Styrene ceiling temperatures

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. 

An interesting feature of the styrene-S02 system, —which indeed is true of all SO2 copolymerizations with comonomers capable of homopolymerizing—, is the existence of a ceiling temperature above which the formation of alternating units, SMS, is forbidden. The number fraction of M sequences of length n is... 

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

The behavior shown in Figure 5.5 is typical of systems that have two stable steady states. The realized steady state depends on the initial conditions. For this example with a() = 1, the upper steady state is reached if T0 is greater than about 398 K, and the lower steady state is reached if T0 is less than about 398 K. At the lower steady state, the CSTR acts as a styrene monomer storage vessel with Tout Tin and there is no significant reaction. The upper steady state is a runaway where the reaction goes to near completion with Tout Tin + ATadiabatic- (In actuality, the styrene polymerization is reversible at very high temperatures, with a ceiling temperature of about 625 K.)... 

In exoenthalpic and exoentropic polymerizations, AGP° eventually becomes positive above a temperature known as the ceiling temperature (AGP° = AHP°/TCASP° > 0). For example, the ceiling temperature of bulk styrene is =400° C, whereas that of methyl methacrylate is =200° C. In contrast, AGP° becomes positive below a floor temperature when the polymerization is endoenthalpic and endoentropic (AGP° = AHp°/TfASp° > 0), such as in the polymerization of Sg (T/ = 160° C). 

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 method utilizing monomers of low ceiling temperature e.g., a-methyl-styrene, yielding highly reactive carbanions is based on the same principle. It ensures a quantitative conversion of a slow initiator into a highly reactive species that, in turn, rapidly initiates polymerization of the required monomer. 

The temperature at which a dynamic equilibrium is reached between the formation and the decay of monomer macroradicals is called a ceiling temperature. For certain monomers, there are published ceihng temperatures, heats, and entropy of polymerization (28,29). Their values are, for example, 150°C for MAH, 200°C for methacrylate, 400°C for acrylate and styrene (28). It should be noted that these values are typical of reactions occurring at a constant (atmospheric) pressure and monomer concentration (usually 1 mol). The peak temperature rises with monomer concentration and pressure. That is why MAH was observed to homopolymerize at an extrusion temperature above 160°C (30). 

Figure 22-2. Free radical copolymerization of Ma (methyl methacrylate or methyl acrylate) with Mb (a>methoxystyrene) at 60 C. Since the copolymerization is carried out above the ceiling temperature of a-methoxy styrene, no di-, tri-, etc. sequences of this monomeric unit can be formed because of the rapid depolymerization. The copolymerization parameter rs is then equal to zero, and the resit is a simple alternating copolymerization. (After data by H. Liissi.)...

Values of AHp for common monomers are summarized in Table 3.4. ASp, difficult to measure experimentally, is typically in the range of — 100 to — 140 J mol K . Table 3.4 also contains estimates of Tc calculated for [M] = 1 mol Depropagation of ethylene, vinyl acetate and acrylates does not occur under typical polymerization conditions. Styrene has a slightly lower ceiling temperature than acrylates and depropagation must be considered at the upper range of temperatures used commercially [15]. The addition of a methyl... 

The glass-transition temperature depends on the mobility of the chain segments and can therefore be raised by stiffening the chain (see Section 10.5.3). Thus, a-methyl styrene forms a polymer that, in contrast to poly-(styrene), does not deform at lOC C, because of a glass-transition temperature of 170°C. However, since the thermodynamic ceiling temperature for for the polymerization/depolymerization equilibrium is also simultaneously lowered (see Section 16.3), poly(a-methyl styrene) degrades more easily than poly(styrene), so that it is not so easy to work by injection molding. 

The depolymerization can be prevented by incorporating monomeric units with higher thermodynamic ceiling temperatures into the polymer. Thus, a-methyl styrene/methyl methacrylate copolymers have achieved a certain commercial importance as heat-stable, transparent polymers for special applications. 

If the results for butylmethacrylate are compared with reactive extrusion of styrene the maximum conversion is much lower in the case of butylmethacrylate. Styrene could be polymerized up to a conversion of almost 99%, while in the case of butylmethacrylate the highest conversion was 96.3%. This limitation indicates the importance of the ceiling temperature as described by Dainton (14). It is known in literature (15) that methacrylates possess a relatively low ceiling temperature, which means that the influence of thermodynamic limitations is most pronounced for these components. Bywater (16) found for methylmethacrylate an equilibrium monomer concentration of 0.3mol/l at 132°C, which was independent of the amount of polymer formed after reaction. This imphes... 

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