Heat of polymerisation

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

Boiling point Specific gravity Refractive index Specific heat Heat of polymerisation... 

Freeder, B. G. et al., J. Loss Prev. Process Ind., 1988, 1, 164-168 Accidental contamination of a 90 kg cylinder of ethylene oxide with a little sodium hydroxide solution led to explosive failure of the cylinder over 8 hours later [1], Based on later studies of the kinetics and heat release of the poly condensation reaction, it was estimated that after 8 hours and 1 min, some 12.7% of the oxide had condensed with an increase in temperature from 20 to 100°C. At this point the heat release rate was calculated to be 2.1 MJ/min, and 100 s later the temperature and heat release rate would be 160° and 1.67 MJ/s respectively, with 28% condensation. Complete reaction would have been attained some 16 s later at a temperature of 700°C [2], Precautions designed to prevent explosive polymerisation of ethylene oxide are discussed, including rigid exclusion of acids covalent halides, such as aluminium chloride, iron(III) chloride, tin(IV) chloride basic materials like alkali hydroxides, ammonia, amines, metallic potassium and catalytically active solids such as aluminium oxide, iron oxide, or rust [1] A comparative study of the runaway exothermic polymerisation of ethylene oxide and of propylene oxide by 10 wt% of solutions of sodium hydroxide of various concentrations has been done using ARC. Results below show onset temperatures/corrected adiabatic exotherm/maximum pressure attained and heat of polymerisation for the least (0.125 M) and most (1 M) concentrated alkali solutions used as catalysts. 

ABu (PIB=). This, the t-Bu+ affinity of a terminal methylene group in polyisobutene, is taken as of the same order of magnitude as the heat of polymerisation of isobutene, -12 kcal. 

The difficulties associated with heat transfer can be overcome, and higher molecular weight polymers obtained, by the use of an emulsion system. The heat of polymerisation is readily dissipated into the aqueous phase and the viscosity of the system changes only slightly during the reaction. 

The technologies that have been developed for the production of polyolefins, olefin homopolymers and copolymers are slurry, solution and gas-phase polymerisation bulk polymerisation of propylene in the liquid monomer as a special case of the slurry process has also emerged. The fundamental differences in the various olefin polymerisation processes reflect the different approaches that have been devised to remove the substantial heat of polymerisation. In addition, processes can be operated in a batch or a continuous mode. In the batch process the reagents are loaded into a polymerisation vessel, the polymer forms and the vessel is emptied before a new charge of reagents is introduced. In the continuous process, the catalyst precursor, activator and other necessary... 

The gas-phase polymerisation of ethylene is typically carried out at 85-100 °C under a pressure of 20-25 atm. The ethylene monomer circulates, thus removing the heat of polymerisation and fluidising the catalyst bed. In order to keep the temperature of the process at values below 100 °C, gas conversion is maintained at 2-3 per pass [37], In the production of polypropylene by the gas-phase process, usually the polymerisation temperature is lower and ranges from about 50 to 85 °C, and the pressure range from ca 15 to 40 atm [553]. 

The fluorinated ethenes CF2=Cp2, CF2=CFH, CF2=CH2, CF2=CFC1 and CF2=CFBr each form homopolymers in conventional free-radical initiation procedures [220] and it is notable that the heat of polymerisation for tetrafluoroethene is much greater than for ethene [2]. Indeed, tetrafluoroethene and trifluoropropene are relatively dangerous monomers to handle because of the risk of explosive polymerisation. In marked contrast, quite drastic conditions are required in order to form a homopolymer from hexafluoropropene (HFP) [221], although commercially successful copolymers of CF2=CFCF3 with CF2=CF2 (i.e. FEP) and with CF2=CH2 (Viton rubber) have been developed. 

Lachinov and co-workers [52] have performed a more detailed study of the kinetics of block radical polymerisation of perfluoroalkylmethacrylates (FMA) in the solid state. The main method of investigation of the kinetics of FMA polymerisation was isothermic calorimetry [58]. Due to the absence of data on the heat effects of their polymerisation in the reference literature, these values were measured [52]. The values of AQ and glass transition point (T ) of polymers formed are shown in Table 8.3. Obviously heat of polymerisation of monomers of the fluoroacrylate sequence is quite close to heat of polymerisation of non-substituted monomers of the AMA sequence [59], and a significant influence of the length of the fluoroalkyl radical on this parameter is absent [52]. In accordance with the Polyani-Semenov rule, the present result makes it possible to consider that chain propagation constant of FMA with the accuracy of the pre-exponential multiplicand being equal to each other [57]. 

Table 8.3 Heats of polymerisation of perfluoroalkyl methacrylates, T, PFMA ...

Typical synthetic water soluble flocculants are polyelectrolytes that are most often based upon copolymers of acrylamide or substituted derivatives. Acrylamide monomer is obtained industrially by the catalytic hydration of acrylonitrile and occurs in both crystaUine form and in aqueous solution. The 50% aqueous form is the preferred form for polymerisation systems in which water can be tolerated. Acrylamide has a very fast propagation rate and a high exothermic heat of polymerisation. 

Polymerisation is carried out via a semi-batch (semi-continuous) process that allows reactants to be added during the polymerisation. It is usual for at least one of the redox initiators to be added as a metered feed over the course of the reaction. Other reactants can be added in order to control desired properties such as molecular weight distribution. Temperature control over the course of the polymerisation is also possible, particularly as the oil phase helps to dissipate the heat of polymerisation. Overall the semi-batch polymerisation technique is more versatile than the batch process and, as a consequence, it is possible to better manipulate polymer properties via this route. 

Monomer Wt.(g) Heat of Polymerisation (cal/mole) Molecular Weight Specific Heat... 

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