Cellulose‐Derived Functional Polyacetal by Cationic Ring‐Opening Polymerization of Levoglucosenyl Methyl Ether

Abstract The unsaturated bicyclic acetal levoglucosenyl methyl ether was readily obtained from sustainable feedstock (cellulose) and polymerized by cationic ring‐opening polymerization to produce a semicrystalline thermoplastic unsaturated polyacetal with relatively high apparent molar mass (up to ca. 36 kg mol−1) and decent dispersity (ca. 1.4). The double bonds along the chain can undergo hydrogenation and thiol–ene reactions as well as crosslinking, thus making this polyacetal potentially interesting as a reactive functional material.

The exploitation of fossil-based resources has given comfort and wealth to society at the expense of increasing atmospheric carbon dioxide concentration and other environmental hazards.T he rise in carbon dioxide concentration increases carbohydrate concentration, thereby reducing the overall content of protein in plants. [1] Moreover,the plastic industry is particularly dependent on fossil-based resources that exist in limited amounts,a nd the produced non-degradable plastics create many environmental problems.I ti st herefore important to move towards renewable feedstocks,v alorization of biomass,a nd environmentally degradable systems. [2] In this respect, biologically sourced polymers have been of interest among the scientific community to tackle the above-mentioned problems. [3] Cellulose,being the most abundant product of biomass on earth, is an attractive renewable,non-edible resource for the production of many value-added chemicals such as sugars, lactic acid, levulinic acid, or furans. [4] Another molecule with relatively complex bicyclic structure that can be obtained through the pyrolysis of cellulose is levoglucosenone (1, Scheme 1). [5] Nowadays,l evoglucosenone is produced in industrial quantities (50 tons per year) by the Circa Group Ltd.,A ustralia, and the derivative dihydrolevoglucosenone (Cyrene) has been launched as an environmentally friendly solvent to replace dipolar aprotic solvents like N-methyl-2pyrrolidone (NMP). [6] Levoglucosenone is used for the synthesis of chiral therapeutic agents and molecules with fixed and known stereochemistry, [7] however it has not yet entered the field of polymers.
Free radical or anionic polymerizations of 1 has only produced oligomers at best. [8] Its alcohol derivative levoglucosenol (2,S cheme 1), on the other hand, was found to polymerize through ring-opening olefin metathesis polymerization (ROMP) to yield an amorphous thermoplastic polyacetal. [8] Levoglucosenol should, on af irst glance,a lso polymerize via the acetal functionality through cationic ring-opening polymerization (CROP). In fact, molecules with similar or related structures,t hat is,a nhydrosugars [9] and bicyclic ketals, [10] have been polymerized successfully through Lewis acid-catalyzed CROP.H owever,a ttempts to polymerize levoglucosenol (2)t hrough CROP failed. We therefore decided to mask the hydroxy function of levoglucosenol by methylation to yield the levoglucosenyl methyl ether 3 (IUPAC name:4-methoxy-6,8-dioxabicyclo[3.2.1]oct-2-ene;S cheme 1). CROP of 3 would then give the linear unsaturated polyacetal 4 with the proposed chemical structure shown in Scheme 1, which is potentially degradable [11] and could be further modified or crosslinked. [12] It is worth being mentioned that 3,l ike its precursor 2, [8] can also be polymerized through ROMP (preliminary data, not shown).
Theo verall synthetic procedure for the levoglucosenyl methyl ether 3 is shown in Scheme 1. Levoglucosenone (1)is reduced by sodium borohydride in water, and the resulting levoglucosenol (2)isthen deprotonated with sodium hydride and methylated with methyl iodide to yield 3 (see the experimental procedures in the Supporting Information). Purification of 3 was achieved by distillation, and the overall yield was 81 %. Thechemical structure of 3 was confirmed by nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS;see Supporting Information) to be (1S,4S,5R)-4-methoxy-6,8dioxabicyclo[3.2.1]oct-2-ene (major isomer, 96 %). Notably, the synthesis of 3 is far less complicated and tedious than that of other sugar-based monomers for ROP. [13] Scheme 1. Synthesis of levoglucosenyl methyl ether 3,s tarting from levoglucosenone 1 via levoglucosenol 2,and polymerization through CROP to yield the polyacetal 4.
First attempts to polymerize 3 involved the use of triflic acid (CF 3 SO 3 H, TfOH) and boron trifluoride etherate (BF 3 ·OEt 2 ). Polymerizations were conducted in dichloromethane (DCM) solution at room temperature or 0 8 8Cf or 24 h and were quenched with triethylamine;r esults are summarized in Table 1. TfOH appeared to be avery efficient initiator for the CROP of 3.M onomer conversion (x p )r eached more than 90 %u nder the chosen conditions,t hough as lightly higher molar mass polymer 4 [M n app = 18.6 kg mol À1 , = 1.4; by size exclusion chromatography (SEC)] was obtained at lower temperature.The attempted polymerizations of 1 and 2 with TfOH in DCM solution at room temperature failed; either no reaction occurred or yet unidentified organic compounds were produced.
Polymer 4 was found to be soluble in DCM, chloroform, tetrahydrofuran (THF), and acetonitrile but insoluble in diethyl ether,d imethyl sulfoxide (DMSO), methanol, and water. Its chemical structure and optical activity were confirmed by NMR and circular dichroism (CD) spectroscopy (see Figure 1a nd the Supporting Information). Importantly, the polymer chains contain exclusively one sequence isomer (as evidenced by the sharp singlet signals in the 13 CNMR spectrum, Figure 1b)a nd the double bonds were fully retained.
