1 Introduction

The total amount of resins and fibers produced every year has been constantly increased since 1950 with a compound annual growth rate (CAGR) of about 2.5 times the CAGR of the global gross domestic product, and exceeded the total of 9 billion ton already in 2017 [1-3], with a global production of plastics that, in 2022 alone, reached 400.3 Mton [4]. At the end of their life, the management of these materials is problematic leading, especially in developing countries, in prevailingly landfilling or leakage with impact on the environment [1-3], in terms of pollution and effects on the climate change [5]. The impressive growth of plastics production, moreover, is expected to prevail on the efforts to mitigate plastic pollution that have been recently set up by many Countries in the World [6]. Plastic waste represents a problem also by an economical point of view: it was reported that 95% of plastic packaging material value is lost to the economy after a short first use [7]. Hence, recovery of plastic is mandatory, but plastics recovered via mixed waste collection mainly go to landfill or energy recovery and only a separate plastic collection makes recycling the best option, albeit with some intrinsic limits [8]. Plastics recycling occurs via mechanical, dissolution/precipitation and chemical routes [9-12]; up to now mechanical recycling has been prevailingly applied [13]: even if economic and relatively fast, it has revealed remarkable drawbacks such as the need of preprocessing and cleaning steps, and the thermomechanical degradation of many polymers due to multiple processing steps. It is acknowledged that, differently from mechanical recycling, the chemical recycling to monomer or to low molecular mass chemicals can allow the so-called upcycling (i.e., the conversion of low value chemicals to higher value ones) and an ideal circular economy in the polymer and plastic field, as it represents an efficient way to transform plastic waste into added-value intermediates and products with a significant reduction in the environmental impact of plastic use [14-19]. In this context, several protocols, including pyrolysis, photoreforming, gasification, bioconversion, mechanical and chemical reprocessing, have been developed to transform plastic waste in valuable products [20-24].

Thus, the upcycling of plastic can be considered as a multidisciplinary approach in which a wide variety of depolymerization methods and techniques are applied [25].

Poly(ethyleneterephthalate) (PET) is one of the most diffused polymers on the market, with a worldwide market volume in 2022 of about 22.5 Mton: it finds large application on fibers and thermoformed packaging. The global warming potential of end-life PET in landfill was estimated of about 45 kgCO₂ eq. kg⁻¹ [26]. Recycling of PET is thus increasingly performed, and it is fundamental to lower its environmental impact. It was estimated that PET bottles and fibers, produced from waste, could reduce by 60% the greenhouse gas emissions and by 85% the fossil resource scarcity reduction [27].

PET can be recycled by mechanical or chemical procedures [26-29], with mechanical processes currently being the most widely used. Unfortunately, as with all polyesters, PET partially hydrolyzes during processing due to moisture present in the environment and therefore a straight recycle is hardly achieved. This leads to the necessity of finding ways to make the molecular weight come back to the original values to limit PET downcycling during processing. Chemical recycling, on the other hand, includes hydrolysis, glycolysis, ammonolysis, and aminolysis and can allow for the production of fine chemicals to be used in many different fields [30-40].

In the last years, mathematical models have been developed to better understand the kinetics of these recycling methods, depending on the reaction conditions, such as the use of catalysts, the kind of solvents, and heating modes, aiming to better compare the depolymerization and/or degradation performances of the various methods and their suitability by the circular economy point of view [41, 42].

It was reported that, among the chemical recycling techniques, aminolysis, hydrogenolysis, and glycolysis using propylene glycol have the lowest global warming potential [26]. Aminolysis of recycled PET (R-PET) consists of its reaction with amines, leading to the formation of the corresponding diamides of terephthalic acid [43-47]. The reaction is highly versatile and can be performed in the presence or in the absence of catalyst. As the diamides can be further used as building blocks to produce new added value materials, this process represents an upcycling [40]. In particular, ethanolamine was used to prepare BHETA, a dihydroxy diamide that has been used in the synthesis of polyurethanes [48, 49], of polyesters [50], as additive in asphalt [51], as corrosion inhibitor [52], as additive in hot melt adhesives [53] or in the synthesis of plasticizers [54] (Figure 1a).

Tetrahydroxy terephthalamides were also obtained by aminolysis of PET with diethanolamine and used in coatings [55]. Very recently imidazole or substituted β-hydroxyamine have been used in the aminolysis of PET [56, 57].

