Propylene oxide polymerization stereoselectivity

The vast majority of papers reporting stereoselective epoxide polymerization focus on isospecific propylene oxide polymerization. For clarity, this chapter is organized by the type of metal of the catalyst active center. The three most commonly used metals for discrete stereoselective epoxide polymerization catalysts are aluminum, zinc, and cobalt, and research using these metals forms the foundation of this chapter. 

Sigwalt and coworkers noted higher stereoselectivity in the polymerization of propylene sulfide using a (f )-3,3-dimethyl-l,2-butanediol/ZnEt2 system when compared to a similar chiral alcohol/ZnEtz system. Based on these results, Sigwalt as well as others have applied this system to propylene oxide polymerization however, the observed stereoselectivity was actually lower than that for the polymerization of propylene sulfide. This lower stereoselectivity was presumably due to the... 

One of the key features of the stereoselective coordination catalysts for propylene oxide polymerization is that, while they can give Isotactic polymer with a very high ratio of Isotactic to syndiotactic sequences (e.g. > 370) (34). in the same reaction mixture a large part of the polymer is amorphous. Even vdien R-propylene oxide is used as starting material, much of the product is amorphous polymer of low optical rotation containing many S-propylene oxide units (30). 

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. ... 

Inoue,S., Tsukuma.I., Kawaguchi,M., Tsuruta,T. Synthesis of optically active polymers by asymmetric catalysts. VI. Behavior of organozinc catalyst systems in the stereoselective polymerization of propylene oxide. Makromol. Chem. 103,151 (1967). 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that contain high ratios of isotactic to syndiotactic sequences. Laige portions of amorphous materials, however, are also present in these same products. These amorphous portions contain head-to-head units that are imperfections in the structures. For every head-to-head placement, one (R) monomer is converted to an (S) unit in the polymer. This shows that at the coordination sites abnormal ring openings occur at the secondary carbon with an inversion of the configuration and result in head-to-head placements. Also, erythro and threo isomers units are present. The isotactic portion consists almost exclusively of the erythro isomer while the amorphous fraction contains 40-45% erythro and 55-60% threo ... 

Special relationships apply, however, when configurationally different but constitionally identical monomers are copolymerized. An example of this is the polymerization of D- and L-propylene oxide mixtures. Here, it is important to distinguish sharply between stereospecific and stereoselective polymerizations. The meaning of these two terms is not the same as the corresponding meaning of stereospecific and stereoselective reactions in low-molar-mass organic chemistry. 

Near the time of Natta s discovery of stereoselective alkene polymerization, Baggett and Pruitt reported that iron (III) chloride was capable of forming poly(propylene oxide) that could be divided into amorphous as well as crystalline materials using solvent fractionation." Soon thereafter, Natta et al. and Price et al. provided evidence that the crystalline material was isotactic poly(propylene... 

Well-defined [A(,A( -bis(2-hydroxybenzylidene)-(lR,2/ )-l,2-cyclohexane diamine] (R-salcy) aluminum complexes (e.g.. Figure 24.1, 4) have been used as stereoselective epoxide polymerization catalysts. Polymerization of racemic propylene oxide in the presence of 5 mol% 4 yields 70% conversion to poly(propylene oxide) after 62 h. The remaining unreacted monomer exhibits an optical rotation of 4-1.85°, which corresponds to an ee of 15% (Scheme 24.7). The modest r-factor... 

In 2005, Coates and coworkers reported the first highly active and selective catalyst system for the isoselective polymerization of racemic propylene oxide (Scheme 24.The complex [(salph-t-Bu)CoOAc] (19) exhibits excellent activity, regioselectivity, and stereoselectivity for the formation of isotactic poly(propylene oxide). This is the first example of isotactic poly(propylene oxide) generation from racemic propylene oxide without concomitant atactic byproduct. Notably,... 

Hasebe, Y. Tsuruta, T. Mechanism of stereoselective polymerization of propylene oxide with [ Me0CH2CH(Me)0Zn0CH(Me)CH20Me 2 EtZn0CH(Me)CH20Me 2] as initiator. Makromol. Chem. 1988,189, 1915-1926. 

The chirality (asymmetry) of the methlne carbon In propylene oxide and In Its polymer has provided many extra experimental parameters to use In studies of the mechanism of stereoselective polymerization by coordination catalysts. The first proposal of the coordination polymerization scheme to explain stereoselective or-olefln and olefin oxide polymerization arose from the propylene oxide studies (26.27). 

The conclusion was that with some initiator systems, abnormal ring cleavage can occur in the polymerization of propylene oxide. However, with some initiator systems, notably dialkylzinc/water or some alcohols, the initiation sites are strongly stereoselective, producing stereospecific polymer of narrow molecular weight distribution. The selectivity appears to occur in the monomer coordination step at the active initiator site. 

Recently, a new crystalline organozinc enantiomorphic initiator for the stereoselective polymerization of propylene oxide was reported (107). The complex was obtained by reaction of diethylzinc and (dl)-l-methoxy-2-propanol, and it had the following composition ... 

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