Ring-Opening Metathesis Polymerization of Unsaturated Carbohydrate Derivatives: Levoglucosenyl Alkyl Ethers

Carbohydrate Derivatives: Levoglucosenyl Alkyl Ethers

Article

Ring-Opening Metathesis Polymerization of Unsaturated

Tapas Debsharma, Bernd Schmidt, Andr é Laschewsky, and Helmut Schlaad*

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sı Supporting Information

ABSTRACT: A series of biomass-derived levoglucosenyl alkyl

ethers (alkyl = methyl, ethyl, n-propyl, isopropyl, and n-butyl) were

synthesized and polymerized by ring-opening oleﬁn metathesis

polymerization using the Grubbs catalyst C793 at room temper-

ature. Polymerizations were successfully performed in conventional

solvents such as 1,4-dioxane and dichloromethane as well as in

polar aprotic “green” solvents such as 2-methyltetrahydrofuran,

dihydrolevoglucosenone (Cyrene), and ethyl acetate. The

prepared polyacetals with degrees of polymerization of ∼100

exhibit Schulz−Flory-type molar mass distributions and are

thermoplastic materials with rather low glass transition temperatures in the range of 43−0 °C depending on the length of the

alkyl substituent. Kinetic studies revealed that the polymerization proceeded rapidly to a steady state with a certain minimum

monomer concentration threshold. When the steady state was reached, just about half of the [Ru] catalyst had been eﬀective to

initiate the polymerization, indicating that the initiation step was a slow process. The remaining catalyst was still active and did no

longer react with monomers but with in-chain double bonds, cutting the formed polymer chains into shorter fragments. In the long

term, all catalyst was consumed and propagating [Ru] chain ends were deactivated by the elimination of [Ru] from the chain ends to

form inactive chains with terminal aldehyde groups.

INTRODUCTION

The demands for sustainable polymers are growing with the

ever-increasing concerns of the environmental impact of a

interfere with the catalyst. Following an alternative strategy,

carbohydrate monomers derived in four steps from glucose and

galactose, consisting of endocyclic alkene and terminal alkyne

moieties, were polymerized by a living cascade ene-yne

■

−3

fossil-resource-based economy.

Biomass-derived polymers

metathesis approach.

have the potential to overcome various concerns of crude oil-

derived polymers, such as limited availability, biocompatibility,

and degradability issues. Among the renewable sources, which

are explored for the synthesis of biobased polymers, cellulose is

the most abundant one on earth and therefore highly attractive

as a nonedible feedstock. An interesting unsaturated bicyclic

carbohydrate molecule produced from (waste) cellulose

feedstock in just one step is levoglucosenone (compound 1

in Scheme 1), which nowadays is made available in larger

quantities by the Circa Group Ltd., Australia, and which can be

easily reduced to levoglucosenol (2a). While the polymer-

ization of levoglucosenone failed, we have recently succeeded

to polymerize levoglucosenol by ring-opening oleﬁn metathesis

The ROMP of levoglucosenol was demonstrated to produce

high-molar mass polyacetals (3a), which exhibited thermo-

plastic behavior and were easily degradable by acid-catalyzed

hydrolysis. So far, the ROMP of levoglucosenol was only

successful with the mono-ortho-substituted N-heterocyclic

carbene derivative of the second-generation Grubbs catalyst

C793, while other ruthenium-based catalysts with sterically

more demanding ligands were not eﬀective. The polymer-

ization exhibited a nonliving character, as seen by incomplete

monomer conversions, nonlinear correlations between polymer

molar mass and conversion, and Schulz−Flory-type polymer

molar mass distributions. The polyacetals formed were chiral

but amorphous materials, supposedly due to a statistical

distribution of sequence isomers along the polymer chain,

polymerization (ROMP). Polymerization of any carbohydrate

derivatives by ROMP, taking advantage of unsaturated

carbohydrate heterocycles, was unreported prior to this, albeit

metathesis polymerization has been known for decades

applied to even functional monomers, including carbohydrate-

−9

and

Received: December 22, 2020

Revised: March 1, 2021

Published: March 12, 2021

functionalized norbornenes or norbornene derivatives of

1,12

levoglucosenone.