Serinol (2-amino-1,3-propanediol) is a nontoxic and biodegradable amine that occurs in sugar cane. It can be prepared both from fossil-based derivatives [58] and from natural ones such as glycerol [58-60]. It is used in medicinal chemistry [61], and was used by some of the authors as a building block for the regioselective synthesis of imines and oxazolidines [62], and for the Paal–Knorr synthesis of pyrrole compounds [63, 64].

The use of serinol for the aminolysis of postconsumer PET could be of great interest for the sustainability of the entire upcycling process. Moreover, serinol has two primary hydroxyl groups, and this allows us to obtain, from the PET aminolysis, a novel molecule, a diamide containing four hydroxy groups, that can be used as a polyfunctional molecule.

In this work, serinol was for the first time used for the aminolysis of postconsumer PET, through a green synthesis, in the absence of solvents and catalysts, to obtain a tetrahydroxy terephthalamide: terephthalic acid serinol bis-amide (TASBA). This molecule was tested as a multifunctional initiator in the synthesis of PLA. The two main steps of the research are summarized in Figure 1a,b respectively.

The use of multifunctional initiators in the ring-opening polymerization (ROP) of lactide yields star-shaped polymers that are characterized by unique thermal and rheological features [65, 66]. The properties of the resulting materials depend not only on the nature of the multifunctional initiator but also on its amount. For this reason, different loadings of TASBA were initially tested as initiators in lactide's ROP and the effects of the different quantities on the properties of the final material were investigated.

2 Results and Discussion

2.1 Depolymerization of PET With Serinol

2.1.1 Depolymerization of Bottle-PET

Depolymerization with serinol was performed by investigating the serinol/PET molar ratio, reaction temperature, and time.

2.1.1.1 Effect of the Reaction Temperature

The initial screening was performed by heating postconsumer bottle-PET flakes in serinol (Scheme 1) at different temperatures and with the same serinol/PET ratio (Table 1, entries 1–3), obtaining TASBA and ethylene glycol as co-product. Values of temperature, time, and analytical yield of TASBA in the crude, calculated versus an internal standard, are in Table 1.

TABLE 1. Experimental conditions and analytical yield for the reaction of bottle-PET with serinol. ^a

Entry	Serinol/PET molar ratio b	t (min)	T (°C)	Analyt. yield c (%)
1	6	90	160	61
2	6	50	180	96
3	6	20	200	89
4	4	90	180	98
5	3	120	180	57
6	10	30	180	89
7	10	50	180	99

• ^a Reactions were performed on 250 mg of PET.
• ^b Amine/PET molar ratio based on the repeating unit of PET.
• ^c Yields were calculated using ¹H NMR spectroscopy, details in Section 4 and Supporting Information.

In the experiment performed at 160°C (Table 1, entry 1), a sticky solid was obtained after 90 min. The yield was 61% and did not significantly increase for longer reaction times. Substantially quantitative yield of diamide TASBA was achieved at 180°C within 50 min (entry 2), and the final mixture was homogeneous. In the reaction performed at 200°C (entry 3), the reaction mixture was substantially homogeneous after 20 min, but it turned dark and it was stopped, achieving a lower yield compared to entry 2 in Table 1.

Reactions were then performed exploring different serinol/PET molar ratios and reaction time at 180°C. Reaction with a lower amount of serinol (Table 1, entry 4) gave substantially complete conversion after 90 min. The experiment, performed with a serinol/PET 3:1 molar ratio (entry 5) gave low yields after 2 h. It is to be noted that, in this latter case, after 50 min the reaction mixture turned solid and unreacted PET was still present after the workup. Thus, the serinol/PET ratio of 4:1 seems to be the minimum one to have an almost quantitative yield, making possible an effective mixing of PET flakes with serinol.

Reactions were also performed with a higher serinol/PET molar ratio, that is, 10:1 (Table 1, entries 6 and 7) at 180°C. A yield of about 90% and quantitative yield were obtained at 30 and 50 min as the reaction time, respectively. As the yields were substantially the same (entries 2 and 7), the use of a higher amount of serinol is not convenient.

