Polymerization turnover frequency

Polymerization conditions 1000 equivalents MAO per Ni b Turnover frequency (moles of ethylene polymerized per mole of Ni per hour)... 

To solve the issue of ligand leaching that was encountered in some of the examples above, fluorous polymeric phosphine ligands 15a-c [28] were developed. The rhodium complexes prepared from 15a-c using a 3 1 ratio of P Rh [28b, 29] displayed good turnover frequencies (TOFs) in the case of 15 a, but reaction rates for 15b,c were lower. The catalyst derived from 15 a was recycled seven times without loss of activity, although leaching was not studied quantitatively. 

Lu and coworkers have synthesized a related bifunctional cobalt(lll) salen catalyst similar to that seen in Fig. 11 that contains an attached quaternary ammonium salt (Fig. 13) [36]. This catalyst was found to be very effective at copolymerizing propylene oxide and CO2. For example, in a reaction carried out at 90°C and 2.5 MPa pressure, a high molecular weight poly(propylene carbonate) = 59,000 and PDI = 1.22) was obtained with only 6% propylene carbonate byproduct. For a polymerization process performed under these reaction conditions for 0.5 h, a TOF (turnover frequency) of 5,160 h was reported. For comparative purposes, the best TOF observed for a binary catalyst system of (salen)CoX (where X is 2,4-dinitrophenolate) onium salt or base for the copolymerization of propylene oxide and CO2 at 25°C was 400-500 h for a process performed at 1.5 MPa pressure [21, 37]. On the other hand, employing catalysts of the type shown in Fig. 12, TOFs as high as 13,000 h with >99% selectivity for copolymers withMn 170,000 were obtained at 75°C and 2.0 MPa pressure [35]. The cobalt catalyst in Fig. 13 has also been shown to be effective for selective copolymer formation from styrene oxide and carbon dioxide [38]. 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

Fig. 3 Comparison of the turnover frequency (TOF) for the polymerization of BD with three catalyst systems (1) NdV/DIBAH/EASC, (2) NdA/DIBAH/EASC and (3) NdP/DIBAH/EASC at various molar ratios mdibah/mncI [272], reproduced by permission of Taylor Francis Group, LLC., http //www.taylorandfrancis.com...

The aim of this Section is to discuss the experimental methods, problems, and recent improvements in the determination of the exact nature of the precursor species Yi, Y2, Y3, etc. (i.e., the determination of the initiation mechanism for the ethene polymerization). Facing this topic, we must be aware that, besides the problems related to the determination of the Cr(II) structure (vide supra Section VI.A.2), the identification of the species formed during the initial stages of the reaction has been prevented so far for two other reasons (a) only a fraction of the Cr(II) sites are active in the polymerization under the usually adopted conditions (225), so that almost all the characterization techniques give information about the inactive majority Cr sites and (b) the active sites are characterized by a very high polymerization rate (high turnover frequency, TOF). It is thus clear that any experimental efforts devoted to the detection of the precursor and/or intermediate species must solve these two problems (vide infra Section VI.C). 

All complexes 177-182 showed high catalytic activity in the ROP of D,L-lactide (Scheme 14) and D,L-p-butyrolactone (Scheme 69) [134]. The polymerization reactions proceeded at room temperature with turnover frequencies of up to 31,200 h and turnover numbers of up to 2,400. The reactions occurred in a controlled fashion, giving polymers with relatively narrow polydispersities = 1.08-1.44). 

Among the many examples of asymmetric hydrogenation catalysts that have been developed, chiral complexes prepared from 1,2-dianiines and RuCb/diphosphines provide one example of the most powerful catalysts for this reaction. Polymer-supported fR -BINAP was treated with RuCh and fR,R -l,2-diphenylethylenedi-amrne to give the polymeric chiral complex 180 (Scheme 3.56) this serves as an excellent precatalyst for the asymmetric hydrogenation of aromatic ketones to give the chiral secondary alcohols in quantitative yields with 84—97% ee-values [115]. For example, the asymmetric hydrogenation of I -acetonaphthone with (R,RR)-180 occurred in quantitative conversion within 26 h with 98% ee. The enantioselectivity, turnover number (TON) and turnover frequency (TOF) in this reaction... 

Figure 3 illustrates that the polymerization rate is independent of resin diameter. During 30 min reactions, CALB immobilized on resins 1 to 4 gives turnover frequency (TOF) of e-CL of about 12 s" In contrast, our previous work of CALB immobilized on PMMA resins showed a large dependence of e-CL %-conversion on resin particle diameter. For example, in 30 minutes reaction time, as the particle size decreased from 560-710, 120, 75 and 35 pm, turnover frequency (TOF) of e-CL increased from 3.8 to 5.3, 7.5 and 11.2, respectively. However, by increasing the resin pore size from 300 (resin 4) to 1000 A (resin 5) for 35 pm beads, the TOF reached 28.2 s. As discussed above, increase in resin pore diameter also corresponds to an increase in %-area of beads at which CALB is found (37 to 88%). 

An attractive alternative to improve the general activity of catalytic MIPs is the utilization of transition metal catalysts. In this case, polymeric catalyst with turnover frequencies of >100 h and turnover numbers of >100 can be prepared. For this type of MIP catalysts, the imprinting procedure should be regarded as a method to... 

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