Effect of Acetylation on the Behavior of Hyperbranched Polyglycerols in Supercritical CO₂

Abstract

Processes using CO₂ either as a solvent or as a reactant, for example, in catalyzed chemical reactions, are increasing in interest due to their green characteristics. Hyperbranched polyglycerols have the potential to be used as support for catalysts in these processes, allowing for an efficient separation of the products and the reutilization of the catalyst, but this requires them to absorb CO₂. Acetylating hydroxylated compounds has shown to be an efficient way to increase their CO₂-philicity, and this work aims to understand how acetylation increases the interaction of hyperbranched polyglycerols with different cores with supercritical CO₂. This involves the study of their kinetics of expansion in this media (from 10 to 25 MPa and at 35 °C and 45 °C) and, eventually, their solubility when it happens. The expansion of the acetylated polyglycerols reached up to 66% in volume, while that of non-acetylated ones, in general, do not exceed 10%. Acetylation plays an important role in increasing the expansion of these polymers in the presence of CO₂ and, therefore, in increasing their CO₂-philicity and CO₂ absorption, making them potential materials to be used in biphasic (polymer/CO₂) reaction systems.

1. Introduction

Hyperbranched polyglycerols (HPG-OHs) are polyether polyols formed by glycerol residual units organized in an imperfect dendritic structure [1]. They are easier to prepare than dendrimers but keep the same unique properties, when compared with linear analogs, such as the presence of nanocavities and the high concentration of end groups. The first leads to a larger free volume and a good capacity to incorporate solutes [2,3], and the second leads to a core–shell environment, where the end groups work as a shell, which can be further functionalized, considerably changing the properties of the polymer and its range of applications [4].

Acetylation is a chemical modification that has been used to increase the CO₂-philicity of different materials. A good interaction, and therefore, the absorption of CO₂ in a polymer, results in its swelling. Applications that rely on the sorption of CO₂ in a polymer will be more efficient when intense swelling is observed. For instance, reactions that occur in a CO₂ expanded polymeric phase will be faster when a higher concentration of CO₂ is present in this phase. Also, impregnation of active molecules can be achieved as the process involves an increase in the distance between the polymer chains and lowering their intermolecular interactions, allowing those molecules to be carried by the CO₂ into the polymeric matrix, and to establish intermolecular interactions with the polymer chains so higher loads can be attained. Processes where CO₂ is used as a plasticizer to decrease the melting point and the glass transition temperature of the polymer, such as extrusion, blending, obtention of composites, or foaming, will also benefit from it making them feasible and suitable at lower CO₂ pressures.

The swelling of a polymer by CO₂ is related to specific intermolecular interactions between the polymer and the CO₂, to the polymer chain flexibility, free volume, and weak polymer–polymer and solvent–solvent intermolecular interactions, leading to an increase in both the solubility of CO₂ into the polymer phase and the solubility of the polymer into the CO₂ phase [5,6]. The acetylation functionalizes hydroxyl or analog groups, removing hydrogen bonds from the material, which reduces the intermolecular interactions between the chains. Moreover, despite CO₂ being an apolar molecule, it presents a charge separation of high amplitude and a significant quadrupolar moment, which allows it to undergo specific Lewis’s acid–base interactions with acetyl groups, where the CO₂ oxygen interacts with the α-hydrogen of the acetyl group, and its carbon interacts with the oxygen of the acetyl group, strongly increasing the interaction of the polymer with the fluid [5,6]. Indeed, Nasir and co-workers acetylated various fractions of phospholipids from soybean and found out that the solubility in supercritical CO₂ (scCO₂) increased, resulting in a cloud point of 13 MPa at 40 °C for the most soluble fraction [7]. Potluri and co-workers showed the high solubility of acetylated cyclodextrins in scCO₂ [8], and Lambert and Ingrosso showed the importance of the acetylation of sugars for their solvation in scCO₂ using a molecular dynamics study [9]; Huang and co-workers used peracetylated-β-cyclodextrin as an additive to increase the viscosity of scCO₂ [10], and we showed that this solubility allowed for the complexation of Ibuprofen in the same acetylated cyclodextrin [11]. Indeed, in poly (vinyl acetate), the acetyl groups promote its solubility in scCO₂ and the solubility of CO₂ into the polymer phase [12,13,14,15]; moreover, an increase in the free volume of this polymer by the insertion of branched segments increases its solubility [15].

