Sustainable Polycarbonates from a Citric Acid-Based Rigid Diol and

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Sustainable polycarbonates from a citric acid-based rigid diol and recycled BPA-PC: From Synthesis to Properties Chengcai Pang, Xueshuang Jiang, Yan Yu, Xiaohan Liu, Jingyan Lian, Jianbiao Ma, and Hui Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04421 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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ACS Sustainable Chemistry & Engineering

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Sustainable polycarbonates from a citric acid-based rigid

2

diol and recycled BPA-PC: From Synthesis to Properties

3 4

Chengcai Pang*, Xueshuang Jiang, Yan Yu, Xiaohan Liu, Jingyan Lian, Jianbiao Ma,

5

and Hui Gao*

6

School of Chemistry and Chemical Engineering, School of Material Science and

7

Engineering, Tianjin Key Laboratory of Organic Solar Cells and Photochemical

8

Conversion, Tianjin University of Technology, Binshui West Road 391, Tianjin 300384,

9

China

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*Corresponding

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E-mail address: [email protected]; [email protected]

author: Tel.: +86 22 60214251.

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: Here we present a series of homopolycarbonates (homo-PCs) and

4

copolycarbonates (co-PCs) based on a novel bicyclic diol octahydro-2,5-pentalenediol

5

(OPD) from naturally occurring citric acid and bis(hydroxyethyl ether) of bisphenol A

6

(BHEEB), synthesized by melt polycondensation. The recently developed OPD has

7

shown to be highly rigid and thermal stable building block suitable for the construction of

8

performance polymers. BHEEB, which was obtained from the chemical recycling of

9

BPA-PC, was used to compens ate for the low reactivity of OPD and to modify the

10

brittleness of polycarbonate (PC) solely based on OPD, without compromising other

11

properties. The single crystal of the endo-endo isomer of OPD was deliberately obtained

12

and its absolute stereochemistry was unambiguously identified by single crystal X-ray

13

diffraction for the first time. The polymers had Mn in the 10100−20000 g mol-1 range and

14

gradually decreased with the increasing of OPD content. NMR analyses revealed the

15

random structures of the co-PCs and the molar content of OPD in all cases was lower

16

than its corresponding feeds. Interestingly, in contrast with the semicrystalline

17

poly(octahydro-2,5-pentalenediol carbonate) (abbreviated as pre-POC) prepared in a

18

different protocol in our previous article, poly(octahydro-2,5-pentalenediol carbonate)

19

(abbreviated as POC) in this study exhibited amorphous feature with lower Tg of 74.5 ºC.

20

A “ductile−to−brittle” transition occurred with the increasing of OPD content in the PBC

21

chains, which can be ascribed to their low molecular weights and the low entangled

22

strand density due to the rather stiff polymer chains. This work combines the chemical


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recycling and bio-based polymer together, which would bring a feasible way to satisfy

2

the demands of sustainability.

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Keywords ： rigidity, chemical recycling of BPA-PC, bicyclic diol, citric acid,

4

polycarbonate

5

INTRODUCTION

6

The preparation of polymers from renewable resources is currently receiving

7

considerable attention because of the finite reserves of crude oils and economical,

8

environmental, and social concerns.1-4 However, completely bio-renewable polymers

9

frequently suffer from some limits of mechanical and thermal properties when replacing

10

general engineering plastics, which make it necessary to utilize additional monomers, e.g.,

11

terephthalate acid and bisphenol A (BPA), to obtain essential properties required in

12

general plastics’ applications. Moreover, due to the fact that the price of recycled

13

polymers is generally lower compared to that of virgin materials,5 recycling of

14

commercial plastics to produce polymers has become much more attractive, and more

15

importantly, can effectively reduce the environmental issues caused by the plastic

16

wastes.6 So, from the sustainable, industrial and environmental point of view, the

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synthesis of bio-based polymers should conjugate with the recycling of commercial

18

plastic wastes, especially when novel polymers with more specific and tailored properties

19

can be achieved. For example, bis(hydroxyethyl ether) of bisphenol A (BHEEB),

20

obtained by reaction of BPA-PC with renewable ethylene carbonate, has been used as

21

rigid comonomer to increase the glass transition temperature and thermal stability of



1

terephthalate polyesters.7

2

BPA-PC is a commercially available amorphous material, characterized by high

3

transparency, high glass transition temperature (Tg), impact resistance and tensile strength,

4

which is massively used in electric and electronic, automotive, and optical components.8,

5

9

6

Moreover, the safety of BPA-PC has been frequently questioned due to the toxicity of

7

BPA in the last few years.11 In this regard, it would be desirable to prepare PCs from

8

renewable but less toxic resources having comparable thermal and mechanical properties

9

to that of BPA-PC.

Unfortunately, BPA-PC is considered unsustainable because BPA is non-renewable.10

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Among the renewable sources, several bicyclic diacetalized diols, obtained by

11

acetalization of the intramolecular hydroxyl groups from C4 and C6 sugars, are regarded

12

as suitable for the production of linear rigid polycondensates due to their outstanding

13

rigidity.12 However, the use of these bicyclic diacetalized diols for the fabrication of

14

bio-based PC is challenging due to their high prices caused by the tedious preparation

15

process and their high sensitivity to heat.13 In addition, isohexides, which is composed of

16

isosorbide, isoidide and isomannide,14 are considered as suitable candidates for replacing

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BPA due to their natural origin, attractive rigidity,15,16 safety and chirality.17 Despite the

18

apparent merits, PCs solely based on isohexides still suffer from several drawbacks: (1)

19

relatively low molecular weights polymers were obtained due to the poor reactivity of the

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hydroxyl groups at the 2- and 5- positions.18 (2) the corresponding homo-PC is too