Polymer 4 is as emicrystalline thermoplastic, showing ag lass transition at around 35 8 8Ca nd melting transitions at 40-120 8 8C, and is thermally stable up to around 220 8 8C, as determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA;s ee Figure 2a and the Supporting Information). Thes emicrystalline nature of the polymer was also seen by polarized optical microscopy (POM;F igure 2b). Furthermore,t he polymer was found to degrade quickly,within several hours,inDCM solution in the presence of BF 3 ·OEt 2 and methanol as an ucleophile,a s expected (see the Supporting Information). BF 3 ·OEt 2 also catalyzes the polymerization, but rather high catalyst concentrations (20 mol %w ith respect to monomer) are required to attain high monomer conversion (entry 4inT able 1). Theneed for ahigh loading of BF 3 ·OEt 2 can be explained by the fact that trace amounts of water are crucial to initiate the polymerization. This can be supported by previous studies demonstrating that BF 3 etherate failed to polymerize trioxane in rigorous dry conditions. [14] In addition, the achieved molar mass of the polymer is not linearly related to the amount of catalyst and no polymerization occurred at low catalyst concentration (entry 3i nT able 1). This is likely due to the equilibrium nature of the reaction between water and BF 3 ·OEt 2 .Ahigher amount of BF 3 ·OEt 2 is needed to shift the equilibrium to the right-hand side to produce protons as the initiating species (Scheme 2).
Thep olymerization of 3 is believed to proceed via an oxonium ion through an active-chain-end mechanism (Scheme 2). Thea ctivation of the cyclic ether ring by acid catalyst leads to the opening of the bicyclicr ing followed by stabilization of the anomeric carbocation via the formation of an oxonium ion. Successive attack of the monomer by OCH 2 ,   Table 2) and b) POM image (crossedp olarizers, scale bar = 5 mm) of apolymer film after heating to 120 8 8Cand slowly cooling down to room temperature (crystals started to form at ca. 57 8 8C). which is more nucleophilic than the competing OCH, should essentially lead to the formation of the polymer 4 with at hermodynamically favorable six-membered ring structure. Although TfOH was found to be the more effective initiator,BF 3 ·OEt 2 was easier to handle and therefore chosen as the catalyst for further screening experiments.P olymerizations of 3 with BF 3 ·OEt 2 were conducted at different monomer-to-catalyst ratios (10:1 to 10:3), monomer concentrations (3 or 4 m), reaction temperatures (À50 to 25 8 8C), and times (1.5 to 48 h);r esults are summarized in Table 2. The highest monomer conversion (97 %) and polymer molar mass (28.8 kg mol À1 )w ere obtained with ah igh catalyst loading (30 mol %) at at emperature of À10 8 8Ci nD CM solution (entry 9i nT able 2). Thep olymer 4 exhibited am onomodal molar mass distribution with adispersity of 1.4.
As tudy of the kinetics of the polymerization of 3 (Figure 3a)r evealed that the monomer was quickly con-sumed within less than 1h our,f ollowing pseudo-first-order kinetics,b ut leveled off thereafter.T he apparent numberaverage molar masses (M n app )increased constantly to around 36 kg mol À1 (x p = 78 %) but decreased at very high monomer conversions (x p > 90 %, Figure 3b), probably due to chaintransfer and back-biting reactions (which are often observed for cationic polymerizations of cyclic ethers [15] )p roducing new growing chains. [16] Nevertheless,a ll polymer samples showed amonomodal and fairly narrow molar mass distribution ( Figure 3c and the Supporting Information).
As mentioned above,the polymerization did not affect the olefin functionality of the carbohydrate rings.H owever,w e noticed that polymer 4 underwent crosslinking,e ven when stored at À20 8 8C, which could be avoided by the exclusion of oxygen or the addition of traces of ar adical inhibitor,f or example,butylated hydroxytoluene (BHT). It is thought that the allylic ether units in the polymer can form peroxide with atmospheric oxygen, [17] and this peroxide can potentially act as radical initiator for the olefin crosslinking.F urthermore, the double bonds in 4 are amenable to modification through thiol-ene reactions and hydrogenation, for example (Scheme 3). [18] Ther adical additions of methyl 3-mercaptopropionate using either azobisisobutyronitrile( AIBN) as ar adical source at 80 8 8Co rb enzophenone/UV light at room temperature were quantitative (polymer 5), as indicated by the complete disappearance of olefin protons in 1 HNMR spectra (see the Supporting Information). Thehydrogenation of 4 with H 2 /Pd-C (polymer 6)was almost quantitative,giving 93 %c onversion of double bonds (see the Supporting Information). Spontaneous crosslinking,a se arlier observed    for 4,did not happen. Theseemingly high reactivity of the 1,2disubstituted cis olefin towards crosslinking and functionalization makes polymer 4 potentially interesting as ar eactive functional material.
In summary,l evogluconsenyl methyl ether was obtained in two efficient steps from levoglucosenone (derived from cellulose) and successfully polymerized through CROP with either TfOH or BF 3 ·OEt 2 to near quantitative conversion. Theresulting semicrystalline thermoplastic unsaturated polyacetals exhibited molar masses (M n app )o fu pt o3 6kgmol À1 with ad ispersity of around 1.4. Thep olymer readily underwent crosslinking and chemical modification through radical thiol-ene reactions or hydrogenation. This cellulose-based monomer/polymer system (and derivatives thereof) is potentially interesting to generate ap latform of reactive and degradable (co-)polyacetals or complex macromolecular architectures. [19] Further studies in this line are in progress, together with optimization of the reaction conditions to achieve aliving/controlled (co-)polymerization, preferably by photochemical processes.