HPG-OHs possess hydroxyl end groups that can be easily acetylated, generating a CO₂-philic material. Acetylated hyperbranched polyglycerols (HPG-Acs) will have, therefore, the potential for applications such as a matrix for active molecules’ impregnation in scCO₂ or as a green solvent in CO₂ biphasic reaction systems. Indeed, acetylated polymers have been used as a matrix to impregnate molecules assisted by scCO₂. For instance, Valor and co-workers developed a poly (vinyl acetate)/polypyrrole scaffold impregnated with gallic acid for its controlled release using scCO₂ [16], and cellulose acetate was used to impregnate menthol and vanillin successfully [17]. Their potential to be used in biphasic systems using CO₂ lays on the fact that those systems have a need for green solvents that can absorb CO₂, solubilize reactants, and support/solubilize/impregnate catalysts. HPG-Acs are non-volatile and non-toxic materials and can act similarly to ionic liquids in those systems but in a much greener way. The solubilization of CO₂ has the potential to decrease their viscosity and enhance the transport properties for a reaction to occur, while favoring the desorption of the products. Indeed, the solubility properties can be tuned by varying the pressure and temperature, and scCO₂ can act as a non-reactional phase representing a reservoir for reactants and products enabling an easy reuse of the catalytic/polymer phase [18].

The aim of this study is to understand the impact of the acetylation and polymer structure on the behavior of hyperbranched polyglycerols in supercritical CO₂ through their volumetric expansion. HPG-OHs and HPG-Acs bearing three different cores were chosen in order to provide polymers with slightly different polarities for each class, increasing from the ones with cores based on the polyols in this sequence: dodecanediol < glycerol < tetraethyleneglycol. Moreover, polymers bearing each core were studied in two different molar masses (Mns) in order to evaluate the impact of the molecular size on the behavior of both HPG-OHs and HPG-Acs in scCO₂.

2. Experimental

2.1. Materials

Hyperbranched polyglycerols (HPG-OHs), Figure 1 and

Table 1. Characteristics of the HPG-OHs.

Code	Initiator	Mn (g mol−1)	Đ (Mn/Mw) a	DB b	nOH c
GL-HPG-OH-1	GL	562	1.06	0.41	7.6
GL-HPG-OH-2	GL	932	1.09	0.45	13.5
DD-HPG-OH-1	DD	547	1.06	0.23	6.7
DD-HPG-OH-2	DD	804	1.18	0.36	10.1
TEG-HPG-OH-1	TEG	604	1.08	0.15	7.5
TEG-HPG-OH-2	TEG	855	1.10	0.43	10.9

^a Polymer Dispersity. ^b Degree of branching [19]. ^c Average number of mols of hydroxyl groups per mol of HPG-OH; the method for calculation is presented in the Supporting Information Section.

, were previously synthesized via the Ring Opening Polymerization of glycidol [19] using glycerol (GL), dodecanediol (DD), and tetraethyleneglycol (TEG) as initiators, in two different Mns for each initiator. Acetic anhydride (98%, Merck, São Paulo, Brazil) and dichloromethane (Analytical grade, Vetec, São Paulo, Brazil) were distilled prior to use, and pyridine (98%, Vetec, São Paulo, Brazil) was distilled prior to use and kept over molecular sieves (3 Å, Sigma-Aldrich, Cotia, Brazil). Dimethylaminopyridine (DMAP, 99%, Sigma-Aldrich, Cotia, Brazil) and liquid CO₂ (99.98%, Oxilumen, São Paulo, Brazil) were used as received.

2.2. Analytical Methods

Mass spectroscopy with electrospray ionization (ESI-TOF-MS) was carried out in a MicroTOF QII Bruker spectrometer (Billerica, MA, USA) operating with a collision energy of 30 eV, using methanol as solvent, to determine the Mn of the polymers.

Nuclear magnetic resonance (¹H and ¹³C NMR) analysis was carried on a Varian Inova spectrometer operating at 300 MHz for ¹H and 75 MHz for ¹³C. The acetylation degree (%AD) was determined by ¹H NMR (in DMSO-d₆), using the relative number of hydrogens of the peaks at δ = 1.90–2.15 ppm (CH₃CO, 3H) and δ = 4.30–4.85 ppm (R-OH, 1H), following Equation (1).

$$\mathrm{\%} AD=\left({\displaystyle \frac{\left(\frac{\int 1.90 \mathrm{p}\mathrm{p}\mathrm{m}-2.15 \mathrm{p}\mathrm{p}\mathrm{m}}{3}\right)}{\left(\left(\frac{\int 1.90 \mathrm{p}\mathrm{p}\mathrm{m}-2.15 \mathrm{p}\mathrm{p}\mathrm{m}}{3}\right)+\int 4.30-4.85 \mathrm{p}\mathrm{p}\mathrm{m}\right)}}\right).100\mathrm{\%}$$

(1)

