab(S04)3/H2S04 in toluene. (From data of T. Higashimura, Y. Ohsumi, K. Kuroda, and S. Okamura.).

The term in parentheses in equation (16-122) is only constant when the ratio [Pl]/[Pd] is constant. Such a constancy is only obtained when the two individual concentrations are constant, or if the two individual concentrations change in the same way and to the same extent with time. The latter possibility can be excluded with living polymers, because the condition t/([Pt] + oD/dt = 0 must hold (see also Chapter 18). Consequently, the individual concentrations must remain constant with time. This condition appears always to be fulfilled with what are known as enantiomorphic catalysts (see below). 

Kageyama, H. Miki, K. Tanaka, N. Kasai, N. Ishimori, M. Heki, T. Tsuruta, T. Molecular structure of [Zn(0CH2CH20Me)2(EtZn0CH2CH20Me)g]. An enantiomorphic catalyst for the stereoselective polymerization of methyloxirane. Mo romoZ. Chem., Rapid Commun. 1982, 3, 947-951. 

Yoshino, N. Suzuki, C. Kobayashi, H. Tsuruta, T. Some features of a novel organozinc complex, [ Me0CH2CH(Me)0Zn0CH(Me)CH20Me 2 EtZn0CH(Me)CH20Me 2], as an enantiomorphic catalyst for stereoselective polymerization of propylene oxide. Makromol. Chem. 1988, 189, 1903-1913. 

This mode of steric control is classified into two categories growing chain control or enantiomorphic catalyst site control. 1-D In crystals C With polymerases (soluble or insolubilized enyme catalyst) polynucleotides polysaccharides... 

---