Synthetic polymers cationic polymerization

This polymer has also been obtained synthetically via cationic polymerization of D( + )(3-methyl-p-propiolactone 19). The racemic monomer polymerizes predominantly to the isotactic polymer. 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

The controlled synthesis of polymers, as opposed to their undesired formation, is an area that has not received much academic interest. Most interest to date has been commercial, and focused on a narrow area the use ofchloroaluminate(III) ionic liquids for cationic polymerization reactions. The lack of publications in the area, together with the lack of detailed and useful synthetic information in the patent literature, places hurdles in front of those with limited loiowledge of ionic liquid technology who wish to employ it for polymerization studies. The expanding interest in ionic liquids as solvents for synthesis, most notably for the synthesis of discrete organic molecules, should stimulate interest in their use for polymer science. 

Cationic polymerizations work better when the monomers possess an electron-donating group that stabilizes the intermediate carbocation. For example, isobutylene produces a stable carbocation, and usually copolymerizes with a small amount of isoprene using cationic initiators. The product polymer is a synthetic rubber widely used for tire inner tubes ... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

This article surveys the research work on the synthesis and modification reactions of poly(ethyleneimine) as well as its applications to metal complexation processes. Poly-(ethyleneimine), one of the most simple heterochain polymers exists in the form of two different chemical structures one of them is branched, which is a commercially available and the other one linear which is synthesized by cationic polymerization of oxazoline monomers and subsequent hydrolysis of polyf(/V acylimino)cthylcne]. The most salient feature of poly(ethyleneimine) is the simultaneous presence of primary, secondary, and tertiary amino groups in the polymer chain which explains its basic properties and gives access to various modification reactions. A great number of synthetic routes to branched and linear poly(ethyleneimine)s and polymer-analogous reactions are described. In addition, the complexation of polyfethyleneimine) and its derivatives with metal ions is investigated. Homogeneous and heterogeneous metal separation and enrichment processes are reviewed. 

Alkenes that easily form carbocations are good candidates for cationic polymerization, which is just another example of electrophilic addition to an alkene. Consider what happens when pure isobutylene is treated with a trace of concentrated sulfuric acid. Protonation of the alkene forms a carbocation. If a large concentration of isobutylene is available, another molecule of the alkene may act as the nucleophile and attack the carbocation to form the dimer (two monomers joined together) and give another carbocation. If the conditions are right, the growing cationic end of the chain will keep adding across more molecules of the monomer. The polymer of isobutylene is polyisobutylene, one of the constituents of butyl rubber used in inner tubes and other synthetic rubber products. 

N-Benzyl and iV-alkoxy pyridinium salts are suitable thermal and photochemical initiators for cationic polymerization, respectively. Attractive features of these salts are the concept of latency, easy synthetic procedures, their chemical stability and ease of handling owing to their low hygroscopicity. Besides their use as initiators, the applications of these salts in polymer synthesis are of interest. As shown in this article, a wide range of block and graft copolymer built from monomers with different chemical natures are accessible through their latency. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

In addition to the repeat unit sequence, another area of current interest in polymer structural control (Fig. 1) may be the spatial or three-dimensional shapes of macromolecules. In fact, the recent development of star [181-184] and graft [185] polymers, as well as starburst dendrimers [126], arborols [186,187], and related multibranched or multiarmed polymers of unique and controlled topology, has been eliciting active interest among polymer scientists. In this section, let us consider the following macromolecules of unique topology for which living cationic polymerizations offers convenient synthetic methods that differ from the stepwise syntheses (polycondensation and polyaddition) [126,186,187]. 

For a synthetic polymer chemist the important question is whether the cyclization processes in cationic ring-opening polymerization can be controlled. If the preparation of linear polymer is attempted, then cyclic oligomers are undesirable side products. This is especially important in synthesis of telechelic polymers containing reactive end groups, because macrocycles would be unreactive admixtures. On the other hand, cyclic polymers, if prepared selectively, could be a valuable materials. 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

Although there are some limitations on the molecular weights of the linear polymers which may be obtained by this method, AM polymerization offers an attractive, synthetic route for preparation of functional, medium molecular weight polymers by cationic polymerization of oxiranes. As the process involves the extension of the chain of hydroxyl group containing compound used to initiate the polymerization (initiator), the method is especially well suited for preparation of oligodiols (low molecular weight diols as initiators) and macromonomers (for example, hydroxy-ethyl acrylate as initiator) ... 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

These developments in cationic polymerization of 1,3,5-trioxane are discussed in more detail, because in this system the problems related to the mechanism of cyclization are now well understood. Cyclic oligomers were identified, isolated, their molecular weight distribution was determined, and the plausible explanation for observed distribution was given. From the synthetic point of view, the cationic polymerization of 1,3,5-trioxane offers the possibility of preparing macrocyclic polymers with relatively narrow molecular weight distribution and predictable (within discussed limits) molecular weights. The cyclic polymers can be prepared easily in relatively large quantities and conveniently separated from linear polymer by alkaline hydrolysis of the latter. 

Cationic polymerizations are among the most important synthetic methods in polymer chemistry. They are used to prepare a variety of commodity and specialty polymers. More recently, controlled cationic polymerizations have been used to synthesize novel functional polymers, block copolymers, and macromolecules with new topologies. The importance of reactions involving cationic active species is continuously increasing, and was recently recognized by the awarding of the 1994 Nobel Prize in Chemistry to George Olah. 

The book is divided into eight chapters. The Introduction is a primer for both synthetic polymer chemistry in general, and cationic polymerizations in particular. More advanced readers may go directly to the following chapters. The second chapter covers the reactions of carbenium ions with various nucleophiles and focuses on the ionization of covalent species and the addition of carbenium ions to alkenes, arenes, and other ir-nucleo-... 

Polyisobutene or polyisobutylene, CAS 9003-27-4, with the model formula [-CH2C(CH3)2-]n is another common polymer, which is used in practice mainly for sealants and adhesives and in various copolymers. Polyisobutylene copolymer with a small amount of isoprene is used as a synthetic rubber, the added isoprene making the material vulcanizable. The polymer is typically obtained by cationic polymerization using, for example, BF3 or AICI3 as catalysts. The thermal decomposition of the polymer generates various fragments [76,119-123]. The heating between 288° C and 425° C... 

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