Thermogravimetry (TG/DTA) was performed on a Shimadzu DTG-60/60H thermogravimetric balance (Shimadzu, Kyoto, Japan) under a N₂ stream with a flow rate of 50 mL min⁻¹, at a scanning rate of 20 °C min⁻¹, and scanning temperatures between room temperature and 800 °C. The thermal decomposition values (T_d) were calculated as the tangent of the first mass loss event. Differential Scanning Calorimetry (DSC) was performed using the Shimadzu DSC-60 calorimeter (Shimadzu, Kyoto, Japan), under a N₂ stream with a flow rate of 100 mL min⁻¹, with a scan rate of 10 °C min⁻¹ (16 °C min⁻¹ for the HPG-OHs [19]), and scan temperatures between −100 and 150 ° C, whose acquisition was preceded by a cycle of heating from room temperature to 150 °C and cooling from 150 °C to −100 °C at the same speed [19,20].

2.3. General Procedure for the Acetylation of the HPG-OHs

Acetylated hyperbranched polyglycerols (HPG-Acs) were obtained from HPG-OHs using acetic anhydride. A given mass of the HPG-OH, equivalent to 50 mmol of hydroxyl groups, acetic anhydride (6.64 g, 65 mmol), DMAP (0.98 g, 8 mmol), 20 mL of pyridine, and 60 mL of dichloromethane were introduced in a 250 mL round-bottom flask equipped with a magnetic rod. The flask was placed in an oil bath over a stirring heating place. The reaction media was kept under N₂ and refluxed and stirred for 24 h. The dichloromethane was removed by rotative evaporation, and the crude material was dissolved in ethyl acetate. The solution was washed with HCl 1M until the presence of pyridine in the organic phase was not detected by TLC (Thin Layer Chromatography) and subsequently washed with a saturated solution of sodium bicarbonate, then deionized water. The solution was dried over anhydrous magnesium sulfate for 24 h, and the solvent was removed using a rotating evaporator.

GL-HPG-Ac-1: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.20–4.85 (m, 3.35 H, -CH-OCOCH₃), 4.30–3.90 (m, 3.98 H, and -CH₂-OCOCH₃), 3.90–3.10 (m, 63.92 H, and -OCH₂CH(O-)CH₂O-), and 2.01 (m, 17.07 H, and -OCOCH₃). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 170.7–170.3 (-COCH)), 78.77 and 77.27 (-CHOCH₂), 71.1–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, and -CHOH), 63.64, 63.3–62.2 (-CH₂OH), and 21.1–20.6 (-COCH₃). ESI-TOF-MS: Mn = 1114 g mol⁻¹; Đ = 1.07. AD = 100%.

GL-HPG-Ac-2: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.15–4.85 (m, 0.54 H, -CH-OCOCH₃), 4.85–4.35 (s broad, 0.05 H, and OH), 4.35–3.85 (m, 1.00 H, and -CH₂-OCOCH₃), 3.85–3.10 (m, 6.30 H, and -OCH₂CH(O-)CH₂O-, H₂O (trace)), and 2.01 (m, 3.00 H, and -OCOCH₃). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 170.7–170.3 (-COCH)), 78.75 and 77.27 (-CHOCH₂), 71.4–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, and -CHOH), 63.60 and 62.3 (-CH₂OH), and 21.1–20.6 (-COCH₃). ESI-TOF-MS: Mn = 1499 g mol⁻¹; Đ = 1.03. AD = 95%.

DD-HPG-Ac-1: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.15–4.90 (m, 0.65 H, and -CH-OCOCH₃), 4.90–4.35 (s broad, 0.01 H, and OH), 4.30–3.85 (m, 4.10 H, and -CH₂-OCOCH₃), 3.85–3.10 (m, 27.61 H, -OCH₂CH(O-)CH₂O-, -CH₂CH₂O-, and H₂O (trace)), 2.10–1.95 (m, 7.86 H, and -OCOCH₃), and 1.7–1.15 (m, 20.0 H, and -CH₂CH₂CH₂-). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 171.3–170.3 (-COCH)), 71.8–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, and -CHOH), 64.65 (-CH₂CH₂O- and -CH₂CH₂O-), 62.98 (-CH₂OH), 29.55–25.80 (-CH₂CH₂CH₂-), and 21.1–20.6 (-COCH₃). ESI-TOF-MS: Mn = 1102 g mol⁻¹; Đ = 1.08. AD = 100%.

