100% Atom-Economy Efficiency of Recycling Polycarbonate into

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2018, 6, 8964−8975

100% Atom-Economy Efficiency of Recycling Polycarbonate into Versatile Intermediates Chien-Hsin Wu,†,‡ Li-Yun Chen,†,‡ Ru-Jong Jeng,*,†,‡ and Shenghong A. Dai*,§ Institute of Polymer Science and Engineering, and ‡Advanced Research Center of Green Materials Science and Technology, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan § Department of Chemical Engineering, National Chung Hsing University, 145 Xingda Road, South District, Taichung City 40227, Taiwan Downloaded via UNIV OF SUNDERLAND on October 31, 2018 at 19:55:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: This study demonstrates a simple and convenient two-step one-pot, highly efficient process of recycling poly(bisphenol A carbonate), i.e., PC, into versatile intermediates for polymers such as polyurethanes (PUs). Via a highly efficient and selective amine carbonylation reaction, PC is depolymerized by aliphatic diamines forming hydroxyl-N,N′-diphenylene-isopropylidenyl biscarbamates (hydroxyl DP-biscarbamates) as major interim prepolymers. Both short- and long-chained prepolymers are prepared with their respective diamines, and the prepolymers are chain-extended with commercially available regents such as diisocyanates to produce a variety of PU polymers. Hence, PC is cleaved into pieces of soluble hydroxyl DP-biscarbamates first and then reassembled into new linear polymers without resorting to a separation process. Different from PC-recycling processes reported in the literature, each carbonate group of PC in this new process is fully utilized for making one carbamate group and one hydroxyl terminated intermediate in the absence of catalyst under mild conditions. Most significantly, this process attains 100% atom-economy efficiency and demonstrates the feasibility of converting one functional polymer into another. KEYWORDS: Polycarbonate, Recycling, Intermediate, Polyurethane

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INTRODUCTION Green chemistry and technology have attracted tremendous attention recently in both industrial research as well as academic studies to address chemical safety, atom economy, and material recycling. The recycling of plastics1 has occupied an important part of economic efforts by emphasizing waste management and reutilization of materials to solve the plastic waste dilemma. Polycarbonate (PC) has enjoyed tremendous growth due to its unique properties of transparency, toughness and temperature-resistance. The global production for PC exceeded 4.30 million tons in 2015 and is expected to reach over 7.0 million tons by 2024.2 PC has been applied to optical devices, medical, automotive industry, packing, and building. Nevertheless, the increasing demand of PCs has also created large quantities of production and postconsumer waste. Traditional waste treatments such as landfill or incineration are not environmentally sustainable. To solve this problem, the reuse of waste PC has been practiced by means of regrinding mechanically followed by re-extrusion. However, the recycled PC using these processes generally results in detrimental changes to PC performances.3 Other solutions based on chemical recycling through PC degradation or depolymerization of PC into raw materials has also been demonstrated by several researchers.4−8 Direct © 2018 American Chemical Society

pyrolysis of waste PC can recover some useful monomers. Tagaya et al.9 decomposed PC into phenol, bisphenol A (BPA), and 4-isopropenylphenol (IPP) at temperatures ranging from 230 to 430 °C using subcritical water. It was reported that the addition of Na2CO3 could accelerate the reaction to elevate yield to 67 wt %. However, the recycling of PC with hydrolysis method usually refers to the use of water under high temperature and pressure conditions. The use of microwave irradiation offers many advantages such as rapid heating without contact materials. Achilias et al.10 utilized microwave irradiation to complete PC degradation in a 5 or 10% w/v NaOH solution at 160 °C. Nevertheless, microwave power and pressure control are still challenging issues in terms of feasibility. As an alternative, alcoholysis of PC has drawn much attention to recover waste PC. Sato et al.11 found that more than 40 wt % of PC was decomposed into reusable chemicals including BPA and IPP in the presence of solvents such as tetralin, decalin, or cyclohecanol at 440 °C. The maximum recovery yield of bisphenol A was merely 59.4 wt % with catalyst in nonpolar solvent. Quaranta et al.12 reported a Received: March 23, 2018 Revised: May 27, 2018 Published: May 28, 2018 8964

