Propagation syndiotactic

What is significant about these reactions is that only two possibilities exist addition with the same configuration (D -> DD or L LL) or addition with th< opposite configuration (D DL or L LD). We shall designate these isotactic (subscript i) or syndiotactic (subscript s) additions, respectively, and shal define the rate constants for the two steps kj and k. Therefore the rates o isotactic and syndiotactic propagation become... 

The propagating polymer then terminates, producing an isotactic polypropylene. Linear polyethylene occurs whether the reaction takes place by insertion through this sequence or, as explained earlier, by ligand occupation of any available vacant site. This course, however, results in a syndiotactic polypropylene when propylene is the ligand. 

Puzin et al.h] reported that the tacticity of PMMA prepared in bulk is influenced (slight increase in syndiotacticity) by very small amounts of titanoecnc diehloride (1 O 3 M). Selective complexation of the propagating radical was postulated. 

The lanthanocene initiators also polymerize EtMA, PrMA and BuMA in a well-controlled manner, although syndiotacticity decreases as the bulk of alkyl substituent increases. Reactivity also decreases in the order MMA EtMA > PrMA > BuMA. Chain transfer to provide shorter polymer chains is accomplished by addition of ketones and thiols.460 The alkyl complexes (190) and (191) also rapidly polymerize acrylate monomers at 0°C.461,462 Both initiators deliver monodisperse poly(acrylic esters) (Mw/Mn 1.07). An enolate is again believed to be the active propagating species since the model complex (195) was also shown to initiate the polymerization of MA. 

The enantiomorphic site selectivity of (R)-(—)-(263) allows highly syndiotactic PLA to be prepared via the polymerization of meso-LA (Mn = 12,030, Mn calc= 13,540, Mw/Mn= 1.05).802 The ring opening of meso-LA by (R)-(—)-(263) occurs to produce a syndiotactic propagating chain bound to the metal via an (S)-lactic acid unit.803 Attempts to produce syndiotactic PLA via the ROP of meso-LA using rac-( )-(263) instead afforded heterotactic-biased material. 

Diastereoisomeric transition states calculated for propene primary insertion in a model of the Ewen achiral metallocenes are shown in Figure 1.20. The two possible diastereomeric transition states correspond to si (Figure 1.20a) and re (Figure 1.20b) insertions of the monomer for the case of a si chain (i.e., a growing chain in which the last monomeric unit has been obtained by a cis addition of a -coordinated monomer molecule) and are suitable for like (isotactic) and unlike (syndiotactic) propagations, respectively.142,143... 

All the proposed models for syndiotactic propagation suppose that the active center is a vanadium-carbon bond and that the monomer first coordinates to the metal. Moreover, all of them attribute the stereospecificity to steric factors. However, different driving forces for the syndiospecificity have been proposed. 

According to an early model.13 1 there are two adjacent accessible positions at the catalytic site, each favoring the coordination of the prochiral monomer with one of its two faces if the growing polymer chain alternates between the two positions at each insertion step, syndiotactic propagation is ensured. Due to the successive finding of a chain-end stereocontrol, this model has to be rejected. 

The general mechanistic features of the ethene/CO copolymerisation cycle (Scheme 7.2) are substantially valid also for styrene. In particular, the propagation steps are similar for both alkenes and consist of subsequent alternated migratory insertions of alkyl to CO and of acyl to olefin, with P-chelate and y-chelate resting states. The structures of the first intermediates in the syndiotactic copolymerisation of styrene derivatives with CO have been determined by an in situ NMR study using [(Pr DAB)Pd(Me)(NCMe)]BAr4 as precursor (Scheme 7.21) [38]. 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

In summary, syndioselective initiators exaggerate the inherent tendency toward syndiotactic placement by accentuating the methyl-methyl repulsive interactions between the propagating chain end and incoming monomer. Isotactic placement occurs against this inherent tendency when chiral active sites force monomer to coordinate with the same enantioface at each propagation step. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

---