Chapter 7: Dimerization, oligomerization and polymerization of alkenes and alkynes

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophilic to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. ...
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Chapter 7: Dimerization, oligomerization and polymerization of alkenes and alkynes Chapter 7Dimerization, oligomerization and polymerization ofalkenes and alkynes The annual production of various polymers can be measured only inbillion tons of which polyolefins alone figure around 100 million tons peryear. In addition to radical and ionic polymerization, a large part of thishuge amount is manufactured by coordination polymerization technology.The most important Ziegler-Natta, chromium- and metallocene-basedcatalysts, however, contain early transition metals which are too oxophilic tobe used in aqueous media. Nevertheless, with the late transition metals thereis some room for coordination polymerization in aqueous systems [1,2] andthe number of studies published on this topic is steadily growing.7.1 Dimerization and polymerization of ethylene Coordination polymerization of ethylene by late transition metals is arather slow process especially when the catalyst is dissolved in water. In astudy of the interaction of and (tos = tosylate),both and wereisolated by evaporation of the aqueous phase which had been previouslypressurized with 60 bar ethylene at room temperature for 6 and 18 hours,respectively. Longer reaction times (72 h) led to the formation of buteneswith no further oligomerization. This aqueous catalytic dimerization was notselective, the product mixture contained Z-2-butene, E-2-butene and 1-butene in a 1/2.2/2.2 ratio [3]. The facially coordinating l,4,7-trimethyl-l,4,7-triazacyclononane (Cn)ligand forms stable methylrhodium(III) complexes, such as and (OTf=trifluoromethanesulfonate)and the latter two have rich aqueous chemistry. When dissolved in water, readily coordinates two water molecules to form the 237238 Chapter 7octahedral in which the aqua ligands undergosequential deprotonation in basic solutions with and(Scheme 7.1) [4]. At 24 °C and 15-60 bar ethylene, catalyzed theslow polymerization of ethylene [4]. Propylene, methyl acrylate and methylmethacrylate did not react. After 90 days under 60 bar (thepressure was held constant throughout) the product was low molecularweight polyethylene with and a polydispersity index of 1.6. Thisis certainly not a practical catalyst for ethylene polymerization ( in aday), nevertheless the formation and further reactions of the variousintermediates can be followed conveniently which may provide ideas forfurther catalyst design. For example, during such investigations it wasestablished, that only the monohydroxo-monoaqua complex was a catalystfor this reaction, both and were foundcompletely ineffective. The lack of catalytic activity of isunderstandable since there is no free coordination site for ethylene. Such acoordination site can be provided by water dissociation from and and the rate of thisexchange is probably the lowest step of the overall reaction.The hydroxyligand facilitates the dissociation of and this leads to a slow catalysis ofethene polymerization. Cationic Pd- and neutral Ni-complexes of chelating N-N or P-O ligandscatalyze the polymerization of ethylene in aqueous media with reasonablyhigh acitivity (Scheme 7.2) [5,6,61,62]. In fact, the turnover frequencies areclose to those obtained with the same catalysts in (TOF-s 450 vs. at room temperature). On the other hand, aqueous polymerizationsprovided polymers with much higher molecular mass (e.g. 77700 comparedto 14500, obtained in ). The same kind of branching was found inthese polymers, nevertheless the higher molecular mass was manifested inthe physical apperance - the polymers obtained in the aqueous reactionsDimerization, oligomerization and polymerization of alkenes and 239alkyneswere rubbery solids while polymerizations in afforded viscous oils.Very importantly, the active Pd- and Ni-catalysts are water-insoluble,consequently these aqueous polymerizations were catalyzed by solidparticles of the catalysts suspended in the aqueous phase rather than byhomogeneously dissolved metal complexes. When a palladium catalyst wasmade water-soluble by using a sulfoalkyl-modified diimine ligand noactivity whatsoever was observed. The catalytic activity was similarly lostupon dissolution of the catalysts in the aqueous phase by co-solvents, suchas acetone.7.2 Telomerization of dienes The linear telomerization reaction of dienes was one of the very firstprocesses catalyzed by water soluble phosphine complexes in aqueousmedia [7,8]. The reaction itself is the dimerization of a diene coupled with asimultaneous nucleophilic addition of HX (water, alcohols, amines,carboxylic acids, active methylene compounds, etc.) (Scheme 7.3). It iscatalyzed by nickel- and palladium complexes of which palladium catalystsare substantially more active. In organic solutions givesthe simplest catalyst combination and Ni/TPPTS and Pd/TPPTS weresuggested for running the telomerizations in aqueous/organic biphasicsystems [7]. An aqueous solvent would seem a straightforward choice fortelomerization of dienes with water (the so-called hydrodimerization). Infact, the possibility of separation of the products and the catalyst without aneed for distillation is a more important reason in this case, too.240 Chapter 7 The m ...