DOI: 10.1021/acssuschemeng.8b01326 ACS Sustainable Chem. Eng. 2018, 6, 8964−8975

Research Article

ACS Sustainable Chemistry & Engineering PC alcoholysis protocol in solvent-free conditions. The use of organocatalyst DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) promotes selective quantitative conversion of PC resulting in satisfactory product yields of 60% ethyl methyl carbonate. Nacci et al.13 conjugated the quaternary tetraalkylammonium ionic liquids with the Lewis acid properties of ZnO nanoparticles for the PC depolymerization process. As a result, PC was completely depolymerized at 100 °C after 7 h. This process is capable of converting two industrial wastes of PC and glycerol into two valuable chemicals such as BPA and glycerol carbonate. Oku et al.14 and our group15 have developed more convenient alkoxylation processes to depolymerize PC with a carbonylation agent in glycols at 180 °C. The cyclic carbonate intermediates were obtained from the reaction of PC and ethylene or propylene glycols for achieving further ethoxylation or a propoxylation process. Unfortunately, the use of catalysts was inevitable in order to increase product yield but resulted in carbon dioxide as a byproduct. To establish a more efficient approach for recycling PC without losing its mass or having CO2 problems, the aminolysis method seems to be an attractive alternative having carbonates treated with aliphatic amines.16 Oku et al.17 have described the treatment of waste PC with N,N′-dimethyl-1,2-diaminoethane (DMDAE) to obtain BPA and 1,3-dimethyl-2-imidazolidinone (DMI). However, the costly separation process between them became problematic after depolymerization. Alternative approaches were to convert the carbonate groups of PC into formation of urea or carbamate derivatives. Singh et al.18 treated the waste PC with primary amines, and his depolymerization process was carried out neat in the absence of catalysts. Useful urea derivatives such as N,N′-bis(cyclohexyl)urea were obtained in about 81 wt % yield without resorting to complicated separations. The development of a successful polymer recycling is hinged upon an efficient depolymerization process to produce useful intermediates in high yields and yet avoiding complicated purification processes.19,20 During our recent green chemistry study on nonphosgene route21 to PUs, we found that carbonylation of aliphatic amines with diphenyl carbonate could yield N-phenyl carbamates selectively with only phenol as the coproduct. The key lies in the use of suitable solvents such as anisole or ethylene diethyl ether where N-phenyl carbamates were formed readily and selectively with minimum formation of urea as the byproduct under ambient temperatures. With the realization of this high yield carbamate synthesis, we thus extended our study on aminolysis of polycarbonate (PC) where diphenylene carbonate moieties were present in the PC as the major structural backbones. In the PC aminolysis, we further assumed that all carbonate linkages shall be cleaved into carbamate intermediates but will yield phenolic hydroxyl-DPcarbamate intermediates as products. Hence, if we hypothesized the addition of fresh diisocyanates or diisocyanate-terminated prepolymers into the products of PC aminolysis, we should be able to reconnect these small pieces of hydroxyl-DP-carbamate intermediates into long-chained PU polymers. In theory, PC can be converted into PUs in the two-steps, aminolysis and recombination, in one-pot without isolating or purifying intermediates. This new PC digestion and recycle approach has been tried and successfully realized. The strength of this work is described in Table 1.

Table 1. Strength in This Work contemporary strategies high temperature of PC cleavage mostly catalyzed processes multiple products formation separation needed low atom recovery

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this work mild noncatalyzed PC aminolysis condition without the need of catalyst only hydroxyl terminated intermediates direct use after depolymerization 100% atom utilization convenient entry into PDMS

RESULTS AND DISCUSSION Selective Transformation of PC into Hydroxyl Terminated DP-Carbamate. PC Aminolysis. If aminolysis of PC carbonate groups could selectively yield diphenolic hydroxyl DP-carbamates for PU synthesis as shown in Scheme 1 (step 1), their subsequent applications to PU would be easily accomplished by the addition of diisocyanates linking the hydroxyl groups generated in the PC aminolysis. In other words, the selective transformation of PC into hydroxyl DPcarbamates would allow us to calculate their OH numbers readily. The reason is that the equivalent number of carbonate units in PC polymer chains can be calculated by the sample weight (Wpc) of PC divided by 254 (the repeating unit formula weight of BPA carbonate, i.e., equivalent weight (EW) of BPA carbonate). Based on the model transformation shown in step 1 of Scheme 2, the amine equivalent needed for the total transformation of the carbonate groups and the OH-equivalent produced from the aminolysis would be the same. However, in the case where there are urea byproducts formed along with OH-terminated DP-carbamates during aminolysis, the urea byproducts would affect the precise estimation of hydroxyl equivalents. Hence, a highly selective chemical reaction of PC is essential to the success of our aminolysis scheme. Model Study of PC Aminolysis by Using N-Hexylamine (HA). In our initial aminolysis of the PC effort, HA was employed as the model agent for demonstration of selectivity and optimization studies shown in Scheme 2. It was found that solvent selection played a crucial role in the PC aminolysis, which is consistent with the results found in our previous study of amine carbonylation reaction.22 As the HA-aminolysis of PC was carried out in ester solvents such as butyl acetate, only 3 mol % of the urea byproduct was found after 24 h (Figure S1). Due to poor solubility of PC, the conversion to the carbamate major products was slow. For those aminolyses carried out in high polar solvents such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethylacetamide (DMAc), conversions to urea byproducts was found to be >38 mol % because of subsequent attack of HA to the initial carbamate products (Figure S2).22 The result agrees with a previous report observed by Singh et al.,18 who indicated that urethanes are susceptible to further attacks by amines in polar solvents. As shown in Scheme 2, these reactions also indicated that the excess of amine is detrimental to the selectivity of carbamate formation. We found that using anisole as the solvent not only overcomes the solubility problem of PC in the mixing but also gives the targeted phenolic hydroxyl carbamates in high selectivity. The result was shown in the monitored IR spectra (Figure 1a). Before the aminolysis by HA, PC exhibited a strong carbonate carbonyl stretching at 1775 cm−1. This peak diminished after the addition of about 0.9 equiv of HA to PC. In this case, the IR peak at 1775 cm−1 was reduced, but the 8965


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ACS Sustainable Chemistry & Engineering

Scheme 1. Conversion of PC into New PUs in One-Pot Two-Step Process: (1) Aminolysis Process of PC by Various Amine Agents and (2) Preparation of PUs

Scheme 2. PC Aminolysis by Using HA as Amine Agent (a Model Study): Formation of (1) Carbamates and (2) Undesirable Urea Group Due to Excess Amine Agent

Figure 1. IR spectra of products derived from PC aminolysis (a) with various equivalents of HA and (b) with 1.02 equiv of HDA, diaminosiloxane (EW 800), and jeffamine D-600. (c) PUs prepared from products derived from PC aminolysis.

residual 1775 cm−1 peak was still visible. A new strong adsorption at 1715 cm−1 confirmed that carbamates were major products. At the estimated (HA/ncarbonate) molar ratio of 1.02,

no more carbonate absorption could be discerned. An IR peak of 1645 cm−1 referring to the carbonyl stretching of urea was also insignificant. This evidence suggests that the high selective 8966


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Figure 2. 1H NMR spectra of products of PC aminolysis in 1.02 equiv of HA.

