Capability of isoprene and butadiene as comonomers under metallocene catalysis: Activating active sites with the addition of 1-hexene and propylene

Abstract

The copolymerization of ethylene (E) with cyclic diene has been reported often in the literature. However, to evaluate isoprene (IP) and butadiene (BD) as co- and ter-monomers catalyzed by metallocene are comparatively rare. In this work, we performed a set of co- and terpolymerization of IP and BD by using symmetrical metallocene catalysts Cat-I and Cat-II. The E/IP and E/BD copolymers Cat-I catalyzed activity is lower than PE and E/P and significantly increased, reaching 3.1 × 107gpolymer/molCat*h, but unlikely the insertion of IP and BD were not detected. The active sites of Cat-I are not factional for IP and BD, and they are not even activated with the addition of 1-hexene (1H) and P. The 1,4-insertion of these into catalyst active sites produced a stable π-allyl type species that actively produced dormant sites in the consequent propagation chain, and many other reactions that convert to dormant sites occur. In contrast, the Cat-II bearing silicon bridged moiety shows additional activity with IP and BD and an increasing trend with P and IH, demonstrating an activating feature. To conduct an extensive investigation, we performed a systematic comparison of the proportion of catalytically [Zr]/[C*] active sites and polymerization rate constant (kp) for both catalysts to shed light on the Zn-based chain termination and transfer reactions. The [C*]/[Zr] was significantly decreased with IP and BD, but compared to BD, IP shows a higher level of active sites and molecular weight, demonstrating that chain transfer reactions are faster with BD. Adding P and 1H increased the [C*]/[Zr], reactivating the prior inactive sites of the catalyst that were dormant or not functioning during the PE, E/IP, and E/BD polymers and reducing the chain transfer reactions. Compared to Cat-I, Cat-II demonstrates a higher level of [C*]/[Zr]; furthermore, the addition of 1H, a slight change in the [C*]/[Zr], while P with IP overall higher level (92.92 %) of the [C*]/[Zr] showed that P with IP shows a double activation for Cat-II and activates all those active sites that were dormant. BD significantly reduced the level of active sites to 49 % compared to IP and showed poisoning for the active site that was active for ethylene. The kpE of E/BD and E/IP polymers are lower than the kpE of E/BD/1H, E/P/IP, E/BD/1H, and E/IP/P, indicating P and 1H significantly activated the dormant sites of the E, IP, and BD that were dormant during the copolymerization. Cat-I catalyzed E/IP/1Hpolymers, the kp1H of 1H is 15.8 L/mol·s, and the kpP of P is 83.3 L/mol·s, demonstrating that the P and more involved in the activation of active sites, and kpP of P is more increased with Cat-II, indicating that substituents of the Cat-II make the catalyst more active for P.