Number of Propagation Centers

The concept of an active center in catalytic polymerization is much more meaningful as compared with that referred to catalytic processes in 

According to the data on the number of propagation centers (N) the propagation rate constant (Kp) is calculated  

The value of Kp as a measure of the reactivity of the active centers in the propagation reaction is the most fundamental characteristic of polymerization catalysts. The conclusions on the polymerization mechanism based on the correct values of N and Kp are much more unambiguous than those made when considering only the data on the polymerization activity and molecular weight of a polymer. 

Methods Used for the Determination of the Number of Propagation Centers 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are  

---

As for the dependence of the polymerization rate V on the monomer concentration some authors have also found first-order kinetics (84, 90, 96, 99), but sometimes deviations from the first order were observed (38, 51, 88) that may be connected with a change in the number of propagation centers with monomer concentration. 

If the dissolving of a portion of the polymer takes place, diffusional restriction may occur as a result. Such a case was observed in (98) where a decrease of the polymerization rate (slurry process in cyclohexane) with temperature rise from 75° to 90°C was found despite the increase in the number of propagation centers. At a further increase of the polymerization temperature (>115°C) polymerization becomes a solution process that may also proceed with no diffusional restrictions (94). 

The most obvious reason for the polymerization rate variation with time is the change in the number of propagation centers (88, 89). According to Zakharov et al. (115) this change may be determined by the concurrence of the following reactions in the polymerization medium ... 

Inhibition methods where the number of propagation centers is calculated by considering the quantity of the inhibitor added and the resulting decrease of the polymerization rate. 

B. Number of Propagation Centers in One-Component Catalysts 1. The Chromium-Oxide Catalysts... 

The data on the determination of the number of the propagation centers on chromium oxide catalysts by the inhibition method were given in several papers water (61), carbon tetrachloride (167), and diethylamine (69) were used as inhibitors. It was found that the number of propagation centers is about 10% (61), 1% (167), and 20% (69) of the total content of chromium in the catalyst. 

In Hogan (69) it was supposed that in a highly active catalyst containing 0.01% of chromium all the chromium ions act as active centers. According to this it was calculated that in the catalyst containing 1% of chromium on silica the number of propagation centers reached 10% of the supported chromium. 

Several determinations of the number of propagation centers by the quenching technique have been carried out (98, 111). As a quenching agent methanol, labeled C14 in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X 102 g C2H4/mmole Cr hr at 75° and about 16 kg/cm2) (98, 111, 168) in catalysts on silica was about... 

Fig. 4. Change of polymerization rate and number of propagation centers with polymerization time. Catalyst Cr ( —CjH6)j/Si02 ethylene pressure 6 kg/cm2, temperature 50°C. The symbols A, A, X, O correspond to different polymerization runs. Arrows show the moment of injection of C140.

A proportional rise of the ethylene polymerization rate with the number of propagation centers was observed (see Fig. 5). 

It was found 158,159) that the fall of the rate observed when aluminum-organic compound was added to TiCl2 during ethylene polymerization was due to the decrease in the number of propagation centers. The propagation rate constant remained unchanged. In propylene polymerization the number of atactic propagation centers sharply diminished when the aluminum-... 

Number of Propagation Centers at Maximum Activity of Catalysis... 

In the case of TiCl2 the number of propagation centers do not exceed 0.5% of the number of surface titanium ions this shows that the formation of the propagation centers proceeds at specific points on the surface of the crystalline catalyst (e.g. lateral faces, outlets of the spiral dislocations). 

Maximum Activity Observed for Ethylene Polymerization by Different Catalysts and the Corresponding Number of Propagation Centers... 

Catalyst Kv (liter/mole sec) Normalized activity (g CjHi/mmole M hr-atm)6 Number of propagation centers (mole/ mole M, X100)... 

The data obtained while studying the role of aluminumorganic compounds during polymerization by TiCh (157-159) show that an aluminum-organic co-catalyst can be a reversible coordination inhibitor by itself. The decrease in the number of propagation centers by the addition of aluminumorganic compounds to titanium dichloride seems to be caused by the reversible adsorption of the aluminumorganic compound on the titan-... 

Tetralin, hydrogenation of, 12 Titanium compounds as catalysts, 188 Titanium dichloride, 192, 193 number of propagation centers, 198-200 Titanium trichloride, 193, 194 Toluene in exhaust gases, 67 Transalkylation, 141, 142 Transalkylidenation, 142 Transition metal compounds as catalysts, 174... 

Yu. I. Yermakov and V. A. Zakharov, The Number of Propagation Centers in Solid Catalysts for Olefin Polymerization and Some Aspects of Mechanism of Their Action, in Ref. 9, p. 91. 

Samples of a-TiCls, prepared by reduction of TiCIa with hydrogen, contain a low number of propagation centers. The Cp value of these well crystallized samples (the specific surface area according to BET is 3 m /g) is several per cent of the number of surface titanium ions. The low number of ACs is in agreement with the Cossee and Arlman concept and the experimental data of Rodrigues et al. on the localization of the ACg on the lateral faces and outlets of spiral dislocations on TiCls crystals. 

The effect of polymerization conditions on Cp has been studied mainly for two-component systems based on titanium chloride and vanadium chloride . The number of propagation centers changes with the polymerization time proportionally to the reaction rate and is independent of the monomer concentration (0.2-2 mol/1 at 70 °C). Most interesting is the effect of the polymerization temperature on Cp. It has been found that with rising temperature Cp ingreases (Table 3). In the... 

---