Figure 3. 1H NMR spectra of products of PC aminolysis in 1.02 equiv of HDA.

readily estimate the selectivity of carbamate products to be 98 mol %. This PC aminolysis reaction by HA was tracked closely by the 1H NMR (Figure S5). These results demonstrate the efficiency of the aminolysis process from the transition of the chemical structure as shown in 1H NMR peaks of pristine PC (0.00 equiv), partial aminolysis of PC (0.90 equiv), complete aminolysis of PC (1.02 equiv), and the formation of urea byproducts when HA was in excess (1.25 equiv). Selective Transformation of PC into Hydroxyl N,N′Diphenylene-isopropylidenyl Carbamates Intermediates. Aminolysis by Using n-Hexamethylenediamine (HDA) and Diaminosiloxanes. To investigate the feasibility of transforming PC into polymer intermediates, HDA as a diamine reagent was added into PC to obtain hydroxyl DPcarbamates for PU synthesis using the same PC aminolysis

conversion of PC into BPA and phenolic carbamates products was achieved. The PC aminolysis products, after HA treatment, were further analyzed by the 1H NMR as shown in Figure 2. The pristine PC has an aromatic chemical shift centered at 7.24 ppm in DMSO-d6, which was the peak corresponding to aromatic protons of PC and anisole (Figures S3 and S4). This peak was shifted to a lower field with three splitting patterns after HA treatment, indicating the occurrence of PC depolymerization. Two new peaks at around 7.61 and 9.11 ppm were found and were assigned to be the new NH resonance of carbamate proton 23 and the OH resonance of phenolic proton, respectively. The tiny peak at 5.67 ppm was assigned to the NH proton of the urea byproduct. Based on these 1H NMR assignments and their respective integration areas, we can 8967


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Figure 4. 1H NMR spectra of products of PC aminolysis with 1.02 equiv of diaminosiloxane (EW = 800).

of isopropylidene methyl groups substituted to the phenolic rings in d7-DMF had been shifted to upper fields to 1.75 ppm for the siloxane containing DP-carbamates. In addition, the splitting patterns of the peaks become more tightly packed together with peak separation of only 16.3 cps instead of being 19.6 and 20.3 cps observed for those PC aminolysis products made from HA and HDA, respectively. PC Aminolysis by Using Jeffamines. Jeffamines D-230, D400, D-600, and D-2,000 are the commercial available longchained polypropylene ether diamines with flexible repeating units in their skeletons. They were also tried as our PC aminolysis agents for making phenolic polyols as soft-segment for PU syntheses. In fact, the reactivity of diamines in Jeffamines was far lower than those of HDA and siloxane diamines. This is due to the steric effect since their primary amino groups are attached to the secondary carbons.24 Consequently, slower incomplete conversions of PC were generally observed. Even using the catalyst system of 1,3,5tris[3-(dimethylamino)propyl]-hexahydro-1,3,5-triazine and isobutyric acid25 to promote the transformation, the yields of DP-carbamate products were still low and urea byproducts were found as high as 22 mol % according to the 1H NMR integration analyses (Figures S7 and S8). Further process improvements on the aminolysis of Jeffamines are still needed to rectify the selectivity problems. Carbamate Percent Distribution in the Aminolysis Products. Another attempt to estimate product distribution among the three major types of products in HA aminolysis process by 1H NMR was investigated in our study. This selectivity study tried to gain information on the preference of amine attacks on PC carbonate sites either at the internal or terminal carbonate groups for the HA/PC model system. The molar ratios of the product mixture containing BPA, DP-monocarbamate, and DP-biscarbamate in HA/PC aminolysis were determined by comparing three splitting absorptions in 1H NMR visibly located at 1.48, 1.54, and 1.59 ppm (Figures S9 and S10). Their ratios were obtained by taking the

process. The experiments were carried out by addition of diamine solution into PC anisole solution, and the mixed solution was stirred at 80 °C for 2.5 h. The aminolysis process monitored in IR spectra shown in Figure 1b exhibited the disappearance of the carbonate absorption peak at 1775 cm−1 and an emerging new carbonyl carbamate peak at 1715 cm−1. In this case, the peak at 1775 cm−1 of residual carbonate carbonyl group completely disappeared. This successful transformation of PC into dihydroxyl DP-carbamates was also confirmed by the 1H NMR as shown in Figure 3. The selectivity of the transformation was estimated to be >97 mol %. For an additional extension of this transformation, we also used long-chained diamines as the aminolysis agents. Oku et al.17 found that PC converted to cyclic urea derivatives when treated with dimethyl ethylenediamine or diethyl ethylenediamine. Their results seem to indicate that the cyclization of diamines into 5- or 6-membered ring cyclic ureas occurred preferentially when the short-chain diamines were used in the aminolysis process. On the other hand, the aminolysis with long-chain diamines has not been done, and it seems to be an attractive synthetic route for making flexible PU prepolymers. Diaminosiloxanes were chosen because they have long straight-chain skeletons of polydimethylsiloxane (PDMS) in the center section of the chain along with two terminal propylene diamines as their end-groups. Siloxane linkages are known to impart good flexibility and desired hydrophobicity to the polymer backbones. In our study, diaminosiloxanes with molecular weights of 260 (EW = 130), 860 (EW = 430), 1600 (EW = 800), 3000 (EW = 1500), and 4400 (EW = 2200) were added individually to the PC solution under our standard aminolysis condition. After 2.5 h of reaction at 75 °C, PC was found to be completely converted to BPA and dihydroxy DPcarbamates as shown in Figure 1b. The chemical structure was further confirmed by the 1H NMR as shown in Figure 4 (also see 1H NMR for diaminosiloxane in Figure S6), and the carbamate selectivity was as high as 98 mol % estimated through 1H NMR analysis. It is noted that the 1H NMR peaks 8968