21

brittle.17 (3) severely colored or cross-linked polymers were frequently obtained during


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melt polycondensation, which was caused by the heat labile skeleton composed of two

2

tetrahydrofuran rings.19,

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copolycondensation

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1,4-cyclohexanedimethanol.21 However, due to the intrinsic thermolability of the

5

tetrahydrofuran rings contained in isohexides, effective ways that can avoid discoloration

6

or gelation under high temperatures are still not available.

of

20

The first two drawbacks can be solved simultaneously by isohexides

with

more

flexible

primary

diols,

e.g.,

7

Very recently, a series of random co-PCs based on a compelling bicyclic diol

8

octahydro-2,5-pentalenediol (OPD), which is derived from naturally occurring citric acid,

9

have been developed in our group. Results indicated that OPD is a highly rigid and

10

thermal stable building block suitable for the construction of performance PCs.22

11

However, due to the high stiffness and immobility of chain conformation, the main

12

demerit of solely OPD-based PC is its fragility. In this work, in order to improve the

13

toughness and compensate for the low reactivity of OPD, while at the same time retaining

14

the rigidity of OPD-based PCs, BHEEB is incorporated in the polymer chains by melt

15

polymerization with OPD in different molar ratios. Previous investigations suggested that

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the high impact strength of BPA-PC can be attributed to its capacity of absorbing high

17

impact energy which, in turn, has been correlated with secondary relaxations involving

18

cooperative motions of BPA units in the polymer chains.23 It is worthy to note that

19

although BPA is toxic, however, BHEEB is biocompatible. In fact, ethoxylated BPA has

20

long been used in food packaging applications and restorative dentistry.24 Therefore, the

21

chemical recycling of BPA-PC through the formation of ethoxylated BPA compound is



1

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of great interests from an environmental and industrial point of view.

2

Herein, we describe the green synthetic routes, the structural characterization and

3

the thermal and mechanical properties of the amorphous homo-PCs and co-PCs, prepared

4

by melt polymerization of OPD, BHEEB and diphenyl carbonate (DPC). OPD was

5

chosen in view of its natural origin and excellent rigidity and thermal stability, while

6

using BHEEB, the modified BPA from chemical recycling of commercial BPA-PC,

7

impact resistant polymers with high molecular weights can be possibly obtained. Since

8

our goal lies in the fabrication of bio-based PCs with high Tg values and impact strength,

9

special attention is paid to study the effect of OPD and BHEEB composition on polymer

10

properties, e.g., Tg value and toughness, a systematic structure-thermal and mechanical

11

properties relations study on the relevant polymers has been performed.

12

Experimental Section

13

Materials

14

Dimethyl-1,3-acetone dicarboxylate (98%), glyoxal (40% aqueous solution), glacial

15

acetic acid (99.5%) and BPA-PC (melt index 10−12 g/10min) were purchased from

16

Sigma-Aldrich

17

4-dimethylaminopyridine (DMAP) (99%), lithium acetylacetonate (LiAcac) (99%) and

18

NaBH4 (98%) were purchased from Energy Chemical Co. (Shanghai, China),

19

concentrated hydrochloric acid, methanol (99%) and chloroform (99%) were purchased

20

from Tianjin Chemical Reagent Co. (Tianjin, China) and used without further

21

purification.

(Shanghai,

China).

Ethylene

carbonate


(99%),

DPC

(98%),

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1

General methods

2

Chemical structure and compositions of the PCs were analyzed using 1H-NMR,

3

13C-NMR

4

were carried out using CDCl3 as the solvent at ambient temperature on a Bruker

5

AVANCE III NMR spectrometer operating at 400 and 100.6 MHz respectively, and the

6

internal standard was tetramethylsilane. The molecular weights (Mn, Mw) and the

7

molecular weight distribution (PDI) were determined by gel permeation chromatography

8

(GPC, Waters 2414 system Milford, 4 MA) at 35 ºC with the flow rate set to 1.0 mL

9

min-1. THF and monodisperse polystyrene were used as the eluent and standard,

10

respectively. The concentration of the sample was 4 mg mL-1, and the injection volume

11

was around 30 μL. The thermogravimetric analyses (TGA) was carried out on a Seiko

12

Exstar 6000 TGA quartz rod microbalance and 5−8 mg of sample was needed for each

13

polymer. The sample was then heated from 40 to 600 ºC at a heating rate of 10 ºC min-1

14

with nitrogen gas purging (50 mL min-1). The thermal characteristics of the polymers

15

were investigated using a differential scanning calorimeter (DSC, Netzsch PC-200) under

16

with nitrogen gas purging (50 mL min-1). About 5 mg of sample was heated from 30 ºC

17

to 120 ºC at a heating rate of 10 ºC min-1 and a cooling rate of 30 ºC every time and then

18

the Tg were calculated from the second heating run. Films for mechanical testing

19

measurements with a thickness of 200 μm were prepared by casting from a chloroform

20

solution at a concentration of 100 g L-1. The tensile properties were tested on dumbbell

21

type specimens (12×2×0.5 mm3) measuring the strain at 25 ºC and 3 mm s-1. Dynamic

and Fourier Transform Infrared (FT-IR) spectra. The NMR measurements



Page 8 of 38

1

mechanical analysis (DMA) were carried out using a TA DMA 2928 in a controlled

2

force-tension film mode with a preload force of 0.1 N, an amplitude of 10 μm, and at a

3

fixed frequency of 1 Hz at the range of -140 to 140 ºC in a heating rate of 3 ºC min-1.

