graft-poly(ε-caprolactone) Copolymers by ROMP - ACS Publications

Jun 29, 2016 - ABSTRACT: Well-defined high molar mass poly(1,4-buta- diene)-g-poly(ε-caprolactone) (PBu-g-PCL) graft copolymers were prepared through...
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High Molar Mass Poly(1,4-butadiene)-graf t-poly(ε-caprolactone) Copolymers by ROMP: Synthesis via the Grafting-From Route and Self-Assembling Properties Flavien Leroux, Véronique Montembault,* Sandie Piogé, Sagrario Pascual, Guillaume Brotons, and Laurent Fontaine* Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS−Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans, Cedex, France S Supporting Information *

ABSTRACT: Well-defined high molar mass poly(1,4-butadiene)-g-poly(ε-caprolactone) (PBu-g-PCL) graft copolymers were prepared through the grafting-from route by the combination of ring-opening metathesis polymerization (ROMP) and organocatalyzed ring-opening polymerization (ROP). The synthesis route relies on the ROMP of cis-4benzyloxymethyl-3-hydroxymethylcyclobutene initiated by ruthenium-based Grubbs’ catalysts followed by organocatalyzed ROP of ε-caprolactone initiated by the hydroxyl side groups of the backbone using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst. The reported strategy provides PBu-g-PCL having a strictly poly(1,4-butadiene) backbone with the highest molar mass reported up to now (Mn > 106 g mol−1). Self-assembling properties of the resulting PBu-g-PCL graft copolymer were investigated using small-angle X-ray scattering (SAXS) in toluene solution and in the solid state.

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INTRODUCTION With the advent of metathesis catalysts endowed with a high activity and a good tolerance toward a wide range of functionalities, ring-opening metathesis polymerization (ROMP)1 with Ru-based initiators2 has become a very practical methodology in polymer chemistry to prepare various macromolecular architectures such as block and graft copolymers with controllable molar mass.1,3 ROMP has also attracted growing interest because of its capability of producing polymers that are unable to be prepared by other (ionic or radical) polymerization methods.1 One such polymer is poly(1,4-butadiene) (PBu) that can be obtained via the ROMP of 1,5-cyclooctadiene and cyclobutene derivatives.4,5 In our group, we have used the ROMP approach to prepare well-defined block and graft copolymers having a polybutadiene backbone with a strictly 1,4-type microstructure. We have previously reported the synthesis and ROMP of various cyclobutene-based monomers6−8 and macromonomers (MMs)5 that have been prepared using orthogonal processes such as atom transfer radical polymerization (ATRP),9 reversible addition−fragmentation transfer (RAFT) polymerization,10 click chemistry,10,11 and organocatalyzed ring-opening polymerization (ROP).12 Polyester-grafted polymersespecially polylactide and poly(ε-caprolactone) (PCL)have attracted much attention due to their potential to act as building blocks for nanomaterials synthesis13 and for biomedical applications.14 Driven by our © XXXX American Chemical Society

interest in developing new efficient methodologies to prepare well-defined grafted PBu, we recently investigated the synthesis and characterization of PBu-g-polyesters from cyclobutenyl MMs bearing one or two polyester segment(s) derived from Llactide (LA) or ε-caprolactone (CL).12 While living ROMP of MMs efficiently generates welldefined bottlebrush copolymers with accurate molar mass control and low dispersities using norbornene-derived MMs,15 the synthesis of high molar mass bottlebrush copolymers according to the grafting-through technique using cyclobutenyl MMs is still challenging: the side chain and/or backbone lengths are generally limited by the low conversion of high molar mass MMs.5−12 In order to reach graft copolymers with higher molar masses, we report herein the synthesis of PBu-gPCL according to the grafting-from approach. In the literature, previous reports devoted to the synthesis of PCL-grafted ROMP polymers used the grafting-through approach starting from (oxa)norbornene MMs12,16−18 and the grafting-onto technique via click azide−alkyne cycloaddition19 or nitroxide radical coupling.20 To our knowledge, there has been no study dealing with the synthesis of high-density bottlebrush PBu-gPCL using the grafting-from strategy. Received: April 15, 2016 Revised: June 15, 2016

