Bioinspired All-Polyester Diblock Copolymers Made from Poly(pentadecalactone) and Poly(2-(2-hydroxyethoxy)benzoate): Synthesis and Polymer Film Properties

Abstract

The bioinspired diblock copolymers poly(pentadecalactone)-block-poly(2-(2-hydroxyethoxy)-benzoate) (PPDL-block-P2HEB) were synthesized from pentadecalactone and dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB). No transesterification between the blocks was observed. In a sequential approach, PPDL obtained by ring-opening polymerization (ROP) was used to initiate 2,3-DHB. Here, the molar mass M_n of the P2HEB block was limited. In a modular approach, end-functionalized PPDL and P2HEB were obtained separately by ROP with functional initiators, and connected by 1,3-dipolar Huisgen reaction (“click-chemistry”). Block copolymer compositions from 85:15 mass percent to 28:72 mass percent (PPDL:P2HEB) were synthesized, with M_n of from about 30,000-50,000 g mol^-1. The structure of the block copolymer was confirmed by proton NMR, FTIR spectroscopy, and gel permeation chromatography. Morphological studies by atomic force microscopy (AFM) further confirmed the block copolymer structure, while quantitative nanomechanical AFM measurements revealed that the DMT moduli of the block copolymers ranged between 17.2 ± 1.8 MPa and 62.3 ± 5.7 MPa, i.e. between the values of the parent P2HEB and PPDL homopolymers (7.6 ± 1.4 MPa and 801 ± 42 MPa, respectively). Differential scanning calorimetry showed that the thermal properties of the homopolymers were retained by each of the copolymer blocks (melting temperature 90 °C, glass transition temperature 36 °C).

1. Introduction

Aliphatic and aromatic polyesters are not an invention of humankind – nature has made use of these highly hydrophobic polymers for a much longer time. For example, the biopolyesters cutin and suberin are found in many plant tissues that form barrier layers of the plant to its environment, e.g. leave cuticles, bark, and roots.^[1] Therein, these biopolyester layers are almost always found in combination with natural waxes, which either form layers on top of the leave cuticles, or are built into the cutin and suberin layers themselves. Through these hydrophobic “bionanocomposites”, plants are protected against water loss, fungi and bacteria, and insect-caused damage.^[1] Although ultimately degradable, this molecular structure can render natural materials based on aliphatic and aromatic polyesters surprisingly stable. This is evidenced by the suberin-based material cork: the overall hydrophilicity of cork reduces its water permeability and its degradation rate. In addition, the antimicrobial activity of the waxes found in cork slow down microbial degradation mechanisms. The result is a combination of long-term stability, yet eventual degradability. Cork is also a popular technical material, with applications ranging from sealing materials (bottle corks, technical gaskets), fishing equipment, shoes and textiles, to thermal and noise insulating engineering materials.

The polyester suberin found in cork cells consists of two domains that phase separate into lamellae (Figure 1a):^{[2, 3]} The aliphatic polyester domain consisting of mainly of fatty acid derivatives (diacids, diols, and ω-hydroxyl acids with or without hydroxyl and epoxy substituents) and 14-26% glycerol;^{[2, 4]} the aromatic domain of the lamellae consists mainly of hydroxycinnamic acids, glycerol, and hydroxyl-substituted fatty acids.^{[2, 4]} In other parts of the plant, the aromatic moiety of suberin is rich in highly cross-linked, non-hydrolysable lignin-like phenolic compounds.^{[5, 6]}

Fig. 1.

a) Electron micrographs of suberinized cork cells, together with an illustration of the assumed structure of suberin lamellae;^{[1, 2, 9]} Adapted with permission. b) Structure of the target block copolymer made from a poly(pentadecalactone) block and a poly(2-(2-hydroxyethoxy)-benzoate) block.

Because of their degradability, aliphatic and aromatic polyesters are interesting materials in the context of sustainability. While common engineering plastics like non-degradable polyethylene and polypropylene can be easily recycled, they may bioaccumulate in the form of microplastics if they are not properly disposed of – which is unfortunately a global problem.^[7] Many synthetic polyesters, on the other hand, can be degraded hydrolytically or enzymatically by environmental organisms, which reduces their persistence in the ecosystem. This has been used industrially for the manufacture of degradable polyester for one-way packaging, for example from poly(butylene-co-adipate-co-terephthalate).^[8]

Ring-opening polymerization (ROP) is a relatively recent method to obtain polyesters from cyclic lactones, and even large macrocycles like pentadecalactone can be polymerized via ROP with suitable catalysts to yield poly(pentadecalactone) (PPDL).^[10–17] ROP is a much more elegant method to synthesize polyesters than polycondensation, as it yields products with precisely defined end groups, controlled molecular masses and low polydispersity.^{[18, 19]} In the here presented context, the polyester poly(pentadecalactone) (PPDL) is particularly interesting: first, because pentadecalactone can be obtained from renewable resources,^[20] and second, because of the structural resemblance of PPDL to polyethylene, with a long aliphatic C₁₄ chain and only one heteroatom in the main chain per polymer repeat unit. PPDL is a semicrystalline polymer with about 60% crystallinity, and was reported to have a melting temperature T_m near 100 °C, a glass transition temperature T_g of - 27°C, and a crystal structure similar to both polyethylene and poly(ε-caprolactone).^[21] It thus resembles low density polyethylene,^[17] but has a slightly better solubility in organic solvents, thus allowing for solution processing. For example, it is soluble in chloroform at room temperature. Its hydrolytic degradation can be catalysed by metals, organocatalysts, and enzymes.^[22] Overall, the literature on true all-polyester block copolymers containing PPDL is rather scarce, and mostly restricted to all-aliphatic one. The reason for this are transesterification problems during synthesis. If one of the blocks of an all-polyester block copolymer is not sufficiently stabilized against nucleophilic attack, repeat unit scrambling between the blocks can be observed during synthesis of the second block – something that is not possible in block copolymers with just one polyester block. Specifically for PPDL, a significant degree of transesterification within 1 hour heating at 100 °C has been shown by model experiments.^[17] However, PPDL-block-poly(ε-caprolactone), PPDL-block-poly(ε-decalactone), and PPDL-block-poly(L-lactide) block copolymers could be realized with negligible or no transesterification between the blocks using sequential ROP.^{[16, 23, 24]} Attempts to obtain aromatic-aliphatic block copolymers containing PPDL have been rather limited so far.^[25]