2.1.1.2 Effect of the Reaction Time

The effect of the reaction time on the depolymerization of bottle-PET flakes was thus studied at 180°C, by using serinol/PET in a molar ratio of 4:1 and 6:1. Values of molar ratio, reaction time, and analytical yield are in Table 2: the experiments of entries 1–5 are with a 4:1 molar ratio and those of entries 6–10 are with a 6:1 molar ratio. The yields of TASBA versus time are plotted in Figure 2. As expected, the formation of TASBA was faster with a higher amount of serinol. In all the experiments, in which the yield was lower than 90%, unreacted PET flakes were still present. This suggests that the formation of the diamide is very efficient with respect to the dissolution of PET.

TABLE 2. Depolymerization of bottle-PET at 180°C during time. ^a

Entry	Serinol/PET molar ratio b	t (min)	Analyt. yield c (%)
1	4	20	33
2	4	30	53
3	4	50	76
4	4	70	95
5	4	90	98
6	6	20	40
7	6	30	74
8	6	50	96
9	6	70	99
10	6	90	98

• ^a Reactions were performed on 250 mg of PET.
• ^b Amine/PET molar ratio based on the repeating unit of PET.
• ^c Yields were calculated using ¹H NMR spectroscopy, details in Section 4 and Supporting Information.

2.1.1.3 Recyclability of Serinol

The recyclability of the excess of serinol was tested. Details of the experiments are in the Section 4. The aminolysis was carried out at 180°C for 90 min, with 4:1 as the serinol/PET molar ratio, thus reproducing the experimental conditions of entry 5 in Table 2, which allowed us to achieve an analytical quantitative yield in the crude. Three consecutive runs were performed, TASBA was recovered at the end of each run by work-up, and the isolated yields were collected for each run in Table 3.

TABLE 3. Depolymerization of PET with the recycling of the excess of serinol. ^a

Entry	Run b	Moles of serinol added per mole of PET	TASBA recovered (%)
1	1	4	77
2	2	2	70
3	3	1	62

• ^a Reactions were performed using 2 g of bottle-PET, at 180°C, for 90 min each run.
• ^b Consecutive runs.

After the first run (Table 3, entry 1), 77% of TASBA was recovered after work-up, performed as follows. The crude was refluxed in water, obtaining a clear solution, and TASBA was recovered as precipitate after cooling the solution. The aqueous solution recovered after filtration of TASBA was evaporated and fresh serinol and PET were added to the solid residue, which contained the excess of serinol, ethylene glycol and the residual TASBA. The mixture was heated to 180°C and the second run (Table 3, entry 2) was performed by using half of the amount of serinol with respect to run 1 (2:1 serinol/PET molar ratio). Differently from the first run, the residue after evaporation was liquid also at room temperature. This gave the chance to further reduce the amount of serinol (1:1 serinol/PET molar ratio) (entry 3). As shown in Table 3, the quantity on the recovered TASBA decreased, however not dramatically. After the three runs the overall ratio serinol/PET used was 2.3:1 and the overall yield was 70%. In this way, at regime, it was possible to use serinol in almost stoichiometric amount. Recycling was not further optimized.

2.1.2 Depolymerization of R-PET Powder

Depolymerization was then performed using R-PET powder in place of bottle-PET, on a scale of 10 g and 60 min as reaction time, obtaining TASBA in 70% yields with high purity (~97%); the remaining ~3% was only serinol. A second recrystallization allowed us to obtain the diamide with higher purity (> 99%). Water, recovered by distillation contains only traces of ethylene glycol and can be reused in the process, confirming that can be recycled and reused.

As serinol can be easily recycled, this experiment was performed at 180°C using a molar ratio serinol/PET of 6:1, in order to have a more efficient mixing of the mixture.

2.3 TASBA As Multifunctional Initiator for the ROP of Lactide

TASBA was tested as a multifunctional initiator for the ROP of L-lactide.

2.3.1 ROP

In Scheme 2, the polymerization with this novel initiator is represented.

The synthesis was carried out with different amounts of diamide, namely, 0.1%, 0.5%, and 1% w/w (PLA-TASBA0.1, PLA-TASBA0.5, and PLA-TASBA1.0, respectively) with respect to L-lactide. The polymerizations were carried out in bulk, with Sn(Oct)₂ as the catalyst (details in the Section 4). All samples were compared to a PLA synthesized in the same conditions without using the multifunctional initiator. The samples were characterized through differential scanning calorimetry (DSC), rheological analyses to determine the effects of the increasing amount of TASBA on the final properties of the materials, and size exclusion chromatography to determine the molecular weight.