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DOI: 10.1021/acs.macromol.6b00786 Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Characteristics of PBu Backbones Synthesized upon ROMP of Inimer 1 Mediated by Grubbs’ Catalysts in C2H4Cl2 at 50 °C run

samplea

initiator

[1]0/[I]0

time (h)

convb (%)

DPn,NMR c

M̅ n,NMR d (g mol−1)

M̅ n,SEC e (g mol‑1)

Đe

1 2 3 4 5 6

PBu85 PBu86 PBu198 PBu188 PBu385 PBu330

G2 G3 G2 G3 G2 G3

100 100 250 250 500 500

1 1 2 2 5 5

85 86 79 75 77 66

85 86 198 188 385 330

17444 17648 40496 38456 78644 67424

19300 22700 48000 45200 68900 78800

1.14 1.11 1.23 1.14 1.54 1.38

a

In the sample name, the number in subscript denotes the number of inimer 1 repeating units determined by 1H NMR. bThe inimer 1 conversions were determined by comparing the peak areas of the cyclobutene alkene protons at δa = 5.95−6.05 ppm and the protons of double bonds of the growing PBu at δa′ = 5.10−5.70 ppm from 1H NMR spectra of the crude mixture. cDetermined by 1H NMR analysis from DPn,NMR = conv × [1]0/ [I]0. dDetermined by NMR analysis from M̅ n,NMR = (DPn,NMR × Minimer 1) + Mextr with Minimer 1 = 204 g mol−1 and Mextr = 104 g mol−1. eDetermined by SEC in tetrahydrofuran (THF) with RI detector, calibrated with linear polystyrene standards.

In the present work, we report for the first time the synthesis and characterization of high molar mass PBu-g-PCL (>106 g mol−1), including self-assembling properties in solution and in the solid state using small-angle X-ray scattering (SAXS). We recently reported the first study devoted to the ROMP of cis-4benzyloxymethyl-3-hydroxymethylcyclobutene using highly reactive initiators containing N-heterocyclic carbenes to prepare a series of well-defined PBu having hydroxyl side groups.8 Herein, our strategy relies on the use of the hydroxyl side groups of this platform to initiate the orthogonal ROP process of CL according to the grafting-from route. We demonstrate that this strategy provides a unique access to PBu-g-PCL copolymers with the highest molar mass reported up to now.

are potentially PBu-based ring-opening polymerization (ROP) macroinitiators (Scheme 1). It should be noted that a [1]0/ Scheme 1. Synthesis of Poly(1,4-butadiene)-g-poly(εcaprolactone) Graft Copolymers According to the “GraftingFrom” Strategy

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RESULTS AND DISCUSSION Synthesis of the Poly(1,4-Butadiene)-Based ROP Macroinitiator. Inimer cis-4-Benzyloxymethyl-3-hydroxymethylcyclobut-1-ene (1) was polymerized in dichloroethane (C2H4Cl2) through ring-opening metathesis polymerization (ROMP) using either Grubbs’ second-generation catalyst (G2) or Grubbs’ third-generation catalyst (G3) as the initiator8 for [1]0/[Grubbs’ catalyst]0 ratios up to 250 (Table 1, runs 1−4, Figure 1A,B, and Figure S3A,B in the Supporting Information) to provide poly(1,4-butadiene) (PBu) with narrow molar mass distributions (dispersity Đ = 1.11−1.23). Such well-defined polymers containing pendant hydroxyl in every repeating unit

