Polymer insertion reactions

Polymer Insertion Reactions. It was of interest to us to increase the siloxane content of the synthesized polymers and thus change their properties. Since polymer 3-CI2 contains an -Si-O-Si- linkage, it should be possible to insert cyclic siloxanes into the polymer backbone via the common ring-opening processes used to make polymeric silicones (Scheme 4) (28,29). This method would be an alternative to using long-chain siloxanes in the original polymer synthesis (Scheme 2, x 2). 

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Many other reagents for converting alkenes to epoxides,including H2O2 and Oxone , VO(0-isopropyl)3 in liquid C02, ° polymer-supported cobalt (II) acetate and 02, ° and dimethyl dioxirane.This reagent is rather versatile, and converts methylene oxiranes to spiro-epoxides. ° ° One problem with dimethyloxirane is C—H insertion reactions rather than epoxidation. Magnesium monoperoxyphthalate is commercially available, and has been shown to be a good substitute for m-chloroperoxybenzoic acid in a number of reactions. 

In the reaction of Ni(CNBu )4 and methyl iodide oligomerization of the isocyanide was observed the only isolable nickel complex was (I), shown below. This product is believed to arise through sequential insertions of three isocyanides into a nickel-carbon bond. Upon further treatment with additional isocyanide at a temperature greater than 60° C one obtains a polymer (RNC) presumably through multiple isocyanide insertion reactions. The addition of benzoyl chloride to Ni(CNBu )4 gave two isolable compounds Ni(CNBu )3(COPh)Cl (74%) and (II) (8.2%). This latter reaction, and the isolation of (II) in particular, suggests that the proposed mechanism for polymerization of isocyanides is reasonable. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

The last class of reactions is of importance for the formation of organic molecules and organic polymers. The carbon insertion reactions (Figure 5.11, reaction (i)) are ion-molecule reactions and so are favoured at low temperatures and result in the... 

Many of the older bis-indenyl catalysts are less selective at higher temperatures, which was ascribed initially to a lower selectivity of the insertion reaction itself. More recent work by Busico, based on deuteration studies and again based on very detailed and elegant analysis of 13C NMR spectra of the polymers, has shown that in fact epimerisation of the growing alkyl chain occurs via a series of (3-hydride eliminations and re-insertion reactions [36] involving even tertiary alkyl zirconium species. 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

Cycloproparenes do not form metal complexes with all transition metals. Reaction of cyclopropabenzene with diiron-nonacarbonyl yields polymer, while reaction with cyclopropanaphthalene leads to a stable product formed by metal and carbonyl insertion. Attempts to form cycloproparene-chromium complexes have also failed. ... 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

In both the mechanisms the polymer chain grows from the catalyst surface by successive insertion reactions of the complexed monomer, the R group originally present in the cocatalyst ending up as the terminal group of the chain. 

The final product can be isolated easily as the triphenylphosphine complex. This reaction is also general as far as the acylcobalt carbonyl is concerned, but the yields vary widely depending upon which acetylene is used (34). Presumably, the presence of substituents on the acetylene favors the cyclization step rather than the formation of linear products. The larger the substituents the more favorable the cyclization becomes. If cyclization does not take place relatively rapidly, linear compounds and polymers of acetylene, or of acetylene and CO are probably formed. Thus, these reactions demonstrate the insertion reaction of both acetylenes and ketonic carbonyl groups. 

In addition, when 1-alkenes are used as comonomers, the molecular weights of the polymers tend to become low because the insertion reaction becomes slower and there occurs an additional chain transfer (22). 

An interesting insertion reaction (Scheme 33) has been utilized to prepare polymers which are readily soluble in a variety of nonpolar solvents but which become insoluble upon hydrolysis (71MI11101). 

This heterocycle has been reported to result from a ring expansion reaction of a pendant oxirane with a ketone (75MI11107). The insertion reaction (Scheme 100) was conveniently conducted in high yield by treatment of a solution of a glycidyl methacrylate-containing polymer (208) employing the ketone as solvent. 

Fyfe et al. (354) have combined 31P and 13C CP/MAS NMR studies first to identify the polymer-immobilized catalyst (Scheme 4, compound ii) formed from the precursor i by treatment with Pd(PPh3)4 and, second, to monitor the carbonyl insertion reaction using 13C-enriched CO to yield iii. The use of isotopically enriched CO was required so as to record meaningful signals above those emerging from the carbon-rich polymer background. 

M-C as propagating species, 4, 1008 monomer coordination and insertion reactions, 4, 1010 monomer insertion regio- and stereochemistry, 4, 1015 overview, 4, 1005-1166 regioirregular insertions, 4, 1023 stereocontrol mechanism, 4, 1018 stereocontrol symmetry rules, 4, 1020 stereoregular polymers, 4, 1016 in Ru-Os heterodinuclear compounds, 6, 1046 in Ru-Os mixed-metal clusters, 6, 1064 semiconductor growth, conventional precursors, 12, 2 with silicon, 3, 514... 

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