Propylene polymerization kinetics

Propylene Polymerization Kinetics in Gas Phase Reactors Usii Titanium Trichloride Catalyst... 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Table II summarizes the yields obtained from the CONGAS computer output variable study of the gas phase polymerization of propylene. The reactor is assumed to be a perfect backmix type. The base case for this comparison corresponds to the most active BASF TiC 3 operated at almost the same conditions used by Wisseroth, 80 C and 400 psig. Agitation speed is assumed to have no effect on yield provided there is sufficient mixing. The variable study is divided into two parts for discussion catalyst parameters and reactor conditions. The catalyst is characterized by kg , X, and d7. Percent solubles is not considered because there is presently so little kinetic data to describe this. The reactor conditions chosen for study are those that have some significant effect on the kinetics temperature, pressure, and gas composition.

The kinetic measurements reported in the following sections are concerned with the polymerization of propylene the results obtained with this monomer can, however, be extended to other olefins (e.g., normal butene-1, pentene-1, or branched). For this reason, although we limit ourselves to recording measurements made with one monomer only and with two types of catalytic system, we have given the most general title to this paper. 

The diethylzinc-alcohol (1 2) system was also extensively studied by Tsuruta and his co workers (85,86). Amorphous zinc dialkoxide was concluded to be an active species, because crystalline zinc alkoxide prepared from zinc chloride and lithium alkoxide proved to have only a very small catalytic activity. Based on kinetic studies of the polymerization of propylene oxide with the ZnEt2-CH3OH (1 2) catalyst system, the catalytically active species was concluded to be the complex formed by coordination of one molecule of monomer to the catalyst. In the polymerization of propylene oxide with the catalyst system, it was concluded that the monomer was polymerized by ring opening brought about by cleaving the CH2-0 bond (87). 

A similar kinetic treatment has been utilized by Fontana and Kidder (/6) to explain the details of the cationic polymerization of propylene. 

The polymerization kinetics of propylene, 3-methyl-l-butene, and 4-methyl-l-pentene can be described by Eq. (12) and it is felt that this scheme may be generally valid for cationic polymerization of olefins as there is no reason to suspect that a fundamental difference in polymerization mechanism exists in the case of the three monomers cited above as compared with other cationically polymerizable olefins. 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

A2 is also a known function of T and space velocity since the rate constant K2 is known from the steady state results (eq. 1). The parameters Ai and Af are not known independently however, the ratio Aj/Af equals the adsorption coefficient Kpr of propylene oxide which is a known function of T obtained from the steady state measurements (eq. 1). Since the steady state kinetics indicate that the surface reaction is the rate limiting step it can be concluded that Ai is larger than A2. It was assumed that propylene oxide adsorption is nonactivated and Aj was arbitrarily set equal to be two times larger than A2 at 400°C,for Y =. 002 then Aj was calculated from Af = Ai/Kpro Yp. The numerical simulations indicated that the model predictions are rather insensitive to Aj but are sensitive to the unknown parameters A3 and 0 c Since the Heat of Polymerization of Propylene Oxide is 18 Kcal/mol the parameter 0 was set equal to 0 exp(-18000/RT). 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Polymerization of propylene oxide initiated by t-butoxide salts in dimethyl sulphoxide showed the simple first order kinetics in the initiator as well as in the monomer53. However, chain transfer to the solvent and E2 elimination deprived this system of a living polymer character. The living character of ethylene oxide polymerization in te-... 

In the polymerization of propylene sulfide and 1,2-butylene sulfide mainly tetra-mers were observed. Cycles were formed mostly during the slow degradation process that followed rapid polymerizations. Degradation can also be induced by adding cationic initiators to polymer prepared by other mechanisms, e.g. by anionic processes. Thus, poly(trans-2,3-butene sulfide) is rapidly degraded to equimolar amounts of 3,5,6,7-tetramethyl-l,2,5-trithiacycloheptane and trans-butene 47). Poly(cis-2,3-butene sulfide) forms, however, a mixture of tetramer, trithiacycloheptane derivative and cis-butene 47 . If one is forced to use cationic processes for the synthesis of poly-sulfides, the reaction conditions should be controlled to avoid macrocyclization. If cyclic products are desired, the kinetics of their formation should be studied to determine optimum yields. 

