Polymerization, activation thermal

Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[=C(OMe)Et](CO)5 with BU4NCI (for pent-l-ene)79, and W[=C(OEt)Bu](CO)5 with TiCLt (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands. 

Since the first report on the catalytic behavior of imidazole-containing vinyl polymers (42), a large number of invest ations have been pubIMied. Overberger, pioneer in this field, is still very active, and the r Its obtained by his group have been summarized on several occasions (43-45). This group synthesized 4-vinylimidazole and 5-vinylbenzimidazole for the first tinw, and polymerized them thermally or by using free radical initiators /, 2 (46). 

Parylene polymerization Active species is created in a separate chamber by thermal cracking Substrate in a separate deposition chamber Ts < Tv AE < 0... 

Cationic vesicles typically used for DNA delivery often self-aggregate or bind to plasma proteins in vivo. Wu et al. [104] attempted to improve vesicle stability using a cationic lipid with a cross-linkable acrylamide attached to the headgroup (Fig. 16). Vesicles were polymerized using thermal initiation with AAPD. Compared to monomeric vesicles, polymerized vesicles were less cytotoxic, more resistant to aggregation in serum, and comparable in transfection activity using a vector encoding firefly luciferase. 

Crivello (5) also found that these photocatalysts could be activated thermally when used in combination with copper compounds. For those conditions, Crivello has proposed a cationic polymerization mechanism which does not... 

CrA sites, the most numerous of the three in this preparation, were identified as the most reactive species and comprise the sites that are active in ethylene polymerization, whereas Crc sites were found to be inactive. CrB sites were also thought to exhibit polymerization activity, but distinctions were made. The concentration of these surface species could be varied with the chromium content and with the conditions of the thermal pretreatments [279]. Heating the catalyst under vacuum at 700 °C, which caused a deactivation for ethylene polymerization, was found to convert CrA into Crc whereas CrB remained unaffected. 

Many olefinic addition polymerization reactions, such as that of ethylene or styrene polymerization, occur by free radical mechanisms. The initiation step can be activate thermally or by bond breaking additives such as peroxides. The general reaction scheme is ... 

Clay surface treatment can influence its reaction with the catalyst and in turn determine the stabihty of polymerization active site on the clay surface. As mentioned earher, there are two main types of clay surface treatments thermal and chemical. Thermal treatment at high temperatures may result in loss of sweUabil-ity [96] and reduced efficiency for in-situ polymerization appHcations. 

A polymeric monolith is a continuous porous polymeric rod made from a mixture of an initiator, monomers (including a cross-linking monomer), and a porogen (pore-forming solvent) that are polymerized in situ in a column. Tuning of the porous properties is typically achieved with a mixture of solvents such as toluene, THF, or decanol. The rationale for choosing an initiator depends on the mode of initiation and on solubility aspects. A common initiator is 2,2-azo-bis-isobutyronitrile (AIBN). Most polymerizations are radical polymerizations, activating radical formation either thermally [54] or with UV radiation [55]. Common monomers used in the preparation of polymer monoliths are styrene, (meth)acrylate, and acrylamide-based materials. The formation of the monolith... 

This truly demonstrates the potential of catalyst 4c. While it is inactive for weeks in DCPD at room temperature, it needs 4 h to polymerize 300 equivalent of COD at 90°C, and it can easily be activated thermally to convert 20,000 equivalent of DCPD in less than 3 min. 

Phillips catalysts are based on Cr(IV) supported on sUica and alumina. The true structure of the Phillips catalyst is not well understood. A mixture of chromium oxide and silicon oxide (Cr03/Si02) is used to create the active sites. The catalyst does not require addition of chemical activators before the polymerization, since the active site is produced prior to the polymerization by thermal activation at high temperatures (600°C, for instance) [21, 22]. Phillips catalysts are used in both gas-phase and slurry processes. Polyethylene made with Phillips catalysts... 

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