Free radical polymerization catalyst

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Polymerization. Chloroprene is normally polymerized with free-radical catalysts in aqueous emulsion, limiting the conversion of monomer to avoid formation of cross-linked insoluble polymer. At a typical temperature of 40°C, the polymer is largely head-to-taH in orientation and trans in configuration, but modest amounts of head-to-head, cis, 1,2, and 3,4 addition units can also be detected. A much more regular and highly crystalline polymer can be made at low temperature (11). Chloroprene can also be polymerized with cationic polymerization catalysts, giving a polymer with... 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

The most common chain reaction polymerization is free-radical polymerization. A free radical is merely a molecule with an unpaired electron, which has a tendency to add a supplementary electron in order to form an electron pair which makes it extremely reactive. These molecular complexes could be produced by heat or irradiation, or formed by the addition of a compound, named the initiator (I), for example, dialkyl peroxides (R compounds (R — N = N — R), which are not strictly, catalysts, since they are chemically altered during the reaction [196],... 

The first polymerizations were free radical reactions. In 1933 researchers at ICI discovered that ethene polymerizes into a branched structure that is now known as low density polyethene (LDPE). In the mid- 50s a series of patents were issued for new processes in which solid catalysts were used to produce polyethene at relatively low pressures. The first was granted to scientists at Standard Oil (Indiana) who applied nickel oxide on activated carbon and molybdenum oxide on alumina. Their research did not lead to commercial processes. In the late 40s Hogan and Banks of Phillips were assigned to study the di- and trimerization of lower olefins. The objective was to produce high octane motor fuels. When they tried a chromium salt as promoter of a certain catalyst (Cr was a known reforming... 

When the less hindered 2,4-tolylene diisocyanate is reacted with a phospholene oxide catalyst linear oligomeric carbodiimides are obtained which have been reacted with a variety of nucleophiles to give poly(ureas), poly(acyl ureas), poly(formamidines) and poly-(guanidines) by addition across the N=C=N group. Also, reaction of the oligomeric carbodiimides with acrylic or methacrylic acid affords linear polymers, which can be further polymerized by free-radical type processes. Also, reaction of the carbodiimide oligomers obtained from 2,4-TDI with adipic acid in DMF produces a polyureid. ... 

Polymers derived from triazoles can be formed by three fundamental methods. In the simplest way 3,5-disubstituted triazoles are treated with acetylene to obtain 1-vinyl derivatives which are then polymerized with free radical catalysts. The resulting polymers do not appear to be used on their own but mixed with other polymerized alkenes they facilitate the dyeing of synthetic fabrics. 

Dialkyl peroxides (1), R-O-O-R (R and R are = or primary, secondary, tertiary alkyl, cycloalkyl, aralkyl and heterocyclic radicals Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and Oj) explode on heating... 

When composite resins were first introduced, their polymerization involved free radical initiation by a combination of benzoyl peroxide with tertiary amine activator, de-hvered in two different pastes, known respectively as base and catalyst. Contemporary composites are visible light-activated one-paste systems and the principal photoinitiator used is camphorquinone [70,80], It is used in conjunction with a co-initiator, typically an amine [81], This latter compound does not absorb hght, but is necessary to react with the fight-activated camphorquinone molecule to generate free radicals and hence initiate the chain polymerization. 

We begin with radical initiation because it is most commonly used. In order to activate the monomers, materials that release free radicals are called on, known as the free radical initiators. (It should be stressed at the outset that the term initiator differs from the term catalyst, because the former becomes part of the generated molecule, as will be described later. A catalyst does not participate directly in the chemical reaction, but speeds it up.) In order to xmderstand the mechanism of polymerization via free radical initiation, we write the relevant chemical equations. 

Polyfluorostyrenes are described in many publications. A (3-fluorostyrene can be formed by cationic mechanism [289]. The material softens at 240-260°C. An a,p,(3-trifluorostyrene can be polymerized by free-radical mechanism to yield an amorphous polymer that softens at 240°C [290], Ring-substituted styrenes apparently polymerize similarly to styrene. Isotactic poly(o-fluorostyrene) melts at 265°C. It forms by polymerization with Ziegler-Natta catalysts [291]. The meta analog, however, polymerized under the same conditions yields an amorphous material [291]. 

Practice shows that different monomers respond differently to the various classes of initiators (Table 16-15). Styrene, for example, is polymerized by free radicals produced by the decomposition of dibenzoyl peroxide, by cations formed from BF3 + H2O H [Bp30H] , by anions from UC4H9, or by Ziegler catalysts [e.g., TiCU plus A1(C2H5)3]. Vinyl esters, on the other hand, can only be polymerized free radically in the condensed (fluid) phase, formaldehyde only cationically and anionically, acetaldehyde only cationically, etc. 

Styrene is one of the few substances that can be polymerized equally well free-radically (thermally and with initiators), cationically, anionically, and with complex catalysts. Free radical, cationic, and most anionic polymerizations yield atactic polymers, whereas certain polymerizations of the polyinsertion type yield isotactic polymers. Only the free radical polymerizations are of commercial interest. 

Free-radical polymerization n. A reaction initiated by a free radical derived from a polymerization catalyst. Polymerization proceeds by the chain-reaction addition of monomer molecules to the free-radical ends of growing chain molecules. Major polymerization methods such as bulk, suspension, emulsion, and solution polymerization involve free radicals. The free-radical mechanism is also useful in copolymerization, in which alternating monomeric units are promoted by the presence of free radicals. Lenz RW (1967) Organic chemistry of high polymers. Interscience Publishers, New York. Odian G (2004) Principles of polymerization, 4th edn. Wiley-Interscience, New York. 

One of the most important types of addition polymerization is free radical polymerization. This process is initiated by the action of free radicals (electrically neutral species with an unshared electron). Free radicals for the initiation of addition polymerization are usually generated by the thermal decomposition of organic peroxides or azo compounds. The polymerization of unsaturated polyesters with a peroxide catalyst is an example of a free radical polymerization process. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

PVC is manufactured by three routes bulk (or mass), suspension, and emulsion polymerization using free radical initiators (section 1.8.1). In the bulk polymerization using liquid vinyl chloride monomer (VCM), the polymerization is usually done in two stages at 60 °C. Pre-polymerization to about 10% conversion yields a viscous suspension (PVC is insoluble in VCM) which is then added to a second horizontal reactor (together with more monomer and peroxide catalyst) with slowly rotating agitator blades. The mixture at 25% conversion becomes a powder. 

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