Monomer after homogenization

Figure 13. Electron micrograph of monomer emulsion of Figure 12 with OPB + hexadecanol (HD) immediately after homogenization...

When propylene is polymerized using homogeneous metallocene/methylaluminoxane (MAO) catalysts, several chain transfer mechanisms occur to release free polymer chains. Their relative frequencies are dependent on the polymerization conditions and the catalyst structure. Three chain transfer mechanisms are identified that form chain-end unsaturated polypropylene. These are (1) P-hydride transfer to metal after a primary (1,2-) propylene insertion, (2) f)-hydride transfer to monomer after a primary propylene insertion, and (3) f)-hydride transfer after a secondary (2,1-) propylene insertion. The formations of these chain ends and associated polymer head groups (chain starts) are shown in Scheme 10.1. Mechanism (2) is commonly referred to as chain transfer to monomer. 

Mitsui Toatsu Chemical, Inc. disclosed a similar process usiag Raney copper (74) shortiy after the discovery at Dow, and BASF came out with a variation of the copper catalyst ia 1974 (75). Siace 1971 several hundred patents have shown modifications and improvements to this technology, both homogeneous and heterogeneous, and reviews of these processes have been pubHshed (76). Nalco Chemical Company has patented a process based essentially on Raney copper catalyst (77) ia both slurry and fixed-bed reactors and produces acrylamide monomer mainly for internal uses. Other producers ia Europe, besides Dow and American Cyanamid, iaclude AUied CoUoids and Stockhausen, who are beheved to use processes similar to the Raney copper technology of Mitsui Toatsu, and all have captive uses. Acrylamide is also produced ia large quantities ia Japan. Mitsui Toatsu and Mitsubishi are the largest producers, and both are beheved to use Raney copper catalysts ia a fixed bed reactor and to sell iato the merchant market. 

In the specific case of silica nanoparticles-pH EMA hybrid materials, the synthesis relies on obtaining a fine dispersion of silica nanoparticles (with a mean diameter of 7nm) in HEMA monomers (liquid phase). When a homogeneous solution is obtained, a free radical initiator is added at a concentration based on the weight of the monomer mixture. After the initiator dissolution, the solution can be poured into molds or between two glass plates to obtain monoliths or uniform films, respectively, after being cured at temperatures around 60-85 °C for several hours. 

We have found that the novel chiral self-dimerizahon of V complexes produces an active unsaturated structure with a new chiral conformation on a Si02 surface, and provides highly enanhoselective achve sites for the asymmetric oxidative coupling of 2-naphthol [35, 36]. Table 10.3 summarizes catalyhc activities, selectivities and enantiomeric excess (e.e.) of homogeneous and heterogeneous V catalysts for the oxidative coupling of 2-naphthol. The homogeneous reactions on a representative L-leucine based V monomer did not exhibit the catalyhc activity. Only in the presence of chlorotrimethylsilane as an acidic promoter did the reachon proceed, but conversion was as low as 15% at 293 K, selectivity to BINOL and e.e. to (P)-BINOL were quite low at 73% and 8%, respectively (Table 10.3). Furthermore, loss of activity occurred and no reachon proceeded after 9 days. 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

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