A reason for this might be the

combination of lacking ring strain in common unsaturated

carbohydrate derivatives, such as glycals or glycosyl derivatives,

and the high density of oxygen-containing moieties that may

2021 The Authors. Published by

American Chemical Society

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Scheme 1. Synthesis and Subsequent ROMP of Levoglucosenol 2a (R = H) and Levoglucosenyl Alkyl Ethers 2b−f (R =

Methyl, Ethyl, n-Propyl, Isopropyl, and n-Butyl) Using the Catalyst C793 in Diﬀerent Solvents to Yield the Polyacetals 3a−f

(

Simpliﬁed Structure, Diﬀerent Sequence Isomers Are Not Shown); (a) NaBH , H O, (b) (i) NaH, THF, (ii) Alkyl Iodide,

4 2

THF, and (c) C793, 1,4-Dioxane or Other Solvent, Room Temperature

suggesting a low regioselectivity of the catalyst system.

Notwithstanding the current limitations, the successful

production of levoglucosenol-based polyacetals has opened a

new ﬁeld of thermoplastic polymer materials with a highly

sustainable property proﬁle.

MHz, Bruker AVANCE 500 MHz, or Bruker AVANCE III 600 MHz

^s^p^e^c^t^r^o^m^e^t^e^r^s^a^t^r^o^o^m^t^e^m^p^e^r^a^t^u^r^e^.^S^a^m^p^l^e^s^w^e^r^e^p^r^e^p^a^r^e^dⁱⁿ^C^D^C^l3

^o^r¹^,⁴^-^dⁱ^o^x^aⁿ^e^-^d8 [hexamethyldisilane (HMD) was added as the

internal standard]. H NMR signals were referenced to the solvent

peak at δ 7.26 ppm (CDCl ), δ 3.53 ppm (1,4-dioxane-d ), or δ 0

ppm (HMD).

In this work, we explore other bicyclic monomers based on

levoglucosenol, namely, levoglucosenyl alkyl ethers (2b−f,

alkyl = methyl, ethyl, n-propyl, isopropyl, and n-butyl), and

also “green” solvents for the polymerization via ROMP (see

Scheme 1). We show that, as for levoglucosenol, speciﬁcally

the catalyst C793 enables a polymerization of the levogluco-

senyl alkyl ethers to aﬀord thermoplastic polyacetals (3b−f)

with tunable glass transition temperatures. In analogy to

levoglucosenol, the ROMP of the new monomers also showed

the features of a nonliving polymerization. To clarify the scope

and current limits of the synthesis of levoglucosenol-based

polymers, we exemplarily explored the kinetics and mechanism

of the ROMP of levoglucosenyl methyl ether in more detail.

Size exclusion chromatography (SEC) with simultaneous UV and

RI detection was performed with THF as the eluent (a ﬂow rate of 0.5

−1

mL·min ) at room temperature. The stationary phase was a 300 × 8

mm PSS SDV linear M column (3 μm particle size, molar mass range

10 −10 Da). Solutions containing ∼0.15 wt % polymers were ﬁltered

through 0.45 μm PTFE ﬁlters; the injected volume was 100 μL.

Polystyrene standards (PSS, Mainz, Germany) were used for

calibration.

Thermogravimetric analysis (TGA) was performed with a Mettler

Toledo TGA/SDTA851 in a temperature range from 25 to 900 °C at

−1

a heating rate of 10 °C min under a continuous nitrogen ﬂow of 20

−1

mL min .

Diﬀerential scanning calorimetry (DSC) was measured on a

Mettler Toledo DSC822e in a temperature range from −80 to 150 °C

−

under a continuous nitrogen ﬂow of 20 mL min . Glass transition

temperatures were determined from the third heating curves at a

heating rate of 30 °C min .

EXPERIMENTAL SECTION

■

−1

Chemicals. Levoglucosenone (CAS number 37112-31-5, 99.3%

purity) and dihydrolevoglucosenone (Cyrene, 99.5%) were provided

by Circa Group Ltd, Australia. Sodium borohydride (98%),

iodomethane (99%), iodoethane (99%), 1-iodopropane (99%), 1-

bromobutane (99%), tetrabutylammonium iodide (98%), ethyl vinyl

ether (99%), 1,4-dioxane (99.8%, anhydrous), 2-methyltetrahydrofur-

an (2-MeTHF, 99%, anhydrous), and ethyl acetate (99.8%,

anhydrous) were purchased from Acros Chemicals. Grubbs catalyst

C793 [1,3-bis-(2-methylphenyl)-2-imidazolidinylidene]-dichloro-

benzylidene)-(tricyclohexylphosphine)ruthenium(II) (CAS number

27429-60-5), 2-iodopropane (99%), and tetrahydrofuran (THF,

9.5% anhydrous) were purchased from Sigma-Aldrich. Dichloro-

methane (DCM, analytical grade), methanol (analytical grade), and

cyclohexane (analytical grade) were purchased from Fischer Scientiﬁc.