The suberin lamellae found in the natural material cork remind the polymer chemist of microphase-separated lamellar block copolymer structures. This has inspired us to synthesize aliphatic-aromatic block copolymers based on polyesters, with an aliphatic PPDL block because the repeat units of PPDL are structurally similar to the fatty acid-derived aliphatic components of suberin. However, there is an important difference between the natural material and our synthetic model. In suberin, the aromatic part is crystalline, while the aliphatic part is not due to numerous hydroxyl and epoxy substituents on the fatty acid repeat units, and due to branching via glycerol. In our model, the system is reversed, with a crystalline PPDL block and an amorphous aromatic block. In a previous piece of work, we synthesized the bioinspired all-polyester block copolymer poly(pentadecalactone-block-(3-hydroxy cinnamate).^[25] In this polymer, the PPDL block was combined with an amorphous aromatic poly(3-hydroxy cinnamate) (P3HCA) block.^[25] PPDL could be obtained in relatively high molecular masses (number average molecular mass M_n up to 43,000 g mol^-1) and with a low polydispersity index (PDI in most cases < 1.8). The PDI of P3HCA, on the other hand, was broad (2 to 3), and its polycondensation yielded only a low M_n of around 3,000 g mol^-1.^[25] For this reason, the PPDL-block-P3HCA system did not have a strongly phase separated microstructure, yet it had a structured fibrillar morphology that was distinctly different from either of its parent homopolymers.^[25]

In this paper, we describe the synthesis of PPDL-block-poly(2-(2-hydroxyethoxy)benzoate) (PPDL-block-P2HEB) polymers via a) a sequential approach, and b) a modular approach using 1,3-dipolar Huisgen addition (“click reaction”)^[26–30] to connect the two blocks. The aim was to obtain a series of block copolymers with different volume ratios, and to compare their morphology and film properties. P2HEB at first glance does not seem to be an obvious choice to build a suberin-inspired synthetic polymer, since other aromatic building blocks like hydroxycinnamic acids, phenols, or catechols have a much higher structural likeness to the aromatic domains of that polymer. However, these are notoriously difficult to polymerize with structural precision or even to high molecular masses.^[31–41] Dihydro-5H-1,4-benzodioxepin-5-one, the monomer of P2HEB, has the advantage that it can undergo ring-opening polymerization.^[42] It was therefore hoped that P2HEB blocks could be used to initiate pentadecalactone, or that PPDL blocks would initiate dihydro-5H-1,4-benzodioxepin-5-one, preferentially without transesterification. P2HEB has a glass temperature T_g of 27 °C and has rubbery properties above T_g. Thus, its combination with semicrystalline PPDL should give an interesting combination of properties.

2. Experimental

2.1. Materials

All chemicals were obtained as reagent grade from Sigma-Aldrich (St. Louis, MO, USA), Carl Roth (Karlsruhe, Germany) or Alfa Aeser (Haverhill, MA, USA) and used as received. High performance liquid chromatography (HPLC) grade solvents were purchased dry from Carl Roth. Freshly distilled chloroform and toluene were stored over molecular sieves in the glovebox. 2,3-Dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB) and pentadecalactone (PDL) were sublimated and dried under high vacuum for three days. Propargyl alcohol (PA) and benzyl alcohol (BA) were dried over calcium hydride and distilled under reduced pressure. 3-azidopropanol (AP) was synthesized according to literature procedures and was distilled under reduced pressure as well.^[43] Triazabicyclodecene (TBD) was recrystallized from diethyl ether and dried under high vacuum for three days. Toluene, chloroform, 2,3-DHB, PDL, AP, PA, BA and TBD were stored and handled under nitrogen in a glovebox (MBRAUN, Garching, Germany). Benzyl- and alkyne-functionalized poly(pentadecalactone) (PPDL) was synthesized according to the literature.^[25]

2.2. Instrumentation

Gel permeation chromatography (GPC, in chloroform, calibrated with poly(methyl methacrylate) and polystyrene standards) was performed on PSS SDV 100, 1,000 and 10,000 Å columns (PSS, Mainz, Germany) using a 1260 Infinity RI detector (Agilent Technologies, Santa Clara, CA, USA). NMR spectra were recorded on a Bruker 250 MHz spectrometer (Madison, WI, USA) using CDCl₃ as solvent and tetramethylsilane (TMS) as internal reference. Fourier-transform infrared spectroscopy (FTIR) spectra were taken using a Cary 630 FTIR Spectrometer (Agilent Technologies, Santa Clara, CA, USA). The glass transition temperature and melting temperature were measured by differential scanning calorimetry (DSC). The spectra were recorded on a Phönix DSC 204 F1 device (Netzsch, Selb, Germany) in a temperature range of -20 to 250 °C and with a speed of 5 K/min.

For the formation of polymeric layers on silicon wafers a SPIN150 spin coater (SPS-Europe, Putten, Netherlands) was used, with the spin-coating parameters 3,000 rpm, 1,000 rpm sec^-1, and 30 sec. The thickness of each polymer layer was measured with a SE400adv ellipsometer (Sentech Instruments GmbH, Berlin), and the static, advancing, and receding contact angles were measured using an OCA 20 set-up (Data Physics GmbH, Filderstadt, Germany). For each sample, the average values from measurements at three different positions were taken. Atomic force microscopy (AFM) was used in order to record surface topology images using a Dimension Icon AFM from Bruker (Karlsruhe, Germany). RFESP-75 cantilivers (width: 40 μm, length: 235 μm, spring constant: 4.18 N m^-1, resonance frequency: 76 kHz) and ScanAsyst-Air cantilevers (width: 25 μm, length: 115 μm, spring constant: 0.4 N m^-1, resonance frequency: 70 kHz) were used. Quantitative nanomechanical (PeakForce-QNM) measurements were performed as described in the literature.^[25] The ScanAsyst-Air cantilever had a deflection sensitivity of 94.96 nm V^-1 and the RFESP-75 cantilever had 125.92 nm V^-1. The spring constant for the ScanAsyst-Air cantilever was calibrated using thermal tune and resulted in a value of 0.463 N m^-1. The spring constant of the RFESP-75 was calculated using the Sader method^[44] and resulted in a value of 4.18 N m^-1. The tip radius was determined using an absolute method. This resulted in a RFESP-75 tip end radius of 6.1 nm for measuring the PPDL sample, and the following ScanAsyst-Air tip end radii: 8.5 nm for measuring the PPDL₂₁-triazole-P2HEB₇₉ sample, 7.4 nm for the cantilever used to measure the PPDL₅₂-triazole-P2HEB₄₈ sample, 9.4 nm for the cantilever used to measure the PPDL₆₆-triazole-P2HEB₃₄ sample, and 8.5 nm for the cantilever used to measure the P2HEB sample.

