Centres isotactic

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

The meaning of the notation uu should be explained. As there is never a (local) mirror plane or twofold axis expressing the relationship between two or three units in alkene/CO polymers, meso and rac cannot be applied. Instead one uses u and /, for unlike and like. Unlike means that two subsequent centres have different absolute configurations, thus they form a syndiotactic polymer, while like means that they have the same configuration and thus they form an isotactic polymer (see Figure 12.23). 

In the present paper we report results obtained with t-butyl-and phenyl-magnesium compounds which under suitable conditions give highly isotactic polymers. We are in particular looking to see the extent to which the character of the polymerization is predetermined during the initiation stage when the stable and persistent growth centres are formed, and whether monomer is coordinated to the active centre in these cases. 

The influence "of THF is shown in Figure 1, where the frequencies of the iso-, syndio- and hetero-tactic triads, i, s and h, of each sample can be read off along the appropriate median (e.g. the apex i corresponding to i = 1-0, the base opposite i corresponding to i = 0). The curve represents the Bernoullian triad distribution (h = 2i1/2s1 2). The black circles refer to samples prepared in toluene solution containing the optimum trace of THF (Xthf s . ) for initiation of isotactic growth centres. 

Preliminary results on the kinetics of the polymerization and the efficiency of initiation of the isotactic polymerizations initiated by t-BuMgBr in toluene solution are consistent with the Bateup mechanism proposed for the stereoblock and syndio-tactic-like polymerizations initiated by n-BuMgBr in THF-rich solution — a mechanism which involves initiation and propagation through monomer — active centre complexes (5,8). 

Isotactic polymerization requires the presence of halide, and is critically dependent on the nature of the alkyl or aryl group. At least one of each of these groups must be present in the vicinity of the active centre. The requirement of a minimum amount of THF for the formation of an isotactic growth site implies that this must also be coordinated at this site. We have evidence that monomer is complexed at the active centre, which has been postulated for many years (10,15). 

According to Cossee, the isotactic centre has one and the atactic two vacancies. According to Rodriguez, the organometal is coordinated into one of the two vacancies of the isotactic centre. The addition of donors (diethyl ether, triethylamine, pyridine, etc.) greatly affects the polymerization rate (catalyst activity) as well as the ratio of the stereoregular and atactic product components. Donors affect the structure of active centres and modify it. 

Valuable data on the properties of active centres are obtained from kinetic measurements. They reveal the simultaneous existence of several centre types. The stable centres are active during the whole course of polymerization in addition, some fraction of decaying centres is also present. Isotactic centres exhibit stereoregulating ability and are, moreover, extremely active. Centres may oscillate between active and inactive (dormant) forms and some centres selectively polymerize enantiomers from a racemate. External effects, caused by specific properties of centres, will be discussed in subsequent chapters. In addition, the centres on which dienes are polymerized will be treated in Chap. 5, Sect. 4. The structure of these centres is a function of the coordinated diene, and it is therefore better presented together with propagation. 

In my opinion, an active centre of alkene polymerization in the liquid phase is not a single chemical entity to be visualized by a single (and simple) chemical formula. Probably a set of compounds, of complexes with variable composition, a dynamic system where the effects of individual components are mutually complementary or overlapping is really in play. The same macroscopic effect (centres of equal activity and iso-specific regulating ability) can be obtained with various starting organometals and donors. In such a system, subsystems may exist each of which is externally manifested as an individual active centre (rapid or slow, isotactic, with a tendency to transfer or termination, or living, etc.) [225],... 

The stereochemistry of addition to a free centre is mostly determined by interactions between the monomer and active centre during approach to the transition state. In simple cases, represented by equations (34) and (35) only the two primary components will interact, and Bernoulli statistics with a single probability parameter Pm will predominate. For Pm = 0.5, the propagation rate constants of isotactic and syndiotactic growth, kpj and k, will differ... 

When the approaching monomer is in syndiotactic conformation and is added as such (c), the / carbon is fixed after addition, but the a carbon can rotate (d) and assume isotactic conformation after the addition of the next unit. The effect of the lithium cation is again assumed to be the driving force for the rotation of the potentially syndiotactic unit to the isotactic conformation. Repeating steps (a) and (b) leads to a threo-diisotactic chain, and repeated steps (c), (d) and (e) to an erythrodiisotac-tic chain. The addition of tetrahydrofuran reduces the number of threo-diisotactic units because it competes with carbonyl groups for coordination to Li+. This enhances the amount of monomer approaching the centre by the syndiotactic route. 

Fig. 10. Model of the interaction of an active centre with the end of an isotactic chain helix [109], Insertion (a) preserving (b) disturbing helix symmetry.

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