**2. Pd‐Nps supported onto PLA stereocomplexes: synthesis and characterization**

The synthesis of PLA can be achieved by different synthetic pathways such as polycondensation of lactic acid [31] or through ring opening polymerization (ROP) of lactide (i.e., the cyclic dimer of lactic acid) [32]. ROP is the adopted synthetic methodology, which allows a carboxylic-end functionalization along with a strict control of the resulting molar mass. The ROP can be either performed by cationic, anionic, or carbene-type mechanism or by a coordination insertion mechanism [33], which shows an advancement over the former mechanisms regarding the efficiency of the polymer chain growth and racemization of the stereogenic center of the lactic acid units. In addition, functionalized PLA at the carboxylic chain-end are straightforwardly obtained by using a suitable initiator molecule for the polymerization. ROP can be performed in solution or in the bulk of the monomer except for some glycolides that decompose at the melting temperature [34]. Typically, PLA is polymerized in bulk due to the low melting temperature of the corresponding cyclic monomers. The mechanism of ROP, through a coordination-insertion mechanism, is mainly based on organic salts of Sn(II). In particular, Sn(Oct)2 (Oct = octanoate) is one of the most employed catalysts for PLA synthesis due to its relatively low cost, solubility in the monomer, which is important for bulk reactions, high catalytic activity (yields over 90%) and a low racemization tendency of the starting glycolide. The mechanism regarding the Sn(Oct)2-catalyzed ROP is shown in **Figure 1**.

**1. Introduction**

2 New Advances in Hydrogenation Processes - Fundamentals and Applications

biphasic catalytic systems [17].

containing metal cations, such as Pd(II).

catalytic solution.

**characterization**

In the realm of hydrogenation reactions, the selective hydrogenation of unsaturated carboncarbon bonds is very attractive [1]. Asymmetric and partial hydrogenation reactions are pivotal for the access of important compounds employed as pharmaceuticals, food additives, pesticides, flavors, and fragrances [2]. In this respect, α,β-unsaturated carbonyl compounds present an interesting group of chemicals whose enantio- [3], chemo [4], and regio-selectivities [5] are highly challenging. For instance, Lindlar's [6] and Lindlar types' catalysts [7] are widely used to chemoselectively hydrogenate alkynes to the corresponding alkene. In parallel, a variety of different selective hydrogenation methods have been developed [8]. In particular, bimetallic catalysts [9], noble metal-based catalysts supported onto carbon-based materials [10] or inorganic matrixes [11], in combination with supercritical solvents [12] and "*flow*" reactors [13], have been studied. Also for the partial alkyne hydrogenation, a great deal of catalytic systems were proposed based on the use of noble metals and different approaches like reactions conducted in gas [14] and liquid phase [15], combined with ultrasound techniques [16] and

The interest on polymer-based catalysts applied for hydroformylation reactions [18], hydrotreatment [19], and strong acid/base [20, 21] catalyzed reactions is steadily increasing. Since functional polymer-based catalytic systems generally exhibit notable advantages over traditional catalysts such as the tunable solubility and improved catalytic selectivity [22], the biodegradable polyester, poly(lactide) (PLA) was used as a polymer matrix [23], which is rather straightforwardly end-capped as reported by Helle et al. [24]. The possibility to produce carboxylic end-capped PLA chains functionalized with nitrogen containing moieties, called macroligands by Giachi et al. [25], allowed the synthesis of well-defined macrocomplexes

used to catalyze hydrogenation reactions conducted in the homogeneous phase followed by an easy separation step, as reported by Giachi et al. [26] and Bartoli et al. [27]. Isotactic endfunctionalized PLA with opposite tacticity can be combined to form a supramolecular structure named stereocomplex [28]. Functionalized PLA stereocomplexes show high resistance against hydrolysis and thermal degradation and in addition, a poor solubility in all organic solvents. This latter property was exploited by Petrucci et al. [29] and Oberhauser et al. [30] for catalytic purposes to easily separate PLA-stereocomplex-based catalysts from

In the following sections, synthesis, characterization, and catalytic application of homogeneous and heterogeneous Pd-catalysts supported on end-functionalized PLA were reported.

