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Dual-Initiating and Living Frustrated Lewis Pairs ARTICLE https://doi.org/10.1038/s41467-021-25069-6 OPEN Dual-initiating and living frustrated Lewis pairs: expeditious synthesis of biobased thermoplastic elastomers ✉ Yun Bai1, Huaiyu Wang1, Jianghua He1, Yuetao Zhang 1 & Eugene Y.-X. Chen 2 Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL), an acrylic polymer bearing a cyclic lactone ring, has attracted increasing interest because it not only is biorenewable but 1234567890():,; also exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). However, such property enhancement has been limited to resistance to heat and solvent, and mechanically both types of polymers are equally brittle. Here we report the expeditious synthesis of well-defined PMMBL-based ABA tri-block copolymers (tri-BCPs)— enabled by dual-initiating and living frustrated Lewis pairs (FLPs)—which are thermoplastic elastomers showing much superior mechanical properties, especially at high working tem- peratures (80–130 °C), to those of PMMA-based tri-BCPs. The FLPs consist of a bulky organoaluminum Lewis acid and a series of newly designed bis(imino)phosphine superbases bridged by an alkyl linker, which promote living polymerization of MMBL. Uniquely, such bisphosphine superbases initiate the chain growth from both P-sites concurrently, enabling the accelerated synthesis of tri-BCPs in a one-pot, two-step procedure. The results from mechanistic studies, including the single crystal structure of the dually initiated active spe- cies, detailed polymerizations, and kinetic studies confirm the livingness of the polymerization and support the proposed polymerization mechanism featuring the dual initiation and sub- sequent chain growth from both P-sites of the superbase di-initiator. 1 State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin, China. 2 Department of Chemistry, ✉ Colorado State University, Fort Collins, CO, USA. email: [email protected] NATURE COMMUNICATIONS | (2021) 12:4874 | https://doi.org/10.1038/s41467-021-25069-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-25069-6 xploring sustainable alternatives to the petroleum-based steps of the tri-BCP synthesis into an accelerated, one-pot, two- polymers has attracted intense attention due to the concerns step procedure. Success in developing this new method would E fi 1–5 about accelerated depletion of nite nature resources . then create PMMBL-based TPEs to explore performance- However, to realize the practical application of sustainable poly- advantaged properties, especially mechanical toughness much mers, it is insufficient to just synthesize polymers from biorenew- desired for acrylics, for such biobased polymers. able monomers, the biobased polymer should exhibit superior or Herein, we designed and synthesized four di-initiating orga- performance-advantaged properties to those of the petroleum-based nophosphorus superbases which, when combined with a bulky polymers for real-world applications4,6. As the cyclic analog of organoaluminum LA, enabled the efficient synthesis of well- methyl methacrylate (MMA), renewable methylene butyrolactone- defined tri-BCPs based on biorenewable MMBL at RT in a one- based vinylidene monomers, such as α-methylene-γ-butyrolactone pot, two-step process (Fig. 1). The structural characterization of (MBL) and γ-methyl-α-methylene-γ-butyrolactone (MMBL), are of the key reaction intermediate and kinetic studies of the poly- particular interest in exploring the prospects of substituting the merization revealed that these bisphosphines indeed serve as petroleum-based methacrylate monomers for sustainable polymer covalently linked di-initiators and thus initiate the chain growth production7–10. Compared to PMMA, these renewable methylene from both sides of the di-initiators concurrently. These BCPs butyrolactone-based polymers typically exhibited increased resis- appear to be the first example of MMBL-based tri-BCPs com- tance to common organic solvents, heat, and scratch11–13.Inpar- posed of both linear methacrylate and its cyclic analog (methy- ticular, their glass-transition temperature (Tg)valuesaremuch lene butyrolactone) units by a living polymerization strategy. higher than that of PMMA: 195 °C for poly(MBL) (PMBL); 227 °C Overall, this LP polymerization system utilizing bisphosphine- for PMMBL, 105 °C for PMMA14–18. Despite these enhanced based FLPs enabled the efficient, convenient synthesis of a series resistance to heat and solvent, much like PMMA, P(M)MBL of tri-BCP TPEs containing high Tg (227 °C) PMMBL as the two materials are mechanically brittle, which largely limited their outer hard blocks and low Tg (4 °C) poly(2-ethyoxyethyl metha- practical applications. It is well known that ABA type