[Grubbs’ catalyst]0 ratio of 500 (Table 1, runs 5 and 6) leads to broad dispersity values (Đ = 1.38−1.54) together with an important spreading of the size exclusion chromatography (SEC) traces (Figure 1C and Figure S3C in the Supporting Information). This behavior is related to “backbiting” phenomena, which are favored by the longer reaction times required to reach high conversions of 1.3,7 Synthesis of the Poly(1,4-butadiene)-g-poly(ε-caprolactone). The “grafting-from” approach was used in the modification of PBu-based ROP macroinitiator backbones with poly(ε-caprolactone) (PCL). ROP of ε-caprolactone (CL) was performed following a procedure reported earlier12 by using PBu backbones as macroinitiators and 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst (Scheme 1). TBD was chosen as the catalyst as it proved highly efficiency in mediating the ROP of CL.21 TBD-mediated ROP of CL using each hydroxyl group of the repeating unit of the PBu as initiator functionality was conducted in tetrahydrofuran (THF) at 25 °C for 4−20 h ([CL]0/[OH]0/[TBD]0 = 18−70/1/ 0.35−0.65; Table 2, runs 1−5, Scheme 1). The living PCL grafts were terminated using acetic acid to give the final poly(1,4-butadiene)-g-poly(ε-caprolactone) (PBu-g-PCL) copolymer after purification through a silica column. With high CL conversions (>80%), PBu-g-PCL graft copolymers with well-controlled structures (M̅ n,NMR = (192−1124) × 103 g mol−1, Đ = 1.10−1.35) were obtained as ascertained by their narrow and monomodal SEC curves (Figure 2 and Figure S4). DPn were determined by 1H NMR analysis of the purified PBug-PCL copolymers (Table 2), based on the comparison of the

Figure 1. SEC traces for the (A) PBu86 (Table 1, run 2), (B) PBu188 (Table 1, run 4), and (C) PBu330 (Table 1, run 6) issued from the ROMP of 1 in C2H4Cl2 at 50 °C using G3 as the initiator. B

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Table 2. Characteristics of PBu-g-PCL Graft Copolymers Synthesized upon ROP of CL Mediated by TBD as the Catalyst in THF at 25 °C run

samplea

([CL]0)/[OH]0/[TBD]0

time (h)

conv.b (%)

DPn,NMR c

M̅ n,NMR d (g mol−1)

M̅ n,SEC e (g mol−1)

Đe

1 2 3 4 5

PBu85-g-PCL18 PBu85-g-PCL35 PBu85-g-PCL45 PBu198-g-PCL18 PBu198-g-PCL48

18/1/0.35 35/1/0.65 70/1/0.65 18/1/0.35 70/1/0.65

4 8 16 20 20

>99 >99 85 >99 80

18 35 45 18 48

191864 356594 453494 446792 1123952

128600 232400 296600 248700 423100

1.11 1.10 1.11 1.16 1.35

In the sample name, the first number denotes the number of inimer 1 repeating units and the last number in subscript the number of CL repeating units determined by 1H NMR. bThe CL monomer conversions were determined by comparing the peak areas of the methylene triplet of PCL at δ = 2.30−2.45 ppm and the methylene group of CL at δ = 4.21 ppm from 1H NMR spectra of the crude mixture. cDPn,NMR of the grafts calculated from 1 H NMR spectra of the purified graft copolymer by comparing the peak areas of the aromatic protons of the benzyl group at δ = 7.10−7.45 ppm and the −OC(O)CH2CH2− methylene triplet of PCL at δ = 2.30−2.45 ppm. dDetermined by NMR analysis from M̅ n,NMR = M̅ n,NMR (PBu) + (DPn,NMR (PBu) × DPn,NMR (CL) × MCL) with MCL = 114 g mol−1. eDetermined by SEC in tetrahydrofuran (THF) with RI detector, calibrated with linear polystyrene standards. a

densely grafted copolymers with a strictly poly(1,4-butadiene) backbone to date.5,7,11,12 Thermal Properties of PBu Backbones and Poly(1,4butadiene)-g-poly(ε-caprolactone) Graft Copolymers. The thermal stability of the PBu backbones (Table 3, runs 1 Table 3. Thermal Properties of PBu Backbones, Linear PCL Homopolymers, and PBu-g-PCL Graft Copolymers run

samplea

1 2 3 4 5

PBu85 PBu198 PCL18 PCL38 PBu85-gPCL18 PBu85-gPCL45 PBu198-gPCL18 PBu198-gPCL48

Figure 2. SEC traces for the (A) PBu85 (Table 1, run 1), (B) PBu85-gPCL18 (Table 2, run 1), (C) PBu85-g-PCL35 (Table 2, run 2), and (D) PBu85-g-PCL45 (Table 2, run 3).

