2. Synthesis of Aliphatic Polyesters by ADMET Polymerization and Hydrogenation
Reports on the synthesis of bio-based polyesters by ADMET polymerization, especially using commercially available (Grubbs-type) ruthenium carbene catalysts RuCl
2(PCy
3)
2(CHPh) (
G1; Cy = cyclohexyl), RuCl
2(PCy
3)(IMesH
2)(CHPh) (
G2; IMesH
2 = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), and RuCl
2(IMesH
2)(CH-2-O
iPr-C
6H
4) (
HG2), shown in Scheme 2, are well known. These ruthenium catalysts have been employed
[8] because these complexes can be readily available and do not require treatment with the strict Schlenk technique due to their insensitivities toward water and oxygen (better functional group tolerance)
[29][30][31][32]. More recently, the example using a molybdenum-alkylidene catalyst (
Mo cat.)
[33][34][35], shown below, also demonstrates the synthesis of high molar mass polymers that exhibit good tensile properties
[36].
Scheme 2. Ruthenium-carbene and molybdenum-alkylidene catalysts for the synthesis of aliphatic polyesters by ADMET polymerization.
The synthesis of a bio-based polyester, expressed as
PE1, by the ADMET polymerization of undec-10-en-1-yl undec-10-enoate (
M1), prepared by the reaction of 10-undecenoic acid with 10-undecenol (derived from castor oil) was reported by the group of Meier in 2008
[37]. The resultant
PE1 synthesized by
G2 (0.5 or 1.0 mol%, 80 °C, 24 h, Scheme 3) possessed a rather high molecular weight (
Mn = 22,000, 26,500), and the
Mn values were controlled by the addition of terminal olefins, such as methyl 10-undecenoate and stearyl acrylate
[37]. In contrast, the group reported that the polymerization of bis(undec-10-enoate) with isosorbide (
M2, Scheme 3) conducted at 70–100 °C under bulk conditions yielded rather low-molecular-weight polymers (
PE2)
[38], whereas the
Mn values seemed to improve when the polymerizations were conducted at high temperatures and/or under nitrogen-purge conditions (for the removal of by-produced ethylene). This was probably due to the catalyst decomposition caused by conducting the reaction at 70–100 °C
[39][40][41][42][43][44], because these ruthenium catalysts are known to decompose under these conditions to produce ruthenium-hydride species
[41] and/or nanoparticles
[43], which induce olefin isomerization and/or certain side reactions by the formed radicals
[39][40][41][42][43][44][45].
G2 showed a more significant degree of olefin isomerization compared to
G1 and a higher percentage of isomerization (estimated by GC-MS, after treating the mixture with MeOH-H
2SO
4 under reflux conditions)
[38]. Later, the degree of isomerization was extensively suppressed when the polymerizations were conducted in the presence of benzoquinone
[45].
Scheme 3. ADMET polymerization of castor oil-derived monomers (M1, M2).
The ADMET polymerization of
M1 by
G1 under high-vacuum conditions for two days produced
PE1 (
Mn = 28,000,
Mw/
Mn = 1.9) and a subsequent hydrogenation step (Pd/C, 50 bar H
2, 60 ºC) produced a saturated polyester (
HPE1, PE-20.20, Scheme 4)
[46]. The
Tm value (103 °C) achieved was somewhat low compared to the
HPE1 prepared by the condensation polymerization of 1,20-eicosanedioic acid with eicosane-1,20-diol (
Tm = 108 °C) to form ‘regio-regular’ ester groups, C(O)-O, aligned with the polymer chain (Scheme 4). It was thus suggested that the microstructural control directly affected the thermal property, as described above
[6][14]. ADMET polymerizations of α,ω-dienes with different methylene chain lengths, di(icos-19-en-1-yl)tricosanedioate (
M3) and di(tricos-22-en-1-yl)tricosanedioate (
M4), using
G1 and the subsequent olefin hydrogenation conducted by Ru(CHOEt)Cl
2(PCy)
2 (40 bar H
2, 70 °C, 2 d), prepared from
G1, yielded the corresponding values of PE-38.23 (
HPE3) and PE-44.23 (
HPE4), respectively (Scheme 4)
[47]. The polycondensation of 1,26-hexacosanedioate, prepared by the cross-metathesis of erucic acid, with the corresponding diol (produced by a reduction with LiAlH
4) with Ti(OBu)
4 also produced the corresponding polyester (
HPE5, PE-26.26,
Tm = 114 °C)
[48]. The thermal properties (
Tm values) of the resultant LCAPEs with different methylene lengths, prepared by ADMET
[47] and polycondensation
[48][49] approaches, revealed that the
Tm values achieved a constant value (
Figure 1a)
[47]. A linear relationship between the
Tm values and the number of ester groups in 1000 carbon atoms was observed (
Figure 1b)
[47]. Polyesters PE-26.26, PE-12.26 and PE-4.26
[48], and PE-18,18
[50] were also prepared by polycondensation.