An image of the TASBA-initiated PLA materials compared to commercial PLA is reported in Figure S4.

2.3.2 Rheological Characterization

Melt viscosity curves of the synthesized samples are shown in Figure 3.

Considering PLA as a reference, both PLA-TASBA0.1 and PLA-TASBA0.5 showed the increase of the melt viscosity, with PLA-TASBA0.1 showing the greatest increase. The reduction of the melt viscosity was observed in the case of PLA-TASBA1.0. It is known that the polymerization reaction in the presence of a multifunctional initiator yields a complex mixture of products, comprising both free linear chains and chains bonded to the multifunctional initiator, which are the stars' arms. The formation of star-shaped macrostructures is known to have profound effects on the melt viscosity properties of the materials [66]. A higher quantity of initiator leads to shorter arms and, in turn, to lower viscosity due to the low quantity of entanglements forming between all the macromolecules. At low initiator concentration, the arms of the stars are long enough to provide entanglements, thus causing a high viscosity of the melt. Due to the presence of the central “core,” that is, the multifunctional comonomer (TASBA in this case), the star-shaped polymers can be characterized by more interchain interactions with respect to a linear macromolecule, resulting in higher melt viscosity. Hence, to account for the findings shown in Figure 3, one could consider that alongside promoting the formation of star-shaped macromolecules, TASBA may exert a toughening effect, provided by its rigid aromatic core. The melt viscosities obtained with different amount of TASBA could be considered as a result of the combination of the two separate and opposite effects, arising from the PLA macrostructure and the TASBA toughening effect.

2.3.3 Calorimetric Analysis

DSC analysis was carried out as described in detail in the Section 4. In brief: a first heating (not shown in Figure 4) was performed to cancel the thermal history of the samples, then cooling and heating cycles were made. The superimposed DSC thermograms of the PLA and PLA/TASBA samples, to be seen in Figure 4, show the three cycles.

The temperatures relative to crystallization (T _C), glass transition (T _g), cold crystallization (T _CC), and melting (T _M) are reported in Table 4.

TABLE 4. T _C, T _g, T _CC, and T _M ^a from DSC analysis of PLA and PLA/TASBA samples.

Sample	T C (°C) b	T g (°C) c	T CC (°C) c	T M (°C) c
PLA	115	49	83	163
PLA-TASBA0.1	105	55	98	171
PLA-TASBA0.5	102	—	—	161
PLA-TASBA1.0	—	47	—	140

• ^a Temperature of crystallization (T _C), glass transition (T _g), cold crystallization (T _CC), and melting (T _M).
• ^b Recorded during the cooling scan.
• ^c Recorded during the second heating scan.

Standard PLA showed a well-defined crystallization transition in the cooling step and residual crystallization and melting in the second heating scan. The use of 0.1% of TASBA in polymerization resulted in a reduced crystallization during cooling and in a more evident cold crystallization and melting, shifted at higher temperatures compared to PLA. The increase of the diamide concentration in polymerization to 0.5% led to the complete crystallization of the material during the cooling step, as indicated by the absence of cold crystallization peaks during the second heating scan, and to the decrease of the melting temperature. The 1.0% concentration of TASBA in polymerization caused the increase of the amorphous character of the material. Indeed, the DSC trace did not reveal any crystallization transition and the melting transition was barely visible.

To account for these findings, the macrostructure of the PLA and PLA-TASBA samples and the PLA-TASBA interaction, commented above, should be taken into account. The viscosity of the PLA-TASBA0.1 sample, which is the highest of all the samples, as shown above in Figure 3, reasonably hinders the crystallization in the first cooling. The 0.5% TASBA concentration reasonably leads to lower molar mass arms, hence to lower viscosity (as shown in Figure 3), thus enhancing the crystallization ability of PLA. The interaction of TASBA, in higher amounts, with PLA could account for the lower melting point. The highest amount of TASBA is expected to cause a further reduction of the arm length. This macrostructure and the interaction of PLA with the TASBA molecules could explain the poor crystallization ability and the lower melting point. These findings confirm that complex polymeric architectures do affect the formation of crystalline structures [65, 66].