4

Sythesis of monomers

5

Synthesis of octahydro-2,5-pentalenediol (OPD)

6

Octahydro-2,5-pentalenediol

7

1,3-acetonedicarboxylate and glyoxal, the exhaustive procedure can be found in the

8

reported literature.25

9

Sythesis of BHEEB

(OPD)

was

synthesized

using

dimethyl

10

BHEEB was synthesized from chemical recycling of BPA-PC as per the reported

11

procedure.24 Briefly, BPA-PC (5.1 g), ethylene carbonate (4.22 g, 48 mmol) and sodium

12

phenoxide (0.1 mol% to BPA-PC repeating unit) were added into a three-neck flask fitted

13

with condenser and a thermometer. The flask was heated to reflux at 250 ºC under magic

14

stirring under a N2 atmosphere until the effervescence ceased. The reaction mixture was

15

cooled to room temperature and the polymer was dissolved in CHCl3 and precipitated in

16

methanol. A suspension of the precipitated polymer was dissolved in a 1 mol L-1 KOH

17

for 3 h at reflux. The white suspension was neutralized with diluted HCl and the solvent

18

evaporated under vacuum. The white solid was washed three times with water and the

19

residue was dissolved in CH2Cl2, dried with anhydrous magnesium sulfate and then

20

crystallized in cyclohexane to afford a white solid (5.8 g, yield: 91%). 1H-NMR (400

21

MHz, CDCl3): δ 7.14 (d, 4H), 6.82 (d, 4H), 4.06 (m, 4H), 3.94 (dd, 4H), 2.1 (t, 2H), 1.64


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1

(s, 6H).

2

Synthesis of polymers

3

Poly(octahydro-2,5-pentalenediol carbonate) (POC) and Poly(bis(hydroxyethyl ether) of

4

bisphenol A carbonate) (PBC) were obtained by reacting DPC with OPD or BHEEB,

5

respectively. The POxByC co-PCs were synthesized from a mixture of BHEEB, OPD and

6

DPC with the selected composition, where x and y represent the content of OPD and

7

BHEEB relative to the total amount of diols in the polymer chains, respectively,

8

determined by integration of the 1H NMR spectra (Table 1). The representative

9

preparation of PO29B71C is described below as a generalized polymerization procedure.

10

In a three-neck round flask fitted with a vacuum distillation outlet, OPD (0.91 g, 6.4

11

mmol), BHEEB (4.3 g, 13.6 mmol) and DPC (4.28 g, 20 mmol) were charged. Three

12

cycles of N2 and vacuum were applied to remove any residual air. LiAcac (0.1 wt %

13

based on the diols ) and DMAP (0.1 wt % based on the diols) were chosen as catalysts.

14

Transesterification reactions were carried out under N2 at 130 °C for 10 h.

15

Polycondensation reactions were performed at 230 °C for 2 h under vacuum (0.5 mbar).

16

After completion of the reactions, the resulting polymers were cooled to room

17

temperature and dissolved in chloroform, precipitated in excess of methanol to remove

18

unreacted monomers and formed oligomers. Finally, the precipitation was collected by

19

filtration and dried under vacuum for 24 h.

20

PBC: 1H-NMR (400 MHz, CDCl3): δ 7.13 (d, 4H), 6.81 (d, 4H), 4.49 (q, 4H), 4.18 (t,

21

4H), 1.63 (s, 6H). FT-IR (cm-1): 3043, 2961, 2869, 1750 (C=O), 1599, 1504, 1229, 1182,



1

916, 834, 791, 572.

2

POxByCs: 1H-NMR (400 MHz, CDCl3): δ 7.10−7.20 (d, x-4H), 6.77−6.89 (d, x-4H), 5.21

3

(m, y-1H, endo-exo), 5.08 (m, y-1H, endo-exo), 4.87−5.04 (m, y-2H, endo-endo),

4

4.42−4.54 (q, x-4H), 4.12−4.24 (t, x-4H), 2.71 (s, y-2H, endo-exo), 2.47 (s, y-2H,

5

endo-endo), 2.03−2.31 (m, y-4H, endo-endo; m, y-4H, endo-exo), 1.70−1.83 (m, y-4H,

6

endo-endo; m, y-4H, endo-exo), 1.64 (s, x-6H). FT-IR (cm-1): 3047, 2974, 2870, 1750

7

(C=O), 1605, 1504, 1240, 1173, 916, 824, 781, 561.

8 9

Figure 1. Molecular structure of the endo-endo stereoisomer in the crystal.

10

POC: 1H-NMR (400 MHz, CDCl3): δ 5.19 (m, 1H; endo-exo), 5.10 (m, 1H; endo-exo),

11

4.87−4.99 (m, 2H; endo-endo), 2.71 (s, 2H; endo-exo), 2.44 (s, 2H; endo-endo),

12

2.02−2.35 (m, 4H, endo-endo; 4H, endo-exo), 1.56−1.85 (m, 4H, endo-endo; 4H,

13

endo-exo). FT-IR (cm-1): 2968, 2876, 1750 (C=O), 1474, 1435, 1277, 1101, 952, 799.

14

Results and discussion

15

Synthesis and characterization of OPD

16

The bio-based monomer octahydro-2,5-pentalenediol (OPD) was prepared from


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1

commercially available dimethyl 1,3-acetonedicarboxylate (derived from citric acid) and

2

glyoxal via Weiss-Cook condensation reaction, followed by decarboxylation under acidic

3

conditions, leading to the formation of bicyclo[3.3.0]octane-3,7-dione. Subsequent

4

reduction with NaBH4 afforded the desired bicyclic diol OPD. 1H NMR spectrum shows

5

that OPD is essentially a mixture of two stereoisomers, in which the endo-endo isomer

6

predominated over the endo-exo one (endo-endo:endo-exo = 4:1).

