Step polymerization catalyzed

In the synthesis route from acetyl-CoA to poly(3HB), at least three steps and three enzymes are involved (Fig. 1). The first step is catalyzed by the 3-keto-thiolase (EC 2.3.1.9) which reversibly links two acetyl-CoA moieties to aceto-acetyl-CoA in a Claisen-condensation. The conversion of acetoacetyl-CoA into D-(-)-3-hydroxybutyryl-CoA can be mediated by a reductase (step 2) or via a sequence catalyzed by a reductase (step 4) and two hydratases (steps 5,6). The last step, i.e., the polymerization, is catalyzed by a polymerase (step 3). This... 

The Key Steps in Olefin Polymerization Catalyzed by Late Transition Metals... 

Consider the polyesterification of a diacid and a diol to illustrate the general form of the kinetics of a typical step polymerization. Simple esterification is a well-known acid-catalyzed reaction and polyesterification follows the same course [Otton and Ratton, 1988 Vancso-Szmercsanyi and Makay-Bodi, 1969]. The reaction involves protonation of the carboxylic acid,... 

The overall rate of conversion of e-caprolactam to polymer is higher than the polymerization rate of e-aminocaproic acid by more than an order of magnitude [Hermans et al., 1958, I960]. Step polymerization of e-aminocaproic acid with itself (Eq. 7-57) accounts for only a few percent of the total polymerization of e-caprolactam. Ring-opening polymerization (Eq. 7-58) is the overwhelming route for polymer formation. Polymerization is acid-catalyzed as indicated by the observations that amines and sodium e-aminocaproate are poor initiators in the absence of water and the polymerization rate in the presence of water is first-order in lactam and second-order in COOH end groups [Majury, 1958]. 

Control of the electron-transfer step was also attempted by combining two metal species on a polymer ligand167. We prepared polymer-metal complexes involving both the Cu(II) and Mn(III) ions. The oxidative polymerization of XOH catalyzed by the PVP-Cu, Mn mixed complex or the diethylaminomethylated poly(styrene)(PDA)-Cu Mn mixed complex proceeded 10 times faster than the polymerization catalyzed by either PVP- or PDA-metal complex. The maxima of the activity observed at [Cu]/[Mn] = 1 and [polymer]/[Cu,Mn] moderately small where Cu and Mn ions were crowded within the contracted polymer chain. Cooperative interaction between Cu and Mn was inferred. The rate constant of the electron-transfer step (ke in Scheme 14) for Cu(II) -> Cu(I) was much larger than that for Mn(III) -> Mn(II). The rate constants of the reoxidation step (k0) were polymer-Mn ex polymer-Cu.Mn > polymer-Cu, so the rapid redox reaction... 

Kinetically slow steps in the formation of melanin from DOPA are the formation of dopaquinone from DOPA (step 1, kD), the reaction of dopachrome to dihydroxyindole (step 2), and the polymerization to form melanin (step 3, kM). Step 1 and step 2 proceed with about the same rate in the oxidative coupling polymerization catalyzed by tyrosinase. However, step 1 becomes remarkably slow when a macromolecule-metal complex is used as a catalyst. The copper complex in poly(l-vinylimidazole-co-vinylpyrrolidone) has been found [38] to act as an excellent catalyst and to exhibit the highest activity for melanin formation. The ratio of the rate constants ( m/ d) is approximately 3 (tyrosinase... 

Write the steps in the mechanism of ethylene polymerization catalyzed by a free radical. 

A sequence of chemical reactions is postulated to account for the features observed in the kinetics of BD polymerization catalyzed by NdV/DIBAH/ EASC [178]. From the reaction scheme which is hypothetically put forward 3 to 7 eq. of DIBAH are required for the generation of one active Nd species starting from one equivalent of NdV (Scheme 24). In the first reaction step the reduction of Nd carboxylate to Nd alcoholate is assumed to occur. Experimental evidence in favor of the reduction of Nd-carboxylate comes from NMR studies in which the reduction of the respective lanthanum carboxylate by DIBAH was observed (halide donor EASC) (Windisch, 2006, personal communication). This reduction does not occur when Nd carboxylates are reacted with TMA (Anwander and Fischbach, 2006, personal communication). DIBAH might be capable of reducing carboxylic acids to the respective al-... 

Michalak A, Ziegler T, The Key Steps in Olefin Polymerization Catalyzed by Late Transition Metals. In Computational Modeling of Homogeneous Catalysis, edited by F Maseras, A Lledos (Kluwer Academic Publishers, 2002)... 

The final step to afford the acrylate monomer is the addition of acryloyl chloride to a solution of 13 and triethylamine in methylene chloride to produce the acrylate monomer 14 (Fig. 17). The acrylate polymer 15 was made by a bulk polymerization catalyzed by 2,2 -azobisisobutyronitrile (AIBN) at 110" C for 2d (Fig. 17). 

