Catalysts for the Controlled Polymerization of Conjugated Dienes

International audienceSince its first discovery at the beginning of the 1960s [1], the coordinative polymerization of conjugated dienes has improved continuously, performer better and better. Today, chemists know how to stereospecifically polymerize conjugated dienes, whether in 1,4-cis, 1,4-trans, or 3,4(1,2) fashion. The petro-sourced (nowadays also bio-sourced for a number of them) butadiene, isoprene, and substituted conjugated diene monomers have been the subject of a very large number of studies in this context, more recently joined by natural dienes from the terpene family such as myrcene, farnesene, and ocimene. The industry has greatly helped to improve the performances of the catalytic systems (activity/productivity, selectivity, efficiency in metal catalyst), with the aim of optimizing the preparation of synthetic polymers such as 1,4-cis polybutadiene, which are widely applied in the tires, rubbers, and combined styrene-based resins (ABS, HIPS). Catalysts today cover a wide set of elements among which are metals from groups 4-6, 8-10 [2,3], and rare earths [4,5,6], while, to date, industrial concerns are mainly dominated by four metallic elements—namely neodymium, nickel, cobalt, and titanium [7]. For the synthesis of 1,4-cis polybutadiene, the industry catalysts are generally based on ternary systems, with a pre-catalyst associated to an activator and an aluminum chain transfer agent. The 1,4-trans polydienes are rather synthesized either by means of binary catalytic systems often comprising an alkylmagnesium cocatalyst, or by combination with an aluminum derivative in the case of transition metal systems. The 3,4-polydienes do not exist in the natural state, their preparation was synthetically developed later, and they have recently been shown to be potentially useful for improving tire performance, thanks to their excellent skid resistance and their low rolling resistance [8]. Nowadays, there is a better understanding of the polymerization mechanism and involves allyl-active species, thanks in particular to the support of more and more efficient calculations methods [9,10,11]. Since the beginning of the 2000s, there has also been a tendency for statistical copolymerization of 1,3-dienes with olefin or styrene comonomers to produce statistical, alternating, and block copolymers [12], while access to multiblock and stereoblock copolymers is currently made possible by the innovative approaches of coordinative chain transfer polymerization [13]. A last challenge is about to be solved with the preparation of stereoregular polydienes (and their copolymers) that are also end-functionalized, thanks to the living character of the polymerization. Finally, the future will probably see the development of alternative catalysts made from non-toxic and abundant metals like iron, while an even greater interest can be expected for rare earth catalysts following the discovery of new geological resources of these elements [14].This issue brings together several important aspects of this chemistry, which remains at the forefront of both academic and industrial research interests