RESULTS AND DISCUSSION

■

As reported previously, the ROMP of levoglucosenol 2a was

only successful with the mono-ortho-substituted N-hetero-

cyclic carbene derivative of the second-generation Grubbs

catalyst C793 (Scheme 1). Preferably, the polymerization was

conducted in 1,4-dioxane solution owing to the poor solubility

of the poly(levoglucosenol) 3a in other solvents typically

employed for oleﬁn metathesis, such as DCM or toluene, at

room temperature. While the polymerization of 2a proceeded

rapidly within the ﬁrst 30 min, it leveled oﬀ thereafter at a

maximum monomer conversion of ∼60% (Figure 1A).

Furthermore, polymer 3a exhibited a Schulz−Flory-type

molar mass distribution with a dispersity of around 2 (Figure

1C). These features demonstrated the nonliving character of

the polymerization system. The polymerization kinetics might

indicate that the reaction takes place under thermodynamic

and not under kinetic control as is characteristic for most

chain-growth polymerizations. Alternatively, it could indicate

(

Chloroform-d (99.8% D) and 1,4-dioxane d (99% D) were purchased

from Deutero GmbH and Eurisotop, respectively.

Levoglucosenol 2a was synthesized by the reduction of

levoglucosenone 1 with sodium borohydride. The levoglucosenyl

alkyl ethers 2b−f were synthesized by the alkylation of 2a with the

respective alkyl iodides, see the Supporting Information.

Analytical Instrumentation. Nuclear magnetic resonance

NMR) spectra were recorded with Bruker AVANCE Neo 400

(

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Figure 2. ROMP of levoglucosenyl alkyl ethers 2b−f in 1,4-dioxane at

room temperature ([2b−f] = 4 mmol per mL solvent, [2b−f] /

Figure 1. (A) Time-conversion plot for the ROMP of levoglucosenol

a in 1,4-dioxane ([2a] = 4 mmol per mL solvent, [2a] /[C793] =

[

C793] = 100): (A) time-conversion plots [monomer conversions

100, room temperature; monomer conversions were determined by

were determined by H NMR analysis (solvent: CDCl ) of crude

the H NMR analysis of crude reaction solutions after quenching with

ethyl vinyl ether); the line is to guide the eye. (B) Evolution of the

apparent number-average degrees of polymerization and dispersity

indices (numbers in parentheses), as determined by SEC (eluent:

NMP) with polystyrene calibration, as a function of monomer

conversion. The theoretical degree of polymerization (dashed line) is

calculated by ([2] /[C793] ) × monomer conversion, as it would be

reaction solutions after quenching with ethyl vinyl ether] (lines are to

guide the eye) and (B) evolution of the apparent number-average

degrees of polymerization, as determined by SEC (eluent: THF) with

polystyrene calibration, as a function of monomer conversion. The

dashed line indicates the theoretical degrees of polymerization as

calculated for a living chain-growth polymerization. (C) SEC-RI

traces of precipitated polymers 3b−f obtained after a reaction time of

expected for a living chain-growth polymerization. (C) SEC-RI trace

of polymer 3a obtained after a reaction time of 24 h. Data are taken

from ref 6 and replotted to compare with the polymerizations of

levoglucosenyl alkyl ethers 2b−f.

24 h.

successfully in all these solvents (Figure 3). Evidently, all the

polymerizations of 2b−f in 1,4-dioxane and of 2b in the

various solvents followed virtually the same general kinetics,

which are very similar to that of 2a (Figure 1). The

polymerizations reached maximum monomer conversion

thresholds of about 70−80% within about 60−120 min. The

polymerization rates decreased slightly with an increase in the

size of the alkyl ether substituent of the monomers 2b−f but

were virtually unaﬀected by the selected solvent. In all cases,

the apparent degrees of polymerization were larger than

theoretically expected and nonlinearly correlated with the

monomer conversion. The ﬁnal polymers 3b−f exhibited

Schulz−Flory-type molar mass distributions (Đ ∼ 2) similar to

that observed for 3a.

the occurrence of unimolecular termination reactions, whereby

the maximum monomer conversion would be determined by

the relative apparent rate constant of propagation to

termination (see below). The apparent number-average

molar masses or degrees of polymerization of 3a, as

determined by SEC with polystyrene calibration, were found

to be up to ∼10 times larger than the theoretical values and did

not correlate linearly with the monomer conversion (Figure

B). These observations possibly indicate an incomplete and/

or slow initiation of growing chains by the catalyst in

combination with termination and/or back-biting reactions.