DD-HPG-Ac-2: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.30–4.80 (m, 5.99 H, and -CH-OCOCH₃), 4.80–4.40 (s broad, 0.02 H, and OH), 4.35–3.90 (m, 10.77 H, and -CH₂-OCOCH₃), 3.80–3.15 (m, 71.84 H, -OCH₂CH(O-)CH₂O-, -CH₂CH₂O-, and H₂O (trace)), 2.05–1.95 (m, 31.96 H, and -OCOCH₃), and 1.65–1.05 (m, 20.0 H, and -CH₂CH₂CH₂-). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 170.5–170.3 (-COCH)), 78.84 (-CHOCH₂), 71.8–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, and -CHOH), 64.54 (-CH₂CH₂O- and -CH₂CH₂O-), 62.59 (-CH₂OH), 29.55–25.80 (-CH₂CH₂CH₂-), and 21.3–20.3 (-COCH₃). ESI-TOF-MS: Mn = 1392 g mol⁻¹; Đ = 1.04. AD = 100%.

TEG-HPG-Ac-1: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.20–4.88 (m, 0.52 H, and -CH-OCOCH₃), 4.88–4.55 (s broad, 0.03 H, and OH), 4.40–3.88 (m, 1.08 H, and -CH₂-OCOCH₃), 3.88–3.05 (m, 4.70 H, -OCH₂CH(O-)CH₂O-, -OCH₂CH₂O-, and H₂O (trace)), and 2.05–1.95 (m, 3.00 H, and -OCOCH₃). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 170.7–170.2 (-COCH)), 77.27 (-CHOCH₂), 71.0–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, -CHOH, and -OCH₂CH₂O-), 63.60 and 62.3 (-CH₂OH), and 21.1–20.6 (-COCH₃). ESI-TOF-MS: Mn = 1337 g mol⁻¹; Đ = 1.04. AD = 97%.

TEG-HPG-Ac-2: ¹H NMR (25 °C in d₆-DMSO, δ in ppm): 5.20–4.85 (m, 0.55 H, and -CH-OCOCH₃), 4.85–4.55 (s broad, 0.03 H, and OH), 4.33–3.82 (m, 1.00 H, and -CH₂-OCOCH₃), 3.72–3.35 (m, 4.70 H, -OCH₂CH(O-)CH₂O-, and -OCH₂CH₂O-), 3.31 (s, 0.95 H, and H₂O (trace)), and 2.05–1.95 (m, 3.06 H, and -OCOCH₃). ¹³C NMR (25 °C in CDCl₃, δ in ppm): 170.7–170.15 (-COCH)), 78.74 and 77.27 (-CHOCH₂), 71.50–68.00 (-CH-OCH₂CH, -CH₂-OCH₂CH, -CHOH, and -OCH₂CH₂O-), 63.56, 62.61, and 62.15 (-CH₂OH), and 21.0–20.6 (-COCH₃). ESI-TOF-MS: Mn = 1301 g mol⁻¹; Đ = 1.04. AD = 97%.

2.4. Behavior in Dense CO₂

The behavior of the polymers in dense CO₂ was assessed by volumetric expansion and cloud point experiments using a phase monitor (Thar Instruments SPM20) equipped with a high-pressure variable-volume (5–15.5 mL) view cell monitored by a CCD camera, a mechanical stirrer, a pressure controller, and heaters (Figure 2). The system was fed with CO₂ using a high-pressure syringe pump (ISCO model 260D series). The determination of the volumetric expansion was performed placing a 1 cm long and 0.52 cm in diameter glass tube containing the material being studied in the field of view of the camera of the phase monitor. The system was heated up to the desired temperature and pressurized with CO₂ to the desired pressure. Images were obtained periodically until the stabilization of the volume of the polymer phase was attained (about 100 min). V₀ (volume of the polymer before CO₂ absorption) was determined obtaining images of the system before the pressurization. These images were treated using the software ImageJ v.1.49m [21], converting the height of the material inside the tube to volume for the calculation of the percentage of variation over time (Equation (2), where $V_0^{pol}$ is the initial volume of the polymer and $V^{pol}$ is the volume of the polymer along time). Figure S1 illustrates the determination of the heights. Calibration curves of the height of the polymer phase in relation to its volume were performed for all the conditions studied. Partial solubility was measured as the percentage of weight loss (%) of the samples (Equation (3), where $m_0^{pol}$ is the initial mass of the polymer and $m^{pol}$ is the final mass of the polymer along time). The conditions studied were {10 MPa, 45 °C, and 0.50 g.mL⁻¹}, {15 MPa, 45 °C, and 0.74 g.mL⁻¹}, and {25 MPa, 35 °C, and 0.90 g.mL⁻¹}. The error in volume measurements lies around 2%.

$$\mathrm{\%}s={\displaystyle \frac{(V^{pol}-V_0^{pol}).100\mathrm{\%}}{V_0^{pol}}}$$

(2)

$$\mathrm{\%} weight loss={\displaystyle \frac{(m_0^{pol}-m^{pol}).100\mathrm{\%}}{m_0^{pol}}}$$

(3)

DD-HPG-Ac-1 was also studied through the determination of their cloud pressures at concentrations of 0.1 and 0.5%wt., at 25, 35, 45, 55, and 65 °C. The experiments were performed by adding the appropriate amount of polymer and CO₂ to the phase monitor to obtain the aimed concentrations. The temperature was set and stabilized, and the pressure of the system was increased from approximately 7.0 MPa to 30.0 MPa by decreasing its volume. The volume was then slowly increased to decrease the pressure to observe the cloud point pressures visually. This cycle of pressurization and depressurization was performed at least five times.

