Polymeric route

Polyolefins are, of course, usually synthesized by the catalyzed polymerization of alkenes. Why is an analogous route, polymerization of disilenes, not employed to prepare polysilanes The reason is paradoxical. The energy barrier to polymerization of doubly bonded silicon compounds is simply too low, so that in most cases they polymerize (or oligomerize) immediately when they are generated ... 

This review concerns the synthesis routes, polymerization techniques, doping, orientation, and development of well-defined, highly conducting polymeric materials. 

In this chapter, various photochemical reactions involving CPs and MOFs have been described. These include structural transformations, post-synthetic modification of surfaces to create reactive sites, radicals which are not possible by convention synthetic routes, polymerization on the surfaces, and cis-trans isomerization of the guest molecules in the cavities using the dynamic behavior of the structures. Further, [2+2] cycloaddition reaction has been used as a tool... 

The ROP of strained cydotrisiloxanes and their unstrained homologs, for example, cydotetrasiloxanes, both lead to the same equilibrium state, however, by different routes. Polymerization of unstrained cyclosiloxanes leads to simultaneous formation of the polymer and of cyclic oligomers. The polymer concentration increases monotonically achieving finally its equilibrium value. In contrast, strained cydotrisiloxanes in the first, rapid step of the process are transformed mainly into linear polymers, which are randomized and partially decomposed to cydics in the second, slower step. The system attains the equilibrium state according to general Scheme 1. 

United States The Ziegler route to polyethylene is even more important because it occurs at modest temperatures and pressures and gives high density polyethylene which has properties superior to the low density material formed by the free radical polymerization described m Section 6 21... 

The Hofmann elimination route, of which many versions exist, can be carried out at much lower temperatures in conventional equipment. The PX is generated by a 1,6-Hofmaim elimination of amine from a quaternary ammonium hydroxide in the presence of a base. This route gives yields of 17—19%. Undesired polymeric products can be as high as 80% of the product. In the presence of a polymerization inhibitor, such as phenothiazine, DPXN yields can be increased to 50%. 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

Polyamines can also be made by reaction of ethylene dichloride with amines (18). Products of this type are sometimes formed as by-products in the manufacture of amines. A third type of polyamine is polyethyleneimine [9002-98-6] which can be made by several routes the most frequently used method is the polymeriza tion of azitidine [151 -56 ] (18,26). The process can be adjusted to vary the amount of branching (see Imines, cyclic). Polyamines are considerably lower in molecular weight compared to acrylamide polymers, and therefore their solution viscosities are much lower. They are sold commercially as viscous solutions containing 1—20% polymer, and also any by-product salts from the polymerization reaction. The charge on polyamines depends on the pH of the medium. They can be quaternized to make their charge independent of pH (18). 

The boric and sulfuric acids are recycled to a HBF solution by reaction with CaF2. As a strong acid, fluoroboric acid is frequently used as an acid catalyst, eg, in synthesizing mixed polyol esters (29). This process provides an inexpensive route to confectioner s hard-butter compositions which are substitutes for cocoa butter in chocolate candies (see Chocolate and cocoa). Epichlorohydrin is polymerized in the presence of HBF for eventual conversion to polyglycidyl ethers (30) (see Chlorohydrins). A more concentrated solution, 61—71% HBF, catalyzes the addition of CO and water to olefins under pressure to form neo acids (31) (see Carboxylic acids). 

Hexafluoiopiopylene and tetiafluoioethylene aie copolymerized, with trichloiacetyl peroxide as the catalyst, at low temperature (43). Newer catalytic methods, including irradiation, achieve copolymerization at different temperatures (44,45). Aqueous and nonaqueous dispersion polymerizations appear to be the most convenient routes to commercial production (1,46—50). The polymerization conditions are similar to those of TFE homopolymer dispersion polymerization. The copolymer of HFP—TFE is a random copolymer that is, HFP units add to the growing chains at random intervals. The optimal composition of the copolymer requires that the mechanical properties are retained in the usable range and that the melt viscosity is low enough for easy melt processing. 

The polymerization of A/-(2-tetrahydropyranyl)aziridine with subsequent hydrolysis of the resulting polymers has been described as an alternative route for the synthesis of linear polyethyleneimine (359). Linear polyethyleneimine, in contrast to branched polyethyleneimines, is only sparingly soluble in water at room temperature. 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

With the avadabihty of polymerization catalysts, extensive efforts were devoted to developing economical processes for manufacture of isoprene. Several synthetic routes have been commercialized. With natural mbber as an alternative, the ultimate value of the polymer was more or less dictated by that market. The first commercial use of isoprene in the United States started in 1940. It was used as a minor comonomer with isobutylene for the preparation of butyl mbber. Polyisoprene was commercialized extensively in the 1960s (6). In the 1990s isoprene is used almost exclusively as a monomer for polymerization (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE). 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

Nylon-6 can also be produced from molten caprolactam using strong bases as catalysts (anionic polymerization) this is used as the basis of monomer casting and reaction injection mol ding (RIM). Anionic polymerization proceeds much faster than the hydrolytic route but products retain catalysts which may need to be extracted. 

Alternative synthetic routes to poly(arylene sulfide)s have been pubHshed (79—82). The general theme explored is the oxidative polymerization of diphenyl disulfide and its substituted analogues by using molecular oxygen as the oxidant, often catalyzed by a variety of reagents ... 

As a variation on the base-catalyzed nucleopbilic displacement chemistry described, polysulfones and other polyarylethers have been prepared by cuprous chloride-catalyzed polycondensation of aromatic dihydroxy compounds with aromatic dibromo compounds. The advantage of this route is that it does not require that the aromatic dibromo compound be activated by an electron-withdrawing group such as the sulfone group. Details of this polymerization method, known as the Ullmaim synthesis, have been described (8). 

A method for the polymerization of polysulfones in nondipolar aprotic solvents has been developed and reported (9,10). The method reUes on phase-transfer catalysis. Polysulfone is made in chlorobenzene as solvent with (2.2.2)cryptand as catalyst (9). Less reactive crown ethers require dichlorobenzene as solvent (10). High molecular weight polyphenylsulfone can also be made by this route in dichlorobenzene however, only low molecular weight PES is achievable by this method. Cross-linked polystyrene-bound (2.2.2)cryptand is found to be effective in these polymerizations which allow simple recovery and reuse of the catalyst. 

In the mid-1950s, the Nobel Prize-winning work of K. Ziegler and G. Natta introduced anionic initiators which allowed the stereospecific polymerization of isoprene to yield high cis-1,4 stmcture (3,4). At almost the same time, another route to stereospecific polymer architecture by organometaHic compounds was aimounced (5). 

Free-radical polymerization is the preferred iadustrial route both because monomer purification is not required (109) and because initiator residues need not be removed from polymer for they have minimal effect on polymer properties. 

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