The dimethylaluminum compounds {3-tBu-2-(O)C6H3CHN−R}AlMe2 [R = C6H5 (1); 2,6-iPr2C6H3 (2); C6F5 (3)] were used as initiators in the ring-opening polymerization (ROP) of ε-caprolactone, l-lactide, and d,l-lactide. Compound 3, in combination with 1 equiv of methanol, exhibited a living behavior in the ROP of the cyclic esters. Such a feature allowed the preparation of poly(d,l-lactide-block-ε-caprolactone) and poly(l-lactide-block-ε-caprolactone) copolymers. Random copolymers of ε-caprolactone and l-lactide and of ε-caprolactone and d,l-lactide were also synthesized by compound 3. NMR and DSC characterization confirmed a highly random structure of these copolymers, even in the absence of transesterification reactions. All the materials, characterized by GPC, showed high molecular weight and narrow molecular weight distributions.
The experimental data available in the literature concerning syndiotactic-specific polymerization of styrene and the data reported in this paper, mainly concerning the comparison of the behaviour of different catalytic systems, lead to the tentative suggestion that the true catalytic species might be complex cations similar to those involved in a-olefin polymerization.
Copolymerization of ethylene and styrene in the presence of the
catalytic system
η5-C5Me5Ti(CH2Ph)3−B(C6F5)3
has been investigated. Independently from the feed composition,
basically
alternating ethylene−styrene copolymers are obtained, together with
some polyethylene and syndiotactic
polystyrene, from which the former can be separated by solvent
extraction. NMR analysis of suitably
13C-enriched end groups shows that the regiospecificity of
styrene insertion in the initiation step is largely
secondary. A tentative explanation for the preferentially
alternating comonomer incorporation is proposed.
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