Applying the same conditions as for 2a (see the caption in

Figure 1), the levoglucosenyl alkyl ethers 2b−f, which were

synthesized by the straightforward alkylation of levoglucosenol

The poly(levoglucosenyl alkyl ether)s 3b−f are thermally

stable up to 240−285 °C, as determined by TGA, and are

amorphous with glass transition temperatures in the range of

2a with alkyl iodides (see the Supporting Information), were

polymerized with C793 in the 1,4-dioxane solution to give the

T ∼ 43 to 0 °C, as determined by DSC (see the Supporting

corresponding poly(levoglucosenyl alkyl ether)s 3b−f (Figure

Information ). As expected, the glass transition temperatures of

). The molecular structures of the produced polyacetals 3b−f

3b−f are lower than that of poly(levoglucosenol) 3a (T ∼ 100

were con ﬁ rmed by H, C, and 2D NMR spectroscopy (see

the Supporting Information). Unlike the parent polymer 3a,

the polymers 3b−f exhibited an enhanced solubility and were

soluble not only in 1,4-dioxane but also in DCM, which is a

preferred solvent for metathesis reactions, as well as in a

number of polar aprotic “green” solvents, such as 2-MeTHF,

dihydrolevoglucosenone (Cyrene), or ethyl acetate (Scheme

°C) and they continuously decrease with an increase in the

length of the n-alkyl substituent, that is, T ∼ 43 °C for 3b (R

= methyl), 32 °C for 3c (ethyl), 8 °C for 3d (n-propyl), and 0

°C for 3f (n-butyl). The glass transition temperature of 3e

(isopropyl) is T ∼ 35 °C and thus considerably higher than

that of 3d, which might be attributed to a reduced ﬂexibility of

the polymer chain. Notably, this behavior is analogous to that

). The polymerization of exemplarily 2b could be performed

of poly(alkyl styrene)s or poly[alkyl (meth)acrylate]s.

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concentrations were performed in situ in NMR tubes at room

Article

temperature using the solvent 1,4-dioxane-d . An exemplary H

NMR spectrum of a mixture of 2b/C793 recorded at an

intermediate reaction time is shown in Figure 4 (see also the

Supporting Information ). Monomer conversions were deter-

mined by considering the integrals of characteristic oleﬁnic

proton signals of the monomer 2b (δ 6.0 ppm, 1H, and 5.6

ppm, 1H) and of the polymer 3b (δ 5.7 ppm, 2H). Number-

average degrees of the polymerization of 3b were calculated by

end-group analysis from the relative signal intensities of the

phenyl group (δ 7.4 ppm, 2H) at the α-chain end and of the

oleﬁn protons of the repeat units. The [Ru]CH-phenyl

signal of the catalyst was observed at δ 19.1 ppm (see the H

NMR spectrum of the catalyst in the Supporting Information ).

Multiple signals were detected at δ 18.5−16.5 ppm, which

were attributed to active [Ru]CH chain ends.

The time-conversion plots (Figure 5A,E) obtained for

diﬀerent monomer (2b) and catalyst (C793) concentrations

showed the characteristic features of initial fast monomer

consumption, which leveled oﬀ after ∼100 min approaching a

maximum monomer conversion. The value of the maximum

monomer conversion depended on the initial monomer

concentration (Figure 5A) but was independent of the catalyst

concentration (Figure 5E). These ﬁndings exclude unim-

olecular termination reactions as a possible explanation for this

kinetic behavior, for which it is expected that the maximum

monomer conversion correlates with the catalyst concentration

but not with the monomer concentration. The results rather

suggest that the polymerization proceeded rapidly to a steady

state when having reached a certain threshold value of the

minimum residual monomer concentration. This steady state

could possibly be explained in terms of a reversible

polymerization system with a low ceiling concentration or

temperature, in which polymerization and depolymerization

occur at the same rates. If so, the treatment of an isolated

polymer 3b with the catalyst should result in a cleavage and

activation of polymer chains and, at a suﬃciently low polymer

concentration, reformation of the cyclic monomer 2b. Indeed,

this was observed for a mixture of 3b (0.07 mmol monomer)

