A New Tricrystalline Triblock Terpolymer by Combining Polyhomologation and Ring-Opening Polymerization. Synthesis and Thermal Properties

New tricrystalline triblock terpolymers, polyethyleneblock-poly(ε-caprolactone)-block-poly(L-lactide) (PE-b-PCL-b-PLLA), were synthesized by ROP of ε-caprolactone (CL) and L-lactide (LLA) from linear ω-hydroxyl polyethylene (PE-OH) macroinitiators. The linear PE-OH macroinitiators were prepared by C1 polymerization of methylsulfoxonium methylide (polyhomologation). Tin(II) 2ethylhexanoate was used as the catalyst for the sequential ROP of CL and LLA in one-pot polymerization at 85 C in toluene (PE-OH macroinitiators are soluble in toluene at 80 C). H NMR spectra confirmed the formation of PE-b-PCL-b-PLLA triblock terpolymers through the appearance of the characteristic proton peaks of each block. GPC traces showed the increase in the number average molecular weight from PE-OH macroinitiator to PE-b-PCL, and PEb-PCL-b-PLLA corroborating the successful synthesis. The existence of three crystalline blocks was proved by DSC and XRD spectroscopy. © 2019 The Authors. Journal of Polymer Science Part A: Polymer Chemistry published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019

In this article, we report the synthesis of a new tricrystalline triblock terpolymer polyethylene-block-poly(ε-caprolactone)block-poly(L-lactide) (PE-b-PCL-b-PLLA). Linear PE-OH with 100% functionality 34 macroinitiator obtained from polyhomologation of methylsulfoxonium methylide was used to initiate the ROP of CL and LLA using tin(II) 2-ethylhexanoate (Sn(Oct) 2 ) as the catalyst. The molecular structure of the triblock terpolymer was characterized by 1 H NMR spectroscopy and GPC. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) spectroscopy reveal the existence of three crystalline blocks in the polymer.
Instrumentation 1 H NMR measurements were performed using a Bruker AVANCE III spectrometers operating at 400 or 600 MHz; toluene-d 8 or CDCl 3 were used as the solvents. 1 H NMR spectra were used to calculate the number average molecular weight (M n ) of each block by using the integrals of the characteristic signals from the end groups of each block and methylene protons of PE, PCL, as well as methine proton of PLLA. High-temperature GPC (HT-GPC) measurements were performed with a Viscotek High-Temperature Chromatograph (Malvern) with 2 x PLgel 10 μm MIXED-B, 300 x 7.5 mm columns. TCB was used as eluent at a flow rate of 1.0 mL min −1 and temperature 150 C. The instrument was calibrated with PS standards. DSC measurements were performed with a Mettler Toledo DSC1/TC100 under nitrogen (N 2 ), calibrated with indium (purity > 99.999%). The samples were first heated from 25 C to 200 C at a heating rate of 5 C min −1 to erase the thermal history and then cooled to −20 C at a cooling rate of 1 C min −1 . This cycle was repeated two times while melting and crystallization temperatures (T m and T c ) were recorded. X-ray diffractograms were obtained from XRD Bruker D2 Phaser using Cu Kα radiation. The sample for XRD measurements was deposited on a glass substrate with an approximate size 1.5 cm × 1 cm, preannealed at 180 C for 5 min, and cooled down to room temperature (rt) with a cooling rate~2 C min −1 .
Typical Polymerization Procedure for the Synthesis of PE-OH PE-OH (entry 1, Table 1): PE-OH was prepared according to the procedure published in the literature. 34 In more details, 40.0 g  of trimethylsulfoxonium iodide was added into a biphasic mixture of DCM (280 mL) and water (400 mL), followed by the addition of 61.5 g of tributylbenzylamino chloride. The mixture was kept in a dark place for 24 h. The aqueous phase was extracted and washed with DCM (70 mL × 3). After removal of water on a rotary evaporator, a white needle-like crystal was obtained. The crystal was dissolved in methanol at 50 C and then cooled to room temperature slowly, followed by cooling to 0 C inside a fridge overnight. A needle-like crystal product of trimethylsufoxonium chloride (19.5 g, yield: 84%) with high purity was obtained.
Next, 10.8 g (0.45 mol) of NaH was inserted into a two-necked flask connected with a reflux condenser and Ar/vacuum line. After three subsequent vacuum-Ar circles, 180 mL of freshly distilled THF was injected. It was followed with the addition of 32.0 g (0.38 mol) of dry trimethylsulfoxonium chloride. The mixture was heated and refluxed at 80 C until the gas ceased (5-6 h). The THF was removed under low pressure and followed the transfer of 150 mL of dry toluene into the flask. A 100-mL Schlenk flask was charged with 1 g of the isolated PE-OH and 20 mL of dry toluene. The flask was heated to dissolve PE-OH, and azeotropic distillation was performed between toluene and water three times to dry the PE-OH. The white powder was stored inside the glove box under Ar.
Typical Polymerization Procedure for the Synthesis of PEb-PCL In a glove box under Ar atmosphere, a dry polymerization flask equipped with a stirrer bar was charged with 200 mg dry PE-OH-1, 5.7 mg of Sn(Oct) 2 (0.014 mmol), 0.31 mL CL (2.8 mmol), and 7 mL dry toluene. The flask was sealed and placed in an oil bath at 85 C outside the glove box. The ROP was monitored by 1 H NMR spectroscopy. After 24 h, the conversion of CL reached 96.0%, 3.5 mL of the reaction mixture was withdrawn, quenched with benzoic acid, and precipitated in cold MeOH. PE-b-PCL was isolated by centrifugation and dried in vacuo for 24 h at 40 C. 1