The synthesis of PLA can be achieved by different synthetic pathways such as polycondensation of lactic acid [31] or through ring opening polymerization (ROP) of lactide (i.e., the cyclic

**2. Pd‐Nps supported onto PLA stereocomplexes: synthesis and**

The latter Pd-based macrocomplexes were efficiently

**Figure 1.** Sn(Oct)2-catalyzed ROP of lactide in the presence of a hydroxylic initiator molecule (ROH).

As shown in **Figure 1**, the ROP of lactide starts with a simultaneous coordination of lactide and the initiator molecule to the metal center of the catalyst. This *cis*-coordination of the two reagents is a prerequisite for the nucleophilic attack of the coordinated hydroxyl oxygen atom on the lactide carboxylic carbon atom. Afterwards, the metal center inserts into the sixmembered ring giving an eight member metallacycle. The obtained end-functionalized linear form of the lactide acts as a new initiator in the following polymerization cycle. The ROH, where R contains the new functional group, is incorporated at the carboxylic acid PLA chain end. The presence of water traces in the lactide melt notably influences the final molecular weight due to the competition role of water as an initiator molecule leading to a carboxylic acid end group which coordinates as carboxylate to the metal center. Nitrogen containing initiators, such as pyridine and 2-2′-bipyridine derivatives, were used in order to obtain carboxylic end-functionalized PLA chains. Synthesis conditions and obtained molar masses are compiled in **Table 1**.

The end-functionalized PLA-based macroligands were mainly analyzed by gel-permeation– chromatography (GPC) equipped with UV-Vis and refraction index (RI) detectors and by 1 H NMR spectroscopy. The UV-Vis detector confirmed the presence of the corresponding chromophore in the polymer chain. The 1 H NMR spectra of the macroligands show, apart the typical multiplets stemming from the polymer chain (i.e., doublet for CH3 and quartet for CH of the repeating lactide unit), signals that are typically in the range of aromatic hydrogen atoms. The ratio of the 1 H NMR integrals of these aromatic hydrogen atoms with that of the methyl group assigned to the terminal lactic acid gives the average number molar weight (*M*n) of the polymer obtained, which is in good agreement with the experimental results of GPC analyses, in those cases where the polydispersity of the obtained material is low.


a Theoretical of the molar mass.

b Bulk synthesis, 410 K, 3 h.

c R-PLA (i.e., atactic PLA) bulk synthesis, 410 K, 3 h, R-PLA-P(L or D)LA toluene reflux, 24 h.

dToluene reflux, 24 h.

**Table 1.** Overview of the synthesized macroligands.

The obtained PLA-based macroligands were used to provide the corresponding stereocomplexes upon mixing equimolar CH2Cl2 solutions of functionalized PLA of opposite stereochemistry, followed by evaporation of the solvent. Differential Scanning Calorimetry (DSC) analyses conducted on the obtained material showed clearly the absence of the melting peak of characteristic PLA (ca. 435 K); new endothermic transition at higher values, typically around 495 K, attributable to the melting of PLA-stereocomplex was observed. The melting enthalpy changed in the same sense from around 38.0 (PLA) to 61.0 J/g (PLA-stereocomplex). These data support the formation of a supramolecular structure more thermally stable. Importantly, the type of end-group does not influence the powder X-ray diffraction (PXRD) pattern of functionalized PLA-stereocomplexes, while PLA and the corresponding stereocomplex are featured by a completely different PXRD pattern as shown in **Figure 2**.

The ratio of the 1

a

b

c

Theoretical of the molar mass.

**Table 1.** Overview of the synthesized macroligands.

Bulk synthesis, 410 K, 3 h.

dToluene reflux, 24 h.

H NMR integrals of these aromatic hydrogen atoms with that of the methyl

**Py**-P(L)LA-OH **L1** 5000 **Py**-P(D)LA-OH **L2** 5000

**BipyOH2**-P(L)LA-OH **L3** 10,000 **Bipy2**-P(D)LA-OH **L4** 10,000 **Bipy**-(PLA-P(L)LA-OH)2 **L5** 10,000 **Bipy**-(PLA-P(D)LA-OH) 2 **L6** 10,000

**BipyOH**-P(L)LA-OH **L7** 10,000 **BipyOH** -P(D)LA-OH **L8** 10,000

**Bn**-P(L)LA-OH **L9** 10,000 **Bn**-P(D)LA-OH **L10** 10,000

HOOC-P(D)LA-OH **L12** 10,000

group assigned to the terminal lactic acid gives the average number molar weight (*M*n) of the polymer obtained, which is in good agreement with the experimental results of GPC analyses,

**Initiator Polymer chain ID MW (g/mol)**

**H2O** HOOC-P(L)LA-OH **L11** 10,000

The obtained PLA-based macroligands were used to provide the corresponding stereocomplexes upon mixing equimolar CH2Cl2 solutions of functionalized PLA of opposite stereochemistry, followed by evaporation of the solvent. Differential Scanning Calorimetry (DSC) analyses conducted on the obtained material showed clearly the absence of the melting peak of characteristic PLA (ca. 435 K); new endothermic transition at higher values, typically around 495 K, attributable to the melting of PLA-stereocomplex was observed. The melting enthalpy

R-PLA (i.e., atactic PLA) bulk synthesis, 410 K, 3 h, R-PLA-P(L or D)LA toluene reflux, 24 h.

in those cases where the polydispersity of the obtained material is low.