3. Results and Discussion

3.1. Synthesis of the HPG-Acs

The HPG-Acs were obtained from the HPG-OHs with high purity, as demonstrated by the NMR analyses. The characteristics of the HPG-Acs synthesized are summarized in

Table 2. Characteristics of the HPG-Acs.

Code	Initiator	%AD	Mn (g mol−1)	Đ (Mn/Mw) a
GL-HPG-Ac-1	GL	100	1114	1.07
GL-HPG-Ac-2	GL	95	1499	1.03
DD-HPG-Ac-1	DD	100	1102	1.08
DD-HPG-Ac-2	DD	100	1392	1.04
TEG-HPG-Ac-1	TEG	97	1337	1.04
TEG-HPG-Ac-2	TEG	97	1301	1.04

^a Polymer Dispersity.

. The acetylation degrees (%ADs) were equal or close to 100%. The values are higher for the polymers with lower Mns and lower polarities. Lower molecular masses provide more accessibility to the hydroxyl groups of the HPG-OHs, and lower polarities favor their interaction with the reaction solvent, also helping the accessibility to the hydroxyl groups. Moreover, since the increase in the Mn is given by an increase in the number of glycerol units in the polymer, it also increases the polarity of the HPGs bearing the least polar core, the DD. The Mn of the HPG-Acs increased in relation to their non-acetylated precursors, which can be explained by the addition of the acetyl groups, and also by the loss of the molecules with lower Mns during the purification steps of their syntheses.

3.2. Thermal Characterization

The thermal behavior of the studied HPG-OHs and HPG-Acs is presented in

Table 3. Thermal behavior of the HPG-OHs and HPG-Acs.

Code	Td (°C)	Tg1 (°C)	Tg2 (°C)	Mp (°C)
GL-HPG-OH-1	334.2	−48.4, −31.2	-	-
GL-HPG-OH-2	298.3	−53.2	-	-
DD-HPG-OH-1	183.9	−48.9, −35.6	−16.0	49.1
DD-HPG-OH-2	329.1	−48.2, −30.7	−8.9	32.8
TEG-HPG-OH-1	305.8	−45.6, −27.1	-	-
TEG-HPG-OH-2	330.4	−59.4, −47.2	-	-
GL-HPG-Ac-1	240.5	−31.5	-	-
GL-HPG-Ac-2	337.7	−25.9	-	-
DD-HPG-Ac-1	201.0	−55.5	-	−70–33 a
DD-HPG-Ac-2	230.1	−46.2	-	-
TEG-HPG-Ac-1	237.9	−41.9	-	-
TEG-HPG-Ac-2	219.0	−31.2	-	-

^a Several peaks.

. The HPG-OH samples have a broad range of decomposition temperatures (T_d), from 180 °C to 335 °C. This broad range reflects the increase in the T_d with the increase in the Mn of the polymers bearing the same structure [22]. This increase is not verified by the pair GL-HPG-OH-1 and GL-HPG-OH-2 because, in addition to the difference in Mn, they present differences in their structures. The first possesses a branched cyclic structure while the second one possesses a mix of branched cyclic and non-cyclic structures; those differences are a natural consequence of the reaction mechanism and synthetic methodology. The acetylation of the polymers caused, in general, a decrease in the T_d values due to the introduction of the labile acetyl group into the molecules; however, this is in part balanced by the increase in Mn after acetylation, which contributes to higher T_ds. Despite the lower T_ds for the acetylated polymers, processes involving these polymers can be carried out at temperatures up to 200 °C, an adequate value for a range of applications, including their use as reaction media in the presence of CO₂, whose reactions are normally carried out in low or mild temperatures (35 °C to 60 °C) [23].