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Figure 5. Assignments of 1H NMR characteristic peaks of methyl units between various phenolic derivetives. Chemical structure illustrations of (a) and their structural characterizations of (b) pristine PC, (c) pristine BPA, (d) purified DP-biscarbamate, and (e) mixture of phenolic and diphenolic carbamates of methyl units. Determination the molar ratio of mixtures from 1H NMR spectra of PC aminolysis products in amine agents of (f) 0.90 equiv HA, (g) 1.02 equiv HA, (h) 1.25 equiv HA, (i) 1.02 equiv HDA, (J) 1.02 equiv diaminosiloxane (EW800), and (k) 1.50 equiv jeffamine D-600.

Table 2. Selectivity and Molar Ratio of Raw Products from PC Aminolysis

a Diaminosiloxane of X-22−161A (EW = 800). bJeffamine of D-600 (EW = 264). cSelectivity of urethane were calculated according to the integrals of peaks in 1H NMR spectra. S (%) = δ urethane/(δ urethane + (1/2) δ urea). dMolar ratios calculated according to the integral areas in Figure 5.

low field to high field of 1.54 to 1.40 ppm, respectively. Therefore, the molar ratio between OH terminated DPbiscarbamates, DP-monocarbamates, and BPA were calculated to be in a ratio of approximately 1:2:1 for those products of PC HA aminolysis as shown in Figure 5f−k and Table 2. These relative ratios imply that aminolysis of PC proceeds through a statistically random fashion without any particularly preference for amination of PC at either center or terminal carbonate groups of the PC chain. Similarly, the product ratios of α:β:γ =

integration ratios between the three peaks. These three peaks were assigned to the gem-dimethyl of isopropylidenyl groups substituted between two phenolic derivatives as shown in Figure 5a−e, respectively. Due to the fact that para-biscarbamate groups in Figure 5h have the strongest electronwithdrawing effect than those of phenol groups, the dimethyl peaks of biscarbamate were assigned the lowest chemical peak at 1.59 ppm. Accordingly, methyl protons of 8β (DPmonocarbamate) and those of 8γ (BPA) were assigned from 8969


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ACS Sustainable Chemistry & Engineering Table 3. Formulations, Compositions, and HS Contents of PUs Prepared from Products of PC Aminolysis compositions (grams) samples

diamine (Ew g/eq)a

PCb

amine agentc

P1-AM100 P1-AH100

HDA (EW = 58.1) HDA (EW = 58.1)

3 3

0.70 0.70

P2-SM75 P2-SM47 P2-SM32 P2-SM20 P2-SM15

PAM-E (EW = 130) KF-8010 (EW = 430) X-22-161A (EW = 800) X-22-161B (EW = 1500) KF-8012 (EW = 2200)

3 3 3 3 3

1.56 5.18 9.63 18.06 26.48

P3-AM-L50f P3-AM-G50g

HDA (EW = 58.1) HDA (EW = 58.1)

3 3

0.70 0.70

diisocyanatesd PU-1 1.58 1.06 PU-2 1.58 1.58 1.58 1.58 1.58 PU-3 2.05 2.33

HS (wt %)e

Mw × 104 (g/mol)

PDI

film formality

100.0 100.0

6.2 4.4

5.64 4.27

Χ Χ

74.5 47.0 32.2 20.2 14.7

2.7 3.1 2.9 3.6 2.5

3.95 4.77 4.11 5.23 4.50

Ο Ο Ο Ο Ο

50.0 50.0

5.4 3.3

4.34 4.80

Ο Ο

a Equivalent weights of amines (EW) were calculated by dividing the molecular weight by the number of amines. bPC with equivalent repeat-unit molecular weight of 254.2 g/eq (EW = 254.2). cCalculated equivalent weight of amines to PC is 1.02 for HDA or PDMS. dCalculated equivalent weight of amine agents is 1.05 for MDI (EW = 125.13) and HDI (EW = 84.1). eHard segment contents (HS) of PUs are calculated according to the following formula: HS = (PC(g) + MDI(g) + HDA(g))/(overall PU(g)). fPU-3 based on PCL3000 (EW = 1500, 5.48 g) as soft segment. gPU-3 based on PTMEG 2000 (EW = 1000, 5.74 g) as soft segment.

Figure 6. Thermal properties of PU-1: (a) TGA and (b) DSC.

24.7:50.1:25.2 are also found to be 1:2:1 for HDA aminolysis of PC products as shown in Figure 5i. PUs Derived from Hydroxyl DP-Carbamate Intermediates. As shown in Scheme 1 and Table 3, PU-1 was synthesized based on a short-chained HDA as an aminolysis agent, whereas PU-2 was synthesized based on a long-chained diaminosiloxane as an aminolysis agent. In addition, PU-3 was synthesized by using the digested DP-carbamates as chain extenders via a typical two-step synthetic process. All of the PU products from aminolysis of PC are assigned as P1 (PU-1), P2 (PU-2), P3 (PU-3), and R (reference PUs). These PUs composed of various building blocks are denoted as -XYnn or -XY-Znn, where X is the diamine agent or diol compound used such as HDA (A), diaminosiloxanes (S), or dihydroxyl monomers for PU references including 1,6-hexanediol (HDO, O) or BPA (B); Y is the diisocyanate, MDI (M) or HDI (H); Z is the commercial soft segment component such as polyester diols PCL 3000 (L) or polyether diol PTMEG 2000 (G); and nn is the hard segment (HS) content in wt % calculated by HS % = {1 − [weight of soft segments (PCL3000, PTMEG2000, or diaminosilaxanes)/weight of total polymer]} × 100%. For example, in the PU-3 series, P3-AM-L50 is composed of DP-carbamates by using HDA as the diamine agent for aminolysis process, MDI as diisocyanate, and PCL3000 as soft segment with HS of 50 wt %. Furthermore, the reference samples for P3-AM-L50, such as R-OM-L50 (HDO, MDI, and PCL3000) and R-BM-L50 (BPA, MDI, and PCL3000) with