2.3. Synthesis

2.3.1. Synthesis of poly(2-(2-hydroxyethoxy)benzoate)

Poly(2-(2-hydroxyethoxy)benzoate) (P2HEB) was synthesized via ring-opening polymerization under inert gas. The amount of each reagent used, and the molecular weight of the resulting polymers, as determined by GPC, can be found in Table 1. In the glovebox, 2,3-DHB (2.00 g, 12.2 mmol), BA (6.19 mg, 0.057 mmol) and an equimolar amount of TBD (7.96 mg, 0.057 mmol) were put together in a Schlenk tube and dissolved in chloroform (0.7 mL). The reaction mixture was removed from the glovebox and stirred for three days at 40 °C. Afterwards, 0.1 mL of methanol was added to quench polymerization. For all reactions, ¹H NMR of the crude product was measured to determine the monomer conversion. The crude product was precipitated twice with an excess of methanol. The resulting polymer was dried in vacuo at room temperature for two days.

Table 1. Ring-opening polymerization of 2,3-Dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB) using TBD as a catalyst under various conditions.^a .

Initiator	nTBD b / mmol	n2,3-DHB / mmol	c2,3-DHB / mg mL-1	Mcal. c / g mol-1	Conv.d / %	Mn e / g mol-1	PDIe
Benzyl alcohol	0.173	3.11	1,020	3,000	57	1,800	1.18
	0.011	3.11	1,020	45,000	50	21,000	1.19
	0.011	3.11	1,700	45,000	71	31,000	1.35
	0.057	12.2	2,857	35,000	79	28,000	1.13
Azido propanol	0.300	1.83	2,727	1,000	76f	750f	1.67
	0.250	3.05	2,941	2,000	54	1,100	1.31
	0.080	9.75	2,909	20,000	69	15,000	1.24
	0.100	12.2	3,333	20,000	87	19,000	1.33
	0.053	9.75	2,909	30,000	72	24,000	1.23

^aReactions were performed for three days at 40°C in chloroform.

^bThe same molar amount was used for the initiator.

^cMolecular mass calculated from the ratio of the reactants used.

^dConversion to polymer was determined by ¹H NMR spectroscopy.

^eThe number-average molecular weight M_n and polydispersity index PDI were determined by GPC in chloroform using PMMA standards.

^fConversion and molecular mass of this entry refer to an average of the polymeric and oligomeric species obtained for this reaction.

¹H NMR (250 MHz, CDCl₃, δ): 7.73 (d, J = 7.0 Hz, 1H, C_aromat H), 7.36 (t, J = 7.7 Hz, 1H, C_aromat H and 5H, C_{aromat,initiator} H), 6.86-6.97 (m, 2H, C_aromat H), 5.29 (s, 2H, CH₂-C_{aromat,initiator}), 4.56 (t, J = 4.7 Hz, 2H, CH ₂-O-C=O), 4.26 (t, J = 4.8 Hz, 2H, CH ₂-O-C_aromat), 4.16 (t, J = 4.3 Hz, 2H, CH ₂-CH₂-OH), 3.85 (t, J = 4.3 Hz, 2H, CH ₂-OH); ¹³C NMR (63 MHz, CDCl₃, δ): 165.82, 158.12, 133.54, 131.71, 120.81, 120.63, 114.03, 67.20, 62.82.

2.3.2. Synthesis of block copolymers using poly(pentadecalactone) as macroinitiator

The block copolymer PPDL-ester-P2HEB was synthesized using PPDL as the macroinitiator for polymerizing 2,3-DHB on the end of the aliphatic chain. The amount of each reagent used, and the molecular weight of the resulting block copolymers, as determined by GPC can be found in Table 2. In the glovebox, 2,3-DHB (500 mg, 3.05 mmol), PPDL (50.0 mg, M_n = 2200 g mol^-1, 0.023 mmol) and an equimolar amount of TBD (3.16 mg, 0.023 mmol) were put together in a Schlenk tube and dissolved in chloroform (0.7 mL). The reaction mixture was removed from the glovebox and stirred for three days at 40 °C. Afterwards, 0.1 mL of methanol was added to quench polymerization. The crude product was precipitated twice with an excess of methanol. The resulting polymer was dried in vacuo at room temperature for two days.

Table 2. Ring-opening polymerization of 2,3-DHB using PPDL as a macroinitiator and TBD as catalyst at different reaction conditions.^a .

Mn,PPDL b / g mol-1	PDIb	nPPDL c / mmol	n2,3-DHB / mmol	c2,3-DHB / mg mL-1	Mcalc. d / g mol-1	conv.e / %	Mn b / g mol-1	PDIb	mol%e PPDL	mol%e P2HEB	mass%f PPDL	mass%f P2HEB
2,200	1.88	0.023	3.05	714	22,000	40	9,800	1.28	18	82	24	76
8,900	2.12	0.006	3.05	625	80,000	17	16,000	1.65	33	67	42	58

^aReactions were performed for three days at 40°C in chloroform.

^bNumber-average molecular mass M_n and PDI were determined by GPC in chloroform using PMMA standards.

^cThe same molar amount was used for the catalyst.

^dCalculated molecular mass based on the ratio of the reactants.

^eConversion (relative to 2,3-DHB) and molar ratio was determined by ¹H NMR spectroscopy.

^fCalculated from the molar percentage and molecular mass M of the repeat units (M(PDL) = 240.38 g mol^-1; M(2,3-DHB) = 164.16 g mol^-1).

¹H NMR (250 MHz, CDCl₃, δ): 7.73 (d, J = 7.0 Hz, 1H_P2HEB, C_aromat H), 7.36 (t, J = 7.7 Hz, 1H_P2HEB, C_aromat H and 5H, C_{aromat,initiator} H), 6.88-6.98 (m, 2H_P2HEB, C_aromat H), 5.12 (s, 2H, CH₂-C_{aromat,initiator}), 4.56 (t, J = 4.7 Hz, 2H_P2HEB, CH ₂-O-C=O), 4.26 (t, J = 4.8 Hz, 2H_P2HEB, CH₂-O-C_aromat and 2H_PPDL, CH₂-O-C_P2HEB), 4.16 (t, J = 4.3 Hz, 2H_P2HEB, CH ₂-CH₂-OH), 4.06 (t, J = 6.6 Hz, 2H_PPDL, CH₂-O-C=O), 3.85 (t, J = 4.3 Hz, 2H_P2HEB, CH₂-OH), 2.36 (t, J = 7.5 Hz, 2H_PPDL, CH₂-C=O), 2.29 (t, J = 7.5 Hz, 2H_PPDL, CH₂-C=O), 1.58-1.68 (m, 4H_PPDL, CH₂), 1.22-1.38 (m, 20H_PPDL, CH₂); ¹³C NMR (63 MHz, CDCl₃, δ): 173.98, 165.82, 158.13, 133.54, 131.73, 120.82, 120.65, 114.03, 67.21, 64.38, 62.82, 34.39, 29.62, 29.60, 29.52, 29.47, 29.28, 29.25, 29.16, 28.65, 25.91, 25.01.