Fig. 8 Kinetic curves of propene polymerization with MgCl2/Di/TiCl4/D2-AlEt3, 70°C. Open circles) polymerization of propylene in n-heptane pcsHs = 2.5 atm, Kh = 0.325 mol/(l atm) closed circles) polymerization in liquid propene [CaHg] = 10.5 mol/L open squares) gas-phase polymerization of propylene, pcjHs = 2.5 atm, = 0.13mol/(l atm) closedtriangles)gas-...

Another kinetic method has been proposed [17]. This method allows one to determine the kp value and the number of active sites using the dependence on monomer concentration of the stationary polymerization rate and of the polymerization rate during the acceleration period of the kinetic curve, according to Eqs. (2) and (6). The method has been applied for determination of these kinetic parameters in the polymerization of propylene and ethylene with VCl3/Al(i-Bu)3 catalyst. 

For gas-phase or liquid propylene bulk reactors, the bulk monomer concentration in the reactor must be converted to concentration in the polymer phase surrounding the active sites with a thermodynamic relationship. Generally, a simple partition coefficient such as the one used in Equation 2.136a is used. For diluent slurry reactors, where the monomer is introduced in the gas phase, a partition coefficient such as Herny s law constant must also be used to calculate the concentration of monomer in the diluent which, in turn, is used to estimate the concentration of monomer in the polymer phase surrounding the active sites. Evidently, more sophisticated thermodynamic relationships relating the concentration of monomer in the gas phase, diluent and polymer can be used but, from a practical point of view, are only justified when the polymerization kinetic constants are very well known. Similar considerations apply to calculate the concentrations of comonomers, hydrogen and any other reactant in the system. 

Ethylene polsrmerization, in contrast to the polymerization of propylene and other alpha-olefins, is often affected by diflfiision limitations, which occur if the monomer reactivity in polsonerization is high relative to diffusivity through the catalyst particle. This can result in the formation of an onion particle structiu-e as pol5nnerization first takes place at the external surface of the particle, particle growth occurring step by step as the monomer reaches the inner parts of the catalyst particle. This mechanism of particle growth is associated with a kinetic profile in which an initial induction period is followed by an acceleration period, after which, in the absence of chemical deactivation, a stationary rate is obtained. 

Thermodynamics is, similarly, well suited for the description of processes which lead to changes of the molecular structure, as just seen for phase changes. A reaction with unfavorable thermodynamics expressed by a positive AG does not occur. However, with a negative AG, a reaction may still fail kinetically, while another mechanism may succeed. A typical example is the preparation of polypropylene. Although the polymerization of propylene is possible thermodynamically, it was not achieved until the work of Ziegler (139) and Natta (140), who discovered the catalyzed mechanism with favorable kinetics. Thus, much effort has been devoted to understand the kinetics of polymerization (118). Early work concentrated on predicting molecular masses and their distribution. In this section the thermodynamics of polymerization is briefly discussed. Most attention is paid to addition (chain) polymerization, but the theory is also applicable to condensation (stepwise) polymerization. The subject is extensively reviewed (141-145). 

Tsuruta found that the optically pure complex [(/ -salcy)Co] (15) was active for epoxide polymerization (Scheme 24.15) when activated with ALEt3. Although the system exhibited no enantioselectivity for the polymerization of propylene oxide, it was moderately selective (r = 1,5) for the kinetic resolutions of tert-butyl ethylene oxide and epichlorohydrin (Scheme 24.15). 

In a study of the cationic polymerization of propylene, but 1 ene and ds-but-2-ene, Puskas et alP concluded that the kinetics could be explained by a rate-controlling initiation step rather than the 30 year old Fontana-Kidder mechanism involving slow propagation. 

The most commonly and effectively used metal alkyls for propylene polymerization employing magnesium chloride - supported catalysts are invaricibly trlalkylaluminium confounds, dialkylaluminium halide conpounds giving much lower activities. In general the polymerization kinetics shown by catalysts of this type are strongly affected both by the trlalkylaluminium to titanium ratio and by the type of alkylaluminlum compounds which is used. 

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