Polymeric catalytic system

In this review, contributions of selected experimental and molecular modeling studies to the elucidation of even fine details relative to the stereospecificity of polymerization catalytic systems are outlined. 

Successively optically active polymers were obtained by the same group starting from non-asymmetric monomers by coordinated anionic (93), and cationic (85) processes. Table 1 shows monomers polymerized, catalytic systems used and molar optical activity of the polymers obtained, referred to one monomeric unit. 

The pooled encoded format was also used by Boussie and co-workers (179, 180) to prepare and screen Pd- and Ni-diimine ligand complexes as olefin polymerization catalytic systems. The successful application of deconvolutive or encoding methods to medium-large SP catalytic system pool libraries should become routine in the future. Careful validation of the synthetic scheme, the screening, and the detection methods will ensure the applicability of HTS of SP catalytic pool libraries to each specific transformation investigated. 

The literature [101, 102] cites the hydrogenation and isomerization of allylbenzene in the presence of palladium complexes on polymeric supports having oxygen-containing coordination groups like -OPh, -COOH, -OH. It was shown that the activity and selectivity of synthesized polymeric catalytic systems were affected by both the electron-donor properties of graft coordination groups and the solvent type. In the presence of the catalyst, the isomerization of allylbenzene to 1-propenylbenzene mainly yields the trans isomer. 

Most of the polymeric-catalytic systems used for the dihydroxylation or aminohydroxylation of C=C bond is issued from alkaloid, mainly cinchona which has been proved to be the best ligand for this type of reaction. 

The first coordination epoxide polymerization catalytic system was reported by Pmitt and Baggett in 1955. It was based on iron tricbloride. Since that time, metal-based catalysts bave been widely exploited for epoxide polymerization. Tbe most studied systems are those based on zinc or aluminum derivatives. A first group consists of diethylzinc or trialkylaluminum associated to a cocatalyst, wbicb is generally water or an organic compound (alcohol, amine, and other compoimds) that reacts with the alkyl metal to form in situ new metal derivatives as the true catalytic system exploited (see Table 6). For a detailed review on the coordination polymerization of epoxides, see Kman. ... 

Before coordination polymerization was discovered by Ziegler and applied to propene by Natta, there was no polypropylene industry. Now, more than 10 ° pounds of it aie prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in chemistry Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular- polymerization. 

By quenching the polymerization with C1402 or Cl40 the determination of the number of propagation rate constants was found to be also possible for the two-component catalytic system TiCl2 + AlEt2Cl 158, 159). In contrast to alcohols, carbon dioxide and carbon monoxide under polymerization conditions react only with titanium-carbon active bonds and do not react with inactive aluminum-polymer bonds. 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

This section presents catalytic systems that have been studied for either the syndiesis of monomers or polymerization. In addition, some new catalysts, working in smoodi conditions, which could be used for high-performance polymers syndiesis are discussed. 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

External recycle reactor Polymerizations, catalytic reactions Very useful for viscous mixtures Equipment cost can be high (for viscous systems and for high pressure operations)... 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

The direct synthesis of poly(3-sulfopropyl methacrylate)-fr-PMMA, PSP-MA-fr-PMMA (Scheme 27) without the use of protecting chemistry, by sequential monomer addition and ATRP techniques was achieved [77]. A water/DMF 40/60 mixture was used to ensure the homogeneous polymerization of both monomers. CuCl/bipy was the catalytic system used, leading to quantitative conversion and narrow molecular weight distribution. In another approach the PSPMA macroinitiator was isolated by stopping the polymerization at a conversion of 83%. Then using a 40/60 water/DMF mixture MMA was polymerized to give the desired block copolymer. In this case no residual SPMA monomer was present before the polymerization of MMA. The micellar properties of these amphiphilic copolymers were examined. 

A combination of ATRP and ROP was employed for the synthesis of PLLA-fr-PS block copolymers and PLLA-fr-PS-fo-PMMA triblock terpoly-mers [120]. Styrene was initially polymerized using the functional initiator /3-hydroxyethyl a-bromobutyrate, HEBB, and the catalytic system CuBr/bpy. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Asymmetric ECH with [Rh(L)2(Cl)2]+ complexes containing chiral polypyridyl ligands has been attempted, in homogeneous media (L = (7)-(12)) and at carbon electrodes coated with polymer films prepared by electropolymerization of [Rh(13)2(Cl)2]+ -61 62 The latter catalytic system gave the best results in terms of turnover number (up to 4,750) and enantiomeric excess, (ee) when applied to the hydrogenation of acetophenone (ee 18%) and 2-butanone (ee 10%).62 Polymeric materials derived from the complexes [RhI(bpy)(COD)]+ 36 and [Pd(bpy)2]2+33have also been applied to the ECH reaction. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Scheme 4. Zirconium catalytic systems useful for polymerization of MMA...

---