Figure 3. ROMP of levoglucosenyl methyl ether 2b in diﬀerent

solvents (1,4-dioxane, DCM, 2-MeTHF, Cyrene, and ethyl acetate) at

room temperature ([2b] = 4 mmol per mL solvent, [2b] /[C793] =

00): (A) time-conversion plots [monomer conversions were

determined by H NMR analysis (solvent: CDCl ) of crude reaction

solutions after quenching with ethyl vinyl ether] (lines are to guide

the eye) and (B) evolution of the apparent number-average degrees of

polymerization, as determined by SEC (eluent: THF) with

polystyrene calibration, as a function of monomer conversion. The

dashed line indicates the theoretical degrees of polymerization as

calculated for a living chain-growth polymerization. (C) SEC-RI

traces of precipitated polymer 3b obtained after a reaction time of 24

Kinetic and Mechanistic Study. Remarkably, all polymer-

izations of 2a−f in various solvents showed very similar

features. We therefore chose to examine the kinetics of the

polymerization of 2b with C793 in 1,4-dioxane, as a

representative example, in more detail by H NMR spectros-

and C793 (0.014 mmol) in 1,4-dioxane-d (0.5 mL) by

copy. Kinetic experiments for diﬀerent monomer and catalyst

analysis with H NMR spectroscopy and SEC (see the

Supporting Information ).

Figure 4. H NMR spectrum (400 MHz) of the polymerization mixture of 2b (2 mmol)/C793 (0.4 mmol) in 1,4-dioxane-d (0.5 mL) after a

reaction time of 90 min at room temperature. [Signal at δ 0.0 ppm originates from hexamethyldisilane (HMD) as the internal standard.]

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Figure 5. ROMP of levoglucosenyl methyl ether 2b with C793 in 1,4-dioxane-d at room temperature for diﬀerent monomer (A−D) and catalyst

concentrations (E−H): (A,E) time-conversion plots (monomer conversions were determined by H NMR analysis), (B,F) consumption of the

catalyst with time, as determined by the depletion of the H NMR signal of the catalyst at δ = 19.1 ppm, (C,G) evolution of the number-average

degree of polymerization, as determined by H NMR end-group analysis, as a function of monomer conversion (the theoretical degrees of

polymerization, as calculated for a living chain-growth polymerization, are indicated by dashed lines in the respective colors), and (D,H) SEC-RI

traces of the crude polymer 3b produced at the end of the kinetic experiments (the peak at 11.8 mL is from the catalyst).

According to the evolution of the [Ru]CH signal with

time, the rate at which the catalyst was consumed to initiate

the polymerization of 2b was comparatively slow with a half-

value period of 100−500 min (Figure 5B,F). The rate of

initiation depended on both the initial monomer and catalyst

concentrations: it was the faster, the higher was the monomer

concentration (Figure 5B), and the lower was the catalyst

concentration (Figure 5F). Amazingly, only about half of the

catalyst had been used for the initiation of chains when the

polymerization came to a stop at the maximum monomer

conversion. The remaining catalyst was still reactive and was

slowly consumed, however, without noticeable consumption of

monomer. These ﬁndings further support our earlier

hypothesis (see above) that the polymerizations reached

steady-state conditions. Supposedly, if no more with the

monomer, the catalyst then reacted with the in-chain double

bonds of the produced polymer, resulting in a cleavage of

chains into smaller fragments, thus equilibrating the molar

mass distribution. Notably, smaller polymer fragments could

also be produced by inter- or intramolecular equilibration

reactions of the active [Ru] chain ends with in-chain double

bonds. An intermolecular exchange reaction between two

active chains produces two chains with diﬀerent lengths but

the same end groups, while an intramolecular back-biting

releases an active linear chain and an inactive macrocycle

lacking the benzylidene end group. However, the reaction of

the catalyst with an in-chain double bond of a macrocycle

would lead to a regeneration of an active linear chain carrying

benzylidene and [Ru] end groups.

The slow initiation and the fragmentation processes must

both be reﬂected in the evolution of the number-average

degree of polymerization with monomer conversion (Figure

5C,G). The degrees of polymerization, as determined by H

NMR end-group analyses, were much higher than those

calculated for a living process by ([2b] /[C793] ) × monomer

conversion because the actual concentration of actively

growing polymer chains was substantially lower than the

nominal catalyst concentration. With the ongoing reaction

time and continuous reaction of the catalyst with either

monomers (chain growth) or polymers (e.g., by the cleavage of

a linear chain or opening of a macrocycle), the average degree

of polymerization decreased. Finally, that is, at full con-

sumption of the catalyst, it matched the theoretical value

(

based on the assumption that all chains carry one benzylidene

end group). According to SEC analysis (Figure 5D,H), the

polymer 3b produced at the end of the kinetic experiments

exhibited the expected monomodal Schulz−Flory-type molar

mass distributions with a dispersity of Đ ∼ 2.