2.3.3. Synthesis of poly(2-(2-hydroxyethoxy)benzoate using azidopropanol as initiator

Azide-functionalized poly(2-(2-hydroxyethoxy)benzoate was synthesized via ring-opening polymerization under inert gas. The amount of each reagent used, and the molecular weight of the resulting polymers determined by GPC can be found in Table 1. In the glovebox, 2,3-DHB (1.60 g, 9.75 mmol), AP (5.39 mg, 0.053 mmol) and an equimolar amount of TBD (7.42 mg, 0.053 mmol) were put together in a Schlenk tube and dissolved in chloroform (0.5 mL). The reaction mixture was removed from the glovebox and stirred for three days at 40 °C. Afterwards, 0.1 mL of methanol was added to quench polymerization. For all reactions, ¹H NMR of the crude product was measured to determine the monomer conversion. The crude product was precipitated twice with an excess of methanol. The resulting polymer was dried in vacuo at room temperature for two days.

¹H NMR (250 MHz, CDCl₃, δ): 7.73 (d, J = 7.0 Hz, 1H, C_aromat H), 7.36 (t, J = 7.7 Hz, 1H, C_aromat H), 6.87-7.01 (m, 2H, C_aromat H), 4.56 (t, J = 4.7 Hz, 2H, CH₂-O-C=O), 4.26 (t, J = 4.8 Hz, 2H, CH₂-O-C_aromat and 2H, CH₂-CH₂-CH₂-N₃), 4.16 (t, J = 4.3 Hz, 2H, CH₂-CH₂-OH), 3.85 (t, J = 4.3 Hz, 2H, CH ₂-OH), 3.38 (t, J = 6.9 Hz, 2H, CH ₂-N₃), 1.93 (q, J = 6.4 Hz, 2H, CH ₂-CH₂-N₃); ¹³C NMR (63 MHz, CDCl₃, δ): 166.28, 165.83, 158.11, 133.52, 131.71, 120.81, 120.64, 114.02, 71.67, 67.20, 62.82, 61.64, 60.89, 48.12, 28.16.

2.3.6. Synthesis of the block copolymer PPDL-triazole-P2HEB

The amount of each reagent used and the molecular weight of the resulting polymers as determined by GPC can be found in Table 3. In the glovebox, PPDL-alkyne (M_n = 6.10 kg mol^-1, 76.3 mg, 0.013 mmol PPDL chains), P2HEB-azide (M_n = 24.0 kg mol^-1, 100 mg, 0.004 mmol P3HCA chains), Cu^IOAc (0.511 mg, 0.004 mmol) and an equimolar amount of TBD (0.580 mg, 0.004 mmol) were put together in a Schlenk flask and dissolved in dry chloroform (1 mL). The reaction mixture was stirred for two days at ambient temperature. Afterwards, it was removed from the glovebox and the crude product was precipitated twice into an excess of ethanol. Reactions conducted with an excess of PPDL species were dissolved in dichloromethane and unreacted PPDL was filtered off. When the reaction was performed with an excess of P2HEB species, the resulting polymer mixture was dissolved in chloroform and precipitated in ice cold dichloromethane in order to remove unreacted P2HEB chains. The resulting block copolymers were dried in vacuo at room temperature for two days.

Table 3.

Reaction parameters for the copper-catalyzed azide-alkyne cycloaddition reaction of P2HEB-azide and PPDL-alkyne using Cu^I(OAc) as a catalyst and TBD as a base.^a

PPDL-alkyne			P2HEB-azide			CuI(OAc)	PPDL-triazole-P2HEB
Mn b / g mol-1	PDIb	n / mmol	Mn b / g mol-1	PDIb	n / mmol	nc / mmol	Mn b / g mol-1	PDIb	mol%d PPDL	mol%d P2HEB	mass%e PPDL	mass%e P2HEB
6,100	2.45	0.0153	750	1.67	0.0459	0.0153	7,900	1.74	79	21	85	15
21,000	1.72	0.0238	15,000	1.24	0.0714	0.0238	32,000	1.43	66	34	74	26
22,000	2.16	0.0110	24,000	1.23	0.0330	0.0110	48,000	1.47	57	43	66	34
30,000	1.62	0.0200	19,000	1.33	0.0600	0.0200	44,000	1.41	52	48	61	39
6,100	2.45	0.0125	24,000	1.23	0.0041	0.0041	30,000	1.23	21	79	28	72

^aReactions were performed for two days at ambient temperature in chloroform (1-2 mL).

^bNumber-average molecular mass and polydispersity index were determined by GPC in chloroform using PMMA standards.

^cThe same molar amount was used for TBD.

^dMolar ratios were determined by ¹H NMR spectroscopy.

^eCalculated using the molar percentage and molecular mass of the repeat units, with M(PDL) = 240.38 g mol^-1 and M(2,3-DHB) = 164.16 g mol^-1.

