Copolymerization Cyclohexene oxide

A similar catalytic system was found to copolymerize cyclohexene oxide and CS2 to afford copolymers with molecular weight ranges of 14,000-34,000 Da with narrow molecular weight distributions [70]. Of interest, the exchange of sulfur and oxygen atoms in both the copolymer and cyclic products was observed during the process, with the cyclic product enriched in sulfur atoms and the copolymer enriched in oxygen atoms (10). 

An alternative method for generating enriched 1,2-diols from meso-epoxides consists of asymmetric copolymerization with carbon dioxide. Nozaki demonstrated that a zinc complex formed in situ from diethylzinc and diphenylprolinol catalyzed the copolymerization with cyclohexene oxide in high yield. Alkaline hydrolysis of the isotactic polymer then liberated the trans diol in 94% yield and 70% ee (Scheme 7.20) [40]. Coates later found that other zinc complexes such as 12 are also effective in forming isotactic polymers [41-42]. 

It is also possible to desymmetiize a meso epoxide in the alternating copolymerization. Thus, asymmetric alternating copolymerization of cyclohexene oxide with CO2 catalyzed by a dimeric zinc complex provides a polycarbonate in which the diol unit is optically active with 80% ee. (See Scheme 4.24.)... 

However, there are numerous reported instances of stereocontrol by a site-control mechanism involving chiral metal catalysts. That is, Nozaki and coworkers first illustrated the asymmetric alternating copolymerization of cyclohexene oxide and CO2 employing a chiral zinc catalyst derived from an amino alcohol (Fig. 2a) [13-16]. This was soon followed by studies of Coates and coworkers utilizing an imine-oxazoline zinc catalyst (Fig. 2b) [17]. Both investigations provided isotactic poly(cyclohexene carbonate) (Fig. 3) with enantiomeric excess of approximately 70%. 

Furthermore, size-exclusion chromatography (SEC) analyses generally reveal a bimodal distribution of molecular weights of the copolymers. Concomitantly, MALDI-ToF mass spectral measurements exhibit two sets of peaks corresponding to copolymer end groups of -OH and -X. For example, utilizing a (salen)CrCl/bis (triphenylphosphme)iminium chloride ([PPNjCl) catalyst for the copolymerization of cyclohexene oxide and carbon dioxide, the two copolymers illustrated in Fig. 6 were observed [26]. 

We have utilized somewhat less-effective optional approaches to copolymer purification with attendant catalyst recovery. One of these methods involved the replacement of the f-butyl substituents on the 5-position of the phenolate ligands with poly(isobutylene) (PIB) groups, as illustrated in Fig. 14 [39]. Importantly, this chromium(III) catalyst exhibited nearly identical activity as its 3,5-di-t-butyl analog for the copolymerization of cyclohexene oxide and carbon dioxide. The PIB substituents on the (salen)CrCl catalysts provide high solubility in heptanes once the copolymer is separated from the metal center by a weak acid. 

Tan C-S, Chang C-F, Hsu T-J (2002) Copolymerization of carbon dioxide, propylene oxide and cyclohexene oxide by a yttrium-metal coordination catalyst system. In CO2 conversion and utilization. ACS Symp Ser 809 102-111... 

Hsu T, Tan C (2002) Block copolymerization of carbon dioxide with cyclohexene oxide and 4-vinyl-1-cyclohexene-1,2-epoxide in based poly(propylene carbonate) by yttrium-metal... 

Darensbourg DJ, Wildeson JR, Yarbrough JC, Reibenspies JH (2000) Bis 2,6-difluorophen-oxide dimeric complexes of zinc and cadmium and their phosphine adducts lessons learned relative to carbon dioxide/cyclohexene oxide alternating copolymerization processes catalyzed by zinc phenoxides. J Am Chem Soc 122 12487-12496... 

For the copolymerization of epoxides with cyclic anhydrides and curing of epoxy resins, Lewis bases such as tertiary amines are most frequently used as initiators. In this case, terminal epoxides react with cyclic anhydrides at equimolar ratios. The time dependence of the consumption of epoxide and anhydride is almost the same for curing 35-36> and for model copolymerizations 39,40,45). The reaction is specific 39,40) to at least 99 %. In contrast, the copolymerization with non-terminal epoxides does not exhibit this high specificity, probably because of steric hindrances. The copolymerization of vinylcyclohexene oxide or cyclohexene oxide is specific only to 75-80 % and internal epoxides such as alkylepoxy stearates react with anhydrides only to 60-65 %. On the other hand, in the reaction of epoxy resins with maleic anhydride the consumption of anhydride is faster 65the products are discoloured and the gel is formed at a low anhydride conversion 39). Fischer 39) assumes that the other resonance form of maleic anhydride is involved in the reaction according to Eq. (33). 

Scheme 8.2 Mechanistic aspects of cyclohexene oxide/C02 copolymerization in the presence of (salen)CrX and cocatalyst (Nuc = anion).

Table 8.1 Catalytic activity for the copolymerization of cyclohexene oxide and C02 in the presence of one equivalent of PPNX cocatalyst.3...

Figure 8.25 Cr(PIBsalen)Cl utilized for the copolymerization of C02 and cyclohexene oxide.

Figure 3.9. Copolymerization of COz and cyclohexene oxide catalyzed by Zn complexes in liquid C02 and sc C02.