The main features of the polymerization mechanism, as

revealed by the kinetic studies, are summarized in Scheme 2.

The initial formation of propagating [Ru] chains from 2b and

C793 is a slow process, but once initiated, the polymerization

proceeds rapidly to a steady state with a certain minimum

monomer concentration threshold. Yet, the remaining catalyst

is still reactive and does not further react with the monomer

but with in-chain double bonds to cut polymer chains into

shorter fragments. The ﬁnally obtained polymer 3b exhibits the

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Scheme 2. Proposed Simpliﬁed Reaction Scheme for the

ROMP of 2b and Cleavage of Polymer Chains by the

Reaction of In-Chain Double Bonds with the Catalyst

C793

Note: the addition of the catalyst or active chain end to the

monomer or in-chain double bond can produce two regioisomers and

here, only the tentatively preferred one is shown.

Figure 6. H NMR spectra (400 MHz), selected regions of 9−10 and

17−20 ppm, taken during the polymerization of 2b (1 mmol) with

theoretically expected degree of polymerization and a Schulz−

Flory-type molar mass distribution.

C793 (0.04 mmol) in 1,4-dioxane-d (0.5 mL) at room temperature

(corresponding to the blue curve in Figure 5E).

In order to assess the long-term activity of [Ru]−alkylidene

chain ends, we added a second monomer batch (either 2b,

norbornene, or cyclooctene) to a polymerization mixture after

all catalysts had been consumed. In these experiments,

however, we did not detect an increase of molar mass or a

change in the molar mass distribution (data not shown),

suggesting that all [Ru] species were deactivated at this point.

The deactivation of [Ru] chain ends could also be

standing that other (however not yet identiﬁed) pathways

cannot be safely excluded.

As reported in the literature, catalyst deactivation may occur

from the resting state of the catalyst, for instance, through

thermal dimerization or by reaction with water, alcohols, or

oxygen.

18−20

These catalyst deactivation pathways have in

common that [Ru]-hydride species are formed. These, in

their turn, may catalyze an isomerization of double bonds.

recognized in the H NMR spectra taken during the

22,23

polymerization of 2b at high catalyst loading (2b 1 mmol/

C793 0.04 mmol; blue curve in Figure 5E), which are shown in

Figure 6. Three proton signals were initially detected at δ

A possible catalyst deactivation pathway, which has been

exclusively observed for second-generation catalysts with

sterically less hindered N-aryl substituents such as the C793

catalyst used here, involves an activation of the ortho-C−H

bond of the N-aryl substituent and transfer of the benzylidene

6.5−18.5 ppm, which could be attributed to diﬀerent

propagating [Ru]−alkylidene species. These proton signals

started to disappear after the steady state of the polymerization

had been reached (after about 100 min) and were no longer

detected at >1200 min. Intriguingly, parallel to the

disappearance of the [Ru]−alkylidene signals, three sets of

proton signals showed up at δ 9.2−9.8 ppm, a range which is

very characteristic for the carbonyl protons of aldehyde groups.

For very high catalyst loading, that is, [2b] /[C793] = 5, and

24−26

ligand to the N-aryl substituent.

The [Ru] complexes

resulting from this decomposition pathway are catalytically

inactive because the [Ru]−alkylidene unit is irreversibly lost.

Alternative catalyst deactivation mechanisms have been

proposed starting from the metallacyclobutane intermediate

of the catalytic cycle, which can be attacked by bases or may

thus higher amount of chain ends, the presence of aldehyde

undergo β-hydride elimination.

groups could be clearly conﬁrmed by H− C HSQC NMR

spectroscopy showing a direct correlation between proton

signals at δ 9.2 −9.8 ppm and carbonyl signals at δ 187−190

ppm (see the Supporting Information ). By the integration of

the proton signals [Ru]CH− (t = 5 min) and −CHO (t =

Based on these considerations, we tentatively propose the

[Ru]−alkylidene deactivation pathways outlined in Scheme 3

to rationalize the observed chain growth termination. In their

catalytically active state, [Ru]−alkylidenes are 14-electron

complexes and therefore highly Lewis acidic (pathway A). This

inherent Lewis acidity has earlier been proposed as a trigger

mechanism for a semipinacol rearrangement following a cross

828 min) (Figure 6), it can be estimated that at least 75% of

the active chain ends were transformed into aldehydes. Hence,

transformation of the [Ru]-active chain end into an aldehyde

seems to be the predominant deactivation pathway, notwith-

metathesis reaction. Coordination of the Lewis acid to the

acetal oxygen atom in the β-position to [Ru] facilitates the

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The Supporting Information is available free of charge at