7

The exo-exo stereoisomer, however, is not obtained since the reduction of the

8

intermediate diketone from the endo faces is unfavorable owing to its steric hindrance.

9

The synthetic approach and the NMR spectroscopic data of OPD can be found in our

10

previous article.22 To further explore its molecular structure, the single crystal of the

11

endo-endo diastereoisomer was deliberately obtained by repeatedly recrystallization from

12

methanol. Single crystal X-ray (Figure 1) revealed its characteristic V-shaped bicyclic

13

frame composed of two cis-fused cyclopentane rings similar to isohexide, thus a fairly

14

high degree of rigidity can be expected. Because the two cyclopentane rings contained in

15

the molecular skeleton are not planar, the dihedral angle between them cannot be given

16

precisely. Alternatively, it can be described in terms of torsion angles, the torsion angle

17

of C4−C5−C1−C8 is 119.2o, and the torsion angle of C2−C1−C5−C6 is 128.4o. More

18

detailed information about the endo-endo stereoisomer can be found in Table S1 in the

19

Supporting Information.

20

Synthesis and chemical structure analysis of co-PCs

21

Homo-PC POC and PBC based on OPD and BHEEB, respectively, as well as co-PCs



Page 12 of 38

1

from OPD and BHEEB, were prepared following a conventional procedure including

2

transesterification and polycondensation reactions as shown in Scheme 1. In order to find

3

the optimal reaction condition, the preparation of the targeted PO29B71C was chosen as

4

the model reaction and conducted with variable conditions including reaction

5

temperatures, stoichiometric ratio of monomers, and catalyst. When OPD was used as a

6

comonomer, the reaction temperature had a significant effect on the component of the

7

resulting polymers. In fact, at temperatures exceeding 250 ºC, the content of OPD units in

8

the resulting polymers were always lower than the initial feeds. Moreover, the longer

9

reaction time, the lower content of OPD was obtained. This phenomenon is identical to

10

our previous observations22 and can be interpreted H OH + y HO

x HO H

O

O

O O O

OH + （ x+y（

1. Transesterification, 130 °C 2. Polycondensation, 230 °C O

H O

O O

H

x

O

O

O

O

y

11 12

Scheme 1. Polymerization reactions leading to PCs

13

using a competitive transesterification mechanism. Compared with BHEEB, the

14

secondary hydroxyl groups in OPD are relatively unreactive, the reaction of DPC with

15

BHEEB has priority over that between DPC and OPD, part of the unreacted OPD or the

16

“kicked” OPD produced in the transesterification process is inevitably eliminated at high

17

temperature under vacuum. Consequently, a 10% molar excess of OPD was used to

18

compensate the expected losses, prepolymerization reactions were performed


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1

under a slow nitrogen flow for a period of 10 h at 130 ºC, while polycondensation

2

reactions were carried out for 2 h at 230 ºC.

3

With the aim to estimate the effects of the catalysts on both molecular weight and

4

product appearance, besides the mostly used Zn(OAc)2, SnOBu2, Ti(OBut)4 and Sb2O3,

5

the combination of LiAcac and DMAP were also used and the catalytic activity was

6

assessed. Results suggested that the combination of LiAcac and DMAP are good

7

catalysts to afford polymers with satisfied molecular weights and product appearance.

8

SnOBu2, Ti(OBut)4 and Sb2O3 were all sluggish, while the use of Zn(OAc)2 afforded

9

severely colored polymers.

10

Based on the above results, homo-PCs PBC and POC, as well as co-PCs from OPD,

11

BHEEB and DPC, were successfully synthesized. Noteworthy, as an eco-friendly

12

alternative to phosgene for carbonylation process, DPC can be produced

13 14

Figure 2. GPC traces of synthesized PCs

15

from the phenol generated as a by-product of the polycondensation process via

16

phosgene-free methods.26 All of the obtained PCs were slightly yellow, optically clear

17

products and soluble in CHCl3 and THF at room temperature. The molecular weights of



Page 14 of 38

1

all the obtained polymers were estimated using GPC (Figure 2, the baselines and the

2

solvent peaks are tailored) and the data are listed in Table 1. Obviously, all

3

polycarbonates showed moderate to good molecular weights and unimodal molecular

4

weight distributions. The Mn values consistently decreased with increasing OPD content,

5

which can be mainly ascribed to its low reactivity. Besides, the bulky ring structure of

6

OPD and high boiling point of byproduct phenol (181.9 ºC) was another causing factor,

7

because the emission of phenol became more and more difficult as the degree of

8

polymerization and melt viscosity increased during the polycondensation process at 230

9

ºC under vacuum, while raising the reaction temperatures to facilitate emission in turn led

10

to the deviation of OPD content.

11

The chemical structures of the PCs were characterized using 1H-NMR spectra, as

12

shown in Figure 3. Obviously, all resonances can be correctly assigned to the various

13

protons present in the polymer chains with matching multiplicities, and the peak intensity

14

changes gradually with the variation of the corresponding monomer fraction.

15

Table 1. Molar Composition, Molecular Weight, and Microstructure of co-PCs Molar composition

Microstructure

co-PCsa

feed molecular dyads (mol %)c Lnd Yield PC Mn Mw D BB BO OO nBC nOC Re XO XB XO XB (%) POC 87 100 0 100 0 13100 29300 2.3 PO38B62C 79 42.3 57.7 38.3 61.7 10100 17100 1.7 35.6 51.6 12.8 2.4 1.5 1.08 PO29B71C 75 32.0 68.0 28.6 71.4 12500 21500 1.7 52.4 38.2 9.4 3.7 1.5 0.94 PO16B84C 80 21.6 78.4 16.3 83.7 12700 21700 1.7 PO10B90C 71 10.9 89.1 10.2 89.8 14700 24800 1.7 PBC 86 0 100 0 100 20000 34900 1.8 aMolar composition determined by integration of the 1H NMR spectra. bNumber and weight average molecular weights in g mol-1 and dispersities measured by GPC in tetrahydrofuran against PS standards. cExperimental values obtained by means of the equations using the 13C NMR data mentioned in the text. dNumber-average sequence lengths. eRandomness index of co-PCs statistically calculated on the basis of the 13C NMR analysis.