The glass transition temperatures (T_gs) of the HPG-OHs were identified in two different regions, the first between −60 °C to −27 °C (T_g1), and the second between −16 °C and −8 °C (T_g2). Most polymers presented more than one T_g at the first region. Goodwin and co-workers demonstrated that this class of polymer can present different T_gs due to the presence of structures with different branching degrees because of their synthetic route, which impacts the mobility of the chains and forms different domains [24]. The T_gs at the first region was verified for all polymers and can be attributed to different structures of the HPG-OH: cyclic and inimeric structures (−60 °C to −50 °C) [20,24,25]; initiator-based structures (−30 °C to −20 °C), given that high Mn molecules will present an additional lower T_g up to −80 °C, referring to the core structure that differentiates from the more external groups as the Mn increases [20,24,25]; and hydroxyl polar domains (−53 °C to −10 °C) [26]. DD-HPG-OH-1 and DD-HPG-OH-2 are the only ones to present T_g2 and M_ps due to the dodecanediol (DD) chain capacity to generate crystalline regions in the polymers, which does not happen for the other polymers due to the higher degree of branching (DB) of GL, and the flexibility of their core chains, regarding TEG. The T_g2 can be attributed to the glass transition associated with the DD chain and surrounding segments. The acetylation of the HPG-OHs led to single events of T_g1, which is a reflex of the loss of the hydrogen bond at the terminal groups of the polymers. This also causes an increase in polymer mobility, which results in the disappearance of the T_g2 events and of the M_p of the DD-HPG-Ac-2. Moreover, the new values are a result of a balance of the mobility increase (contributing negatively) and of the Mn increase (contributing positively) resulting from the acetylation; moreover, they are also a result of the polarity and DB of the polymers. The polarity of the polymers are as follows: DD-HPG-Ac-1 < DD-HPG-Ac-2 < GL-HPG-Ac-1 < GL-HPG-Ac-2 < TEG-HPG-Ac-1 ≈ TEG-HPG-Ac-2. TEG-HPG-Ac-1 and TEG-HPG-Ac-2, however, do not present the highest T_gs among the samples, due to their lower DBs (DB is lower for the TEG-HPG-Ac-1). The DBs for the polymers DD-HPG-Ac-1 and DD-HPG-Ac-2 are also low, contributing for these polymers to present the lowest T_gs.

3.3. Behavior in Dense CO₂ of the HPG-OHs

The study of the behavior of HPG-OHs was performed in three different CO₂ densities: 0.50 g.mL⁻¹ {10 MPa, 45 °C}, 0.74 g.mL⁻¹ {15 MPa, 45 °C}, and 0.90 g.mL⁻¹ {25 MPa, 35 °C}. This was necessary because preliminary studies showed that volumetric expansion, because of the solubilization of CO₂ in the polymer, increases as a function of the density of CO₂.

The visual observation of the behavior of these polymers, illustrated in Figure 3, shows that upon contact with the scCO₂, their viscosity reduces, and bubbling is presented upon decompression (Figure 3a). For polymer DD-HPG-OH-2 (Figure 3b), due to its solid nature at the studied temperatures, a change in viscosity was not observed when the system was pressurized, but cracking is observed upon decompression. These behaviors demonstrate that CO₂ is able to solubilize into these polymer phases.

The kinetic study of expansion in scCO₂ allowed for the verification of the evolution of the expansion percentage of the polymers as a function of time and the maximum expansion attained when the system is in equilibrium. The volume of the polymer phase stabilizes, in general, in less than 20 min, but polymers presenting higher solubilities/expansions required longer times. The graphics presenting the kinetics of expansion are presented in the supporting information.

Figure 4 shows the final expansion, in volume percentage, of the polymers studied as a function of the density of the system.

It is observed that the %s are small for all polymers and even negative in some conditions. The solubilization of CO₂ in a polymeric matrix depends on the intermolecular interactions between the polymer chains and between the polymer and solvent, and on the free volume of the polymer [17,27,28,29]. In these systems, the intermolecular interaction between polymer and solvent is weak due to the absence of specific interactions; moreover, the interaction polymer–polymer is strong due to the presence of hydrogen bonds, resulting in low solubilities of CO₂ into the polymers, caused mainly by its penetration in the free volume of the polymeric phase. Indeed, branched polymers present higher free volumes when compared to their linear analogs due to the natural space between the branches and to the difficulty in packing the chains due to their irregular form. It is possible to propose that this solubilization can generate three conditions that influence the expansion: (i) The separation of the polymer chains after the penetration of CO₂ into the free volume regions, causing the decrease in viscosity and contributing to volume expansion; (ii) the retraction of the polar groups of the polymer chains due to the decrease in the polarity of the system, which is especially important for the polymers bearing more hydroxyl groups per volume, i.e., the HPG-OH with lower Mn, contributing to a decrease in the volume of the polymer phase; and (iii) the reorganization of the chains due to the increase in their freedom (mobility) caused by (i), which can lead to their organization and compaction, contributing to a decrease in the volume of the polymer phase. Indeed, CO₂ is known for acting as a plasticizer in polymer matrices [30,31] and by its ability to allow the organization of polymer chains. It is able, for instance, to increase the crystallinity of semi-crystalline polymers [32,33]. The absence of specific interactions between the polymer and solvent and, therefore, efficient solvation, is the basis to understand the low solubility of CO₂ into the polymer phase and the low efficiency of (i) in promoting an increase in expansion.