2.3.4 Size Exclusion Chromatography

The molecular weight of the samples was determined to understand how the addition of TASBA could affect the growth of the chains. As anticipated, TASBA is a multifunctional monomer that could, in principle, act as a multifunctional initiator in the ROP of L-lactide. For this reason, the introduction of different amounts of TASBA in the feed could affect significantly the outcome of the reaction, not only in terms of macromolecular architecture but also molecular weight. Table 5 reports the molecular weight data of the synthesized samples.

TABLE 5. Molecular weight data of PLA and PLA/TASBA samples.

Sample	$$$ \overline{M_{\mathrm{n}}} $$$	$$$ \overline{M_{\mathrm{w}}} $$$	D
PLA	20,000	33,000	1.6
PLA-TASBA0.1	19,400	31,000	1.6
PLA-TASBA0.5	6500	8600	1.3
PLA-TASBA1.0	5000	9000	1.8

As data show, the introduction of TASBA strongly affects the molecular weight of the samples. The introduction of 0.1% of multifunctional initiator has only limited effects, resulting in just a modest decrease of molecular weight. On the other hand, as expected, the molecular weight drops, with increasing loading of TASBA. Remarkably, alongside the decrease of molecular weight, the dispersity decreases as well for PLA-TASBA0.5 in comparison to standard PLA used as reference. A further increase in concentration up to 1.0% resulted in an increase of dispersity. These observations may hint the possibility that 0.5% could be close to the maximum amount of TASBA to be loaded as higher concentrations could have detrimental effects on the properties of the materials.

3 Conclusions

Nowadays, one of the major drawbacks of PET is its global warming potential when its end-of-life is in landfills. The recycling of PET is mandatory and chemical recycling gives the chance for an ideal circular PET economy, leading to monomers and low molecular mass functional molecules. In this context, serinol has been used in the aminolysis of post-consumer PET obtaining a polyhydroxylated diamide that has been successfully used as a multifunctional initiator for PLA, a well-known biopolymer. The high efficiency of the aminolysis, which is performed under atmospheric pressure without the need for catalysts, followed by a simple work-up procedure in which only water is required, together with the fact that serinol is a biosourced and nontoxic amine, allows the whole process to meet the Green Chemistry principles development. Moreover, green metrics such as AE, PMI, and E-factor have been also calculated.

The potential applicability of the diamide as a multifunctional initiator in the ROP of lactide has been demonstrated. Different amounts of diamide result in effects on thermal and rheological properties. These results pave the way for the possibility of applying this strategy for tuning the properties of PLA materials depending on the intended final application, increasing the versatility of this class of polymers. In this regard, as results show, the different amounts of TASBA have significant effects on the thermal and rheological properties of the final materials. Being able to control such properties could eventually lead to new applications for PLA-based materials, widening its array of properties.

4 Experimental Section

Author Contributions

Ada Truscello: conceptualization (equal), data curation (equal), formal analysis (equal), investigation (equal), methodology (equal), software (equal), supervision (equal), validation (equal), writing – original draft (equal), writing – review and editing (equal). Chiara Sartiano: conceptualization (equal), data curation (equal), formal analysis (equal), investigation (equal), methodology (equal), supervision (equal). Maurizio Stefano Galimberti: conceptualization (equal), data curation (equal), formal analysis (equal), investigation (equal), methodology (equal), supervision (equal), validation (equal), writing – original draft (equal), writing – review and editing (equal). Cristian Gambarotti: conceptualization (equal), data curation (equal), formal analysis (equal), funding acquisition (equal), investigation (equal), methodology (equal), resources (equal), supervision (equal), validation (equal), writing – original draft (equal), writing – review and editing (equal). Stefano Gazzotti: conceptualization (equal), data curation (equal), formal analysis (equal), investigation (equal), methodology (equal), software (equal), supervision (equal), validation (equal), visualization (equal), writing – original draft (equal), writing – review and editing (equal). Marco Aldo Ortenzi: conceptualization (equal), data curation (equal), formal analysis (equal), investigation (equal), methodology (equal), software (equal), supervision (equal), validation (equal), visualization (equal), writing – original draft (equal), writing – review and editing (equal).

Conflicts of Interest

The authors declare no conflicts of interest.