peak integration areas of the −OC(O)CH2CH2− methylene triplet of the PCL at δ = 2.30−2.45 ppm (labeled (a) in Figure S7) against the aromatic protons of the benzyl group at δ = 7.10−7.45 ppm (labeled (f) in Figure S7). The experimental DPn values were in the same range as the theoretical DPn values calculated from [CL]0/[OH]0 molar ratio and conversion of CL. The number-average molar masses determined by SEC (M̅ n,SEC) using RI detector increased with increasing [CL]0/ [OH]0 molar ratios (Table 2, runs 1−5). In this graft system, side chains are formed from multiple initiating sites along the backbone. Such branching architecture in a graft copolymer will lead to relatively smaller dynamic volume compared to a linear copolymer of the same molar mass, thus causing M̅ n,SEC to be underestimated (Table 2, M̅ n,SEC vs M̅ n,NMR ).22 A well-known drawback of the grafting-from strategy is that the grafting density cannot be easily controlled. Although the length and the dispersity of the grafts cannot be determined through hydrolysis of the PCL grafted chains of the PBu-g-PCL copolymers, SAXS analysis (vide inf ra) shows an excellent agreement between the calculated and measured SAXS long lamellar d-spacing, indicating that the dispersity of the PCL chains is low in the PBu-g-PCL copolymer. The bottlebrush architecture of the obtained PBu-g-PCL was further corroborated by SEC using multiangle light scattering (MALS) detection. PBu198-g-PCL48 sample (Table 2, run 5) MALS exhibited an absolute molar mass M̅ w,SEC of 1 026 000 g mol−1 (Đ = 1.26, Figure S10) that matched well the M̅ n,NMR value ( M̅ n,NMR = 1 123 952 g mol −1 ). This PBu 198 -g-PCL 48 copolymer exhibits the highest molar mass value of all known

6 7 8

Tgb (°C)

Tcb (°C)

Tmb (°C)

ΔHcb (J g−1)

ΔHmb (J g−1)

Tdegradationc (°C)

χcd

65 67 44

43 43 −e −61f −e

29 33 16

46 51 43

88 90 59

88 91 60

260 290 330 350 333

−58f

25

52

69

64

350

47

−e

18

43

76

73

325

54

−59f

26

52

66

64

354

47

In the sample name, the first number in subscript denotes the number of inimer 1 repeating units and the last number in subscript the number of CL repeating units determined by 1H NMR. bGlass transition temperature Tg, crystallization temperature Tc, melting temperature Tm, enthalpy of crystallization ΔHc and enthalpy of fusion ΔHm measured by DSC. cTemperature at 5% mass loss determined by TGA. dDegree of crystallinity calculated by comparison with the reported enthalpy of fusion for the parent polymer crystal: χc = ΔHm/ ΔHm 100% crystalline polymer, where ΔHm 100% crystalline PCL28 = 140 J g−1. eNot detected. fGlass transition temperature measured by DSC where the polymers were heated to 100 °C and then cooled to −150 °C at a rate of 10 °C min−1. a

and 2) was evaluated by thermogravimetric analysis (TGA) (Figure S11). A one-step degradation process is seen in both backbones PBu85 and PBu198 with a decomposition temperature in the range from 260 to 400 °C.23 Similarly, the TGA curves of the PBu-g-PCL copolymers present only one-step obvious degradation (Table 3, runs 5−8, and Figure S12) with a temperature range between 325 and 354 °C. TGA analyses showed that there is no significant influence of the length of the PBu backbone. The thermal stability increases with the length of PCL chain in both PCL-based MMs12 and PBu-g-PCL graft copolymers (Table 3, run 3 vs run 4; run 5 vs run 6; run 7 vs run 8) as already evidenced for PCL homopolymers.24 The C