It is expected that the polymers bearing the core DD, with lower polarity and lower interaction among their chains, present higher solubilization of CO₂, and that more polar polymers would present a lower solubilization of CO₂. Indeed, it was observed that DD-HPG-OH polymers presented a higher expansion than TEG-HPG-OH polymers due to higher CO₂ solubility in the former. However, their expansions were lower than those for the GL-HPG-OH polymers due to their better capacity for organization and compaction, caused by their lower DB. In general, the expansion increases with the increase in the density of the system, due to the increase in the solvent power of CO₂. However, this trend was not observed for the polymer GL-HPG-OH-1, showing that the rearrangement of the chains and the retraction of the hydroxyl groups are potentialized for this polymer.

3.4. Behavior in Dense CO₂ of the HPG-Acs

The study of the expansion of HPG-Acs was performed using the same conditions used for HPG-OHs: 0.50 g.mL⁻¹ {10 MPa, 45 °C}, 0.74 g.mL⁻¹ {15 MPa, 45 °C}, and 0.90 g.mL⁻¹ {25 MPa, 35 °C}. The kinetic of expansion was followed and is presented in the supporting information. During preliminary expansion experiments, the polymer DD-HPG-Ac-1 presented a complete solubilization in the scCO₂ phase and, because of this, it was studied using only the cloud point experiment instead.

The HPG-Acs along time, presented an increase in expansion that reached a maximum value (%s_max) before decreasing and stabilizing (%s_final). The time needed for the polymers to reach the %s_max was reported as t_{%s max}. Moreover, the weight loss (%) of the samples, referring to the solubilized mass, was determined. The results of the expansion experiment are presented in

Table 4. Expansion results for the HPG-Acs.

Polymer	Condition	%smax (%)	%sfinal (%)	t%s max (min)	Weight Loss (%)
GL-HPG-Ac-1	0.50 g.mL−1, 10 MPa, 45 °C	41	40	22	6.7
	0.74 g.mL−1, 15 MPa, 45 °C	66	61	67	9.8
	0.90 g.mL−1, 25 MPa, 35 °C	57	53	66	15.0
GL-HPG-Ac-2	0.50 g.mL−1, 10 MPa, 45 °C	23	23	21	1.8
	0.74 g.mL−1, 15 MPa, 45 °C	29	28	65	9.1
	0.90 g.mL−1, 25 MPa, 35 °C	56	50	96	25.1
DD-HPG-Ac-2	0.50 g.mL−1, 10 MPa, 45 °C	59	58	61	6.1
	0.74 g.mL−1, 15 MPa, 45 °C	57	54	43	12.0
	0.90 g.mL−1, 25 MPa, 35 °C	1	−7	10	37.8
TEG-HPG-Ac-1	0.50 g.mL−1, 10 MPa, 45 °C	33	33	88	6.5
	0.74 g.mL−1, 15 MPa, 45 °C	36	35	60	7.9
	0.90 g.mL−1, 25 MPa, 35 °C	41	28	65	21.2
TEG-HPG-Ac-2	0.50 g.mL−1, 10 MPa, 45 °C	45	44	47	−0.6
	0.74 g.mL−1, 15 MPa, 45 °C	38	38	77	4.7
	0.90 g.mL−1, 25 MPa, 35 °C	48	48	91	10.3

.

The variation in the volume of the polymers along time (Table 4, third and fourth column) can be attributed to three processes that occur simultaneously: (i) A significant solubilization of CO₂ into the polymeric phase, which increases the volume of this phase, (ii) the partial solubilization of the polymers in scCO₂, which contributes to the decrease in the volume of the polymer phase, and (iii) the rearrangement of the polymer chains, which contributes to the decrease in the volume of the polymer phase. It is also important to note that because of the dispersity (Đ) of these polymers, the lower Mn chains will have higher solubilities in scCO₂ than the higher Mn ones. After the polymer phase reaches the %s_max, process (ii) becomes more significant than (i), which explains the decrease in the observed %s. Process (iii) could not be clearly evaluated due to the predominant contributions of processes (i) and (ii).

