Stereoblock polybutadienes by means of switchable cobalt catalysts

Diene-based polymers, specifically cis-1,4 polybutadiene and cis-1,4 polyisoprene, have found and still find main application in the preparation of elastomeric compounds for the production of tires. In order to broaden and diversify the application spectrum of the conjugated diene polymers, it would be desirable to be able to rely on polymeric materials having features different from those of the simple elastomer. The recent discovery and introduction, inspired by Porri's 1,3-dienes polymerization mechanism [1], of a new generation of catalysts based on transition metal and lanthanide complexes having a well defined structure and containing ligands with P and/or N and/or O as donor atoms [2], able to i) afford polymers with different structures from different monomers, ii) exhibit living features, iii) modify their type of selectivity during polymerization while maintaining the living nature, resulted to be extremely useful for this purpose. The strategies adopted to modify the selectivity during the polymerization runs are of different type, ranging from adding a specific ligand to changing the cocatalyst (i.e., different type of aluminum alkyl), from varying the Al/Mt molar ratio to modifying the polymerization temperature. We are reporting in the present communication on two synthetic routes for the synthesis of stereoregular di-block polybutadienes [3] consisting of two stereoregular polymer blocks having different structures and properties: a first block with a highly cis-1,4 structure, amorphous, with a Tg value below -100°C, and a second block with a syndiotactic 1,2 structure, crystalline, with Tm in the range 60-140°C. According to the first synthetic procedure (Figure 1), butadiene is first polymerized with a catalytic system obtained by combining a cobalt complex with ligands (L1) (e.g., PtBu3, PtBu2Me), characterized by a cis-type selectivity, with formation of an amorphous cis-1,4 polybutadiene block. Figure 1. Scheme of formation of stereoregular di-block polybutadienes with cis-1,4/1,2 structure (L1 = PtBu2Me or PtBu3; L2 = PRPh2, R = Me, Et, nPr, iPr, tBu, Cy) 3. Subsequently a second ligand L2 is introduced (generally an aromatic phosphine of the type PRPh2, in which R = alkyl or cycloalkyl group) which replaces L1 on the cobalt atom, causing a drastic change of the catalytic selectivity, from cis-1,4 to syndiotactic 1,2, with the formation of a second block of crystalline polybutadiene with a 1,2 syndiotactic structure (the syndiotacticity degree and therefore the crystallinity vary according to the nature of the R group on the phosphorus atom). Having the initial catalytic system "living features", and being these characteristics maintained even after the addition of the ligand L2, the two polymeric blocks result joined together through a single junction point, thus allowing to have within the same polymer chain the characteristics of both an elastomeric and thermoplastic material, with a considerable advantage for the ultimate properties of the polymer obtained. Completely analogous stereoregular diblock polybutadienes can also be obtained following a second synthetic way, in which butadiene is polymerized with a catalyst obtained by combining a cobalt complex supported by a mixed P&O ligand with MAO; the change in selectivity is in this case obtained by varying the MAO/Co molar ratio during polymerization (Figure 2). Figure 2. Polymerization of 1,3-butadiene with Co(P&O) based catalysts The presence of an oxygen donor atom within the phosphine ligand structure, associated to an increase of the concentration of MAO up to MAO/Co = 1000, likely causes the whole phosphine to migrate on the aluminum atoms, with generation of a cis-1,4 specific catalytic center, quite similar to the one obtained by reacting naked CoCl2 with MAO, a well-known cis-specific catalyst. All the polybutadienes obtained were analyzed by different analytical techniques (FT-IR, NMR, DSC, GPC, RX, AFM, DMA) which clearly supported the effective formation of stereoregular di-block polymers and pointed out the improved properties of these new thermoplastic elastomers with respect to commercial butadiene rubber (BR).