The polymerization of vinylidene fluoride in supercritical carbon dioxide was studied in a continuous stirred tank reactor using diethylperoxydicarbonate as the free radical initiator. Experiments were carried out to investigate the effect of inlet monomer concentration, temperature, average residence time, and agitation on the polymerization rate, the average molecular weights, and the molecular weight distribution of the poly(vinylidene fluoride). A homogeneous kinetic model that includes inhibition due to chain transfer to monomer predicted the polymerization rates reasonably well. However, imperfect mixing, rather than a chemical effect, may have caused the apparent inhibition observed at high monomer concentrations. At inlet monomer concentrations greater than about 1.5 M, broad and bimodal molecular weight distributions were observed. An extended homogeneous kinetic model that includes chain transfer to polymer predicted the polydispersities reasonably well. This model also predicted a region of inoperability that matched the experimental results. However, the extended homogeneous model could not account for the bimodal distributions.

Since the initial reports by Ziegler 1 on the "aufbau" reaction, or stepwise insertion of ethene into the aluminum-carbon bond of alkylaluminum compounds, it has been widely believed that because of the accompanying displacement reaction the products of this reaction were limited to oligoethenes, with few reports of the preparation of high molecular weight polyethene at an aluminum center. Martin has described the preparation of polyethene by exposing ethene to a heptane solution of either bis-(dichloroaluminum)ethane or trialkylaluminum over several days. 2 Recently, Jordan and Gibson have reported the polymerization of ethene using chelated alkylaluminum complexes activated by a Lewis acid. 3 On the other hand, the polymerization of higher alkenes, such as propene, by an aluminum-based system has neWer been reported. Herein, we report that high molecular weight, linear homo-and copolymers of ethene and propene can be prepared, in the absence of any transition metal species, via a catalyst system consisting of simple alkylaluminum compounds activated by Lewis acids.The homo-and copolymerization of ethene and propene were carried out in 125 mL glass-lined reactors and our results are summarized in Tables 1-3. As can be seen, several systems based on the combination of an alkylaluminum compound and a Lewis acid are effective. Of note is the observation that methylaluminoxane (MAO) can act as either one of the two components (presumably because of the presence of small amounts of trimethylaluminum in the commercial sample). Several features of these systems are of interest. First, the molecular weights are high and 1 H and 13 C NMR spectroscopy indicates that the polymers are highly linear. The linearity of the polyethene formed was further supported by its high melting point (T m > 135°C). The polypropene is atactic. For the ethene-propene copolymer, the melting point was found to decrease with increasing propene content in the copolymer.Narrow polydispersities (approximately 2) were observed for the polyethene and polypropene formed, suggesting a single-site catalyst. Finally, in the polymerization of ethene, the polymer molecular weight was found to increase with increasing reaction time (first entry in Table 1), indicating some degree of "livingness" to the system. This may be due to the lack of d-orbitals on aluminum that are necessary for chain transfer through facile -hydrogen abstraction ( -hydrogen abstraction from neutral aluminum alkyls occurs only at elevated temperatures). The transfer of a growing polymer chain from a transition metal center to aluminum to form a stable aluminum-terminated polymer has been reported. 4A critically important issue that must be addressed for all transition metal-free polymerization systems is whether trace amounts of transition metal impurities are actually responsible for the polymerization. For reasons given below, this appears to be unlikely for the present systems. First, the two components used in our systems were analyzed for Ti, Zr, and V by AA spectroscopy and we...

The surfactant‐free precipitation polymerization of vinylidene fluoride (VF2) in supercritical carbon dioxide was studied in a continuous stirred autoclave. The polymerization temperature ranged from 65 to 85°C, the average residence time in the reactor varied from 10 to 50 min., and the pressure was between 210 and 305 bar. Diethyl peroxydicarbonate was used as the initiator. The fractional conversion of monomer varied from 7 to 26%, the number‐average molecular weight of the polymer was between about 14,000 and 79,000, and the weight‐average molecular weight was between about 21,000 and 700,000. In many cases, the polymer exhibited a bimodal molecular‐weight distribution, especially at high monomer concentrations.

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