Active centers types

Chien s kinetic model [48,49], unlike Ewen s model described above, is for the systems in which more than one active species is present. The model assumes the presence of multiple active center types, chain transfer to MAO, chain transfer by /3-H elimination (see p. 801), and first-order deactivation reactions of active centers. Chien applied the model in the study of ethylene polymerization with Cp2ZrCl2/MAO catalyst and propylene polymerization with Et(Ind)2ZrCl2/MAO and Et(H4lnd)2ZrCl2/MAO catalysts. 

With the exception of a few solution processes such as one used to make ethylene-propylene copolymers, traditional CCC ( Ziegler-Natta ) catalysts, which were used to make all linear polyethylenes until the advent of the metallocene catalysts, have multiple active center types and therefore yield polymers having a moderately broad MWD and CCD. Techniques used to control the distribution include blending, use of mixed catalysts or cocatalysts, and the use of staged batch reactors or multiple, cascaded continuous reactors. These techniques complicated the already poorly-defined MWD due to the heterogeneity of the catalyst, and as a result, the distribution could not be reliably modeled or described using the standard equations presented in Chapter 3. 

The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

It is evident, that all of the abovementioned relates to the reaction of cycle opening type and the formation of active centers (radicals) at molecule ends ... 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

A variant of the common Langmuir type models is obtained if it is assumed that both alkene molecules are chemisorbed (Langmuir adsorption) on the same active center. If it is further assumed that there are two different adsorption steps, the following set of reaction equations for the initial stages of the metathesis of propene is obtained ... 

Here Nx is the active center deactivated by X and P is the polymer molecule. 02, C02, C2H2 appear to be X-type inhibitors. 

Here Ny is the active center deactivated by Y. H20 is likely to be a Y-type inhibitor. To explain the steady-state period of polymerization it may be assumed that some quantities of Y are adsorbed on the support surface. 

The protein from D. desulfuricans has been characterized by Mbss-bauer and EPR spectroscopy 224). The enzyme has a molecular mass of approximately 150 kDa (three different subunits 88, 29, and 16 kDa) and contains three different types of redox-active centers four c-type hemes, nonheme iron arranged as two [4Fe-4S] centers, and a molybdopterin site (Mo-bound to two MGD). Selenium was also chemically detected. The enzyme specific activity is 78 units per mg of protein. 

Among several applications, Fe-based hydrogenases play a central role in the stepwise reduction of CO2 to methane. This process is accomplished through various types of Fe-hydrogenases however, in most of these enzymes, the active center is either a binuclear Fe-Fe- or an Ni-Fe-complex. Although the exact... 

Heterogeneous recombination of active particles and their interaction with molecules of the adlayer are simplest processes of this type. The rates of such reactions as functions of surface coverage by the specified reagents are fully determined by the rate of their surface diffusion towards active centers. In a number of cases, the rate of lateral diffusion is determined not only by the type of diffusing particle, but also (sometimes, predominantly) by the composition and state of the solid substrate surface. Taking into account the role played by the composi-... 

The collision must be sufficiently energetic that enough energy is available to break the chemical bond linking the two bromine atoms. This type of reaction is called an initiation reaction because it generates a species that can serve as a chain carrier or active center in the following sequence of elementary reactions. 

Both of these reactions involve the production of two active centers where there was only one before. When reactions of this type occur to a significant extent, the total number of active centers present in the system can increase very rapidly, since a multiplication effect sets in as the chains propagate. The growth of chain carriers in a branched chain reaction is pictured below. 

Triblock terpolymers PS-b-PBd-b-P2VP and PBd-b-PS-b-P2VP, where PBd is polybutadiene (mostly 1,2-PBd), were prepared in order to study the microphase separation by transmission electron microscopy, TEM and SAXS. In the first case the triblocks were synthesized by the sequential addition of monomers in THF using s-BuLi as the initiator [26]. For the second type of copolymers, living PBd-b-PS diblocks were prepared in benzene at 40 °C in the presence of a small quantity of THF in order to obtain the desired 1,2-content and to accelerate the crossover reaction as well. DPE was then added to decrease the nucleophilicity of the active centers in order to avoid side reactions with the THF, which in combination with benzene was the solvent of the final step. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

In the first family, the metal is coordinated by one molecule of the pterin cofactor, while in the second, it is coordinated to two pterin molecules (both in the guanine dinucleotide form, with the two dinucleotides extending from the active site in opposite directions). Some enzymes also contain FejSj clusters (one or more), which do not seem to be directly linked to the Mo centers. The molybdenum hydroxylases invariably possess redox-active sites in addition to the molybdenum center and are found with two basic types of polypeptide architecture. The enzymes metabolizing quinoline-related compounds, and derivatives of nicotinic acid form a separate groups, in which each of the redox active centers are found in separate subunits. Those enzymes possessing flavin subunits are organized as a2jS2A2, with a pair of 2Fe-2S centers in the (3 subunit, the flavin in the (3 subunit, and the molybdenum in the y subunit. 

This assumption is implicitly present not only in the traditional theory of the free-radical copolymerization [41,43,44], but in its subsequent extensions based on more complicated models than the ideal one. The best known are two types of such models. To the first of them the models belong wherein the reactivity of the active center of a macroradical is controlled not only by the type of its ultimate unit but also by the types of penultimate [45] and even penpenultimate [46] monomeric units. The kinetic models of the second type describe systems in which the formation of complexes occurs between the components of a reaction system that results in the alteration of their reactivity [47-50]. Essentially, all the refinements of the theory of radical copolymerization connected with the models mentioned above are used to reduce exclusively to a more sophisticated account of the kinetics and mechanism of a macroradical propagation, leaving out of consideration accompanying physical factors. The most important among them is the phenomenon of preferential sorption of monomers to the active center of a growing polymer chain. A quantitative theory taking into consideration this physical factor was advanced in paper [51]. 

This monomer concentration Ma in the formalism of the quasi-homogeneous approximation, unlike M a, refers to the whole volume of the two-phase system. The aforementioned quantities are connected by the simple relationship Ma = flM a where y01 stands for the volume fraction of the a-th phase in miniemulsion. An analogous relation, Ra = sdaR a, exists between the concentrations Ra of the a-th type active centers in the entire system and those R a in the surface layer of the a-th phase. This layer thickness da has the scale of average spatial size of the a-th type block, which hereafter is presumed to be small as compared to the average radius of miniemulsion drops. Apparently, in this case, the curvature of the interphase surface can be neg-... 

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