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temperatures of PBu-g-PCL graft copolymers are similar to those of PNB-g-PCL graft copolymers reported in the literature.16−18,26,27 The crystallinities of the linear PCL-based MMs (Table 3, runs 3 and 4) and PBu-g-PCL graft copolymers (Table 3, runs 5−8) were calculated by comparison with the reported enthalpy of fusion for the parent polymer crystal (ΔHf 100% crystalline PCL = 140 J g−1).28 The results show a significant decrease of the enthalpy of fusion of the PBu198-gPCL48 compared to the linear PCL38 homopolymer (91 J g−1 for PCL38 and 64 J g−1 for PBu198-g-PCL48), resulting in a lower crystallinity. This low mobility and capability for crystallization can be explained by the specific architecture of graft copolymers, resulting in a restriction of conformational freedom, compared to free chains of PCL.27 Morphology of Diluted PBu198-g-PCL48 Particles. Small-angle X-ray scattering (SAXS) intensity curves were collected from PBu198-g-PCL48 batch at a concentration of 10 g L−1 in toluene. Figure 4A shows a typical SAXS intensity curve, I(q) (with a log−log scale for scattering vector q in Å−1 and absolute intensities in cm−1), for PBu198-g-PCL48 at 10 g L−1. The SAXS data from the dispersed sample can be well fitted assuming a core−shell particle for its form factor calculation P(q) due to the solvent-induced self-organization of the polymers. Corresponding fitted parameters are reported in Table 4, namely, the core radius (Rc), its log-normal distribution width (δc), and core electron density (ρc); the shell thickness (tS), its Gaussian distribution width (δS), and shell electron density (ρS). A constant background (counting for toluene scattering) and two additional parameters were also used for the structure factor, S(q), calculation using the hardsphere volume fraction (ϕHS) and radius (RHS fixed here to Rc + tS). Since the form factor, P(q), calculation depends on scattering contrasts with the solvent, we fixed the solvent value to its calculated value for the fit corresponding to pure toluene ρM = 0.284 electrons/Å−3 (or equivalently scattering length density ρbM = 8.0026 × 10−6 Å−2). We also tried to fit the data using a simplest homogeneous spherical particle (core model) with only one contrast with solvent, but it did not fitted the data as well. The presence of a thin shell covering the particles (here ∼9 nm in thickness) with a slightly higher electron density (+0.003 e/Å3) than toluene reduces the scattering signal at smallest angles measured (toward the so-called Guinier plateau regime) while it increases the scattering signal at intermediate and larger angles measured, where a q−4 slope

differential scanning calorimetry (DSC) technique was invited to obtain the glass transition temperature (Tg) of PBu backbones (Table 3, runs 1 and 2, and Figure S13). PBu85 and PBu198 backbones exhibit the same Tg of 43 °C. This value is higher than those reported for PBu issued from ROMP of 3,4-disubstituted cyclobutenes with ether and ester functions, which are −4 and 21 °C, respectively.23 This high Tg of 43 °C can be caused by the presence of hydroxyl groups promoting hydrogen interactions. DSC analysis was also used to determine the thermal properties of PBu-g-PCL graft copolymers and those of linear PCL MMs,12 and the results are listed in Figure 3 and Table 3.

Figure 3. DSC traces of PBu198-g-PCL48 (full line) (Table 3, run 8) and PCL38 (dashed line) (Table 3, run 4).