When the values of the %s for the HPG-Acs (and the solubilization of DD-HPG-Ac-1 in scCO₂) are compared with the values of %s for the HPG-OHs, it is evident that acetylation played a significant role in increasing CO₂ solubility by increasing its interaction with the polymers. Firstly, by reducing the interaction of polymer–polymer through the elimination of the hydrogen bonds of their hydroxyl groups, as evidenced by the DSC analysis. Secondly, by creating specific Lewis’s acid–base interactions between polymer and CO₂ [5]. Indeed, the acetyl group is recognized as a group that promotes CO₂-philicity, therefore promoting the solubility of materials in CO₂ and the solubility of CO₂ into a phase of the material [7,34,35]. The acetylation decreases the polarity of the polymers, but their order of polarity remains the same as for the HPG-OHs (except for the polymers TEG-HPG-Ac-1 and TEG-HPG-Ac-2, which have similar polarities since their Mn became alike after acetylation). Higher polarities represent stronger polymer–polymer interactions and, therefore, lower CO₂-philicities.

Figure 5 shows the %s_max of HPG-Acs for the different densities tested. The %s_max reached up to 66% and, in general, increases with the increase in the density of CO₂ and the decrease in their polarities and Mns. However, as the expansion of the polymer phase occurs simultaneously with its solubilization, the weight loss of the samples needs to be evaluated together with the %s_max. This allows us to have a better understanding of the system and of the samples that do not follow that tendency. For instance, the less polar polymers, DD-HPG-Ac-2 and GL-HPG-Ac-1, present a decrease in their %s_max for higher CO₂ densities, which is in agreement with their higher weight loss under these conditions. Indeed, preliminary tests (Supplementary Materials, Figures S68 and S69) showed partial solubility for HPG-Acs, evidenced by their cloud points. As observed for the expansion experiment, the polymers presented higher weight losses for lower polarities, and the condition of the higher weight loss was the one of higher density {0.90 g.mL⁻¹, 25 MPa, 35 °C}, because, when increasing density, the solubilizing capacity of the sc-CO₂ increases.

The time needed for the polymers to reach %s_max decreases as their CO₂-philicities increase, evidencing that a greater interaction of CO₂ with the polymer phase leads to faster processes. Wind et al. showed the kinetics of swelling of polyimides and also attributed faster swelling to a greater suitability to CO₂ absorption [36].

Figure 6 shows the phase diagram obtained for DD-HPG-Ac-1 for the cloud point experiment. It is shown that the cloud points present higher values of pressure as the temperature increases, ranging from to 4.3 MPa at 25 °C to 13.5 MPa at 65 °C. This occurs because at higher temperatures, higher pressures are needed to generate densities that allow for enough solvation for the solubilization of the polymer. Indeed, Fortunatti-Montoya et al. showed that an increase in pressure from ca. 9.4 to 11.4 MPa is needed to solubilize 56 mg of triacetyl glycerol (the lowest Mw analog of our studied polymers) in 1 g of scCO₂ when the temperature increased from 42 °C to 50 °C [37]; Gregorowicz et al. studied a higher Mw acetylated hyperbranched polyglycerol and showed that the cloud points present an increase in pressure from around 70 to 110 MPa when the temperature increases from around 32 °C to 152 °C [38]. The cloud points presented in this work for different concentrations have similar values for the same experimental conditions, showing that precipitation is efficient at the cloud point.

4. Conclusions

The acetylation of hyperbranched polyglycerols was shown to be an efficient way to increase their CO₂-philicity for all the polymers studied. This increase was reflected in the increase in both polymer expansion and polymer solubility in scCO₂ media. The expansion of the acetylated polymers reached up to 66% of their initial volume, while for non-acetylated ones, in general, the expansion did not exceed 10% and was negative in some conditions due to the plasticizing effect of CO₂, which allowed for the accommodation of the polymer chains. All acetylated polymers presented solubility in scCO₂; it was complete for the polymer bearing a dodecanediol core and lower Mn, and only partial for the others. Indeed, not only acetylation, but also the structure of the polymers, were relevant for their interaction with CO₂. The use of polar and apolar cores and different Mns allowed for the evaluation of polymers with different polarities, and it was shown that decreasing their polarities lead to lower polymer–polymer interactions, which contributed to higher CO₂-philicities and, therefore, a higher expansion/solubilization in scCO₂. Therefore, the characterization of the behavior of those polymers in scCO₂ evidenced relevant structural characteristics that contributed to the solubilization of the CO₂ into the polymer phase and the solubilization of the polymers in the CO₂ phase, as well as the role of the density of the system on these processes. Indeed, increasing the density of the system caused an increase in the expansion/solubilization of the acetylated polymers.

HPG-Acs are therefore potential materials for biphasic (polymer/CO₂) reaction systems in processes using CO₂ either as a solvent or as a reactant, and in temperatures up to at least 200 °C, where good absorption of CO₂ into the second phase of the system is required, such as catalyzed chemical reactions. The data presented gives a basis to understand the role of process conditions and to select or design an HPG-Ac that would best fit a given process as a reaction media, support for catalysts, or even co-solvent or phase transfer agent.