Centres, active isotactic

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. 

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... 

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.

When monomer coordination to the active centre is prevented, polymerization cannot occur. Coordination is a reversible reaction strongly solvating agents deactivate centres in a ratio very close to 1 1 (acetylenes, allene, ketene, tetrahydrofuran, ROH, H20, COS, CO, C02, R3N) but weaker donors must be present in some excess in order to cause total inhibition. It appears that isotactic and atactic centres of Ziegler-Natta polymerizations, particularly centres with TiCl3 and Et2AlCl, exhibit a different ability to coordinate donors (different acidity). 

In reality various direct experimental evidences exist to support such an hypothesis first of which, in the case of polypropylene, is the well known heterogeneity according to stereospecific properties of centres Actually, the stereoregular polypropylene isotactic fraction and the amorphous one generally show broad MWD too, thus proving the probable existence of different active centres although similar in stereoregulating ability... 

For example, Soga et al. > found, by examining propylene polymerization in toluene in the temperature range 0-65 °C with an apparently soluble catalyst such as tetrabenzylzirconium, that the isotactic index and the polydispersity of the polymer increased as the polymerization temperature increased. Furthermore, the value of Q > 30 was much greater than that predicted for homogeneous active centres. From these results the authors concluded that the catalyst should be made up of small invisible colloid-type particles. The same interpretation is valid for the surprising results (Q 40) obtainedfor polyethylene, in Isopar solution at 200 °C, with the originally soluble tetrabenzylzirconium-water catalyst. 

The comparable values of the polydispersity index for both isotactic and atactic polymers led Keii to conclude that the same active centre distribution exists according to the ratio between propagation and transfer rate constants. Furthermore, the low values of the polydispersity index would suggest good active centre homogeneity. 

It has been established from the polymerization of l-dj-deutero-propene that the double bond opens in the cis direction [71], in conformity with the proposed mechanism. A repetition of this process with the monomer molecules approaching the active centre in the same direction would give the isotactic polymer while alternate approach to mirror im e positions would give the syndiotactic form. 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

The uncertainty about the mechanism has been resolved very elegantly by Zambelli et al [39] who prepared isotactic polypropylene using catalysts in which the initial active centre consisted of Ti- CH3 groups. The NMR... 

The role of the organometallic cocatalyst has been discussed in detail by Yermakov et al. in terms of formation of active centres and of equilibrium chemisorption reactions at the newly-formed centres for TiCla, TiCla-AlEtaCl, and TiCls-AlEtaCl systems. In a later publication it is concluded from Cp values and C-n.m.r. spectra that the catalytic properties (reactivity and stereospecificity) of the propagation centres, are not directly influenced by the cocatalyst. However the cocatalyst indirectly influences the overall rate of polymerization and the polymer isotacticity due to the complexation reactions altering the number of propagating centres and the ratio of stereospecific to non-stereospecific centres. 

It is known [14,15] that, on the surface of the heterogeneous Ziegler-Natta catalysts promoting the isotactic polymerisation of propene, active centres are also present which may give rise to the formation of significant amounts of V-rich sequences (syndiotactic or syndiotactoid ). As a matter of fact, syndiotactic polypropene was isolated for the first time as an impurity from samples of isotactic polypropene prepared in the presence of catalyst systems such as, for example, a- or y -TiClj in combination with A1(C2H5)2F or LiC4H9 [16]. 

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