DSC traces for the PBu85-g-PCL45 and PBu198-g-PCL48 (Table 3, runs 6 and 8) show a single Tg around −60 °C, which is similar to the Tg of linear PCL38 homopolymer (Table 3, run 4) and is in agreement with the literature for graft PCL-based copolymers.25,26 The melting temperature (Tm) of PCL-based structures between 43 and 52 °C (Table 3, runs 3−8, and Figure 3) prevents the Tg of the PBu backbone from being observed.26,27 Endothermic and exothermic peaks are observed due to the melting and cold-crystallization of PCL crystalline phase in the PBu-g-PCL graft copolymers (Figure 3). The crystallization temperatures (Tc) and Tm of PBu-g-PCL graft copolymers increased with the length increase of the grafted PCL chains (Table 3, run 5 vs run 6; run 7 vs run 8), which should be attributed to the crystalline imperfection of the short chain length of PCL segments.25 The characteristics thermal

Figure 4. SAXS intensity curves (I(q)) for PBu198-g-PCL48: (A) dispersed in toluene at 10 g L−1 and plotted in absolute intensity scale (cm−1) with the model fit described in the text on top of the data; (B) for two thin polymer films obtained after slow toluene evaporation (in arbitrary scale of the intensity). D

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Macromolecules Table 4. SAXS Best Fit Parameters from the Core−Shell Form Factor P(q) Model Rc (nm)

ρbc (e/Å−3)

tS (nm)

ρbs (e/Å−3)

δc log-normal

δs Gaussian

5.6 ± 0.4

0.325 ± 0.03

9.0 ± 2

0.287 ± 0.05

0.18 ± 0.03

0.14 ± 0.05

chains each one composed of two PCL repeating units along the c cell axis equal to 1.726 nm in the P212121 space group29 (Figure 5). PCL chains with more than 15 units are known to

would be expected for a smooth core sphere without shell (in the so-called Porod regime and after background subtraction). The P(q) calculation is plotted in Figure 4A, including the particle polydispersity broadening and smearing effects due to instrumental resolution. Note that at such high dilution the data are not affected by the particle−particle interactions (in that sense, we did not reach a precise determination of ϕHS from our fit, even though we obtained from the fit a hard-sphere volume fraction of 0.1, in agreement with the sample concentration). Since we fitted the data within the frame of a simple core− shell model, we do not intended to establish a detailed profile of the side chains distributions but whether if PBu198-g-PCL48 self-assembles in micelles or aggregates with a dense PBu core due to its solvophobic interactions with toluene while the side chains (PCL and benzyl) would remain swollen. The core electron density of 0.325 e/Å−3 obtained from the fit is fully consistent with a PBu core (0.327 was expected for pure PBu198 at 1.01 g cm−3), and the shell fits well with a highly swollen PCL domain with electron density close to the solvent value. (Pure toluene and pure PCL would correspond to 0.284 and 0.376 e/Å−3, respectively, for dPCL = 1.145 g cm−3, so we estimate that the shell, as depicted from X-rays here, contains more than 90% of toluene in volume.) Also for contrast reasons, we do not see the PCL side chain far away from the PBu core and highly swollen. The scattering mainly comes from the core and it would represent nearly 28 self-assembled pure PBu polymer backbones, to give such a volume. The number of self-assembled polymers per micelle reduces to 12 if we include the presence of the small benzyl side chains in the core volume calculation and to 7 including the first graft PCL unit. Otherwise, assuming that each scattering particle is a single molecule, we estimated the number of PCL repeating units that would have to be included in the core in order to reach the measured SAXS core volume to n = 20. Following this hypothesis, we would expect a core electron density of ≃0.37 e/Å−3, much higher than the measured one. Thus, we concluded that the aggregates in toluene are made of few self-assembled polymer chains with a core mainly composed of PBu. Self-Assembling of PBu198-g-PCL48 Melts in Crystallized Lamellar Phases after Solvent Evaporation. We also prepared pure PBu198-g-PCL48 films from an extremely slow and gentle evaporation of toluene in order to form a thin film that wetted well the quartz surface support (transparent X-ray quartz walls of 10 μm thickness were used). After total removal of toluene under vacuum over days, the polymer films thickness were roughly estimated to range between 10 and 90 μm depending on the concentration of evaporated solution. The SAXS measurements were carried out perpendicularly to the quartz supported films, and we measured a powder averaged diagram since the samples crystallized with small lamellar domains randomly arranged. The raw intensity data scales with the thickness, and all curves showed a broad but pronounced correlation peak at q1 = 0.048 Å−1, a second weaker peak at q2 = 0.104 Å−1, and a much weaker peak at q3 = 0.206 Å−1 only visible for the thicker film (Figure 4B). After an isothermal crystallization at 20 °C, PCL is expected to crystallize in an orthorhombic unit cell including two parallel

