Polymerization centers

The principal differences between cationic and anionic polymerizations center around the following points ... 

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

A further modification of the active-center model was based on the consideration that the vacancy in the active center is strongly shielded by the polymer chain, mainly by the CH2 and CH3 groups of the second monomer unit. As a result, the vacant site is blocked and inaccessible for olefin coordination. Kissin et al. suggest a polymerization center with two vacancies one shielded and the other open for complexation.344,345 After each insertion step the end of the growing polymer chain flips from side to side and the two vacant sites are alternately available for alkene coordination. 

Ten years later Napper and Alexander, in studying the kinetics of vinyl acetate polymerization in the presence of anionic, cationic and nonionic emulsifiers arrived at the same conclusions as Priest s, although they did not cite his work (13). They also observed an acceleratory effect of added emulsifier like that found by Baxendale et al. (8), but they seemed unaware of that work as well. They showed that when the charge on primary particles (due to initiator fragments) was opposite to that of the emulsifier, the rate of polymerization was slower than that in the absence of emulsifier. This was presumably due to the greater instability of the colloid formed and the consequent production of fewer polymerizing centers. 

Chains with monodisperse molecular weight distribution (Mw/Mn = 1.00) can occur in idealized conditions when all polymerizing centers initiate instantaneously and chain termination is absent. In these cases the catalyst is actually an initiator. These living polymerizations are quite rare among transition metal catalysts. More often, random chain termination leads to many chains formed per metal atom. A Schulz-Flory most probable distribution of polyalkene molecular weights (Mw/Mn = 2.00) is the result. In cases when more than one type of active site is present, bimodal or multimodal distributions of molecular weights result (Mw/Mn > 2.00). 

The work is divided into three parts. The first (Chapters 3 and 4) covers the catalyst (solid components) with particular regard to preparation, structure and role of the components. The second part (Chapter 5) deals with the reactions which take place between catalyst and cocatalyst and give rise to the formation of polymerization centers. The third part regards the polymerization and, apart from a discussion of number and nature of the active centers, reports the hypotheses about the most probable reaction mechanisms as a function of various kinetic parameters. 

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

From the above results, it is clear that the rate decay must be attributed to a chemical deactivation of the polymerization centers with time. Different mathematical expressions have been proposed, for those catalyst systems most widely studied in the literature, in order to express the law of the decay. For propylene polymerization with TiCyMgCl2—AlEt3/EB or with TiCyEB/MgCl2 - Al Et3/EB, Spitz 45-97) proposed an expression of the following type ... 

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

On the basis of these results, the Lewis base has been assumed by several authors 50.67,69,81.100,113.115,117) selectively poison the less stereospecific polymerization centers, through a reversible complexation of the Lewis base to the coordinatively unsaturated active sites. 

Table 7, Number of polymerization centers and values of propagation and transfer rate constants for ethylene polymerization... 

The kinetics and mechanism of olefin polymerization can be elucidated from the dependences of the number, reactivity and selectivity of the active centers (active sites, polymerization centers) on ... 

M being a fraction of polymerization centers covered by the coordinated monomer (then the kp dimension is s 1). 

If diffusion phenomena are not involved, the formation and deactivation of polymerization centers should reflect in rate-time dependences, other conditions being constant. Rate acceleration period of very widely differing lengths is often observed, followed either by a more or less steady rate or by a deceleration (rate decay) period. As for the polymerization center deactivation, it is quite important to know whether a macromolecule or a metal-polymer bond is formed due to this reaction (see Sect. 4). 

Anionic polymerizations must be carried out in the absence of water, oxygen, carbon dioxide, or any other impurities which may react with the active polymerization centers. Glass surfaces carry layers of adsorbed water which react with carban-ions. It is necessary to take special precautions, such as flaming under vacuum, to remove this adsorbed water in laboratory polymerizations. Anionic reactions are easier to carry out on a factory scale, however, because the surface-to-volume ratio is much less in large reactors. 

Kinetic curves of the polymerization of these monomers in the presence of both emulsifiers with their initial concentration close to CMC are shown in Fig. 9. As can be seen the initial polymerization rate, at the same rate of radical formation (the same concentrations of initiator and the same temperature), decreases from MA to BA in accordance with the decrease of the chain propagation rate constant in these monomers (Bagdasar yan, 1966). Subsequently, however, the polymerization rate of the MA becomes much lower than the rate of other monomers, a fact which apparently is associated with the colloid behavior of the system (reduction of the number of polymerization centers as a result of flocculation). 

Majority opinion now favors the concept that the d-orbitals in the transition element are the main source of catalytic activity and that chain growth occurs at the Ti-alkyl bond, which acts as the polymerization center, the function of the aluminum alkyl being only to alkylate TiCls. The monometallic mechanism presented below are mainly based on the ideas of Cossee and... 

This step is assumed to be the insertion of the first monomer molecule, M, into a transition metal-carbon bond in an active center, Cat-R, resulting in the formation of a polymerization center Cat-P ... 

In this scheme, all polymerization centers are regarded as equally active and having the same propagation rate constant, kp, independent of their geometric location and of their degree of polymerization. However, since it is established that all centers are not equally active, the A p in this scheme must be regarded as an average. 

At any instant there will be polymerization centers generally represented by Cat—P (n = 1, 2, ) and different initiation centers Cat—R, Cat—R" and Cat—R " [cf. Eqs. (9.18)-(9.20)]. Thus the total concentration of active centers, C, is given by the sum of the concentration of polymerization centers, C, and the concentrations of different initiation centers, Q, at which initiation takes place at that particular moment ... 

Under stationary state conditions only the steady state with respect to the concentration of polymerization centers need be considered-. A steady state thus implies the following conditions ... 

The situation for stereoregular polymerization is quite similar to the cases discussed in Problem 9.5, if it is postulated that the dimeric alkylaluminum molecules are adsorbed on TiCls sites to give rise to polymerization centers by the following equilibrium process ... 

If the polymerization centers react with the adsorbed monomer molecules then the Langmuir-Hinshelwood rate equation [Eq. (P9.5.13)] should be used and one would obtain the rate expression in the stationary zone as... 

Rationalize the above kinetic behavior assuming Langmuir-type adsorption and formation of polymerization centers from Al2Ets (represented by A2) and propylene monomer on the surface of TiCla. 

Stage I may be considered as the period during which the polymerization center is established on the surface of titanium trichloride from Al2Etg and the olefin monomer. Al2Ete adsorbs reversibly on the surface S to form a surface complex C. Assuming that the adsorption equilibrium is of the Langmuir type [cf. Eq.(P9.5.4)],... 

The polymerization center C may be formed irreversibly by the attack on the surface complex C by a propylene monomer M from the solution. Then, the formation rate of the center can be expressed by... 

Note that the formation of the polymerization center [Eq. (P9.8.4)] is assumed to be irreversible and that of the surface complex [Eq. (P9.8.3)] to be reversible because the polymerization center is more stable than the surface complex. 

If the polymerization occurs by the Rideal mechanism, that is, the rate determining step of the polymerization is the attack of a propylene monomer from solution onto the polymerization center, the rate Rp can be given by... 

Since S is an intermediate entity in the formation of the polymerization center, [S ]oo — 0. This follows from the reasoning that after a sufiSciently long interval of time, monomer molecules would have reacted completely with sdl the potential polymerization centers by the irreversible reaction in Eq. (P9.9.2). Moreover, since both [S] and [S]oo are large numbers and it may be assumed that only a few of the active sites participate in polymerization, [S]oo — [S] — 0. Therefore, Eq. (P9.9.6) reduces to... 

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