16

weightb

In Figure 3, signals at δ 7.13, 6.81, 4.49, 4.18 and 1.63 ppm were assigned to protons 11,


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1

12, 9, 10 and 13 of the BHEEB moiety, respectively. For all co-PCs, the signals attributed

2

to H9 (marked with an asterisk, Figure 3), whose peak density increased with the

3

increasing of the OPD content, are split into two main peaks, indicating the different

4

chemical environment caused by the stereoisomers bearing two different secondary

5

hydroxy groups. With regard to the OPD moiety, more complicated peak split was

6

observed, apart from the increased complexity caused by the coexistence of the

7

endo-endo and endo-exo isomers of OPD, the stereoirregularity of the PC (i.e., endo-endo,

8

endo-exo, exo-exo, endo-B, exo-B) results in peak split and hence significant peak

9

overlapping in the 1H NMR spectrum. For the sake of distinction, protons of the

10

endo-endo and endo-exo isomers are numbered using Arabic numerals

11

and English letters, respectively. In the case of POC, the signals of methyne hydrogen

12

atoms g and c in the endo-exo isomer appeared at 5.19 and 5.01 ppm, respectively, while

13

the methyne hydrogen atoms 3 (7) of the endo-endo isomer gave distinctively different

14

chemical shifts at around 4.87−4.99 ppm. In contrast, in the 1H-NMR spectra of co-PCs,

15

signals at 5.19 and 5.01 ppm attributed to the endo-exo isomers almost



1 2

Figure 3. 1H NMR spectra of the obtained PCs

3

disappeared because their molar fractions were rather low. The bridge protons, exocyclic

4

protons and endocyclic protons in the molecular skeleton of OPD, regardless of the types

5

of stereoisomers, exhibited overlapped multiplets at around 2.45, 2.26 and 1.69 ppm,

6

respectively. Integration of the peaks corresponding to H (g/c, 3/7) and H (11) allow us to

7

quantify the composition of the co-PCs in such units. Data provided were given in Table

8

1, where it can be seen that the content of OPD in all cases was lower than its

9

corresponding feeds; in other words, an excess of OPD is

10

necessary to obtain co-PCs with desired compositions. The molar ratio of the

11

endo-endoto endo-exo isomers in POC was analyzed by integration of the proton signals

12

arising from H(g)/H(c) and H(3)/H(7), as shown in Figure S1 in the Supporting

13

Information. The obtained result is 4.03:1, which is quite approach the initiative value of

14

4:1, indicating that the difference in reactivity between the two isomers is negligible,

15

which is consistent with our previous results.22


Page 16 of 38

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

FTIR analysis was used to further investigate the structural difference among the

2 3

Figure 4. FT-IR spectra of PCs

4

synthesized polymers with various OPD molar fractions (Figure 4). All FT-IR spectra

5

show identical strong absorption band at 1750 cm-1, corresponding to the typical C=O

6

stretching vibration of the carbonate linkages. The different types of diols on the both

7

side of the carbonyl groups, BHEEB or OPD, seems have negligible effect on the

8

absorption frequency. Compared to POC, the co-PCs showed more complex absorption

9

pattern. The absorption peaks at 1612 as well as 1516 cm-1, and 833 cm-1, which are

10

absent in the FTIR spectra of POC, are attributed to the C=C stretching vibration and

11

=C-H bending vibration in the benzene ring of BHEEB, respectively.27

12

In addition, the bands at 1240 cm-1 were assigned to the asymmetric stretching of C-O-C

13

in BHEEB moiety, which is in good accordance with the reported literature.28

14

The microstructures of all the co-PCs were determined by 13C NMR spectra (Figure

15

5). Because the carbonyl carbons are more sensitive to sequence effects at the dyad level

16

than any other carbon atoms, these signals in the range of 154.4−155.1 ppm were



1

analyzed to clarify the microstructure of co-PCs as far as distribution of BHEEB and

2

OPD units along the polymer chain is concerned. As a consequence of the occurrence of

3

three dyad types and also of the two orientations of the endo-exo

4 5

Figure 5. Extended 13C NMR spectra of the carbonyl carbon region in various co-PCs

6

OPD unit, this signal is expected to be split into at least six multiple peaks in the

7

expanded 13C NMR spectrum. Nevertheless, for PO38B62C and PO29B71C, only five peaks

8

with enough resolution can be discerned, arising from the five types of diol-dyads BB,

9

BO (BOendo, BOexo) and OO (OendoOendo, OendoOexo), where B and O represent BHEEB

10

and OPD units, respectively. The sixth peak attributed to the OexoOexo diol-dyad, however,

11

is not observed because of its low content. By the same reason, for PO16B84C, only four

12

peaks were present in the carbonyl carbon region, corresponding to the BB, BOendo, BOexo

13

and OendoOendo diol-dyads. With regard to PO10B90C, in which the OPD content is the

14

lowest, only two peaks corresponding to the BB and BOendo were detected. To figure out

15

the dyad contents as precisely as possible, the

16

PO29B71C were chosen and integration of the relevant peaks were performed. Based on

17

these values, the number-average sequence lengths (n) and the degree of randomness (R)

