Fully-reversible and semi-reversible coordinative chain transfer polymerizations of 1,3-butadiene with neodymium-based catalytic systems

Abstract

Coordinative chain transfer polymerization (CCTP) of 1,3-butadiene was assessed by employing several traditional Ziegler–Natta type Nd-based catalytic systems. Both the types of alkylaluminum as CTA and chloride donor as third component significantly affected the chain transfer characteristic of the CCTP systems. Among the catalytic systems examined, Nd(OiPr)₃/Al(iBu)₂H/Me₂SiCl₂ and Nd(OiPr)₃/Al(iBu)₂H/Al₂Et₃Cl₃ systems exhibited the highest catalytic efficiency, yielding 6–10 polymer chains per Nd atom in the presence of 20 equiv. CTA. Kinetic examination revealed that Nd(OiPr)₃/Al(iBu)₂H/Me₂SiCl₂ and Nd(OiPr)₃/Al(iBu)₂H/Al₂Et₃Cl₃ catalytic systems proceeded with fully- and semi-reversible chain transfer reactions, respectively. Quantitative formation of polymers was observed in each step of the 1,3-butadiene seeding polymerization, indicating the living mode of the two catalytic systems. Moreover, the triblock copolymers, PBD-b-PIP-b-PBD and PBD-b-PIP-b-PCL, were successfully synthesized with Nd(OiPr)₃/Al(iBu)₂H/Me₂SiCl₂ catalytic system.

Introduction

Ziegler–Natta type catalytic systems based on neodymium salts represent an important class of catalysts and have been utilized industrially for 1,3-congjugated dienes polymerization in a large capacity due to their high catalytic activity and excellent stereoselectivity [1], [2], [3]. However, those catalysts are still suffering from the poor controllability over molecular weight, resulting in the polymers with rather broad molecular weight distribution (MWD) [4], [5]. It is reported that high cis-1,4 polydienes coupled with narrow MWD are desirable for high abrasion resistance, low heat buildup and high tensile properties [6], [7]. Living polymerizations [8], [9], [10], including anionic polymerization [11], [12], living coordination polymerization [13], [14], [15], [16], can feasibly realize controlled polymerization of 1,3-conjugated dienes. However, these living polymerization processes produce only one polymer chain per active metal centre, so searching for adaptive approaches to synthesize polydienes with controlled polymerization behaviors more expediently and economically is still imperative.

As an efficient alternative to living polymerization, the new concept of coordinative chain transfer polymerization (CCTP) has gathered an upsurge in research interest recently [17], [18], [19], [20]. Besides sharing some similar features with living polymerization, CCTP preserves various unprecedented advantages such as atom economy, controlling of statistical copolymers composition [21], [22], [23], [24], [25], [26], and design of new polymeric materials via chain shuttling polymerization [18], [27], [28], [29], [30]. CCTP involves the use of a single transition metal-based catalyst and a chain transfer agent (CTA). The heart of CCTP is the highly efficient, rapid and reversible chain transfer reaction between active transition metal-based propagating centers and CTA which is usually in the form of main group metal alkyl such as ZnR₂, MgR₂, and AlR₃ [31], [32]. In comparison with classical living polymerization, chain transfer to CTA allows the growth of several polymer chains per active species which has grabbed much attention from industrial point of view. Arriola et al. has derived a mathematical model and put forward three kinds of simulations in olefin polymerizations, that is, fully reversible chain transfer, semi-reversible chain transfer and irreversible chain transfer [32], [33]. Experiment examples for irreversible chain transfer [34] and reversible chain transfer polymerization [35], [36] suitable for the simulations have been accumulated, however, that for semi-reversible chain transfer polymerization has not been disclosed yet.

Tremendous advances of CCTP have been made in the field of olefin polymerization in the past decade, and innovative features such as well-controlling over the microstructure and architecture of polymers have emerged [37], [38]. The concept of CCTP was introduced to the field of conjugated dienes polymerization only recently and mainly focused on isoprene polymerization. For instance, Cp*La(BH₄)₂(THF)₂/EtMg(nBu) [25], [39], [40], [41], rac-[{Me₂C(Ind)₂}Y(1,3-(SiMe₃)₂–C₃H₃)]/ZnEt₂ (Al(iBu)₃ or Mg(nBu)₂) [26], and bis(allyl)[(C₅Me₄–C₆H₄-o-NMe₂)Gd (η³-C₃H₅)₂]/AlR₃ catalytic systems [42] can serve as efficient catalysts for CCTP of isoprene. In addition to these sophisticated lanthanide complexes based catalyst systems, traditional Ziegler–Natta type neodymium (Nd)-based catalytic systems are developed for the CCTP of isoprene by our group [43], [44]. However, the CCTP of 1,3-butadiene remained less explored. Trans-1,4 stereoselective chain transfer polymerization of butadiene was reported by using Nd(O-2,6-t-Bu₂-4-Me-Ph)₃(THF)/Mg(n-Hex)₂ system [45]. Cis-1,4 stereoselective chain transfer polymerization of butadiene was achieved by using Nd(vers)₃ based catalyst systems, however the polymerization rate was highly decreased in transfer conditions [46], [47]. Recently, chain shutting copolymerization of styrene, isoprene, and 1,3-butadiene was reported by using two different scandium catalysts with Al(iBu)₃ as chain shuttling agent [48]. Hence, it is necessary to explore new catalytic systems for CCTP of 1,3-butadiene to get a deep understanding of the polymerization mechanism.

In this contribution, we examined the ability of several typical Ziegler–Natta Nd-based catalytic systems for the CCTP of 1,3-butadiene. The effects of various chain transfer agents based on alkylaluminum and chloride donors on the chain transfer ability of catalytic systems were firstly assessed. The kinetic experiments were designed and carried out to examine the polymerization process with different catalytic systems. We report notably herein fully-reversible and semi-reversible coordinative chain transfer polymerizations of 1,3-butadiene, which have never been reported as far as we are aware. Moreover, the living metal-polybutadienyl species could further initiate block copolymerization, affording PBD-b-PIP-b-PBD and PBD-b-PIP-b-PCL triblock copolymers.