Figure 5. Sketch of the large scale lamellar structure giving the SAXS peaks and crystal structure of the PCL chains.

fold, and it has been shown that the lamellar d-spacing then varies between 12.8 and 14.9 nm while the number of repeating units varies from 16 to 700.30 In our case, NPCL = 48 so we expected to be in this regime where the number of folds (nfolds) can be determined from the corresponding lamellar d-spacing using31 n folds Lc,PCL =

1 2(n folds + 1)

× c × NPCL

We find that nfolds = 2 gives a excellent agreement with the measured SAXS long lamellar d-spacing along the c-axes of the cell structure (Lc,PCL2 = 13.8 nm and 2π/q1 = 13.2 nm). Note that the small difference between these values comes from the fact that the equation neglects the chain bonds forming the Uturns and tilt angles in the crystallographic cell unit. Moreover, PCL have been grafted here onto a PBu backbone, which certainly implies that several PCL repeating units do not participate to the crystallized structure and rather form a thin amorphous zone close to the backbone and to the benzyl side chains. In the absence of solvent we can estimate the volume fractions of polymers from their estimated volume densities, and we obtained 96% for the side chains and 4% for the PBu backbone. The second Bragg peak measured (q2) certainly contains contributions from the second order of the first one (at 2q1). Since both are quite broad, we cannot exclude that it also contains contributions from a superstructure corresponding to a repeating period of 6.06 nm. The third peak would be in that case its second order since we find that q3/q2 = 1.98. We can hypothesize that superstructures from the lateral organization of the PCL crystallized chains (along a- and baxes) exist due to their fixed connection to the backbone, depending on its configurations and conformations (cis/trans and PCL/benzyl sequence along PBu backbones). For sure, no diffraction peaks from the P212121 cell structure are expected in the SAXS q-range window (the first P212121 Bragg peak appears at 2θ = 10.242° for hkl = 002). E

DOI: 10.1021/acs.macromol.6b00786 Macromolecules XXXX, XXX, XXX−XXX

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CONCLUSIONS We have developed an efficient strategy to prepare PBu-g-PCL copolymers through the grafting-from route, starting from welldefined PBu having hydroxyl side groups obtained by the ROMP of cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene initiated by ruthenium-based Grubbs’ second- and thirdgeneration catalysts. This strategy utilizes the organocatalyzed ROP of CL and provides well-defined PBu-g-PCL copolymers with the highest molar mass reported up to now. These polymers were thoroughly characterized by NMR, SEC, TGA, and DSC to reveal their structure, sizes, molar masses, and thermal properties. The self-assembling properties of the prepared PBu-g-PCL copolymers in solution and in the solid state were investigated using small-angle X-ray scattering (SAXS). In toluene solution, the graft copolymer formed aggregates with a core mainly composed of PBu. In the solid state upon isothermal crystallization at room temperature, PCL side chains fold while they form an orthorhombic molecular network, building up a self-assembled lamellar phase with large scale d-spacing’s. The results of this study are important for the synthesis of well-defined polydiene-g-polyester copolymers and for the self-assembling properties of such graft copolymers. They are expected to be useful in the design and synthesis of new brush copolymers that can serve as building blocks for nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00786. Materials, detailed experimental conditions and procedures, additional polymer characterization data (NMR, SEC, DSC, TGA), and SAXS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (V.M.). *E-mail [email protected] (L.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mireille Barthe and Alexandre Bénard for SEC analyses and Amélie Durand and Corentin Jacquemmoz for NMR analyses.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00786 Macromolecules XXXX, XXX, XXX−XXX