Scalable and continuous access to pure cyclic polymers enabled by ‘quarantined’ heterogeneous catalysts

Abstract

Cyclic polymers are topologically interesting and envisioned as a lubricant material.However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recycle a used catalyst for the synthesis of cyclic polymers. Unfortunately, it is demanding because of the catalyst's vulnerability and inseparability from polymers, which depreciates the practicality of the process. Here, we develop a continuous process streamlined in a circular way that polymerization, polymer separation, and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. It is enabled by introducing silica-supported ruthenium catalysts and a newlydesigned glassware. Also, different depolymerization kinetics of the cyclic polymer from its linear analogue is discussed. This process minimizes manual labor, maximizes security of vulnerable catalysts, and guarantees purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased reusability of precious catalysts (≥415,000 turnovers).The economy of lubrication is encumbered by a high replacement cost of lubricants in many applications 1,2 . One of its countermeasures is to increase the lifespan of lubricants, thereby decreasing the replacement frequency. The most common synthetic lubricant by far is polyalphaolefin 3-5 , which gradually loses its viscosity due to permanent chain scissions over time 6 . Cyclic hydrocarbon polymers similar to polyalphaolefins or mineral oils (e.g. polyethylene, polypropylene, polybutadiene, etc.) 7,8 are tribologically interesting because the initial chain scission of cyclic topology increases its viscosity by producing an opened linear topology with a higher chain volume [9][10][11] . This feature of cyclic polymers is envisioned as a viscosity modifier to prolong the lubricant lifetime. Since our discovery of the ring expansion route to cyclic polymers in 2002 11 , we [12][13][14][15][16] and other groups [17][18][19][20][21][22][23][24][25][26] have done exciting research on more functionalized and purer cyclic polymers.One of the most important needs is the development of a practical synthetic process to produce pure cyclic polymers on a larger scale for testing in many applications. To date, all the reported synthetic protocols were operated on a milligram scale in solution by homogeneous catalysis, which was accompanied by rigorous ex situ processes for polymer purification without actual catalyst recovery (Fig. 1a) [24][25][26][27] . Nonetheless, the residual metal catalysts remained at a few hundred ppm level 12 ; in this conventional process, it is difficult not only to obtain an uncontaminated white polymer but also to recycle the precious metal catalysts.

Topology Matters: Conformation and Microscopic Dynamics of Ring Polymers

“…In the past decades a huge number of different techniques was developed for the synthesis of ring-shaped polymers covering a large variety of different monomers. Mainly two synthetic strategies were employed: the ring closure technique and the ring expansion technique − (Figure ). In the ring closure approach, first linear precursors are synthesized using living or controlled polymerization techniques, which also allow for equipping the chain ends with functional groups.…”

Functionalized Cyclic Polymers and Network Gels

“…Recently, Grubbs et al reported the silica-supported ruthenium heterogeneous catalyst for the bulk REMP of cyclopentene, allowing catalyst recovery. 45 These catalysts have provided access to cyclic variants of commercially relevant and historically important polymers, such as poly(propylene), 14 poly(norbornene), 39−42 poly(methyl-1-pentene), 46 poly-(phenylacetylene), 47−49 and poly(acetylene). 50 Interestingly, upon doping, cyclic poly(acetylene) showed conductivity on the higher end of nonstretch aligned linear poly(acetylene), and cyclic poly(methyl-1-pentene) exhibited higher T g compared to its commercial linear counterpart while maintaining remarkable optical clarity.…”

“…Subsequent advances have led to homogeneous tantalum, tungsten, − molybdenum, ruthenium, and vanadium , catalysts suitable for scalable cyclic polymer synthesis by REMP. Recently, Grubbs et al reported the silica-supported ruthenium heterogeneous catalyst for the bulk REMP of cyclopentene, allowing catalyst recovery . These catalysts have provided access to cyclic variants of commercially relevant and historically important polymers, such as poly­(propylene), poly­(norbornene), − poly­(methyl-1-pentene), poly­(phenylacetylene), − and poly­(acetylene) .…”

Improved Trianionic Pincer Ligand Synthesis for Cyclic Polymer Catalysts

“…Tailored transition metal catalysts featuring a tethered metal–carbon multiple bond, or in one case a metallacyclobutane, are initiators for ring expansion metathesis polymerization (REMP). − Cyclic alkenes, most commonly norbornene, − , undergo ring expansion with tethered-metal carbon double bonds to give cyclic polymers. REMP of cyclic alkenes provides cyclic polymers with stereoregular chains, − bottlebrushes, , gels, , and/or cross-linked networks .…”