13C

NMR spectra of PO38B62C and


Page 18 of 38

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1

were estimated using the following equations,29 where NBC/OC, NBC/BC and NOC/OC

2

represent molar fraction of dyads, calculated from the integral intensities of signals of

3

carbonyl carbon from BO (or OB), BB, and OO dyads, respectively. nBC and nOC are the

4

number-average sequence length of BHEEB carbonate (BC) unit and OPD carbonate

5

(OC) unit, respectively. Data obtained are summarized in Table 1. Results indicated that

6

the value of R is close to 1 in both cases, leading to the conclusion that the microstructure

7

of PO38B62C and PO29B71C was essentially random. This conclusion was in good

8

agreement with those statistically calculated for the random co-PCs based on OPD and a

9

series of aliphatic or alicyclic diols reported in our previous paper,22 and can be

10

reasonably extended to PO16B84C and PO10B90C in this study.

11

𝑛BC =

12

𝑛OC =

13

𝑁BC/OC + 2𝑁BC/BC 𝑁BC/OC 𝑁BC/OC + 2𝑁OC/OC

𝑅=

𝑁BC/OC

1

𝑛BC +

1

(1) (2)

𝑛OC(3)

14

Thermal properties

15

The thermal stability of the synthesized PCs was comparatively investigated using

16

thermogravimetric analysis (TGA). TGA and TGA derivative curves of all polymers,

17

measured from 50 to 600 ºC under an inert nitrogen atmosphere, are shown in Figure 6.

18

The values of T5% and Td of the PCs are presented in Table 2. As expected, PBC exhibited

19

the highest thermal stability among all the polymers due to the excellent thermal stability

20

of BHEEB: thermal decomposition started to be detectable at about 295 ºC with onset



Page 20 of 38

1

temperature (measured for 5% of weight loss) as high as 350 ºC. A

2

two-step pyrolysis profile was observed for PBC, where the second one appears as a

3

shoulder of the first peak, with maximal decomposition rates at 395 and 422 ºC,

4

respectively. The first step can be attributed to the decomposition of carbonate linkage,

5

and the second step should be mainly caused by the degradation of the ether linkage

6 7

Figure 6. TGA traces of PCs at a heating rate of 10 °C min-1

8

in BHEEB. In contrast, POC showed a notable reduction in the thermal stability with a T5%

9

value 74 ºC lower than that of PBC. Furthermore, its thermal decomposition was found to

10

happen in a single step with the maximum rate at 312 ºC. Although thus far we have been

11

unsuccessful in elucidating this relatively lower thermal stability, it is reasonable to

12

assume

that

McLafferty-type

rearrangement

similar


to

the

thermolysis

of

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

isosorbide-containing PC is involved, as reported by Lee.30 With regards to co-PCs, as

2

expected, the thermal stability is tightly dependent on the molar ratios of BHEEB and

3

OPD units in the polymer chains. In fact, the replacement of BHEEB by OPD caused an

4

evenly decreasing effect in both onset and maximum decomposition rate temperature.

5

Furthermore, decomposition was found to proceed along three well differentiated stages,

6

which can be attributed to the weight loss of OC, BC short sequence, and ether linkage in

7

BHEEB, respectively.

8 9

Figure 7. Second heating DSC curves of PCs measured at a heating rate of 10 ºC min-1

10

In addition, the thermal behaviors of these PCs were determined by DSC at a

11

heating rate of 10 ºC min-1. The second DSC heating curves measured from 30–120 ºC

12

are shown in Figure 7, the thermal data are summarized in Table 2. A remarkable feature

13

of all samples is their amorphous structures with single-step glass transition temperatures.

14

PBC and POC have the lowest and the highest Tg values among all the polymers, which

15

were 54.7 ºC and 74.5 ºC, respectively, POxByCs have intermediate values between PBC

16

and POC that increased steadily with the increasing of OPD molar fraction (Figure 8).

17

For PBC, its amorphous feature is understandable because of the low packing ability of



1

BHEEB, which was caused by the presence of isopropyl groups and the rotation of

2

benzene rings in it. For co-PCs POxByCs, their amorphous feature is an expected result

3

considering the poor crystallization ability of the corresponding homo-PC (PBC and

4

POC). Moreover, their random structures determined by microstructure analysis is also

5

responsible for their amorphous features, because the regularity of the polymer chains

6

plays an important role in the crystallization.31 In the case of POC, the amorphous

7

characteristic and relatively lower Tg value is in sharp contrast with the results exhibited

8

by its analogue (abbreviated as pre-POC) reported in our previous article.22 In that case,

9

pre-POC

10 11

Figure 8. The variation tendencies of PCs’ Tg

12

composed of a mixture of two stereoisomers (endo-exo:endo-endo = 1:9) prepared in a

13

different synthetic protocol, was found to be semicrystalline with Tm and Tg of 109.1 and

14

80.4 ºC, respectively. Given that the main structural difference between POC and

15

pre-POC lies in the molar fraction of endo-exo isomers, which are 20 mol% and 10 mol%,

16

respectively, we speculate that the amorphous feature observed for POC find its origin in

17

the higher proportion of the endo-exo isomers. In fact, OPD is a conformationally


Page 22 of 38

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1

restricted diol that hard to crystallize due to its V-shaped bicyclic frame and the

2

puckering of the cyclopentane rings in it.22 Moreover, the molecular symmetry is another

3

important factor. Homo-PC based on the endo-endo isomers is prone to crystallize.

4

However, the incorporation of the endo-exo isomers will generate stereoirregular

5

structures due to its asymmetry, thus dramatically depress the crystalline ability of the

6

polymer chains: when the endo-exo isomer content is below 10 mol%, pre-POC is

7

semicrystalline; when this value is increased to 20 mol%, amorphous characteristic is

8

observed for POC because this proportion is high enough to prevent crystallization.