Article

Scheme 3. Proposed Reaction Pathways for the

Deactivation of [Ru]−Alkylidene Chain Ends via Lewis

Acid-Assisted Hydrolysis of the Terminal Acetal (Pathway

A) or β-Hydride Elimination/Reductive Elimination and

Acetal Hydrolysis (Pathway B)

CONCLUSIONS

■

A series of levoglucosenyl alkyl ethers 2b−f were synthesized

and polymerized by ROMP using the Grubbs catalyst C793.

Polymerizations could be successfully performed not only in

conventional solvents such as 1,4-dioxane and DCM but also

in polar aprotic “green” solvents such as 2-MeTHF, Cyrene,

and ethyl acetate. All produced polyacetals had apparent

degrees of polymerization of ∼100 and broad Schulz−Flory-

type molar mass distributions with a dispersity of ∼2. They

were amorphous and showed thermoplastic behavior. Their

glass transition temperatures could be modulated by the choice

of the alkyl substituent, continuously decreasing from 100 to 0

C with an increase in the length of the substituent for

polymers 3a−f. These ﬁndings demonstrate the high potential

of levoglucosenol-based polymers as sustainable plastic

materials.

Until now, the polymerizations of 2a and 2b−f were limited

to the use of the catalyst C793, showing nonliving character.

While the propagation step is quite fast, the initiation of the

polymerization is comparatively slow. Apparently, the polymer-

ization reaches rapidly a steady state at monomer conversions

of 60−80% before the catalyst has been reacted completely.

The residual catalyst then does not react with monomers but

with in-chain double bonds, thus cutting the formed polymer

chains into fragments and equilibrating the molar mass

distribution. Finally, the polymers exhibit the expected molar

mass with a Schulz−Flory-type molar mass distribution. In the

long term, all propagating [Ru] chains are deactivated by the

elimination of the [Ru] from the chain ends under the

formation of inactive chains with terminal aldehyde groups.

Future investigations will be devoted to a deeper analysis of

the polymerization mechanism including the identiﬁcation and

fate of the propagating [Ru] species. The ultimate goals are to

accelerate the initiation step (i.e., the reaction of the catalyst

with the monomer) and to prevent deactivation of [Ru] chain

ends, which would enable the synthesis of block copolymers or

of even more complex polymer architectures. Furthermore, a

detailed analysis of the polymer microstructures, that is,

distribution of regioisomers and cis/trans ratio of in-chain

double bonds, is on the way.

nucleophilic attack of water (traces of water in the reaction

mixture or from moisture), yielding a hemiacetal. From this

hemiacetal, a [Ru]−carbene diol is rapidly cleaved oﬀ,

furnishing the polymer chain with an aldehyde group at the

former [Ru]−alkylidene terminus. The released [Ru]−carbene

diol species should further decompose, for instance, through β-

hydride elimination and reductive elimination, under the

formation of a [Ru]−H complex and hydroxyacetone. As

mentioned before, the released [Ru]−H complex may catalyze

double-bond isomerization, which could lead to the formation

of a conjugated aldehyde end group. Alternatively, the [Ru]−

alkylidene species may undergo β-hydride elimination in

competition to chain propagation (pathway B). The resulting

ASSOCIATED CONTENT

■

[

Ru]-hydride species may then undergo reductive elimination

sı Supporting Information

of an inactive [Ru] species and formation of an enol ether.

This enol ether should be easily hydrolyzed in the presence of

the Lewis acidic [Ru] fragments to furnish an aldehyde-

terminated polymer (same as before) and hydroxyacetone as a

byproduct. Such a catalyst deactivation mechanism has earlier

been proposed for oleﬁn metathesis reactions with allyl

https://pubs.acs.org/doi/10.1021/acs.macromol.0c02821.

Procedures for the synthesis of monomers 2b−f and

polymerization reactions; H NMR spectrum of catalyst

0,31

C793; H, C NMR, and ESI-MS spectra for 2b−f; H,

13 1 1 1 13

alcohols.

In any case, although deactivation or termination

C, H− H COSY, and H− C HSQC spectra and

of the propagating [Ru] chains occurred, this was responsible

for neither the incomplete monomer conversions nor the

broad molar mass distributions.

