Ceiling temperature of polymerization

Fluorothioketones are more difficult to polymerize. There are two reasons. First, agents that promote polymerization also catalyze dimerization to dithi-etanes, which is a very fast reaction. Second, ceiling temperature of polymerization is low with the result that polymer decomposes back to monomer as it is being isolated. However, poly(hexafluorothioacetone) can be formed at very low temperatures by initiation with dimethylformamide or BF3 etherate, even though at — 78° C the only product isolated is 2,2,4,4-tetrakis(trifluoromethyl)-l,3-di-thietane. 

In most cases, the growth of polymeric chains is accompanied by volume contraction. Therefore external pressure tends to shift the monomer polymer equilibrium in favour of the polymer or, in other words, it increases the ceiling temperature of polymerization (lowers 7 ). This analysis can be refined by means of the known thermodynamic relations. The change in enthalpy with pressure is described by the thermodynamic equation of state... 

Polymerization in gas phase must cope with larger entropy change than polymerization in liquid phase. Therefore, polymerization of gas phase monomers such as olefins is carried out in superatmospheric pressure and/or in the presence of heterogeneous catalyst. Polymerization in gas phase in low pressure (in vacuum) does not occur easily due to the limitation of the ceiling temperature of polymerization, and there are only few cases in which the deposition of polymeric material from gas phase starting material occurs in vacuum. Those main exceptional cases are plasma polymerization and parylene polymerization. 

Conventional polymerization does not occur in gas phase, particularly in vacuum, because of the limitation set by the ceiling temperature of polymerization, and there are only a few cases in which the deposition of polymeric material occurs in vacuum. Those exceptional cases are plasma polymerization and Parylene polymerization, which is also a vacuum polymerization coating process using a gaseous monomer. The common denominators for these two processes are 1) the polymerizations yield solid-state polymer (in the form of film in most cases) from a gas phase monomer in vacuum and 2) the polymer formed by the processes... 

The type and combination of the halogen atoms in the trihalomethy1 group would also be a determining factor for the rate of polymerization of the perhaloacetaldehyde and for the location of the ceiling temperature of polymerization. 

Initiation of perhaloacetaldehyde polymerization must be done above the polymerization threshold temperature (at one molar monomer solutions, this is the ceiling temperature of polymerization) in order to provide complete mixing of initiator and monmer prior to polymerization- To form a homogeneous mixture is particularly important, when the initiation equilibrium is very much on the side of the addition of the initiator to one mole of monomer (Effective initiation), because the polymer which forms rapidly, precipitates and occludes unused initiator. Growing polymer ends are also occluded and the polymerization comes to a standstill both effects cause the polymers to be formed in low yield and/or tow molecular weight-... 

Ue have been able to determine the ceiling temperature of polymerization of the perhaloacetaldehyde polymerization by determining the threshold tempera-tu-... 

In Table 3 the ceiling temperature of polymerization of all the polyaldehydes are listed together with spectral characteristics of the perhaloacetaldehydes. It may be seen that the T of fluorosubstitu-ted acetaldehydes are higher tha those of chloro- and bromosubstituted perhaloacetaldehydes. The "mixed" perhalaloacetaldehydes have T s somewhere in between those of the three halo-substituted perhaloacetaldehydes. 

It appears that there is a linear relationship of the contribution of temperature increment of each of the C—X bonds of the trihalomethy1 groups to the value of the ceiling temperature of polymerization for each of the perhaloacetaldehydes. Some of the measurements of the threshold temperatures of the more volatile perhaloacetaldehydes are still in the process of being refined. Our calculations and final values will be presented at a later time. 

---