50% HS from commercial raw materials were synthesized to study the structure−property relationship of PU products. Preparation of PUs Directly from DP-Carbamates Using a Short-Chained Aminolysis Agent (PU-1). The mixtures of BPA and hydroxyl DP-carbamate intermediates obtained in the aminolysis of PC (as shown in step 2 in Scheme 1 and Table 3) were used as the prepolymers for our subsequent preparation of PU-1. As indicated in the formulations in Table 3, the diisocyanate, either MDI or HDI, was directly added into the crude products in anisole solution at 75 °C followed by the addition of dry DMAc and tin catalyst (T-9) to obtain PUs with high molecular weights. The phenolic hydroxyl groups from the products of PC aminolysis reacted with an equal equivalent of diisocyanate within 3 h in the formation of PU-1. The polymerization progresses were monitored by inspecting complete disappearances of the −NCO absorption at 2270 cm−1 in IR spectra. Powder-like PU-1 was isolated after removal of the solvent in the PTFE dish. As shown in Figure 1c, the PU1 product of P1-AM100 exhibited IR urethane absorption at 1716 cm−1 indicating a successful transformation into PUs. The molecular weights of PU polymers ranged from 40 000 to 65 000 for P1-AM100 and P1-AH100, respectively, based on the GPC analysis (Table 3). The thermal properties of PU polymers were characterized by thermogravimetric analysis (TGA) as shown in Figure 6a, and phase behaviors were inspected by the second round heating (at 10 °C/min) using DSC as shown in Figure 6b. In 8970


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Figure 6a, P1-AM100 (280−290 °C) has higher peak temperatures of maximum thermal degradation than that of P1-AH100 (270−280 °C) from their derivative thermogravimetric analysis (DTG) profiles. The results of TGA revealed that the PU-1 chain extended with aromatic MDI has a slightly higher thermal stability than that for P1-AH100 made from aliphatic HDI. For PU-1 prepared from aminolysis PC, HDA derived intermediates were rigid short DP-carbamate prepolymers, and they turned into the HS chains of PUs after MDI or HDI additions. Both PU-1 products are powder-like solids with glass transitions (Tg) of 45.0 and 29.2 °C for P1-AM100 and P1AH100, respectively. No melting temperatures were found in DSC as shown in Figure 6b. This implies that the crystallization of PU-1 is seemingly hindered structurally by the existence of two different urethane blocks, one from products of PC aminolysis and the other from MDI addition. Furthermore, the BPA structural feature possesses isopropylidenyl blocks in the PU product skeletons, and they also contribute to their lack of crystallinities26 through disturbance of their chain−chain packing. In the other words, the PUs made from carbamate diols of PC aminolysis will yield mostly amorphous materials. In order to prove the above statements, reference PU-1’s such as R-OH100, R-OM100, R-BH100, and R-BM100 were synthesized for comparison (Figure 6b). R−OH100 and ROM100 were derived from reacting aliphatic 1,6-hexanediol with MDI or HDI, respectively. Tms’s of both PU-1’s were distinctively present at 187.6 and 160.1 °C in the respective DSC thermograms. On the contrary, reference PU-1’s such as R-BH100 and R-BM100 with BPA as the diol raw materials exhibited only a Tg at around 30 °C. Based on the above, we conclude that BPA plays a decisive factor in thermal phase transition of PUs. The presence of the BPA structure in PUs will significantly suppress the crystallinity leading to amorphous products.26 Preparation of PUs Directly from DP-Carbamates Using a Long-Chained Aminolysis Agent (PU-2). By using the longchained diamines as a PC aminolysis agent, DP-carbamates with flexible repeating units of PDMS (via diaminosiloxanes) in their skeletons have been synthesized. The mixtures of BPA and DP-carbamates with flexible PDMS units obtained in the aminolysis of PC (as shown in step 2-1 in Scheme 1 and Table 3) were used as the prepolymers for the subsequent preparation of PUs. The synthesis of PUs was followed immediately by direct addition of MDI to the crude products of PC aminolysis solution at 75 °C. Subsequently, dry DMAc was added to the solution to obtain PUs with high molecular weights. It was reported that the syntheses and processing of siloxane PU previously were generally met with some difficulties due to

