Methylstyrene, ceiling temperatures

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

The stoichiometric reaction of y -diisopropenylbenzene [3748-13-8] with two moles of j -butyUithium in the presence of triethylamine has been reported to produce a useful, hydrocarbon-soluble dilithium initiator because of the low ceiling temperature of the monomer (78,79) which is analogous in stmcture to a-methylstyrene however, other studies suggest that oligomerization occurs to form initiators with functionahties higher than two (80). 

This method was first applied by McCormick27 and by Bywater and Worsfold11 to the system a-methylstyrene/poly-a-methyl-styrene, and the free energy, entropy and heat of polymerization as well as the ceiling temperature were determined. Similar studies concerned with the system styrene/polystyrene are being carried out in our laboratories. 

In the copolymerization of isopropenylferrocene with a-methyl-styrene at 0°C, using varying molar ratios of isopropenylferrocene and a-methylstyrene, traces of polymer formation were obtained only at a 30/70 ratio of the two monomers, as shown in the data in Table III. Because a-methylstyrene has a much lower ceiling temperature than styrene, we also decided to use styrene as a comonomer under conditions similar to those employed with a-methylstyrene. The reaction temperature for the copolymerization with a-methylstyrene was 20°C. 

The ceiling temperature for styrene pylene 300°C, for methyl methacrylate a-methylstyrene 61°C. 

In the case of a-methylstyrene with a high ceiling temperature of the polymerization, trimer or tetramer were produced in the solution at room temperature, and after electrolysis for a desired time the cell was brought into a dry ice-methanol bath in order to complete polymerization. Yields of polymer of high molecular weight were almost quantitative. The colored solution at the cathode did not distribute to the anode side through the sintered glass disk over a few hours. In the anodic solution no solid polymer was observed. 

Polymerization leads to a contraction in the volume of the system so that the equilibrium of a monomer-polymer system shifts in the direction of the reaction as the hydrostatic pressure increases. This was demonstrated by Weale (40) in his studies of a-methylstyrene polymerization under high pressure. The ceiling temperature increased from 61° C at 1 atm to 170° C at 6480 atm. 

The ceiling temperatures listed in Table 3 refer to bulk conditions, whereas the equilibrium monomer concentrations refer to polymerizations performed at 25° C. As discussed in Section F, the equilibrium monomer concentration varies with the polymerization temperature, just as the ceiling temperature depends on the monomer concentration. Table 4 demonstrates that the equilibrium monomer concentration of a-methylstyrene... 

However, the equilibrium monomer concentrations of disubstituted alkenes is measurable. The equilibrium constants for dimerization, tri-merization, and polymerization of a-methylstyrene have been determined as a function of temperature under anionic conditions [12] similar values should be obtained under cationic conditions. Unfortunately, the equilibrium position can t be determined directly under cationic conditions due to the irreversible side reactions of isomerization and indan and spirobiindan formation (Section II. A). The equilibrium monomer concentrations of isobutene and isopropenyl vinyl ethers should also be relatively high, albeit lower than those of a-methylstyrenes. However, the true equilibrium can t be reached with these monomers due to irreversible side reactions, and reliable data are therefore not available. Nevertheless, the ceiling temperature of isobutene polymerization is apparently between 50 and 150° C. 

A major limitation of a-methylstyrene in free-radical polymerizations is its very low ceiling temperature of 61 °C.347 As a result, AMS is utilized commercially only in radical copolymerization. Nonetheless, it is among the most active CCT monomers with Cc = 9 x 105 at 50 °C for 9a as CCT catalyst.348 This value is relatively unchanged at 40 °C. This high value reflects the low kp = 1.7 M 1 s 1 so that kc = 5 x 105 M-1 s 1. 

Figure 6.15 Logarithm of equilibrium monomer concentration plotted against reciprocal ceiling temperature for a-methylstyrene (Problem 6.39).

While for many alkene monomers the position of the propagation-depropagation equilibrium is far to the right under the usual reaction temperatures employed (that is, there is essentially complete conversion of monomer to polymer for all practical purposes), there are some monomers for which the equilibrium is not particularly favorable for polymerization. For example, a-methylstyrene in a 2.2 M solution will not polymerize at 25°C and pure a-methylstyrene will not polymerize at 61°C (see Table 6.14). In the case of methyl methacrylate, though the monomer can be polymerized below 220° C, the conversion will be appreciably less than complete. For example, the value of [M]g at 110°C is found to be 0.139 M [3] which corresponds to about 86% conversion of 1 M methyl methacrylate. Since Eqs. (6.195) and (6.196) contain no reference to the mode of initiation, they apply equally well to ionic and ring-opening polymerizations. Thus the lower temperatures of ionic polymerizations often offer a useful route to the polymerization of many monomers that cannot be polymerized by radical initiation because of their low ceiling temperatures. 

H5P, an a-methylstyrene derivative, seems to have a low ceiling temperature and consequently did not homopolymerize but underwent copolymerization with styrene, methyl methacrylate, and n-butyl acrylate. Based on the homopolymerization attempts, it appears that 2H5P is present as isolated monomer units in these copolymers. The co-polymerization parameters of 2H5V and 2H5P with styrene, methyl methacrylate, and n-butyl acrylate have also been determined. The results are shown in Figure 3 The copolymerization experiments were done to 5 conversions. 

The reactions of monomers with aromatic radical anions or directly with alkali metals can be used to prepare oligomeric dianionic initiators from monomers such as a-methylstyrene which have accessible ceiling temperatures (T = 61 °C) as shown in Scheme 7.5 (R = CH3) [54], Dimers or tetramers can be formed depending on the alkali metal system, temperature, and concentration. 

As AWp is negative, a rise in temperature will cause [A(,] to increase, thus at 405 K, methyl methacrylate has a value of [M ] = 0.5 mol dm , whereas a-methylstyrene will not polymerize at all. Ceiling temperatures then refer to a given monomer concentration, and it is more convenient to refer it to a standard state. This can either be referred to pure liquid monomer or a concentration of 1 mol dm typical examples for pure liquid monomers are given in Table 3.6. Whereas the ceifing temperature alters with monomer concentration, it is also sensitive to pressure. As AH and AS are both negative, an increase in is obtained if -AS can be decreased. 

FRP leads to the formation of statistical copolymers, where the arrangement of monomers within the chains is dictated purely by kinetic factors. However, reactivity of a monomer in copolymerization cannot be predicted from its behavior in homopolymerization. Vinyl acetate polymerizes about 30 times more quickly than styrene (see Table 4.2), yet the product is almost pure polystyrene if the two monomers are copolymerized together in a 50 50 mixture. a-Methylstyrene cannot be ho-mopolymerized to form high-MW polymer due to its low ceiling temperature (see Table 4.6), yet is readily incorporated into copolymer at elevated temperatures. These and other similar observations can be understood by considering copolymerization mechanisms and kinetics. 

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