¹H NMR (250 MHz, CDCl₃, δ): 7.73 (d, J = 7.0 Hz, 1H_P2HEB, C_aromat H), 7.59 (s, 1H_triazole, H-C=C), 7.36 (t, J = 7.7 Hz, 1H_P2HEB, C_aromat H), 6.88-6.98 (m, 2H_P2HEB, C_aromat H), 5.13 (s, 2H_PPDL, CH ₂-C=C), 4.56 (t, J = 4.7 Hz, 2H_P2HEB, CH ₂-O-C=O), 4.43 (t, J = 7.0 Hz, 2H_P2HEB, CH ₂-N), 4.26 (t, J = 4.8 Hz, 2H_P2HEB, CH ₂-O-C_aromat and 2H_P2HEB, CH ₂-CH₂-CH₂-N₃), 4.16 (t, J = 4.3 Hz, 2H_P2HEB, CH ₂-CH₂-OH), 4.06 (t, J = 6.6 Hz, 2H_PPDL, CH ₂-O-C=O), 3.85 (t, J = 4.3 Hz, 2H_P2HEB, CH ₂-OH), 3.65 (t, J = 6.6 Hz, 2H_PPDL, CH₂-OH), 2.36 (t, J = 7.5 Hz, 2H_PPDL, CH₂-C=O), 2.29 (t, J = 7.5 Hz, 2H_PPDL, CH₂-C=O), 2.04 (q, J = 6.4 Hz, 2H_P2HEB, CH ₂-CH₂-N), 1.58-1.68 (m, 4H_PPDL, CH ₂), 1.22-1.38 (m, 20H_PPDL, CH ₂); ¹³C NMR (63 MHz, CDCl₃, δ): 173.98, 173.59, 165.85, 158.13, 142.47, 133.54, 131.71, 124.52, 120.82, 120.65, 114.04, 71.67, 67.23, 64.37, 63.03, 62.83, 61.75, 60.91, 57.04, 47.24, 34.38, 34.08, 29.61, 29.58, 29.51, 29.45, 29.25, 29.14, 28.63, 25.91, 25.00.

2.4. Polymer film formation

Polymer films were formed via spin coating of the respective polymer solution at a concentration of 20 mg mL^-1 on silicon wafers pre-functionalized with triethoxy benzophenone silane (3EBP-silane), which has been synthesized as described in the literature.^[45] The wafers were pre-functionalized as described in the literature.^[46] The resulting polymer layers were put on a 120 °C hot plate for five minutes to delete the processing history of the sample. Afterwards, the samples were heated at 90 °C for 30 minutes and stored overnight at room temperature.

3. Results and Discussion

Study design

The aim of this work was to obtain bioinspired block copolymers with an aliphatic poly(pentadecalactone) (PPDL) block and an aromatic poly(2,3-dihydro-5H-1,4-benzodioxepin-5-one) (P2HEB) block via ring-opening polymerization (ROP) (Scheme 1). In a sequential approach (Scheme 1b), a PPDL macroinitiator was used to initiate the ROP of the aromatic block. In a modular approach (Scheme 1c and d), the two polymer components were obtained separately using functionalized initiators (carrying an azide or alkyne group, respectively) and combined via 1,3-dipolar azide-alkyne addition (click reaction). Films cast from these polymers were then analyzed for their morphology and nanomechanical properties.

Scheme 1. Polymer Synthesis.

a) Homopolymerization of 2,3-dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB) via ring-opening polymerization in chloroform (TCM), catalyzed by triazabicyclodecene (TBD). b) Sequential approach: 2,3-DHB was polymerized using poly(penta-decalactone) (PPDL) with an OH end group as ROP macroinitiator. c) Homopolymerization of poly(2-(2-hydroxyethoxy)benzoate (P2HEB) using azido propanol (AP) as initiator, yielding azide-functionalized P2HEB (P2HEB-azide). d) Modular approach: alkyne-functionalized PPDL group (PPDL-alkyne, obtained after literature procedures)^[25] was reacted with P2HEB-azide via 1,3-dipolar addition of azide to alkyne (“click”-reaction) using a copper catalyst.

Sequential approach

The synthesis of the aliphatic poly(pentadecalactone) (PPDL) by ROP was reported previously.^[25] Thus, PPDL with an OH end group was obtained, which is a suitable macroinitiator for ROP. 2,3-Dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB) was chosen as a monomer for the aromatic polymer block since this lactone could also be polymerized by ROP and thus potentially initiated by PPDL. The reaction solvent for the sequential approach was chloroform, which was the only solvent found in which PPDL could be dissolved over a broad molecular mass range, which also dissolved 2,3-DHB. Prior to the block copolymer synthesis, the homopolymerization of 2,3-DHB in chloroform was optimized (Scheme 1a; initiator: benzyl alcohol, catalyst: triazabicyclodecene (TBD), Table 1). The polymers thus obtained was isolated by precipitation into methanol.

In these reactions, monomer conversion strongly depended on the concentration of the reaction mixture. At a higher concentration of 2.9 mg mL^-1 2,3-DHB in chloroform, a high conversion of 79% could be reached. With a concentration of only 1.0 mg mL^-1, only 50% of the monomer was converted into polymeric species. The polydispersity index was relatively low (1.13 to 1.35) and did not seem to depend on the monomer concentration. The ¹H NMR spectrum of the obtained poly(2,3-dihydro-5H-1,4-benzodioxepin-5-one) (P2HEB, Figure S1, Supporting Information) matched the literature data,^[42] and the peaks found in the FTIR spectrum (Figure S2, the Supporting Information) were also in agreement with the expected polymer structure. The glass transition temperature of the aromatic homopolymer polymer (determined by differential scanning calorimetry, DSC, Figure S3, Supporting Information) was at observed 38 °C. No additional melting temperature was found, indicating that the polymer is amorphous.

Based on these results, polymerization of 2,3-DHB was initiated with a PPDL macroinitiator haring a M_n of 2,200 g mol^-1 and 8,900 g mol^-1, respectively (Scheme 1b, Table 2). The lower M_n macroinitiator was used to enable more detailed end group analytics. Since lower 2,3-DHB concentrations had to be used in these reactions to avoid precipitation of the PPDL macroinitiator, an overall low M_n of aromatic block was also expected. GPC characterization of the reaction products indicated that the molecular mass of the obtained block copolymer had indeed increased compared to its macroinitiator, thus confirming a successful initiation of the aromatic monomer by PPDL. As expected, the conversion remained low (40% and 17%, respectively, depending on the monomer concentration). To remove the unconverted 2,3-DHB monomer, the crude product was precipitated twice with an excess of methanol. Afterwards, the resulting polymer was dried in vacuo at room temperature for two days.

