Flash polymerization

In solution polymerizations catalyzed by Lewis acids, the polymerization frequently does not begin immediately after addition of the initiator, and there is an induction period that cannot be completely eliminated even by careful purification of the starting materials. In contrast, some cationic polymerizations proceed so quickly (flash polymerization), even after dilution, that conversion is already complete after a few minutes (e.g., isobutylene. Example 3-16). 

Bulk Polymerization of e-Caprolactam with Anionic Initiators (Flash Polymerization)... 

Further applications of flash polymerization in microflow systems will hopefully appear in the field of polymer science and technology in the future. 

Hydrocarbon Monomers.—The flash polymerization on mixing of isobutylene and EtAlClj was studied by Russian workers. A series of publications by a group of Italian workers describes detailed examination of the structure of homo and copolymers of several alkenes and dienes. This enabled polymerization mechanisms to be explained in terms of relative reactivities of the intermediate carbenium ions towards different monomers and their rearrangements involving hydride and methide shifts. Monomers studied were propylene, but-l-ene, isobutylene/ butadiene, isobutylene/rrans-l,3-pentadiene, isobutylene/isoprene, and propylene/but- 1-ene. ... 

Polymerization of vinyl ethers initiated by an A -acyUminium ion pool has demonstrated that the molecular weight and the molecular weight distribution can be controlled by using the flow microreactor system (Fig. 10.5) [4, 5]. An iV-acyh-minium ion generated and accumulated by the cation pool method was used as an initiator, which was mixed with a vinyl ether at high speed using a micromixer. The polymerization proceeded in a flow microreactor and was complete within a short residence time. An amine was then introduced using a micromixer to terminate the polymerization. Thus, this polymerization can be called flash polymerization. 

Fig. 10.9 Schematic illustrations of a conventional living polymerization and b living polymerization using a flow microreactor (flash polymerization)...

In contrast, the flash polymerization using a flow microreactor system involves no additive and thus does not form an equilibrium between the active species and the dormant species (Fig. 10.9b). All the polymer ends form active species. Thus,... 

The relatively low flash points of some acrylates create a fire hazard. Also, the ease of polymerization must be home in mind in ah. operations. The lower and upper explosive limits for methyl acrylate are 2.8 and 25 vol %, respectively. Corresponding limits for ethyl acrylate are 1.8 vol % and saturation, respectively. All possible sources of ignition of monomers must be eliininated. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

A process of polymerization of isomerized a-pinene or turpentine with vinylbenzenes has been disclosed (105). a-Pinene or turpentine is isomerized by flash pyrolysis at 518 5° C in a hot tube reactor to yield a mixture of predominantly dipentene and i7t-alloocimene... 

A simplified flow diagram of a modern H2SO4 alkylation unit is shown in Eigure 1. Excess isobutane is suppHed as recycle to the reactor section to suppress polymerization and other undesirable side reactions. The isobutane is suppHed both by fractionation and by the return of flashed reactor effluent from the refrigeration cycle. 

Polymeric Ma.teria.Is, The single-ply membranes are made from a wide variety of polymers. The following is a brief description of those polymers and their characteristics. There are three thermosetting-type elastomeric membranes as of this writing (1996) neoprene, CSPE, and EPDM. Neoprene is stiU used where oil resistance is needed. Eor instance. Hydrotech uses neoprene flashings, the base of which is hot-set in mbberized asphalt (see ElASTOL RS, SYNTHETIC-POLYCm.OROPRENE). 

The impact of cold GR-S was quite pronounced. The U.S. government edicted that all of the emulsion SBR plants switch to the cold process. This required addition of refrigeration capacity in these plants as well as other significant changes, such as insulation of reactors, improved vacuum to reduce oxygen that retards polymerization, and the heating of latex in blowdown tanks to aid in the disengagement of butadiene when transferred to the flash tanks. 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Another important ether is vinyl ether, a colorless liquid with the characteristic ether odor. Its molecular formula is CjHjOCjHj. Vinyl ether has a flash point of -22°F and an ignition temperature of b80°F. It is highly toxic by inhalation and is used in medicine and in the polymerization of certain plastics. 

Fire Hazards - Flash Point Not flammable Flammable Limits in Air (%) Not flammable Fire Extinguishing Agents Not pertinent Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Toxic oxides of nitrogen may form in fire Behavior in Fire Sealed containers may burst as a result of polymerization Ignition Tenqterature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

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