4 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 2.** PXRD patterns of isotactic PLA (A) and of the corresponding stereocomplex (B).

The 2,2′-bipyridine or pyridine end-group of the macroligands easily react with Pd(OAc)2 (OAc = acetate) in a 1:1 and 2:1 molar ratio, respectively, giving the expected *cis*-coordination of the nitrogen atoms to Pd(II) in case of 2,2′-bipyridine or *trans*-coordination in case of pyridine. Pd(OAc)2 is a highly suitable Pd(II) precursor since it is soluble in noncoordinating solvents, for instance CH2Cl2, which is the solvent of choice for suspending PLAstereocomplexes. In addition, coordinated Pd(OAc)2 is easily reduced in the presence of hydrogen, giving the acetic acid as a removable side product from reaction medium.

The coordination of either the end-functionalized macroligands or stereocomplexes to Pd(II) was proved by 1 H NMR spectroscopy conducted in CD2Cl2. The use of CDCl3 to characterize Pd-acetate macrocomplexes was avoided due to the easy protonation of acetate by trace amounts of acidity present in CDCl3. The occurred coordination of the macroligands to Pd(II) was proved by 1 H NMR, UV-Vis spectroscopy and by comparison of this latter spectroscopic data with that obtained from an analogous model compound (i.e., molecular species without the polymer part), which is exemplified in **Figure 3**.

**Figure 3.** UV-Vis spectra of (A) Pd(OAc)2**(**BipyOH**)**, (B) BipyOH, (C) Pd(OAc)2 **L7** , and (D) **L7** .

For the synthesis of well-defined Pd-nanoparticles (Pd-NPs), the size and shape of which strongly influence their catalytic activity and chemoselectivity [35], different synthetic approaches were applied:


Transmission electron microscopic (TEM) analyses carried out on Pd-NPs supported onto PLA-stereocomplexes clearly confirmed the influence of the synthetic procedure on the size of NPs and their distribution and the effect of the nitrogen-containing functional groups on the dispersion of Pd-NPs on the polymer-based support as shown in **Figure 4**.

**Figure 4.** Comparison of TEM analysis of Pd@**L7/8** obtained through MSV technique (A), average diameter of 2.0 nm, reduction of Pd precursors (B), average diameter of 3.8 nm, and Pd@**L11/12** obtained through MSV technique (C), which showed a strong aggregation.

The thermal gravimetric analysis (TGA) of **L11/12** and Pd@**L11/12** showed that the presence of Pd-Np strongly affected the thermal stability of stereocomplex structures [30] as shown in **Figure 5**.

**Figure 5.** Comparison of TGA of **L11/12** (A) and Pd@**L11/12** (B).

**Figure 3.** UV-Vis spectra of (A) Pd(OAc)2**(**BipyOH**)**, (B) BipyOH, (C) Pd(OAc)2 **L7**

6 New Advances in Hydrogenation Processes - Fundamentals and Applications

the type of solvent chosen, which acts as a ligand.

approaches were applied:

showed a strong aggregation.

at 305 K.

For the synthesis of well-defined Pd-nanoparticles (Pd-NPs), the size and shape of which strongly influence their catalytic activity and chemoselectivity [35], different synthetic

**•** Synthesis of Pd-NPs by coordination of Pd(OAc)2 to the functional groups of PLA-stereocomplexes, followed by metal reduction in the presence of hydrogen pressure (*p* = 1.5 MPa)

**•** Synthesis of Pd-NPs by the MVS technique [36], which has the great advantages over the former synthesis approach that the final metal content can be adjusted upon choosing the desired metal concentration of the solvated metal particles, the size of which are defined by

Transmission electron microscopic (TEM) analyses carried out on Pd-NPs supported onto PLA-stereocomplexes clearly confirmed the influence of the synthetic procedure on the size of NPs and their distribution and the effect of the nitrogen-containing functional groups on the

**Figure 4.** Comparison of TEM analysis of Pd@**L7/8** obtained through MSV technique (A), average diameter of 2.0 nm, reduction of Pd precursors (B), average diameter of 3.8 nm, and Pd@**L11/12** obtained through MSV technique (C), which

dispersion of Pd-NPs on the polymer-based support as shown in **Figure 4**.

, and (D) **L7** .

> The decomposition process of **L11/12** started at 552 K instead of Pd@**L11/12** that did not show a meaningful loss of weight up to 575 K. So, not only PLA-stereocomplexes prevent the aggregation of Pd-NPs but the latter increase the thermal stability of the stereocomplex structure.