In Figure 2a, the proton NMR spectrum of the PPDL macroinitiator (M_n = 2,200 g mol^-1) and the resulting block copolymer (M_n = 9,800 g mol^-1) are shown. The signals of the aliphatic macroinitiator match the ones found in literature,^[25] with additional end group signals (δ = 7.36 ppm and 5.12 ppm: benzyl end group; δ = 3.65 ppm: CH₂ next to the OH end group). The end-group signal at 3.65 ppm vanished after copolymerization (Figure 2b), indicating the loss of the OH end group, as expected. Additional new signals characteristic for poly(2,3-dihydro-5H-1,4-benzodioxepin-5-one) (P2HEB) repeat units appeared. The signal intensity of the P2HEB chain end matched the one from the PPDL chain end, which also indicated a successful block copolymer synthesis. Importantly, no additional signals due to undesired transesterification between the two polyester blocks appeared. In the proton NMR spectrum of the block copolymer initiated with the higher M_n PPDL initiator, which had an overall M_n of 16,000 g mol^-1, the end group signals were below the detection limit (using standard measurement protocols). Figures 2c and d show the GPC elugrams of the two block copolyesters, together with their respective PPDL macroinitiators. The lower M_n copolymer has a relatively sharp maximum, with a slight shoulder on the high molecular mass flank of the elugrams (Figure 2c). Notably, the low molecular mass peaks originating from the PPDL initiator vanished, and the peak maximum shifted towards higher molecular masses. In the case of the higher M_n copolymer, the peak maximum only slightly shifted to higher molecular masses, potentially due to the low 2,3-DHB conversion (17%). However, the oligomeric peaks of the higher M_n PPDL macroinitiator also vanished, which is a strong indication that these peaks initiated P2HEB chains. This is plausible, as shorter polymer chains statistically have a higher probability to find a reaction partner than longer chains, where the reactive chain ends may be buried inside the polymer coil. Thus, for both GPC elugrams, the shift to higher molecular masses is more strongly observed at the lower M_n flank than at the higher M_n flank. Overall, both GPC and NMR results indicate the successful block copolymer formation. FTIR spectra (Figure S4, Supporting Information) further confirm this result. The DSC curves of the block copolyesters are shown in Figure S5 (Supporting Information). The smaller molecular mass copolymer does not feature a sharp melting point, but a broad melting area between 40 - 80 °C with several peak maxima, each corresponding to crystalline domains of PPDL with different sizes, or possible structural rearrangements during melting. This can be attributed to the low molecular mass and relatively broad polydispersity of the PPDL macroinitiator used to synthesize this copolymer, where the short oligomers hamper the formation of crystallites of similar sizes. The DSC curve of the other block copolymer, with a higher M_n of each block (Figure S5b, Supporting Information) had a single melting temperature at around 89 °C, which is characteristic for the PPDL block. A very small glass transition temperature can also be seen at around 22 °C, which is lower than the T_g of the aromatic P2HEB homopolymer. Thus, overall, the thermal properties of each homopolymer could be retrieved in the block copolymer.

Figure 2.

Characterization of the copolymers obtained by the sequential approach: ¹H NMR spectra of a) the aliphatic macroinitiator PPDL (M_n = 2,200 g mol^-1, PDI = 1.88), and b) the PPDL-ester-P2HEB copolymer (M_n = 9,800 g mol^-1, PDI = 1.28) recorded in CDCl₃ at 250 MHz. c) and d) GPC elugrams of the PPDL-ester-P2HEB copolymers (black line) and their corresponding aliphatic macroinitiators PPDL (dashed line). Two copolyesters with a molecular mass of c) 9,800 g mol^-1 (M_{n,macroinitiator} = 2,200 g mol^-1) and d) 16,000 g mol^-1 (M_{n,macroinitiator} = 8,900 g mol^-1) were synthesized.

In summary, the sequential approach to obtain all-polyester block copolymers from PPDL and P2HEB worked successfully, and had the big advantage that the target block copolymer could be obtained in only 2 steps. On the other hand, the poor solubility of higher molecular weight aliphatic homopolymer prohibited the polymerization of 2,3-DHB at higher concentrations, thus the M_n of the second block stayed low. A higher molecular mass PPDL macroinitiator would require even more solvent in the reaction mixture. Thus, both the M_n of the PPDL block and the P2HEB block that can be obtained via the sequential approach are limited.

Modular approach

An alternative reaction path to obtain the target block copolymers is shown in Scheme 1c to d. In this approach, both homopolymers - functionalized with suitable reactive end groups - are synthesized and characterized separately, and then connected by a copper-catalyzed 1,3-dipolar azide-alkyne cycloaddition (“click reaction”). With this approach, the molecular mass limit accessible with the sequential approach has been overcome. PDL polymerization was initiated with propargyl alcohol as described previously.^[25] This yielded PPDL with an alkyne end group (PPDL-alkyne, Scheme 1d). Aromatic lactone 2,3-DHB was initiated with 3-azidopropanol (AP), yielding P2HEB with an azide end group (P2HEB-azide, Scheme 1c). The reaction parameters for these polymerizations have been included in Table 1. Again, conversion depended on the concentration of the reaction mixture and varied between 54 and 87%. Using AP as initiator, P2HEB-azide polymers with a M_n from 750 to 24,000 g mol^-1 were accessible, with the usual decrease of PDI at higher molecular mass. In the proton NMR spectrum of P2HEB-azide (Figure 3a), all characteristic signals expected for a P2HEB homopolymer were found, in addition to a set of new signals originating from the azide end group (triplet at δ = 3.38 ppm from the α-CH group next to the azide, quintet at δ = 1.93 ppm from the neighboring β-CH₂ group). In the corresponding FTIR spectrum (Figure S6, Supporting Information), all characteristic signals of a P2HEB homopolymer were present, in addition to a strong band at 2097 cm^-1 originating from the N=N=N stretching vibration.

Figure 3.

Characterization of block copolymers obtained by the modular approach: ¹H NMR spectra of a) the azide-functionalized aromatic polymer P2HEB-azide with M_n = 1,100 g mol^-1 and PDI = 1.31, and b) the block copolymer PPDL₇₉-triazole-P2HEB₂₁ with M_n = 7,900 g mol^-1 and PDI = 1.74 in CDCl₃ at 250 MHz. GPC elugrams of the block copolymers c) PPDL₇₉-triazole-P2HEB₂₁ and d) PPDL₂₁-triazole-P2HEB₇₉ (both shown as black lines), and the corresponding functionalized homopolymers PPDL-alkyne (dashed line) and P2HEB-azide (dotted line).

The thus obtained P2HEB-azide and PPDL-alkyne homopolymers were connected by copper-catalyzed 1,3-dipolar addition (click-reaction). To obtain block copolymers with different block ratios, but similar overall molecular masses, various combinations of P2HEB-azide and PPDL-alkyne were allowed to react with each other (Table 3). In each case, an excess of one of the blocks was used. The homopolymers were dissolved in chloroform in a Schlenk flask inside the glove box, where the appropriate amounts of Cu^IOAc and triazabicyclodecene were added. The reaction mixture was stirred for two days at ambient temperature. To purify the block copolymers, the crude product was precipitated into an excess of ethanol. Further work-up depended on the reaction stoichiometry: Reactions conducted with an excess of the PPDL block were suspended in dichloromethane. In this solvent, unreacted PPDL did not dissolve and could be removed by filtration. Reaction mixtures containing an excess of the P2HEB block were dissolved in chloroform and precipitated into ice cold dichloromethane in order to remove unreacted P2HEB.