9

Concerning the Tg for POC, note that it has a Mn of 13100 g mol-1, which is about twice

10

of the value for pre-POC (Mn = 6700 g mol-1), a higher Tg is expected because Tg is

11

directly influenced by the polymer chain length.32 However,

12 13

Table 2 Thermal and mechanical properties of the synthesized PCs measured by TGA, DSC and DMA TGAa PCs

T5%

POC PO38B62C PO29B71C PO16B84C PO10B90C PBC aT

5%

(ºC)a

276 277 282 283 310 350

DSC Td

(ºC)a

312 294/378 296/383 301/384 306/394 395/422

Tg

(ºC)b

74.5 65.8 65.2 63.7 59.4 54.7

DMA E′ (20 ºC) (MPa)

Tg (ºC)c

−d − − 925 1063 1241

− − − 68.1 64.5 60.1

= temperature of 5% mass loss, Td = temperature for maximal decomposition rate. bTg = glass transition

temperature obtained from DSC during second heating at 10 ºC min-1. cDetermined using a TA DMA 2928 in a controlled force-tension film mode with a preload force of 0.1 N, an amplitude of 10 μm, and at a fixed frequency of 1 Hz at the range of -140 to 140 ºC in a heating rate of 3 ºC min-1. dNot determined due to their brittleness.

14

a significantly lower Tg was observed for POC (74.5 ºC vs. 80.4 ºC). Obviously, the

15

endo-exo isomers impart less rigidity to the polymer chains than the endo-endo isomers

16

and are thus detrimental to the Tg of the polymer. To further elucidate this apparent



Page 24 of 38

1

discrepancy, comparisons of a relevant series of isomeric polycondensates based on

2

isosorbide

3

2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO, cis-/trans-) were conducted and similar

4

thermal transition tendencies were observed. Storbeck reported the preparation of

5

aromatic polyesters from 2,5-furandicarboxylic acid and isohexides and indicated that the

6

isomannide-based polyester has a higher Tg than the corresponding isosorbide-based

7

polyester with similar molecular weight. However, the reason was not analyzed.33 Nash

8

et.al

9

2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO, cis-/trans-) and 1,3-propanediol, and

10

claimed that the cis-rich CBDO-based copolymer has higher Tg than the corresponding

11

trans-rich CBDO-based copolymer. Investigations on the structure-property ascribed the

12

higher Tg for the former to the formation of randomly coiled polymer chains, which was

13

caused by the kinks due to the molecular geometry associated with the cis-cyclobutane

14

ring.34 Similarly, the endo-endo and endo-exo isomer of OPD can be regarded as cis- and

15

trans-, respectively. The endo-endo isomer, having both carbonate linkages on the same

16

side of the bicycle frame, tends to induce kinked polymer chains, this could result in

17

more rigid, bent and sterically hindered structures, consequently stiffens the polymer

18

similar to corrugation. Given that the molar content of the endo-endo isomers in POC is

19

lower than that in pre-POC, we believe this could be the reason explaining why POC in

20

turn afforded lower Tg, despite having much higher Mn than the latter.

21

Dynamic mechanical analysis (DMA)

(endo-exo),

synthesized

a

isomannide

series

of

terephthalate


(endo-endo)

copolyesters

and

from

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

To gain insight into the effect of composition changes on the viscoelastic property, PBC,

2

PO10B90C and PO16B84C were all investigated by DMA. For PO29B71C, PO38B62C and

3

POC with relatively high OPD content, DMA tests were not performed due to their

4

brittleness. The thermo-mechanical data are recorded in Table 2, the storage modulus (E′)

5

and the loss factor tan δ as a function of temperature are shown in Figure 9 (a) and (b),

6

respectively. A drastic decrease in the E′ for all the tested samples was observed after

7

heating above Tg, further confirming their amorphous structures. Generally speaking, the

8

value of E′ tend to increase with increasing the rigid monomer content provided that the

9

samples have similar Mn.17 However, PO10B90C exhibited nearly identical E′ curve to that

10

of PBC in the whole temperature range, which can be ascribed to the combined effects of

11

its lower Mn compared to the latter and the low molar content of OPD units. In the case of

12

PO16B84C, a much lower E′ was observed, despite having the highest OPD content among

13

them, this is



1 2

Figure 9. Variation of (a) E′, and (b) tan δ with temperature for PBC, PO10B90C and PO16B84C.

3

understandable because its Mn is the lowest and this value is not enough to afford E′

4

comparable to PO10B90C and PBC. In Figure 9 (b), all samples showed relaxation peaks

5

above 50 ºC, which can be ascribed to the transition from glass to rubber, the temperature

6

where the tan δ reached the maximum was defined as the Tg of the polymer samples.

7

Obviously, the Tg values determined by DMA are slightly higher than that from DSC

8

(Table 2), which can be correlated with the heat transporting hysteresis for large-scale

9

samples in DMA. As expected, the Tgs shifted to higher temperatures with increasing

10

OPD content, which was in good accordance with the results from DSC. The increased Tg

11

reflected by DMA can be interpreted by the increased rigidity induced by OPD, which

12

increased the motion resistance of the chain segments. Moreover, the tan δ curves for


Page 26 of 38

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1

PO10B90C and PBC show low temperature transitions at around 40 ºC. The reason for this

2

inconspicuous transition is not obvious to us, the oligomers that contained in the resultant

3

polymers may be the reason and further investigation should be carried out.