The deactivation of [Ru]−alkylidene chain ends resulting in

the formation of terminated chains with aldehyde end groups is

ascertained; however, it is not clear by which pathway A or B.

Pathway A (tentatively favored) oﬀers the possibility that the

TGA/DSC curves for polymers 3b−f; H NMR spectra

and SEC traces for the polymerizations of 2b−f in 1,4-

dioxane and for 2b in DCM, 2-MeTHF, Cyrene, and

ethyl acetate; H NMR spectra for in situ kinetic

experiments for the polymerization of 2b in 1,4-dioxane-

d ; H− C HSQC spectrum of a crude reaction mixture

b/C793 at high catalyst loading for the identiﬁcation of

terminal aldehyde groups; and H NMR spectra and

[

Ru] chain ends remain active when the polymerization is

performed under rigorously water-free conditions. Following

pathway B, the deactivation would occur even in the complete

absence of water or moisture.

SEC traces for the degradation of polymer 3b with C793

(PDF )

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Macromolecules 2021, 54, 2720−2728

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Article

13) Bhaumik, A.; Peterson, G. I.; Kang, C.; Choi, T.-L. Controlled

AUTHOR INFORMATION

■

Living Cascade Polymerization To Make Fully Degradable Sugar-

Based Polymers from D-Glucose and D-Galactose. J. Am. Chem. Soc.

Corresponding Author

Helmut Schlaad − Institute of Chemistry, University of

Potsdam, 14476 Potsdam, Germany; orcid.org/0000-

14) Debsharma, T.; Yagci, Y.; Schlaad, H. Cellulose-derived

019, 141, 12207−12211.

002-5824-7204 ; Email: schlaad@uni-potsdam.de

functional polyacetal by cationic ring-opening polymerization of

levoglucosenyl methyl ether. Angew. Chem., Int. Ed. 2019, 58, 18492−

Authors

18495.

Tapas Debsharma − Institute of Chemistry, University of

Potsdam, 14476 Potsdam, Germany; orcid.org/0000-

002-1296-7396

(15) Gao, H.; Harmon, J. P. An empirical correlation between glass

transition temperatures and structural parameters for polymers with

linear and branched alkyl substituents. J. Appl. Polym. Sci. 1997, 64,

Bernd Schmidt − Institute of Chemistry, University of

Potsdam, 14476 Potsdam, Germany; orcid.org/0000-

07−517.

(16) Hong, S. H.; Wenzel, A. G.; Salguero, T. T.; Day, M. W.;

Grubbs, R. H. Decomposition of Ruthenium Olefin Metathesis

Catalysts. J. Am. Chem. Soc. 2007, 129, 7961−7968.

002-0224-6069

André Laschewsky − Institute of Chemistry, University of

Potsdam, 14476 Potsdam, Germany; Fraunhofer Institute of

Applied Polymer Research IAP, 14476 Potsdam, Germany;

orcid.org/0000-0003-2443-886X

(17) Blanco, C. O.; Sims, J.; Nascimento, D. L.; Goudreault, A. Y.;

Steinmann, S. N.; Michel, C.; Fogg, D. E. The Impact of Water on

Ru-Catalyzed Olefin Metathesis: Potent Deactivating Effects Even at

Low Water Concentrations. ACS Catal. 2021 , 11 , 893 −899.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.macromol.0c02821

(18) Dinger, M. B.; Mol, J. C. Degradation of the First-Generation

Grubbs Metathesis Catalyst with Primary Alcohols, Water, and

Oxygen. Formation and Catalytic Activity of Ruthenium(II)

Monocarbonyl Species. Organometallics 2003, 22, 1089−1095.

(19) Beach, N. J.; Lummiss, J. A. M.; Bates, J. M.; Fogg, D. E.

Reactions of Grubbs Catalysts with Excess Methoxide: Formation of

Novel Methoxyhydride Complexes. Organometallics 2012, 31, 2349−

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

(

356.

We would like to thank Sascha Prentzel, Akhil Dev, Dirk

Schanzenbach (TGA/DSC), Ines Starke (MS), Angela

Krtitschka, Matthias Heydenreich, and Heiko Möller (NMR)

for their support and help in this project. Financial support was

provided by the University of Potsdam.

20) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.;

Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. Synthesis

and Activity of Ruthenium Alkylidene Complexes Coordinated with

Phosphine and N-Heterocyclic Carbene Ligands. J. Am. Chem. Soc.

21) Higman, C. S.; Plais, L.; Fogg, D. E. Isomerization During

003, 125, 2546−2558.

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