Types of Active Centers

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51 . 

The existence of a further type of active centers was demonstrated by Pino and Rotzinger93 by polymerizing ethylene with a MgQ2-supported catalyst in the presence of an electron donor. A comparison of the ethylene and propylene kinetic curves shows that, while propylene polymerization is characterized by the well known rapid decrease in rate, the ethylene polymerization rate increases reaching a constant value after about 30 min. This has been attributed to the existence of active 

The presence of different active species has also been evidenced in typical catalysts for the ethylene polymerization. For example, based on the determination of the number of active centers and their relative propagation constants, Bohm 129) concluded that in the TiCl4 + Mg(OC2H5)2—AlEt3 catalysts there are at least two types of species with extremely different kp values (2900 1/mol. sec and 80 1/mol. sec). The more rapid propagation centers would only make up 2 % of the total. 

Reichert149) studied the ethylene polymerization with Mg(OC2Hs)2 + TiCl4 — Al(octyl)3 in a plug flow reactor. Oligomers and polymeric fractions were obtained indicating the presence of more than two active species on the catalyst surface. 

The broadening of the MWD observed in polyethylene as well as in polypropylene isotactic and atactic fractions constitutes further evidence for the existence of different active centers 53). 

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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). 

Solid state lasers are those whose active medium consists of an insulating material activated by an optically active center. Three different types of active center have usually been used as active laser centers rare earth ions, transition metal ions, and color centers (see Chapter 6). 

Kinetics of Addition Polymerization. As the name suggests, addition polymerizations proceed by the addition of many monomer units to a single active center on the growing polymer chain. Though there are many types of active centers, and thus many types of addition polymerizations, such as anionic, cationic, and coordination polymerizations, the most common active center is a radical, usually formed at... 

Free radicals are certainly the most commonly used type of active centers. They are produced by decomposition of a suitable molecule, an initiator, either thermally or by irradiation. Typical initiators are given in Table 2.15. Homolytic scission of a a bond leads to two free radicals. Depending on the selected initiator, two identical or two different radicals can be formed. 

Table 1 Full energies En calculated for different types of active centers with n- and a-coordinated growing polymer chains (adopted from [94])...

With the balance for the number of available forms of active centers of a catalyst, the concentrations and, correspondingly, the thermodynamic mshes of the reaction complexes catalytic intermediates)—that is, the thermalized intermediate compounds of the reactant molecule (or molecular fragment) with the active center—appear interrelated through these balance relations in respect of each type of active center. The balance is of primary importance to the kinetics of the stepwise transformations and causes a number ofpecuHarities of the stationary kinetics of the stepwise processes. This makes the kinetic description of the catalytic transformations differ considerably from the descrip tion of the preceding schemes of noncatalytic reactions. For example, in the simplest catalytic stepwise transformation of substance R to substance P,... 

As applied to catalysis, the microkinetic analysis of catalytic reactions is used most often. This is an instrument of an idealized description of com plex catalytic processes without consideration of the mass transfer that can affect considerably the observed kinetics of the catalytic transformations. The microkinetic analysis with the necessary consideration of the active sites balance for all types of active centers of the catalyst, even though it has several drawbacks, can provide important information about the potential influence of the very different thermodynamic factors. 

The balance equation for all types of active centers of the catalyst is here... 

Propagation is the most important elementary reaction in which a macro-molecular chain is formed. Control in new carbocationic polymerizations, in which well-defined polymers are prepared, might be explained by new mechanisms of propagation and new types of active centers involved. However, as discussed briefly in Section IV.B.4, we believe that only two types of species with different degrees of ionization are involved sp3-hybridized dormant species and sp2-hybridized carbenium ions [Eq. (43)] ... 

Group ab) is split into three subgroups (Sect. 4.1.2.1-4.1.2.3). Their features can be characterized conveniently using an example of idealized living Z-N polymerization with a single type of active centers. Table 1 shows how various stoppers... 

Let us consider several types of active centers, their numbers being Cf. Modification of Eq. [2] gives ... 

The number of isotactic active centers (Cp) in propylene polymerization using one-component catalysts is close to the number of atactic active centers (Cf) (see Table 2). The kp values for the two types of active centers are also similar (with the exception of the catalyst containing ethyl benzoate). 

The effect of the last monomeric unit of the growing polymer chain on the stereospecificity of the olefin addition has been confirmed by the calculation of the energy of non-bonded interactions and by quantum-chemical calculations (see section 5.2). Corradini et al. have analyzed the possibility of the it-complex formation on the octahedral titanium ions located on different faces of a- andy-TiCla. The possibility of the coordination by both faces of the propylene molecule was studied. It was shown that active centers on the lateral faces of a-TiCls and y-TiClj may be regioselective (primary insertion of propylene) ruther than stereospecific (no predominant CjHs coordination by one face). In the case of active centers located on the edges of the layered modifications of TiClj, CsHg is coordinated with the more accessible (outward) coordination sites of the titanium ions predominantly the polymer chain is then located on the less accessible (inward) octahedral site. This position of the polymer chain results in a fixed orientation of the first carbon-carbon bond of the polymer chain due to its non-bonded interaction with the TiClj surface. This may explain the predominant coordination of propylene molecules by one face and the stereospecificity of such type of active centers. 

Homogeneous Homogeneous with more types of active centers Heterogeneous... 

Multiple Active Center Polymerizations. As mentioned above, it has been suggested that the two peaks observed in polymers with bimodal molecular weight distributions result from two different types of active centers present reacting simultaneously. 

This result can be Interpreted either by assuming that only one type of active species Is present - presumably cryptated Ion pairs - or by assuming that If there are different types of active centers, they might have the same reactivity. 

A homogeneous catalyst can be made as a single-site, that is, only a single type of active center is present in a homogeneous system, under proper conditions. A single-site stereospecific catalyst [44] can produce polymers with sharp melting transitions (T ) and markedly narrow molecular weight distribution Mj/Mn <2). 

For a system with i types of active centers, the polymerization rate of the ith species is... 

Only the conjunction of different techniques can give a full characterization of the type of active center ... 

The only type of active centers in the polymerization of ethylene oxide is the free ion pair. There are only a few free ions, and their activity only slightly exceeds that of ion pairs. For a concrete system in dimethyl sulfoxide (Na counterion) the ratio of rate constants is equal to 2. 

In a series of papers58,66,675, it was established that this process represents a special type of copolymerization in which a statistic copolymer is formed in spite of the fact that formaldehyde and dioxolane are polymerized on different types of active centers. 

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