Methacrylates, ceiling temperatures

Copolymerizations of other monomers may also be subject to similar effects given sufficiently high reaction temperatures (at or near their ceiling temperatures - Section 4.5.1). The depropagation of methacrylate esters becomes measurable at temperatures >100 °C (Section 4.5.1).96 O Driseoll and Gasparro86 have reported on the copolymerization of MMA with S at 250 °C. 

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

Because of the general lack of quantitative thermodynamic (ceiling temperature Tc) and kinetic (kp, kp/k[J 5) data for the polymerization of the captodative olefins, it is impossible to draw firm conclusions about the importance of electronic factors on their polymerizability. If we compare them with other 1,1-disubstituted olefins by replacing the heteroatom O, S, or N by a CH2 and check the polymerizability of the resulting olefins, we find that the latter are in fact also difficult to polymerize as shown in Table 8 [77], Only methyl acrylates and methacrylates give high polymers easily. The polymerizability decreases rapidly with the steric hindrance of the substituent. 

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). 

Thus the values shown in Table 1.8 are for standard conditions and represent just one of a series of ceiling temperatures for various monomer concentrations above which polymer formation is not favoured. Thus, in a bulk polymerization reaction the ceiling temperature may change with conversion in such a way that complete conversion is not achieved. For example, if methyl methacrylate is polymerized at 110°C the value of [M]c calculated from the above equation is 0.139M and this will be the monomer concentration in equilibrium with the polymer. The polymer, when removed from the monomer, will have the expected ceiling temperature as given in Table 1.8 and will depolymerize only if there is a source of free radicals to initiate the depolymerization (Section 1.4.1)... 

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. 

Calculate A [M]e and k p at different temperatures and hence determine the ceiling temperature for pure methyl methacrylate (density 0.940 g/cm at... 

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 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). 

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. 

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.)...

Decomposition can be looked upon as the reverse reaction of synthesis. Polymers with a ceiling temperature can simply show a reverse of the polymerization reaction as seen in Fig. 3.30. Figure 3.49 represents typical thermogravimetry traces for the decomposition of poly(methyl methacrylate), PMMA, and polytetrafluoroethylene,... 

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... 

The most important commercial exploitation of this phenomenon has been the development of positive radiation resists for use in the semiconductor industry. A number of methacrylates and butene sulfones have been developed for these applications (42-44). Another important class of materials which undergo large-scale degradation of the main chain are the aliphatic polysulfones. For example, in 1981 Bowmer and O Donnell (45) examined the 5delds of a number of aliphatic polysulfones as a fimction of temperature and discussed these results in terms of the change in the equilibrium between polymerization and depolymerization as the ceiling temperature is approached. This aspect of radiation chemistry has had far-reaching consequences for our modem society. 

Almost normal behaviour has been found in polymerizations of some unusual acrylates and methacrylates. " Ort/io-substitution in phenyl methacrylate generally reduces the tendency to polymerize and depresses the ceiling temperature. Several papers on ionized monomers refer to effects of the ionic strength and polarity of the medium. " ... 

Most monomers are above their free-radical ceiling temperature and only a single monomer unit is often added onto the polymer backbone. This is notably the case with maleic anhydride, acrylic acid, and methyl methacrylate. Styrene is an exception and one graft polymerize long polystyrene chains onto polyolefins [223]. 

When the temperature applied approaches or exceeds the ceiling temperature, a homolytic rupture occurs that may result in the total depolymerization and regeneration of the monomer for several polymers. Methyl methacrylate is recovered in this way from waste of the corresponding polymer ... 

For thermodynamic reasons, some adhesives may not cure at all above a critical temperature, which is known as the ceiling temperature, and may only partially cure at lower temperatures. This particularly applies to adhesives which harden by polymerization of monomers with C=C bonds, such as acrylates, methacrylates, and cyanoacrylates. 

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