its immiscibility of PDMS diols/or diamines with traditional PU raw materials and solvent.27 However, by using our twostep, one-pot process, PDMS based PUs such as PU-2 series were synthesized easily without complications. The siloxane modified prepolymeric carbamate diols from PC aminolysis appeared more compatible with other reagents than pure siloxane reagents, and the syntheses of elastomeric PUs did not encounter any problems. PU thin-films were prepared, and the products were cast into PTFE dish resulting in isolation of low-colored transparent films. As shown in Figure 2c, the product samples exhibited an IR peak of urethane absorption at 1742 cm−1 for P2-SM32 in the absence of the isocyanate peak at 2265 cm−1. The molecular weights of PU polymers range from 25 000 to 35 000 for PU-2 based on the GPC analyses. Thermal properties were measured by TGA as shown in Figure 7a, and phase behavior were measured by the second round heating (at 10 °C/min) using DSC as shown in Figure 7b for the PU-2 samples. In the TGA thermograms of Figure 7a, DTG peak temperatures shifted from approximately 265 °C for the hard segment-rich P2-SM75 sample to higher temperatures at ca. 510 °C for the soft segment-rich P2SM15 sample. When compared to the PU polymers comprising ether or ester based soft segments, the PU polymers with PDMS based soft segments exhibited higher DTG peak temperatures. For the DSC investigations (Figure 7b), the glass transition temperatures are proportional to the hard segment contents, showing a Tg of 27.7 °C for P2-SM75 and a Tg of −14.1 °C for P2-SM47. For the polymers with hard segment contents lower than 47 wt %, the glass transition temperatures were not pronounced due to the association with a much lower Tg (usually lower than −80 °C) for the PDMS soft segment.28 Preparation of PUs via a Two-Step Process Using Hydroxyl DP-Carbamates as Chain Extenders (PU-3). The properties of PUs are highly dependent on the types of the building blocks, and their relative ratios between the hard- and soft-segment contents. Since the HS contents already have been set by using PC derived DP-carbamate prepolymers of this study as shown in step 2 in Scheme 1 and Table 3, the possibility for manipulating PU properties was explored by mixing the crude DP-carbamates products as chain extenders for segmented PU syntheses in a two-step process. These elastomeric PUs were prepared from reacting soft segment PCL 3000 or PTMEG 2000 with diisocyanates in PU formulations. PCL 3000 or PTMEG 2000 employed in this synthesis are both well-known partially crystalline diols. Therefore, new PU-3 polymers with 50 wt % PC based hard segment contents were reacted with these diols to form new PUs as shown in Table 3. All of the 8971


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Figure 9. Performances of PUs: (a) P2-SMS32 exhibited a transmittance as high as 90% in the range of 400−800 nm of visible light in UV−vis spectra and good hydrophobicity with a contact angle (CA) of as high as 105 ± 3° and (b) UTM properties of P3-AM-G50 and P3-AM-L50.

phase-mixed behaviors. Thus, this leads us to conclude that the prepolymers of PC aminolysis (BPA and BPA containing carbamates) will have overriding effects on the resulting PU performances. This phenomenon is similar to the case in PU-1 and PU-2. Based on the above, different from the traditional PUs, these PUs (PU-1, PU-2, and PU-3) derived from the components of PC aminolysis would have part of urethane linkages consisting of BPA and diisocyanates. Because of this, some of the PU samples exhibited only one glass transition temperature without showing any sign of crystallinity. Performance of PUs from Products of PC Aminolysis. The properties of PU polymers are highly dependent on the PC depolymerized products from using different diamines agents, since the lengths and the types of the diamine’s backbones are responsible for the rigidity or flexibility of new PU prepolymers. Both PU-1 samples (P1-AM100 and P1-AH100) were powderlike solids without any film forming tendency. The use of this symmetrical short-chain HDA compound as aminolysis agent would bring about rigid DP-carbamate prepolymers for PUs. For PU-2 and PU-3 series prepared in this work, all of the samples could be cast into films for property evaluation as shown in Figure 9. The PU samples in the PU-2 series made from diaminosiloxane-containing prepolymers and MDI are elastomeric materials that could be cast into films readily. It was observed that the PDMS based PUs were found to be transparent and hydrophobic.29 As demonstrated in Figure 9a, films made from P2-SM32 exhibited the transmittance as high as 90% in the range of 400−800 nm along with good hydrophobicity with a contact angle (CA) as high as 105 ± 3° (CA < 90 o for ordinary polyether or polyester based PU30). These characteristics were a result of amorphous PU hard segments from the PC derived structures as mentioned earlier.

polymerizations were monitored by IR spectra of the PU products until no isocyanate absorption was present. PU thinfilms were prepared by removal of the solvent in the PTFE dish. As shown in Figure 2c, the product samples exhibited an IR peak of urethane absorption at 1725 cm−1 indicating the successful transformation into PUs. The molecular weights of PU polymers range from 30 000 to 55 000 based on the GPC analyses for P3-AM-L50 and P3-AM-G50. All elastomeric PU-3 samples prepared in this approach could be cast into films for properties evaluation. Thermal properties were measured by TGA as shown in Figure 8a, and phase behaviors were measured by second round heating (at 10 °C/min) using DSC as shown in Figure 8b. In Figure 8a, both PUs based on reused DP-carbamates (P3-AML50 and P3-AM-G50) exhibited similar thermal stability with two distinct peaks in the DTG thermograms. The first peak at around 275 °C is attributed to the degradation of the BPA-MDI segment of PU. The second peak at around 400 °C is attributed to destruction of polyester (PCL 3000) or polyether (PTMEG 2000) portions of PUs. In the DSC thermograms of Figure 8b, only one glass transition, Tg = −13 °C for P3-AM-L50 and Tg = −39 °C P3AM-G50, could be observed for PU-3. Unlike PU-1, their glass transition temperatures are below room temperature because PU-3 is composed of 50 wt % soft segments, which normally exhibited a glass transition at about −50 °C for pristine PCL 3000 or PTMEG 2000. Surprisingly, the melting of these semicrystalline soft-segments of PCL 3000 and PTMEG 2000 after being incorporated in the PU was not observed. It appears that the linkages of those diols with the components of PC aminolysis having multiple DP-carbamates segments have formed well phase-mixed morphologies. During the DSC investigation, R-BM-L50 and R-BM-G50 have exhibited similar 8972