Poly(bisphenol-A-carbonate) under pseudoideal reaction conditions was investigated, and the cyclic polycarbonate was obtained as the main product. In the system, the interface of the water/toluene mixture might have favored the cyclization reaction between the polar end groups [88]. Cyclic carbonates during the (Salen)CrCl catalyzed CCh/cyclohexene oxide copolymerization process in the presence of ionic initiators was also obtained [89]. The cyclic carbonate is produced via the backbiting mechanism, and the process is assumed to take place via a metal alkoxide (polymer chain) intermediate. Subsequent ring-opening of the cyclic carbonate with concomitant formation of polyether and CO2 was fast at the reaction temperatures from 80 to 100 °C). 

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

Both catalytic systems, alkoxides and carboxylates, are often described as efficient catalysts for the copolymerization of CO2 and epoxides but some drawbacks which hamper a widespread industrial utilization need to be pointed out. The phenoxides, though displaying good selectivities, have up to now only been tested with model substrates, e. g., propylene- and cyclohexene oxides, and the carboxylates, though active, present low-to-fair selectivities. Cyclization... 

Kim, S. M. Kim, C.-S. Ha, D.-W. Park, Synthesis and cyclohexene oxide/carbon dioxide copolymerizations of zinc acetate complexes bearing bidentate pyridine-alkoxide ligands, Macro-molec. Rapid Commun. 25 (2004) 888. 

Thus far, the discussion of polymerizations conducted in carbon dixiode has centered on systems where CO2 acts only as a solvent for the polymerization. However, there are also examples of polymerization systems where CO2 acts as a comonomer. Most notable among these in the context of this chapter is the coploymerization of CO2 and epoxides. The copolymerization of propylene oxide and carbon dioxide was conducted in SCCO2 using a heterogeneous zinc catalyst [142]. Additionally, Beckman and co-workers have shown that a soluble, fluorinated ZnO-based catalyst can be effectively utilized to promote the copolymerization of CO2 and cyclohexene oxide [143]. These examples indicate that supercritical carbon dioxide can be viable as both a solvent comonomer in polymerization reactions. 

Copolymerization of cyclohexene oxide and CO2 proceeds with an immobilized chromium phorphyrin complex, although the molecular weight of the resulting copolymer 47 is limited to the oligomer level (Scheme 87). 

Figure 6. An example of a potential use of a switchabie solvent the copolymerization of cyclohexene oxide and CO 2- In the new route, after the polymer precipitate is removed by filtration, the ionic liquid is easily distilledfrom the Cr catalyst, and both the solvent and the catalyst are then recycled.

An optically active polycarbonate (271) has been synthesized by asymmetric synthesis copolymerization of cyclohexene oxide and CO2 with optically active Zn catalysts 272 and 273. " Through hydrolysis of the polymer giving 1,2-dihydroxycyclohexane, the absolute configuration was found to be R,R). The optical purity of the polymer was estimated to be 72% ee and 80% ee with the catalysts 272 and 273, respectively. 

IR analysis indicated no substantial improvement in polymer to cyclic carbonate selectivity. The best result achieved was 2.3 in the case of 1,2-butylene oxide compared to 2.0 for propylene oxide copolymerization in dioxane (the only literature result for this catalyst). (134) The polymer produced (in the case of cyclohexene oxide) was of moderate molecular weight and highly polydisperse (N i = 13,3(X), Mw = 68,800, Mw/Mn = 5.2 using polystyrene standards). 

Zevaco and co-workers reported the use of aluminum isopropoxide in RO copolymerization of cyclohexene oxide (CHO) with carbon dioxide [4]. Their results showed that the optimum condition was at 70°C and CO pressures of 114 bar with a substrate to catalyst molar ratio of 300 1 and a CO to substrate molar ratio of 2 1. Zevaco et at claimed that their catalytic... 

Beckman and coworkers [11, 58] reported that inexpensive poly(ether carbonate) (PEC) copolymers have been reported to be soluble in CO2 under moderate conditions and could function as building blocks for inexpensive surfactants, but numerous practical difficulties remain. These hydrocarbon systems involve PECs synthesized by aluminium-catalyzed copolymerization of cyclic ethers with CO2 (i.e.. Ml = ethylene oxide, propylene oxide, cyclohexene oxide M2 = CO2). These copolymers were found to be soluble in liquid CO2 at concentrations of 0.2-1.5% (w/v) at ambient temperatures and pressures in the range 120-160 bar - that is, significantly above the liquid-vapour pressure for CO2. These statistical copolymers were generated from very inexpensive feed-stocks and are thus appealing as building blocks for cheap surfactants. The enhanced solubility of these copolymers with respect to poly(propylene oxide) is speculated to arise, at least in part, from specific... 

A number of metal-catalyzed polymerizations have utilized CO2 as both a solvent and as a reagent in the reactions. Precipitation copolymerization of either propylene oxide (83) or cyclohexene oxide (84) with CO2 in SCCO2 has been catalyzed using heterogeneous zinc catalysts. Copolymerizations of CO2 and propylene oxide formed PCs with a molecular weight of about 10 g/mol and incorporation of CO2 at greater than 90% (eq. (7)). A small percentage of propylene carbonate by-product was also observed. 

In a recent report, Nozaki and coworkers isolated presumed intermediates in the asymmetric alternating copolymerization of cyclohexene oxide with CO2P Reaction of a 1 1 mixture of ZnEt2 and (5)-a, a-diphenylpyrrolidine-2-yl-methanol (11, Scheme 24.13) yielded dimeric 12, which was structurally characterized by X-ray diffraction studies. At 40 °C and 30 atm CO2, 12 catalyzed... 

SCHEME 24.13 Chiral zinc catalysts for the asymmetric, alternating copolymerization of cyclohexene oxide and CO2. 

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