Scheme 4. Synthesis of linear polyesters (LCAPEs).
Figure 1. Plots of melting temperature (Tm) vs. number of (a) methylene units (x) in diol and (b) ester groups per 1000 C (methylene) in PE-23.x.
The one-pot synthetic method used for the bio-based aliphatic polyesters by ADMET polymerization and subsequent hydrogenation was demonstrated (Scheme 5)
[51]. The polymerization of bis(undec-10-enoate)s with isosorbide (
M2), isomannide (
M6), 1,3-propanediol (
M7), and 1,4-cyclohexanedimethanol (
M8), derived from castor oil and glucose in chloroform by
G2 or
HG2 under a reduced pressure at 50 °C produced unsaturated polymers (expressed as
PE2 and
PE6–
PE8, respectively)
[51]. The
Mn values in the produced polymers (
Mn = 11,900–15,900) were somewhat higher than those reported previously (
Mn = 4400–8400), conducted at 70–100 °C
[38], and the
Mn values did not change, even under rather scaled-up conditions
[51]. One reason for the obtainment of high-molecular-weight product could be that the degree of the catalyst decomposition was significantly suppressed by conducting the polymerization at 50 °C (and the polymerization was conducted under a continuously reduced pressure)
[51].
Scheme 5. One-pot synthesis of bio-based polyesters by Ru-catalyzed ADMET polymerization and hydrogenation.
As described above (Scheme 4) and below
[52], conventional olefin hydrogenation requires a high hydrogen pressure and high temperature after the isolation of unsaturated polyesters after ADMET polymerization
[46][47][52]. In contrast, one-pot hydrogenation under rather mild conditions (1.0 MPa, 50 °C, 3 h) was demonstrated following the addition of a small amount of Al
2O
3 (ca. 1 wt%) to the reaction mixture (Scheme 5). The completion of the olefin hydrogenation was confirmed by DSC thermograms (uniform compositions) due to the difficulty (accuracy of the integration of olefinic protons) of obtaining the
1H NMR spectra. No significant differences in the
Mn and
Mw/
Mn values were observed before/after hydrogenation
[51].
As shown in
Figure 1b, the melting temperatures (
Tm values) of the polyesters are influenced by the methylene unit number (n). As shown in Scheme 6, the copolymerization of
M1 with undeca-1,10-diene (UDD) followed by olefin hydrogenation (H
2 40 bar, 110 °C, 2 d) produced various LCAPEs with different chain lengths (ranging from 0.9 to 52.6 ester groups per 1000 carbon atoms), expressed as H
2-poly(
M1-
co-UDD)
[52]. A linear correlation of the melting temperatures (
Tm values) with the average number of ester groups per methylene unit was thus demonstrated, whereas the ester group was incorporated in a random manner. A similar trend was observed in the copolymerization of
M2 with 1,9-decadiene (DD) and the subsequent one-pot hydrogenation
[53]. The saturated polymers possessed
Tm values in the range of 71.7–107.6 °C, depending on the molar ratios of
M2 and DD.
