Syndiotactic addition steps

Mono-Cp titanium catalyst systems are also suitable for the polymerization of polar and non-polar olefinic monomers. The reduction of a mixture of Cp TiMe3 and Ph3C[B(C6F5)4] with zinc produces a catalyst for the syndiotactic homopolymerization of styrene. The same catalyst mediates the polymerization of methyl methacrylate to poly(methyl methacrylate) (PMMA) with >65% of syndiotacticity. This system is also effective for the co-polymerization of styrene/methyl methacrylate upon optimal conditions. A new polymerization mechanism to explain the characteristics of the polymers is proposed based on sequential conjugate addition steps.541... 

Figure 1. Schematic representation of syndiotactic and isotactic addition steps.

Recently, this type of deuterated monomer method was applied to the copolymerization of MM A and styrene in the presence of BC13 at low temperature, which gives a heterotactic-rich alternating copolymer.245,246 Using styrene-/3-dj, Goto et al. revealed that the addition of MMA to the styrene radical is syndiotactic-specific while the addition of styrene to the MMA radical is isotactic-specific, and that the alternation of these addition steps gives rise to the formation of a heterotactic-rich alternating sequence. 

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... 

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

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. 

When methyl iodide is replaced by 2-vinylpyridine in these reactions we have in effect the propagation step of polymerization or oligomerization, and indeed reaction proceeds in the same selective fashion. In the case of lithium counterion >95 % of isotactic trimer is formed by the next addition. Epimerization of the selectivity methylated product using K+OBu " in dimethylsulphoxide at 25 °C for 2 weeks yields a final mixture of approximately 1 2 1 of isotactic, heterotactic, and syndiotactic trimers. Thus the trimerization process must be kinetically controlled. 

Reductions with metal hydrides are often preliminary steps for additional reactions. For instance, a product of UAIH4 reduction of syndiotactic poly(methyl methacrylate) can be reacted with succinic anhydride and then converted to an amide " ... 

The essential polymerization step is a repetitive free radical addition to the monomer double bonds, forming chains of carbon atoms constructed of imits —(—CH2—CHR—)— or —(—CH2—CRR —)— linked together predominantly head-to-tail (the substituted carbon atom is denoted as the headX Since the free radical is essentially sp hybridized, very limited control of tacticity is observed, with many monosubstituted monomers providing atactic poljuners and disubstituted like methacrylates showing (thermodynamic) preference for syndiotacticity. 

Fig. 15. NMR partial spectra in pyridine at 100 °C and 220 MHz of syndiotactic MMA-MAA copolymers prepared by a first (trace I) and an additional (trace II through V) hydrolysis step. Degree of hydrolysis increases from trace I to V, I = 63, II = 53, III = 41, IV = 29 and V = 15 mole% MM A in copolymer. The copolymer of trace I is of nearly random monomer distribution, while the copolymers of traces II through V possess blocklike distributions.

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