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polystyrene as standards (Mn = 4000, 10 720, 200 000, and 300 000 from Varian Inc.). The glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) were measured under a N2 atmosphere using a differential scanning calorimeter (DSC, TA Instruments, TA-Q20) operated at a heating rate of 10 °C min−1. Thermogravimetric analysis (TGA) was carried out using a Q50 thermogravimetric analyzer (TA Instruments) operated under nitrogen atmosphere at a heating rate of 10 °C/min. Tensile strength measurements were carried out using a Gotech testing machine (AI-3000, Gotech Detection Device Co., Ltd., Taiwan) with a cross-head speed of 100 mm/min. Tensile specimens of SPUs were made according to ASTM D638 specification. General Procedure for Aminolysis of PC into BPA and DPCarbamates. Aminolysis of PC with HA. In a typical aminolysis procedure, PC (3 g, EW = 254.2) was dissolved in 15 mL of anisole at 85 °C in a 250 mL two necked flask equipped with a magnetic stirrer, reflux condenser, thermometer, and nitrogen inlet. Subsequently, the temperature was cooled to 75 °C, and a diluted solution of 1.02 equiv HA (1.22 g, EW = 101.2, 12.0 mmol in 15 mL of anisole) was added dropwise into the polymer solution. The reaction was kept at the same temperature and carried out for 2.5 h until the disappearance of the carbonate functional group, which was determined by IR or 1H NMR spectra. 1H NMR (400 MHz, DMSO-d6): δ(ppm) = 0.83(t, 3H), 1.22 (m, 2H), 1.40 (s, 2H), 1.44−1.64 (m, 6H), 2.99 (t, 2H), 6.40−7.30 (m, 8H), 7.61 (m, 1H), 9.10(s, 1H). FT-IR (KBr): absence of 1775 cm−1 (carbonyl stretching of carbonate), 1715 cm−1 (carbonyl stretching of urethane), absence of 1645 cm−1 (carbonyl stretching of urea). Aminolysis of PC with HDA. PC (3 g, EW = 254.2) was conducted according to the investigated condition of HA in PC. At first, PC was dissolved in the 15 mL of anisole at 85 °C in a 250 mL two-necked flask equipped with a magnetic stirrer, reflux condenser, thermometer, and nitrogen inlet. Subsequently, the temperature was cooled to 75 °C, and a diluted solution of 1.02 equiv HDA (0.70 g, EW = 58.1, 6.0 mmol in 15 mL anisole) was added dropwise into the polymer solution. The reaction was kept at the same temperature and carried out for 2.5 h until the disappearance of the carbonate functional group, which was determined by IR or 1H NMR spectra. 1H NMR (400 MHz, DMSO-d6): δ(ppm) = 1.25 (m, 2H), 1.41 (m, 2H), 1.44−1.64 (m, 6H), 3.02 (t, 2H), 6.55−7.30 (m, 8H), 7.68 (m, 1H), 9.12 (s, 1H). FT-IR (KBr): absence of 1775 cm−1 (carbonyl stretching of carbonate), 1717 cm−1 (carbonyl stretching of urethane), absence of 1645 cm−1 (carbonyl stretching of urea). Aminolysis of PC with Diaminosilaxanes. Similar procedures were performed by the more solvent for dissolving PC (3 g, EW = 254.2) in 30 mL of anisole at 85 °C in a 250 mL two necked flask equipped with a magnetic stirrer, reflux condenser, thermometer, and nitrogen inlet. Subsequently, the temperature was cooled to 75 °C, and a diluted solution of 1.02 equiv diaminosiloxane (9.63 g, EW = 800 in 30 mL of anisole) was added dropwise into the polymer solution. This reaction was kept at the same temperature and carried out for 2.5 h until the disappearance of the carbonate functional group, determined by the IR or 1H NMR spectra. 1H NMR (400 MHz, DMSO-d6): δ(ppm) = 0.27 (m, 66H), 0.77 (t, 2H), 1.74−1.84 (m, 6H), 3.29 (t, 2H), 6.77−7.48 (m, 8H), 7.73 (s, 1H), 9.47 (s, 1H). FT-IR (KBr): absence of 1775 cm−1 (carbonyl stretching of carbonate), 1717 cm−1 (carbonyl stretching of urethane), absence of 1645 cm−1 (carbonyl stretching of urea). General Procedure for Preparation of PUs from PC Aminolysis. Polymerization in One-Pot by Using DP-Carbamate Intermediates as PU Prepolymers (PU-1 and PU-2). The as-prepared mixture of BPA and DP-carbamates was subjected to a vacuum to remove the anisole. Subsequently, 10 mL of dry DMAc as solvent was added into the viscous slurry and then the diisocyanates of MDI or HDI with 1.05 equiv of aminolysis agents were poured into the solution. The reaction mixture was stirred at 70 °C with T-9 as the catalyst. After 3 h of heating, the polymer solution was cast into an aluminum pan of 5 cm in diameter, and the solvent was removed in a circular oven for 24 h to obtain PU (PU-1 and PU-2) thin films with thicknesses that ranged from 200 to 280 um. Direct preparation of

Through the incorporation of polyols like PCL3000 or PTMEG 2000, the resulting PUs exhibited elastomeric properties with high elongations of ∼1100% and 750%, as shown in Figure 9b. Both polymers (P3-AM-L50 and P3-AMG50) in the PU-3 series exhibited lower stress at break but larger elongation. This is because the PC digested products consist of multiple DP-carbamates prepolymers. Therefore, these PU products behaved mostly like elastomers with high elongation but generally deficient in yield stress. On the other hand, due to the lack of crystallinity, a high transparency of these PU samples could be realized with ease. With the understanding of chemistry and properties of materials from PC aminolysis, we are now targeting the syntheses of a variety of functional polymers through choices of specialty PCs.

