Stereoselectivity polymerization reactions

Allcock and coworkers " have carried out extensive work on inclusion polymerization within cyclotriphosphazene host structures (Figure 15), reporting numerous stereoselective polymerization reactions. For example, polymerization of 1,4-divinylbenzene (4-vinylstyrene) in its bulk phase produces an insoluble cross-linked polymer, whereas polymerization of 1,4-divinylbenzene guest molecules within the inclusion compound formed with host molecule J (Figure 15) produces a soluble, linear noncross-linked polymer. According to the authors, this inclusion polymerization reaction represents one of the few ways of producing... 

The use of catalysts for a Diels-Alder reaction is often not necessary, since in many cases the product is obtained in high yield in a reasonable reaction time. In order to increase the regioselectivity and stereoselectivity (e.g. to obtain a particular endo- or exo-product), Lewis acids as catalysts (e.g. TiCU, AICI3, BF3-etherate) have been successfully employed." The usefulness of strong Lewis acids as catalysts may however be limited, because they may also catalyze polymerization reactions of the reactants. Chiral Lewis acid catalysts are used for catalytic enantioselective Diels-Alder reactions. ... 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Chakraborty and Chen reported the syntheses and structures of organzinc compounds bearing chiral trans-1,2-(HNSiMe3)2-cyclohexanediyl ligands for the stereoselective polymerization of methacrylates. Depending upon the reaction conditions, Scheme 62, either the fully deprotonated trimeric bis(amido)methylzinc complex 80, or a dimeric amido(amino)methylzinc complex 81 were isolated.140... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

Inspired by the ability of cationic ansa-zirconocene complexes to effect stereocontrolled alkene polymerization reactions, Jordan has recently reported the stereoselective insertion of simple alkenes into both the (ebi)Zr(r 2 -pyrid-2 -yl) and (ebthi) Z r (r 2 -pyr id - 2 -yl) systems [113]. As shown in Scheme 6.36, treatment of rac-(ebi)ZrMe2 114 with nBu3NH+BPh4 in the presence of 2-picoline affords the (ebi)Zr(q2-pyrid-2-yl) complex 115 (the derived B(C6F5) derivatives may also be prepared and are in fact reported to be more convenient to use). 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

From a kinetic point of view processes (1) and (2) are characterized by equal macroscopic reaction rates for both enantiomers (v = Vj), while for (3) v Vj (in the extreme case, one of the two rates is zero). At the microscopic level, for a stereoselective polymerization... 

Polymerizations that yield tactic structures (either isotactic or syndiotactic) are termed stereoselective polymerizations. The reader is cautioned that most of the literature uses the term stereospecific polymerization, not stereoselective polymerization. However, the correct term is stereoselective polymerization since a reaction is termed stereoselective if it results in the preferential formation of one stereoisomer over another [IUPAC, 1996]. This is what occurs in the polymerization. A reaction is stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products. This is not what occurs in the polymerization since the starting material does not exist in different configurations. (A stereospecific process is necessarily stereoselective but not all stereoselective processes are stereospecific.)... 

The first reported instance of stereoselective polymerization was probably the cationic polymerization of isobutyl vinyl ether in 1947 [Schildknecht et al., 1947]. A semicrystalline polymer was obtained when the reaction was carried out at —80 to —60°C using boron tri-fluoride etherate as the initiator with propane as the solvent. The full significance of the polymerization was not realized at the time as the crystallinity was attributed to a syndiotactic structure. X-Ray diffraction in 1956 indicated that the polymer was isotactic [Natta et al., 1956a,b], (NMR would have easily detected the isotactic structure, but NMR was not a routine tool in 1947.)... 

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. 

When nonpolar solvents are employed, polymerization proceeds by an anionic coordination mechanism. The counterion directs isotactic placement of entering monomer units into the polymer chain. The extent of isotactic placement increases with the coordinating power of the counterion (Li > NaK. Cs). The small lithium ion has the greatest coordinating power and yields the most stereoselective polymerization. Increased reaction temperature decreases the isoselectivity. 

The versatility of polymerization resides not only in the different types of reactants which can be polymerized but also in the variations allowed by copolymerization and stereoselective polymerization. Chain copolymerization is the most important kind of copolymerization and is considered separately in Chap. 6. Other copolymerizations are discussed in the appropriate chapters. Chapter 8 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of initiators and polymerization conditions. In the last chapter, there is a discussion of the reactions of polymers that are useful for modifying or synthesizing new polymer structures and the use of polymeric reagents, substrates, and catalysts. The literature has been covered through early 2003. 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Although copper reagents, hahdes and triflates, are widely used in atom-transfer polymerization reactions (ATRP) [63], these processes do not fall under the category of Lewis acid-mediated reactions. Sherrington and co-workers have shown that a vinyl monomer coordinated to a chiral copper Lewis acid (122) undergoes stereoselective polymerization (Sch. 29) [64]. A chiral block-copolymer 124 was prepared under radical conditions. 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

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