tri-block crylate) (PEEMA) as the soft midblock. Tensile testing results copolymers (tri-BCPs) composed of hard end block A and soft revealed performance advantages of the biobased PMMBL TPEs midblock B can self-assemble into thermoplastic elastomers (TPEs) over the petroleum-based PMMA TPEs, especially under high with a wide range of applications by combining thermoplastics’ working temperature conditions. processability with elastomers’ ductility and toughness19–21. Hence, we envisioned that renewable PMMBL could be employed as the Results and discussion high T hard segments for TPEs, which would not only solve the g Synthesis of iminophosphine superbase di-initiators.A brittleness issue, but also produce high-temperature TPEs by taking straightforwardmethodwasdevelopedforthesynthesisoffourbis advantage of its high T . However, it remains as a challenge to g (imino)phosphine superbases, with the same imidazoline framework synthesize PMMBL-based TPEs containing a soft methacrylate [P(NIiPr)Ph] but varied in the length (spacing) of the alkyl bridging block, because after the completion of MMBL polymerization, group -(CH ) -: n = 2, μEt;3,μPr;4,μBu;6,μHex (Fig. 1). Taking μBu the growing chain end is not nucleophilic enough to initiate the 2 n – [P(NIiPr)Ph] as an example, commercially available bisphosphine polymerization of less reactive methacrylate monomers22 24. 2 precursor Ph PC H PPh was treated with Li to generate the cor- Although PMBL-based TPEs are known, they were synthesized by 2 4 8 2 responding dilithium salt [PhPC H PPh]2-Li+ (THF) ,whichwas atom transfer radical polymerization (ATRP) involving pre- 4 8 2 4 subsequently reacted with PCl at −78 °C to furnish Ph(Cl)PC H P synthesis and isolation of Br-(soft-block) polymer-Br macro- 3 4 8 – (Cl)Ph; the ensuing reaction of this dichloride intermediate with the initiators for subsequent ATRP of MBL25 27.Sofar,weareunaware deprotonation product of imine (NIiPr)H treated by n-butyllithium of reports on PMMBL-based TPEs achieved by living polymeriza- (nBuLi) afforded the target product (Fig. 2a and Supplementary tion strategy. Information). NMR spectra confirmed the successful synthesis of all Owing to their unique, unquenched reactivities, frustrated four bis(imino)phosphine superbases (Supplementary Figs. 1–27), Lewis pairs (FLPs) not only demonstrated well-established uti- – and two of them, μBu[P(NIiPr)Ph] and μHex[P(NIiPr)Ph] ,havealso lities in small molecular chemistry28 36 but also exhibited unique 2 2 – been structurally verified by single-crystal X-ray diffraction applications in polymer synthesis37 47. By selecting a suitable (Fig. 2b, c). organoaluminum Lewis acid (LA) with appropriate steric hin- drance and acidity, we achieved the first example of living poly- merization of methacrylates by an authentic FLP48. The field of Characteristics of bisphosphine-based di-initiators in the Lewis pair (LP) polymerization has grown considerably over the model MMA polymerization. To identify the most efficient LP – past decade49 51. Recently, by fine-tuning of both the steric and system that can also prevent the backbiting chain-termination electronic properties of the organophosphorus superbase as the side reactions48,52, we examined the synergistic effects of the μHex i paring Lewis base (LB) in the FLP, we successfully synthesized combination of C6-bridged di-initiator [P(NI Pr)Ph]2 with ultra-high molecular weight PMMA with number-average five LAs with different acidity and steric hindrance on MMA molecular weight (Mn) up to 1927 kg/mol at room temperature polymerization (Fig. 1). They possessed descending acidity as 52 − (RT) and a series of sequence-controlled multiblock copolymers revealed by Gutmann Beckett method (Al(C6F5)3 (100%) > ≈ i i in 30 min at RT, including a tripentacontablock copolymers (BHT)2AlMe (86%) (BHT)2Al( Bu) (85%) > (BHT)Al( Bu)2 = = = 48,52 (n 53, k 4, dpn 50) with the record number of blocks to (74%) > AlMe3 (71%)) . It turned out that the LP composed of 53 date . It is known that, in a living polymerization system, the Al(C6F5)3 with the highest acidity achieved the highest poly- polymer architectures can be controlled by the structure of the merization rate. However, this LP system gave a low initiating – initiator54 57. Owing to the facile structural tunability of these efficiency (run 1, Table 1) and unsuccessful chain extension organophosphorus superbases58, 59, we hypothesized that speci- (Supplementary runs 1 and 2, Table 3; Supplementary Fig. 37), fically designed bisphosphine superbases could form a FLP with a presumably due to backbiting side reactions resulted from its high suitable LA, which would potentially serve as a dual-initiating and acidity that can effectively
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