Scheme 6. ADMET copolymerization of undec-10-en-1-yl undec-10-enoate (M1) or bis(undec-10-enoate) with isosorbide (M2) with nonconjugated dienes, and subsequent hydrogenation.
The polymerization of bis(undec-10-enoate)s with
D-xylose (1,2-
O-isopropylidene-α-
D-xylofuranose,
M9c), and
D-mannose (
M10) by
G2 was studied under a dynamic-vacuum (0.1 mbar) condition without solvent (bulk) conditions (60–90 °C, 20 h, Scheme 7)
[54]. The molecular weights of the resultant polymers (
PE9c,
PE10) were affected by the polymerization temperature employed and the monomer/Ru molar ratios. Conducting the polymerization at 90 ºC under a low Ru concentration (0.1 mol%) seemed to be the optimized condition (
PE9c: Ru,
Mn = 7.14–7.16 × 10
4,
Mw/
Mn = 2.2–2.3,
PE10:
Mn = 3.24 × 10
4,
Mw/
Mn = 2.4)
[46]. Due to the fact that the polymerization was conducted without a solvent, the PDI (
Mw/
Mn) values were rather high due to the difficulty pf controlling the stirring
[54]. Later, the polymerizations of
D-xylose diester analogs with different methylene lengths (
M9, x = 0, 2, 8, Scheme 7) and the corresponding diether analogs (
M11) were explored
[55]. The
Mn values of the resultant polymers decreased upon decreasing the methylene length, and the monomers did not possess a methylene spacer
[55]. Some polymerization runs failed due to precipitation or the difficulty of performing isolations
[55]. The resultant unsaturated polymers were amorphous, except
PE11a, and both glass transition temperatures (
Tg) increased after reducing the olefinic double bonds by treating them with
p-toluenesulfonyl hydrazide as a reducing agent; most of the resultant saturated polymers (
HPE9 and
HPE11) were amorphous, except
HPE9a and
HPE11a derived from the castor oil (10-undecenoate), suggesting that the placement of the methylene spacer was important (as shown in
Figure 1a and
Figure 2)
[55]. The resultant hydrogenated polymer films, especially the
HPE11a-oriented film, exhibited a good tensile strength (43 MPa) with an elongation at a break of 155%; but, the hot-press film showed a much weaker tensile strength (7.8 MPa) with and improved elongation at the break (667%)
[55].
Figure 2. Plots of melting temperature (Tm) vs. number of ester groups per 1000 C (methylene units) in the hydrogenated copolymers, H2-poly(M1-co-UDD)s.
Scheme 7. ADMET polymerization of α,ω-dienes containing D-xylose, D-mannose, vanillin, and eugenol as the monomer units.
The syntheses of polyesters containing vanillin (
PE12)
[56] afforded high-molecular-weight
PE12 (
Mn = 10,000,
Mw/
Mn = 1.6) possessing a
Tg value of 4 °C (Scheme 7), whereas the polymerization of 4-allyl-2-methoxyphenyl 10-undecenoate (
M13) by
G2 produced amorphous high molar mass polymers with low PDIs (
Mw/
Mn) with
Tg at −9.6 °C
[57]. The ADMET polymerization of
M13 in the presence of 5-formylbenzene-1,2,3-triyl tris(undec-10-enoate) produced rather high molar mass network polymers
[57].
The polymerization of trehalose bis(10-undecenoate) (
M14) by
HG2 (4.0 mol%) in THF at 45 ºC for 24 h (Scheme 8) produced semicrystalline polymers (
PE14) possessing high molecular weights with unimodal molecular-weight distributions (
Mn = 13,200,
Mw/
Mn = 2.1) with higher
Tm values (156 °C)
[58]. Both the molecular weights and melting temperatures (
Tm values) of the resulting copolyesters with undec-10-en-1-yl undec-10-enoate (
M1) decreased with the increase in the percentage of
M1 [58].
Scheme 8. Synthesis of bio-based copolyesters with different molar ratios.