In Figure 3b, the proton NMR spectrum of PPDL₇₉-triazole-P2HEB₂₁, which is the block copolymer with the lowest M_n of all the copolymers synthesized, is shown as a representative for the block copolymers obtained by the modular approach. For this reaction, a low molecular weight P2HEB-azide (M_n = 750 g mol^-1, PDI = 1.67) was used, consisting mainly of oligomeric species. Those smaller chains gave NMR signals at slightly higher ppm values than the known polymeric P2HEB signals. Therefore, the characteristic signals of the P2HEB block seem to be comparatively broad (see Figure 3a). Characteristic signals from each polyester block are visible in the spectrum, indicating that both species must be present. The end group signals of both the azide chain end of P2HEB-azide and the alkyne signal of PPDL-alkyne vanished, including the signal at 3.38 ppm from the CH₂ group next to N3 and the signals at 4.68 ppm and 2.47 ppm from protons of the alkyne group of PPDL-alkyne. Instead, new signals from the newly formed triazole group appeared (singlet at δ = 7.59 ppm, corresponding to the C-H proton in the triazole ring; singlet at δ = 5.13 ppm from the α-CH₂ group of PPDL next to the triazole group; triplet at δ = at 4.43 ppm form the α-CH₂ group P2HEB next to the triazole group; triplet at δ = at 2.04 ppm from the neighboring β-CH₂ group). The intensity of these signals matched the intensity of the end group signals of both the aromatic and the aliphatic block. The latter also corresponded to each other. Overall, this is substantial evidence of successful block copolymer formation and purification. The absence of any further signals indicated that no undesired transesterification took place during the cycloaddition reaction. Due to their higher molecular masses, the other block copolymer spectra did not feature the end group signals. The molar ratio of the two blocks was therefore calculated from the ratio of their characteristic peaks.

The GPC curves of PPDL₇₉-triazole-P2HEB₂₁ and PPDL₂₁-triazole-P2HEB₇₉ are shown in Figure 3c and 3d, together with the corresponding functionalized homopolymers. An oligomeric P2HEB-azide was used to synthesize PPDL₇₉-triazole-P2HEB₂₁, which is clearly visible by the characteristic oligomer peak maxima at the lower M_n flank of the GPC curve. The PPDL-alkyne used had a higher molecular mass, and thus fewer oligomeric signals in the GPC curve. In the block copolymer elugram, the oligomer signals disappeared, and the left flank of the curve shifted to higher Mn, as did the peak maximum. These data indicate successful block copolymer formation. The GPC curve of the block copolymer PPDL₂₁-triazole-P2HEB₇₉ also showed a peak maximum at a lower elution volume than the homopolymers, which indicates a shift towards higher molecular weights. Here, the GPC curve of the block copolymer also was clearly unimodal and did not show any shoulders or oligomeric peaks, which indicates both the successful block copolymer formation and the absence of unreacted homopolymer. In summary, the GPC data further confirms the NMR findings. In the FTIR spectrum of a representative PPDL-triazole-P2HEB block copolymer (Figure S6, Supporting Information), all characteristic signals for the two polyester blocks found. Additionally, the characteristic signals of the alkyne group at 3,307 cm^-1 as well as the signal for the azide group at 2,097 cm^-1 vanished, and new signals corresponding to the triazole groups were observed at 1,636 cm^-1 and 1,463 cm^-1 (C=C and C-H vibrations, overlapping with signals from the homopolymers). The DSC curve of PPDL₆₆-triazole-P2HEB₃₄ with a molecular weight of 32,000 g mol^-1 (Figure S7, Supporting Information) had a weakly visible glass transition temperature at 36 °C, which is characteristic for the aromatic P2HEB block. For the PPDL block, a sharp and intensive melting temperature at 90 °C was observed. Overall, the characterization data of the polymers obtained by the sequential and modular approach are rather similar, indicating that both methods yielded the desired all-polyester block copolymers.

Polymer film formation and characterization

The film properties of the above described block copolymers were investigated and compared to the two homopolymers, as well as a 1:1 homopolymer blend. Polymer films were formed via spin coating using a polymer solution with a concentration of 20 mg mL^-1. The target substrates were silicon wafers pre-functionalized with the hydrophobic 3EBP-silane,^[47] which was used to match the hydrophobicity of the substrate and the polymers, and thereby prevent surface-induced dewetting. After film formation, the layers underwent a heat treatment (5 min on a 120 °C on hot plate to delete the thermal history of the sample, 30 min on a 90 °C hot plate to enable equilibration the polymer chain conformations).

Contact angle measurements confirmed the hydrophobic character of both homopolymers (Table S1, Supporting Information), where PPDL (advancing contact angle 96° ± 2°) was slightly more hydrophobic than the aromatic P2HEB (advancing contact angle 88° ± 2°). The block copolymers varied between those values, with a slight trend to higher contact angles with increasing PPDL content. The polymer film thickness (measured by ellipsometry, Table S1) was around 100 nm for most of the polymers used. Pure P2HEB yielded layers with a higher thickness of 135 ± 2 nm. This could be due to its aromatic structure and amorphous character. In agreement with general spin-coating theory, there was a trend towards higher film thickness with increasing molecular mass for the PPDL-triazole-P2HEB copolymers.