RESULTS AND DISCUSSION
The polyhomologation reaction and subsequential ROPs of CL and LLA from the PE macroinitiator with Sn(Oct) 2 as the catalyst are described in Scheme 1. Sn(Oct) 2 was selected as the catalyst instead of an organic catalyst such as phosphazene superbase to avoid the formation (S,R) meso-lactide monomeric unit during the ROP of LLA at 80 C. 35 The presence of (S,R) meso-lactide in the PLA chain is unfavorable, because it can worsen the thermal properties of PLLA block and lead to an amorphous polymer. 35 Two PE-OH macroinitiators having a different molecular weight (PE-OH-1 and PE-OH-2) were prepared by polyhomologation (entry 1 and entry 4, Table 1). The ROPs of CL and LLA from the macroinitiators are monitored by 1 H NMR spectroscopy. The conversion of CL is determined by the integral ratio of CH 2 OCO protons (h) in the polymer at 4.08 ppm and monomer at 4.15 ppm (Supporting Information Fig. S1). The conversions of CL were 87.3% for PE-OH-1 (entry 2, Table 1) and 96.0% for PE-OH-2 (entry 5, Table 1). The conversion of LLA is determined by comparing the integral ratio of methine proton (k) in the polymer centered at 5.17 ppm with the methine proton in monomer centered at 5.01 ppm (Supporting Information Fig. S2). The conversion of LLA for each block is 94.3% and 95.9% (entry 3 and 6, Table 1), respectively. It is worth noting that the remaining CL was not polymerized during the ROP of LLA. Thus, the resulting third block is a pure PLLA chain. 1 H NMR spectra of the isolated PE-OH, PE-b-PCL, and PE-b-PCL-b-PLLA (entry 1-6, Table 1) are presented in Figure 1 (from PE-OH-1 macroinitiator) and Supporting Information Figure S3 (from PE-OH-2 macroinitiator). The CH 2 OH protons of PE (c) appear at 3.35 ppm, while the CH 2 protons of the main chain (b) were observed at 1.32 ppm. The integral ratio of these protons was used to calculate the M n of PE-OH. After the copolymerization with CL, six new peaks appear in the spectrum. The peak centered at 3.96 ppm (h) is assigned as CH 2 OC O protons of PCL block and merged with the protons (i). The OCH 2 triplet protons appear at 2.11 ppm (d) while the four protons of CH 2 CH 2 O (g) and CH 2 CH2 OC O (e) appear at 1.52 and 1.45 ppm, respectively. The CH 2 CH 2 CH 2 protons (f) arise at 1.23 ppm. Two new protons emerged after the ROP of LLA from PE-b-PCL macroinitiator; and methine proton (CH 3 CHO CO) and methyl ( CH 3 ) protons of PLLA block (k and j) around 5.06  Table 1) with Sn(Oct) 2 as the catalyst (toluene-d 8 , 80 C. The mark * denotes the methyl protons signal from toluene). and 1.37 ppm, respectively. The end-group protons of PLLA (m) are detected at 4.09 ppm, corroborating that PLLA block is covalently linked to PCL block. After melting and annealing of the polymers for 3 min, crystallization behavior of the triblock terpolymers and its precursors was evaluated by applying a cooling scan 10 C min −1 (Fig. 4) for PE-OHs (black), PE-b-PCLs (blue), and 1 C min −1 for PE-b-PCL-b-PLLAs (green). A slow cooling scan (1 C min −1 ) must be applied to facilitate the crystallization of PLLA block. 1 The T c of PE-OH-2 decreases from 110.5 C to 100.6 C in PEb-PCL-b-PLLA-2. This could be due to the presence of PLLA block that crystallizes earlier than the PE block, forcing the  The crystal structures of PE-OHs (black), PE-b-PCLs (blue), and PE-b-PCL-b-PLLAs (green) were analyzed using XRD (Fig. 5). Samples were deposited on top of a glass substrate and heated up to 180 C, annealed for 10 min, and cooled down to room temperature at a roughly 2 C min −1 . The diffractograms show the appearance of the characteristic crystallographic planes on homo, di-, and triblock terpolymers. PE-b-PCL-b-PLLAs exhibit peaks in the range of 2θ = 15 − 25 . PE and PCL blocks are known to form orthorhombic crystal structure, thus giving an identical peak position on the diffractogram. The (200) crystallographic plane of PE and PCL is observed around 23.7 < 2θ < 24.2 , while the (110) crystallographic plane of PE and PCL appears around 21.5 < 2θ < 21.9 . Crystallographic planes of PLLA are identified with the appearance of three peaks; 19.2 < 2θ (203) < 19.4 , 16.8 < 2θ (200/110) < 17.1 , and 15.0 < 2θ (010) < 15.2 , respectively. Thus, the successful synthesis of tricrystalline terpolymer is also confirmed by XRD. Slight deviations on the peak position between the two triblock terpolymers are attributed to the different molecular weights and compositions. Further details will be discussed in a separate paper.

CONCLUSIONS
Tricrystalline triblock terpolymers, PE-b-PCL-b-PLLA, were synthesized by combining polyhomologation with ROP. Polyhomologation produced perfectly linear PE-OH and used as the macroinitiator for the ROP of CL and LLA with Sn(Oct) 2 as the catalyst. 1 H NMR spectroscopy and GPC confirmed the molecular structure of triblock terpolymers, while DSC and XRD revealed the existence of three crystalline blocks. The synthetic approach presented in this work offers a straightforward strategy to synthesize PE-based tricrystalline triblock terpolymer with well-defined microstructure. This new PE-b-PCL-b-PLLA triblock terpolymer is essential for the advancement of polymer crystallization field.