Vinylidene fluoride polymerization

As in any type of polymerization, a batch reaction is not as commercially attractive as a continuous polymerization process that can produce larger quantities of polymer in the same amount of time. The first continuous polymerizations in C02 were reported (Charpentier et al., 1999) with the monomers acrylic acid and vinylidene fluoride. The vinylidene fluoride polymerization was extensively studied at 75 °C, 275 bar. The polymerizations were run with residence times that varied between 15 and 40 min in a continuous-stirred-tank reactor before collection in a filter. The maximum rate of polymerization was determined to be 19 x 10 5 mol L-1s-1. Future research will move toward continuous removal of polymer, recycling of unreacted monomer and C02, and expansion to other monomers. 

While partially fluorinated alkenes such as vinyl fluoride and vinylidene fluoride polymerize with the same facility as tetrafluoroethylene, the latter is unique in the class of the perfluoroalkenes with the respect to the ease of polymerization as will be described later. Perfluoroalkenes such as hexafluoropropylene (HFP) and hexafluorobutadiene polymerize only with great difficulty as the result of the steric inhibition in the propagation step [598]. HFP can be converted to high-molar-mass polymer only at pressures above 1000 atm. The polymerizations are carried out most conveniently in a perfluorinated solvent using perfluorinated free-radical initiators [599]. For the polymerization of hexafluorobutadiene, similar conditions are reported [600]. Due to these very drastic polymerization conditions, these oligomers/polymers are not yet commercially applied. 

G. Laroche, Y. Marois, R. Guidoin, M.W. King, L. Martin, T. How, Y. Douville, Poly-vinylidene fluoride (PVDF) as a biomaterial From polymeric raw material to monofilament vascular suture, J. Biomed. Mater. Res. 29(12) (1995) 1525-1536. 

POLVINYLIDENE FLUORIDE. This product is made by the free-radical chain polymerization of vinylidene fluoride (H2C=CF2). This odorless gas which has a boiling point of —82°C is produced by the thermal dehydrochlorination of 1,1,1-chlorodifluoroethane or by the dechlorination of 1,2-dichloro-l.l-difluoro-ethane. As shown by the following equations, 1,1,1-chlorodifluoroethane may be obtained by the bydroflnorination and... 

Poly vinylidene fluoride is polymerized under pressure at 25-150°C in an emulsion using a fluorinated surfactant to minimize chain transfer with the emulsifying agent. Ammonium persulfate is used as the initiator. The homopolymer is highly crystalline and melts at 170°C. It can be injection molded to produce articles with a tensile strength of 7000 psi (48 MPal. a modulus of elasticity in tension of 1.2 x 105 psi and a heat deflection of 3003F (149°C). 

Most ultrafiltration membranes are porous, asymmetric, polymeric structures produced by phase inversion, i.e., the gelation or precipitation of a species from a soluble phase. See also Membrane Separations Technology. Membrane structure is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc.). Commonly used polymers include cellulose acetates, polyamides, polysulfoncs, dyncls (vinyl chlondc-acrylonitrile copolymers) and puly(vinylidene fluoride). 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

The most common methods of producing homopolymers and copolymers of vinylidene fluoride are emulsion and suspension polymerizations, although other methods are also used.55... 

The inclusion of the relatively large chlorine into the polymeric chain reduces the tendency to crystallize. Commercially available grades include a homopolymer, which is mainly used for special applications, and copolymers with small amounts (less than 5%) of vinylidene fluoride.84 The products are supplied as powder, pellets, pellets containing 15% glass liber, and dispersions. Low-molecular-weight polymer is available as oil or grease. The oil is used to plasticize PCTFE.85... 

Chemically, THV Fluoroplastic (hereafter referred to as THV) is a terpolymer of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and vinylidene fluoride (VDF) produced by emulsion polymerization. The resulting dispersion is either processed into powders and pellets or concentrated with emulsifier and supplied in that form to the market.91 Currently, the manufacturer is Dyneon LLC and there are essentially nine commercial grades (five dry and four aqueous dispersions) available that differ in the monomer ratios and consequently in melting points, chemical resistance, and flexibility. 

There are essentially two methods used for the production of commercial FTPEs. The first is referred to as iodine transfer polymerization, which is similar to the living anionic polymerization used to make block copolymers such as styrene-butadiene-styrene (e.g., Kraton ). The difference is that this living polymerization is based on a free radical mechanism. The products consist of soft segments based on copolymers of vinylidene fluoride (VDF) with hexafluoropropylene (HFP) and... 

Teblina et al. [26] studied the kinetics of radical polymerization of metha-crylic acid in the presence of a hexafluoropropene-vinylidene fluoride copolymer. The polymerization rate increased with conversion. A grafted copolymer was formed by a reaction analogous to eqn. (40) (with F instead of... 

TABLE 1. The effects of polymerization of vinylidene fluoride for one hour at 318 K at varying reaction pressures. 

In subsequent investigations by the author [1] when the Step 1 reaction of the current invention was continued for 120 to 150 minutes, the corresponding polymer had a M of 81,000 daltons and a of 203,000 daltons. Additional critical polymerization reaction scoping studies using vinylidene fluoride are described by the author [2]. 

Lee [3] polymerized vinylidene fluoride in supercritical water, namely ThzO 374 C and f ri20 218.2 atm, using either t-butyl peroxyacetate or t-butylperoxy-2-ethylhexanoate and obtained Mn s exceeding 1 million daltons with a crystalhne content >50%. In an earlier investigation by Lee [4] showed... 

Four perfluorodiacylperoxide free radical initiators were prepared by condensing perfluoroacyl chloride derivatives with hydrogen peroxide and sodium hydroxide. When the product bis(2-fluoro-2-trifluoromethyl-perfluoropropionyl) peroxide was used to polymerize vinylidene fluoride, a 90% conversion was observed. 

The Step 1 product (0.12 mmol) dissolved in 1.2 ml of 1,1,2,-trichloro-1,2,2-trifluoroethane and 20 ml of distilled water were introduced into a 50 ml steel reactor equipped with a magnetic stirrer. The reactor contents were cooled with liquid nitrogen and evacuated to 1 x 10 mbar to remove trace amounts of oxygen. The reactor was next charged with 22 atm of vinylidene fluoride and the reactor temperature, raised to 57°C. Once the autoclave pressure decreased to 15 atm, additional vinylidene fluoride was added to maintain the reaction pressure at 20 atm. The polymerization was stopped after 48 hours, and the product was isolated in 90% yield. 

TABLE 1. Selected perfluoroalkyl peroxides and their effectiveness in initiating the polymerization of vinylidene fluoride. 

---