Catalysts, transfer constants

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

There are many papers which purport to record the effect of counter-ion on such factors as transfer constants, co-polymerisation ratios, etc. It is significant that in most of these studies relatively high initiator concentrations have been used, so that counter-ion effects are more likely but before accepting that the observed effects are indeed due to change of counter-ion (derived from different catalysts or co-catalysts) it must be ascertained that these polymerisations are in fact cationic and not pseudo-cationic - in which case the effects would stem from the different reactivities of different esters (see Section 5). 

Table 2. Transfer constants of catalysts and telogen in the redox telomerisation of chloro-trifluoroethylene with CC14 [32]...

The kinetic law applied to redox telomerisation depends on both the transfer constants of the catalyst and of the telogen but the former one is much greater as shown is Table 2 [32] ... 

Table 6. Values of chain transfer constants for ethylene or propylene polymerization with Ziegler-Natta catalysts... 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

Generally, efficiency of CCT catalysis drops in emulsion polymerization. The following values of CCT chain-transfer constants may be compared with solution and bulk polymerization CcMMA =1100 M 1 s-1, CcEMA = 640 M-1 s 1, Ccn BMA = 520 M s-1, Cc2 EHMA = 400 M 1 s 1 (75 °C, water, 9a).342 In miniemulsion polymerization, the choice of catalyst depends on the choice of initiator (see Table 10).345... 

The chain-transfer constants for styrene polymerization with porphyrinic CCT catalysts are given in Table 4. In addition, the following values can be found in the literature kc= 1.4 x 105 M-1 s-1 (40— 70 °C, 9a),314 and k = 6.4 x 104 M-1 s-1, Cs = 400 (40 °C, 9c).40 The low Cs in styrene systems can be partly attributed to the formation of Co—C bonds, reducing the concentration of active Co11 catalyst. The experimental observation that the molecular weight distribution of the polymer formed in CCT remains constant with conversion is not explained.369... 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

In a typical copolymerization involving monomers that are considered to be good for CCT311 in combination with those that are less effective,430 it is typical that the chain-transfer constant of a given catalyst diminishes.135 It is also observed that there is a period of inhibition of polymerization which is dependent upon the concentration of catalyst and the ratio and... 

Table 12. Copolymerization Chain-Transfer Constants, Cc, for Selected Catalysts...

Table 9.4 Chain Transfer Constants (ktr/kp) for Alkene Polymerization with Various Heterogeneous Ziegler-Natta Catalysts...

Hint Defining [M]o = monomer concentration at the gas-liquid interface, [M] = average concentration of monomer in solution, [cat] = concentration of catalyst, = mass transfer constant (dependent on stirring speed), and = polymerization rate constant, the mass balance for the... 

In the field of ionic polymerization, we should mention the investigators87,114) who calculated the relative chain transfer constants for the polymerization of styrene in benzene solutions and its mixture with 1,2-dichloroethane on Friedel-Crafts catalysts. 

Fig. 2. Hypothetical Br0nsted plot showing how the observed rate of the reaction outlined in Eqn. 7 of text varies with nucleophilicity of the attacking nucleophile and basicity of the catalyst (a) constant nucleophile, increasing basicity of catalyst when proton transfer from the intermediate is thermodynamically favourable increasing the nucleophilicity of the nucleophile increases the rate from (c) to (b).

Since acid-base catalysis always involves the transfer of a proton from the acid catalyst or to the basic catalyst, it is natural to seek a correlation between the effectiveness of a catalyst and its strength as an acid or base, since this strength is a measure of the ease with which the catalyst transfers a proton to or from a water molecule. The most satisfactory.relationship between the rate constant fcj,..and the add di oelation, constant of a inonoWic acid is the equation. 

The efficienqf of a chain transfer catalyst has traditionally been measured by its chain transfer constant (Cs), which is the ratio of the rate constant for chain transfer (feu) to that for propagation (fep). Values of Cs are generally determined from the slope of a plot of 1/DP vs. (CTA)/(M] (the Mayo method. Equation 1.27 [74]), in which DP is the degree of polymerization, CTA is the chain transfer agent, and M is monomer. 

---