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CONCLUSION In this work, a facile methodology for converting PC into versatile intermediates was presented. For this new process, PC was depolymerized fully with primary aliphatic diamines in the absence of catalysts, under mild conditions of less than 80 °C. The highly selective and structurally well-defined mixtures of bisphenol A and phenolic carbamate intermediates were demonstrated, and the products derived were characterized in model aliphatic monoamine. By using short- or long-chain diamines, PC was respectively converted into hydroxylterminated short chain-extenders or prepolymers suitable for further polymer constructions without utilizing an isolation process. With the subsequent addition of reagents such as diisocyanates, a variety of PU polymers were synthesized in one pot. In fact, all carbonate groups of PC were fully utilized in making new phenolic carbamate derivatives, which are completely incorporated into the new PU products to achieve 100% atom-economy efficiency. This two-step process would further point to possible utilization of other PC structures as raw materials for synthesizing new functional polymers with distinct performances.

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EXPERIMENTAL SECTION

Materials. Poly(bisphenol A carbonate), with a Mw of ∼45 000 (PC), was purchased from Sigma-Aldrich. Reagents hexamethylenediamine (HDA), n-hexylamine (HA), 1,6-hexanediol (HDO), 4,4′methylene-bis(phenyl isocyanate) (MDI), hexamethylene diisocyanate (HDI), and stannous octoate (T-9) were all reagent grade and purchased from Sigma-Aldrich. Amino-terminated siloxanes (PAM-E (EW = 130), KF-8010 (EW = 430), X-22−161A (EW = 800), X-22− 161B (EW = 1500), and KF-8012 (EW = 2200)) were provided by Shin-Etsu Chemical Co., Ltd. Jeffamine (D-230, D-400, D-600, and D2000) were provided by Huntsman Corp. Polycaprolactone diol (CAPA 2303, PCL 3000; Mn ≈ 3000) was provided by Perstorp Specialty Chemicals Company. Poly (tetramethylene ether) glycol (PTMEG 2000; Mn ≈ 2000) was purchased from Sigma-Aldrich. 1,3,5-Tris[3-(dimethylamino)propyl]hexahydro-1,3,5-triazine was purchased from Sigma-Aldrich. Solvents including anisole, isobutyric acid, dimethylformamide (DMF), and dimethylacetamide (DMAc) were distilled under reduced pressure over MgSO4 or CaH2 and stored over 4 Å molecular sieves. Other reagents were used as received without further purification. Instrumentations. Infrared (IR) spectra were recorded to identify the chemical structure using a Jasco 4100 FT-IR spectrophotometer with a Jasco ATR Pro 450-S accessory. 1H NMR spectra were taken on a Bruker Avance-400 MHz FT-NMR spectrometer with chloroform-d or dimethyl sulfoxide-d6. Gel permeation chromatography (GPC) was conducted on a ViscotekTM MBLMW-3078 utilizing a MBHMW3078 column with N-methyl-2-pyrrolidone (NMP) as the mobile phase at a rate of 1.0 mL/min (40 °C). GPC was calibrated using 8973


Research Article

ACS Sustainable Chemistry & Engineering PUs from DP-carbamate prepolymers of PC aminolysis results in PU-1 or PU-2. Polymerization in a Two-Step One-Pot by Using DP-Carbamate Intermediates as PU Chain Extenders (PU-3). The polymerization of PUs was carried out in a typical two-step condensation reaction. For T1-AM-L50 as an example, excess (1.05 equiv weight of diamine agents) MDI (2.05 g, 8.2 mmol), PCL 3000 (5.48 g, 1.8 mmol), and catalyst (T-9) were first dissolved in 10 mL of dry DMAc and heated to 70 °C for 0.5 h to obtain isocyanate-terminated prepolymers. Subsequently, this isocyanate-terminated prepolymer was chainextended by the addition of as-prepared phenolic carbamates derivatives from the PC aminolysis (3 g, EW = 254.2; HDA, 0.70 g, EW = 58.1, 6.0 mmol in 15 mL of anisole and additional 5 mL of dry DMAc) at 80 °C for about 3 h. PU thin films (thickness of 250−300 um) were obtained after the removal of solvents in a circular oven at 60 °C. The polymers were analyzed by GPC and IR spectra. FT-IR (ATR): 1741 cm−1 (carbonyl stretching of urethane), 1645 cm−1 (carbonyl stretching of urea). Formulations, compositions, and HS contents of PU from PC aminolysis are compiled in Table 3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01326. Additional data and figures including 1 H NMR investigations of starting materials and intermediates (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chien-Hsin Wu: 0000-0002-7564-1457 Ru-Jong Jeng: 0000-0002-0913-4975 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Great Eastern Resins Industrial Co. Ltd. (GRECO) and Coating P. Materials Inc. for their partial financial aid to S.D.’s group at NCHU. This work was financially supported by the “Advanced Research Center of Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 1062218-E-002-021-MY2 and 107-3017-F-002-001). The authors also wish to thank Ms. Karin D. Kelly, a certified patent lawyer for technical editing.

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REFERENCES

(1) Al-Salem, S. M.; Lettieri, P.; Baeyens, J. Recycling and recovery routes of plastic solid waste (PSW): A review. Waste Manage. 2009, 29 (10), 2625−2643. (2) Polycarbonate Market Size To Reach $25.37 Billion By 2024; http://www.grandviewresearch.com/press-release/globalpolycarbonate-market; Grand View Research: San Francisco, CA; accessed October, 2016. (3) Antonakou, E. V.; Achilias, D. S. Recent Advances in Polycarbonate Recycling: A Review of Degradation Methods and Their Mechanisms. Waste Biomass Valorization 2013, 4 (1), 9−21. 8974


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