Highly Efficient Cationic Polymerization of β-Pinene, a Bio-Based, Renewable Olefin, with TiCl4 Catalyst from Cryogenic to Energy-Saving Room Temperature Conditions

Abstract

Polymers based on renewable monomers are projected to have a significant role in the sustainable economy, even in the near future. Undoubtedly, the cationically polymerizable β-pinene, available in considerable quantities, is one of the most promising bio-based monomers for such purposes. In the course of our systematic investigations related to the catalytic activity of TiCl4 on the cationic polymerization of this natural olefin, it was found that the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4/N,N,N′,N′-tetramethylethylenediamine (TMEDA) initiating system induced efficient polymerization in dichloromethane (DCM)/hexane (Hx) mixture at both −78 °C and room temperature. At −78 °C, 100% monomer conversion was observed within 40 min, resulting in poly(β-pinene) with relatively high Mn (5500 g/mol). The molecular weight distributions (MWD) were uniformly shifted towards higher molecular weights (MW) in these polymerizations as long as monomer was present in the reaction mixture. However, chain–chain coupling took place after reaching 100% conversion, i.e., under monomer-starved conditions, resulting in considerable molecular weight increase and MWD broadening at −78 °C. At room temperature, the polymerization rate was lower, but chain coupling did not occur. The addition of a second feed of monomer in the polymerization system led to increasing conversion and polymers with higher MWs at both temperatures. 1H NMR spectra of the formed polymers indicated high in-chain double-bond contents. To overcome the polarity decrease by raising the temperature, polymerizations were also carried out in pure DCM at room temperature and at −20 °C. In both cases, rapid polymerization occurred with nearly quantitative yields, leading to poly(β-pinene)s with Mns in the range of 2000 g/mol. Strikingly, polymerization by TiCl4 alone, i.e., without any additive, also occurred with near complete conversion at room temperature within a few minutes, attributed to initiation by adventitious protic impurities. These results convincingly prove that highly efficient carbocationic polymerization of the renewable β-pinene can be accomplished with TiCl4 as catalyst under both cryogenic conditions, applied widely for carbocationic polymerizations, and the environmentally benign, energy-saving room temperature, i.e., without any additive and cooling or heating. These findings enable TiCl4-catalyzed eco-friendly manufacturing of poly(β-pinene)s, which can be utilized in various applications, and in addition, subsequent derivatizations could result in a range of high-added-value products.

A Bio-Based Tackifier Synthesized by Room-Temperature Cationic Copolymerization of Isobutene and β-Pinene

“…Many catalytic systems for the cationic polymerization of β-pinene have been studied thus far [4][5][6][7][8][9][10][11][12][13][14]. For example, living polymerization was achieved using a titanium system with ammonium chloride additive [4].…”

“…For example, living polymerization was achieved using a titanium system with ammonium chloride additive [4]. Later, a slightly modified titanium system was reported to promote quasi-living polymerization at ambient temperatures [5]. As for group 13 compounds, aluminum catalysts tend to yield high molecular weight poly(βpinene), especially when a difunctional initiator was applied [6][7][8].…”

Stereoregular poly(2-phenylthiirane) via cationic ring-opening polymerization

“…Initially, a series of Lewis- or Brønsted-acids, which are effective for the cationic polymerization of olefins or other heterocyclic monomers, 24,31 were screened for the CROP of PT (Table S1, ESI†). With a feed ratio of PT/acid = 1000/1 at 25 °C, the CROP of PT exhibits high efficiency, where PT was almost fully consumed within 0.3 h for BF 3 ·Et 2 O-catalyzed ROP of PT.…”