4

Tensile properties

5

To evaluate the influence of the composition on the mechanical properties, tensile tests

6

were performed using the films prepared by casting as described in the Experimental

7

Section, the Young’s modulus, ultimate tensile strength and elongation at break are given

8

in Table 3, the recorded stress-strain curves are depicted in Figure 10. In general

9

polymeric materials, the mechanical property is mainly affected by molecular structure

10

and Mn. In consequence, PBC showed excellent Young’s modulus, ultimate tensile

11

strength, and elongation at break, which were 452.1 MPa, 11.2 MPa, and 19.5%,

12

respectively. The excellent mechanical property of PBC is presumably due to a

13

combination of its high molecular weight, moderate rigidity of molecular chain and

14

superior toughness caused by the BHEEB moiety. However, for POxByC, a

15

“ductile−to−brittle” transition occurred as the OPD content increased, resulting in a

16

substantial reduction in the tensile strength and extensibility. For PO10B90C and PO16B84C,

17

the ultimate strength was 6.3 and 3.5 MPa, the elongation at break was 12.6% and 7.9%,

18

respectively. This phenomenon can be ascribed to two reasons. Firstly, compared to PBC,

19

their lower Mn values in the range of 10000−15000 g mol-1 were not high enough to

20

exhibit satisfactory mechanical strength because it was strongly influenced by Mn.

21

Secondly, the brittleness caused by OPD is a common characteristic of rigid polymers,



Page 28 of 38

1

even at rather large molecular weights.17 In fact, the brittleness of glassy amorphous

2

polymers can be ascribed to the low entangled strand

3 4

Figure 10. Stress-strain curves of polymers at 25 °C and 3 mm s-1

5

density due to the rather stiff polymer chains, which predisposes the glassy polymer to

6

form weak crazes that break down readily to form cracks.35 Future studies will focus on

7

enhancing extensibility for thermally stable, bio-based plastic alternatives.

8

9

Table 3 Mechanical properties of PCs PCs

Young’s modulus (MPa)

Ultimate strength (MPa)

Strain at break (%)

PO16B84C

500.9 ± 11.3

3.5 ± 0.3

7.9 ± 1.3

PO10B90C

339.5 ± 39.0

6.3 ± 0.2

12.6 ± 1.1

PBC

452.1 ± 9.5

11.2 ± 0.9

19.5 ± 3.0

CONCLUSIONS

10

In conclusion, a series of homo-PCs and co-PCs that combine the chemical recycling of

11

BPA-PC with the use of novel bicyclic diol OPD, which is derived from naturally

12

occurring citric acid, has been successfully synthesized. Single crystal X-ray identified

13

the distinctive V-shaped skeleton of the endo-endo isomer of OPD composed of two

14

cis-fused cyclopentane rings similar to isohexides. By melt polymerization of different

15

molar feed ratios of OPD and BHEEB, co-PCs with relatively high molecular weights


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

were obtained in good yields. 1H NMR spectrum showed that the molar ratio of the

2

endo-endo and the endo-exo isomers in POC is close to 4:1, which is identical to the

3

initial value in OPD, confirming pretty similar reactivity. For POxByCs, the OPD content

4

are consistently lower than the initial feed ratio, which was caused by the lower reactivity

5

of OPD compared to BHEEB. Analysis of the dyad sequence distributions using the

6

expanded

7

measurements showed that all samples are amorphous with Tg values in the order of PBC

8

< POxByC < POC. For POxByCs, their Tg values increased steadily with the increasing of

9

OPD molar fraction, further confirming the excellent rigidity of OPD. In contrast with the

10

semicrystalline pre-POC, amorphous characteristic with a lower Tg of 74.5 ºC is observed

11

for POC, which is presumably caused by the variations in molecular geometry around the

12

different isomer sites. With the increasing of the OPD content in POxByC, a

13

“ductile−to−brittle” transition occurred, a result caused by the combined effects of low

14

molecular weights and the low entangled strand density due to the rather stiff polymer

15

chains. We are currently modifying the brittleness of POC using other flexible monomers

16

or polymers, by copolymerization or melt-blending, this will be addressed in our future

17

work.

18

Supporting Information

19

Synthetic routes of OPD from citric acid; 1H-NMR spectrum of POC; Crystal data and

20

structure refinement for the endo-endo OPD.

21

Conflicts of interest

22

There are no conflicts to declare.

23

Acknowledgement

13C

NMR spectra revealed the random microstructures for all co-PCs. DSC



Page 30 of 38

1

We are grateful to the National Natural Science Foundation of China (No. 51503150 &

2

No. 21674080), Tianjin Municipal Natural Science Foundation (No. 17JCQNJC03400 &

3

18JCZDJC37700), Training Project of Innovation Team of Colleges and Universities in

4

Tianjin (TD13-5020), 131 talents program of Tianjin, and the leading talents program of

5

Tianjin Educational Committee.

6

Notes and references

7

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For Table of Contents Use Only

14 15

A series of polycarbonates that combine the chemical recycling of BPA-PC with the use

16

of citric acid-based bicyclic octahydro-2,5-pentalenediol (OPD) were successfully

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high-Tg polymers.


Page 36 of 38

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1



For Table of Contents Use Only

Sustainable polycarbonates from a citric acid-based rigid diol and recycled BPA-PC: From Synthesis to Properties

Chengcai Pang*, Xueshuang Jiang, Yan Yu, Xiaohan Liu, Jingyan Lian, Jianbiao Ma, and Hui Gao*

A series of polycarbonates that combine the chemical recycling of BPA-PC with the use of citric acid-based bicyclic octahydro-2,5-pentalenediol (OPD) were successfully prepared, which showed potential as alternative candidate to replace petroleum-based high-Tg polymers.


Page 38 of 38

Sustainable Polycarbonates from a Citric Acid-Based Rigid Diol and

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