The surface morphology of the thin films was analyzed by atomic force microscopy (AFM) using the peak-force tapping mode (Height images in Figure 4 and Figure 5, Inphase and Quadrature images in Figure S8, Supporting Information). Previously recorded PPDL images, showing the crystalline morphology of that polymer, are included as a reference (Figure 4a).^[25] P2HEB had an amorphous morphology, with a roughness of only 0.3 nm on an area of 1 μm² (Figure 4b). Thus, the homopolymer morphology is in agreement with the thermal properties. The 1:1 blend of PPDL and P2HEB (Figure 4c) showed the expected phase separation of poorly miscible polymers, with spherical, unstructured P2HEB domains having a broad size distribution (height: 20-550 nm, width: 0.2-4.0 μm) embedded into a structured PPDL matrix. The overall sample roughness was 144 nm. Compared to the blend, the block copolymer films had much finer morphological features and an overall roughness of only 3 to 5 nm on a 100 μm² sample area. This absence of macrophase separation further confirms the postulated block copolymer structure. While the pure PPDL sample had a distinct spherulite morphology, the block copolymers with a lower PPDL content (PPDL₂₁-triazole-P2HEB₇₉, PPDL₅₂-triazole-P2HEB₄₈ and PPDL₅₇-triazole-P2HEB₄₃, Figures 5a-e) had a slightly fibrillar nanostructure but no spherulite microstructure. Apparently, the higher P2HEB content prevented the formation of more ordered crystalline domains. PPDL₂₁-triazole-P2HEB₇₉ (Figure 5a) with the highest P2HEB content even has a number of evenly distributed vacancies that were up to 5 nm deep and 120 nm wide. Its roughness was higher than for a pure P2HEB surface, but still as low as 3.0 ± 0.1 nm. The roughness of PPDL₅₂-triazole-P2HEB₄₈ (Figure 5b) on the other hand was 5.4 ± 0.1 nm, a value closer to the roughness of the pure PPDL sample (8.6 ± 0.1 nm). Overall, the morphology of PPDL₅₂-triazole-P2HEB₄₈ looks like lamellar crystals sticking together without a higher level of order. Furthermore, up to 10 nm deep and 150 nm wide, evenly distributed vacancies were also found on this material. The typical lamellar structure often found for block copolymers with a 1:1 volume fraction ratio was not observed here, most likely due to the higher PDI of these polymers (not close enough to 1), and the overall too low molecular masses. PPDL₅₇-triazole-P2HEB₄₃ also showed vacancies which were up to 9 nm deep and 140 nm wide. The surface structure of this polymer layer was very similar to that of PPDL₅₂-triazole-P2HEB₄₈, only showing slightly thinner lamellar crystals due to the slightly higher amount of PPDL. PPDL₆₆-triazole-P2HEB₃₄ and PPDL₇₉-triazole-P2HEB₂₁ (Figure 5d and 5e), the two copolymers with the highest PPDL content, showed spherulite structures which are characteristic for a crystalline polymer. In contrast to PPDL, the diameter of the spherulites was smaller for both block copolymers, and their lamellar crystals seemed finer. The PPDL79-triazole-P2HEB21 spherulite shown in Figure 4gis 44 μm x 29 μm in size, and the PPDL₆₆-triazole-P2HEB₃₄ spherulite in Figure 4g has a size of 27 μm x 50 μm. Spherulites formed from pure PPDL typically had a diameter >250 μm and consisted of densely packed lamellae.^[25] The roughness of the spherulite-forming copolymers shown here was 4.1 ± 0.1 nm and 4.8 ± 0.1 nm, respectively, and thus lower than that of pure PPDL. The vacancies found in these samples were 120 nm wide/9 nm deep, and 160 nm wide/7 nm deep. In summary, all block copolymers showed defined microstructures characteristic for each composition of the copolymer.

Figure 4.

AFM height images and roughness of polymer films made from a) PPDL and b) P2HEB homopolymers and c) a 1:1 homopolymer blend.

Figure 5.

AFM height images and roughness of polymer films made from the PPDL-triazole-P2HEB block copolymers with different molar ratios (a) PPDL₂₁-triazole-P2HEB₇₉, b) PPDL₅₂-triazole-P2HEB₄₈, c) PPDL₅₇-triazole-P2HEB₄₃, d) PPDL₆₆-triazole-P2HEB₃₄ and e) PPDL₇₉-triazole-P2HEB₂₁). The subscript after each block represents its molar percentage.

AFM was further used to quantify the nanomechanical properties of the polymer films obtained using the AFM-QNM mode.^[48] These QNM-AFM pictures can be found in the Supporting Information (Figures S9-S12). From these, the elastic modulus of the samples was calculated using the Derjaguin-Muller-Toporov (DMT) model. P2HEB had an overall DMT modulus of 7.6 ± 1.4 MPa, which proves the softness of this amorphous polymer. In contrast, crystalline PPDL had a stiffer surface (801 ± 42 MPa), as previously reported.^[25] The three block copolymers studied by QNM had DTM modulus values between those of the two reference homopolymers: PPDL₆₆-triazole-P2HEB₃₄, which consisted of a majority of aliphatic PPDL, had a DMT modulus of 62.3 ± 5.7 MPa. The DMT modulus of PPDL₂₁-triazole-P2HEB₇₉, with only 21 molar percent of the aliphatic block, was only 25.6 ± 2.9 MPa. Interestingly, the block copolymer PPDL₅₂-triazole-P2HEB₄₈, with an almost equimolar ratio of both components showed a DMT modulus of 17.2 ± 1.8 MPa, which is lower than that of PPDL₂₁-triazole-P2HEB₇₉. This demonstrates that the surface properties can strongly vary with the block ratio due to the specific morphology formed at each ratio, for example by preferential segregation of one blocks to the air-polymer interface.

Conclusion

In this work, the bioinspired diblock copolyesters PPDL-ester-P2HEB and PPDL-triazole-P2HEB were synthesized – the former by sequential ring-opening polymerization, the latter by a modular approach in which functionalized homopolymers were connected to a via alkyne-azide 1,3-dipolar cycloaddition reaction. Importantly, in both cases, no signs of transesterification between the two ester blocks were observed under the given reaction conditions. Reasons for this were the mild ROP conditions for the synthesis of the P2HEB block in the sequential approach, and in the alkyne-azide addition in the modular approach, respectively. However, it was not studied if transesterification would occur upon heating to higher temperatures. However, previous work with PPDL shows that mild conditions alone are not sufficient to prevent transesterification.^{[25, 37, 49]} Additionally, it seems that the benzene ring in α position to the carbonyl group of P2HEB renders a nucleophilic attack on this repeat unit more difficult. If this can be verified by further work, it would be possible to conclude that all-polyester diblock copolymers without transesterification are available if at least one carbonyl group of the two blocks is sufficiently sterically hindered.

While the sequential approach only led to block copolymers with limited molar masses of the P2HEB block, and thus to limited block ratios, the modular approach gave access to the whole range of desired molar fractions of PPDL-triazole-P2HEB. This enabled a systematic study of changes in the polymer film properties with composition for this sample series. These studies revealed a distinct morphology at each composition, and nanomechanical moduli in between the limiting values of the parent homopolymers. In future work, the barrier properties of these thin films will be evaluated.