Let us consider the polyesterification of a diacid and a diol to illustrate the kinetic behavior of a typical step polymerization. Like simple esterification (Sykes, 1986), polyesterification is an acid-catalyzed reaction that can be represented by a sequence of reactions as shown by Eqs. (5.4)-(5.6). In these equations, the wavy lines (www) are used to signify that these equations apply, irrespective of the size of the molecular species. Equation (5.4) represents pfotohation of oxygen in carbon-oxygen double bond which leads to a more positive carbon atom for subsequent addition of a nucleophile, in this case www-OH [Eq. (5.5)], followed... 

So far, two main mechanistic schemes (1 and 2 in Scheme 8) have been proposed for olefin polymerization catalyzed by group 3 and 4 transition metals. The first of these mechanisms is named after Cossee and substantially occurs in two steps ... 

The mechanism for bacterial synthesis of PHA is not the simple dehydration reaction between alcohol and carboxyl groups. It is more complicated and involves the coenzyme A thioester derivative of the hydroxyalkanoic acid monomer (produced from the organic feedstock available to the bacteria) [Kamachi et al., 2001]. Growth involves an acyl transfer reaction catalyzed by the enzyme PHA synthase (also called a polymerase) [Blei and Odian, 2000]. The reaction is not a step polymerization but is a chain polymerization with the characteristics of a living polymerization [Su et al., 2000] (Secs. 3-15, 5-2g, Chap. 7). 

Equation (2.57) provides a means of determining the relative contribution of the termination and transfer steps. Thus, if kp is largely relative to kt, the molecular weight will be virtually independent of monomer concentration, but if the reverse is true, X will be directly proportional to [A/]. Hence this relation lends itself to a simple experimental test, i.e., a plot of 1/X" (the reciprocal of the initial X value) against 1 /[MY (the reciprocal of the initial monomer concentration). It has actually been found that the polymerization of styrene by SnCU in ethylene dichloride (Pepper, 1949), and of vinyl alkyl ethers by SnCU in m-cresol (Eley and Richards, 1949), showed a dominance of termination over transfer, i.e., X a [M] however, for isobutylene polymerization catalyzed by TiCU in -hexane (Plesch, 1950), the observed polymer molecular weights were independent of monomer concentration, i.e., transfer appeared to predominate. 

Several microorganisms are known to accumulate PHB within the cells as an intracellular storage material for carbon and energy source. In microbes PHB synthesis takes place by a sequence of reactions catalyzed by three enzymes— 3-ketothiolase, acetoacetyl-coA reductase and PHB synthase. The first step is catalyzed by the enzyme 3-ketothiolase (E.C. 2.3.1.16) which condenses acetyl-CoA to acetoacetyl-CoA. This intermediate is reduced to D (-)-P-hydroxybutyryl-CoA by an Nicotinamide Adenine Dinucleotide Phosphate (NADPH)-dependent acetoacetyl-CoA reductase (E.C.1.1.1.36) and the enzyme PHB synthase catalyzes the head to tail polymerization of the monomer to PHB (Figure 17.3). 

Polycondensation. Oligocarbonate with molecular weight of less than 1x10 was formed by lipase CC-catalyzed polycondensation of carbonic acid diphenyl ester with bisphenol A (206). Diethyl carbonate was poljmierized with 1,4-butanediol by lipase CA catalyst (207,208). The successive two-step polymerization, the prepoljTnerization under ambient pressure, followed by the polymerization under vacuum (0.5 mm Hg), produced poly(tetramethylene carbonate) with Mw of more than 4x 10 . 

Rgure 1.11 Porod plots of scattering data of silicates polymerized under a variety of conditions (from a study by Schaefer and Keefer ) (a) two-step acid-catalyzed tetraethoxysilane (TEOS) system (b) two-step acid-and base-catalyzed TEOS system (c) one-step base-catalyzed system TEOS system (W = 1) (d) one-step base-catalyzed system TEOS system (VP = 2) (e) aqueous silicate system. LUDOX . VP is the water/ silica ratio. 

Step-Polymerization Reactions Via Nickel- and Palladium-Catalyzed Carbon-Carbon Bond Formation. 

Scheme 6.45. Free radical polymerization catalyzed by di-ferf-butyl-peroxide [(CH3)3 C-0-0-C(CH3)3]) of (a) ethene (ethylene [CH2=CH2]) at high pressure to yield polyethylene. Step 1 is termed initiation as the initial species needed for polymerization have been formed in this step. Steps 2 and 3 are called propagation since a new radical forms for each that is used. The chain continues. Steps 4 and 5 are examples of tenmnation. All steps that destroy radicals and do not create new ones terminate the reaction. In Part b 2-chloro-l,3-butadiene [chloroprene, CH2=C(C1)CH=CH2] undergoes a similar polymerization to yield neoprene rubber.

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