Well-Defined Polyethylene-Based Random, Block, and Bilayered

Article pubs.acs.org/Macromolecules

Well-Defined Polyethylene-Based Random, Block, and Bilayered Molecular Cobrushes Hefeng Zhang,†,‡ Zhen Zhang,†,‡ Yves Gnanou,‡ and Nikos Hadjichristidis*,†,‡ †

Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, and ‡Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia S Supporting Information *

ABSTRACT: Novel well-defined polyethylene-based random, block, and bilayered molecular cobrushes were synthesized through the macromonomer strategy. Two steps were involved in this approach: (i) synthesis of norbornyl-terminated macromonomers of polyethylene (PE), polycaprolactone (PCL), poly(ethylene oxide) (PEO), and polystyrene (PS), as well as polyethylene-b-polycaprolactone (PE-b-PCL), by esterification of the hydroxyl-terminated precursors (PE, PCL, PEO, PS, and PE-b-PCL) with 5-norbornene-2-carboxylic acid and (ii) ring-opening metathesis (co)polymerization of the resulting macromonomers to afford the PE-based molecular cobrushes. The PE-macromonomers were synthesized by polyhomologation of dimethylsulfoxonium methylide, while the others by anionic polymerization. Proton nuclear magnetic resonance spectroscopy (1H NMR) and high-temperature gel permeation chromatography (HT-GPC) were used to imprint the molecular characteristics of all macromonomers and molecular brushes and differential scanning calorimetry (DSC) for the thermal properties. The bilayered molecular cobrushes of P(PE-b-PCL) adopt a wormlike morphology on silica wafer as visualized by atomic force microscopy (AFM).

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INTRODUCTION Molecular cobrushes have attracted considerable attention in recent years as a result of their large molecular size, high rigidity, and shape-persistent properties.1−5 Up to now, molecular cobrushes have found specific applications in bottom-up self-assembly,6−11 drug delivery,12,13 single molecular morphology visualization,14 and organo-nanomaterials fabrication,4,15,16 etc. For example, photocrystals were generated through the bottom-up self-assembly of diblock molecular cobrushes resulting in colorful films with tunable bandgaps.6−8,11 Organo-nanotubes have also been fabricated from the layered molecular cobrushes.15,16 Similarly to homomolecular brushes, there are three strategies giving access to well-defined molecular cobrushes, i.e., (i) “grafting from”, growing two or more chemically different side branches from a polymeric backbone (multifunctional initiator) through independent living or controlled/living polymerization processes;16−18 (ii) “grafting onto”, coupling the different types of branches to a polymeric chain through highly efficient and independent coupling reactions;19 and (iii) “grafting through”, also known as macromonomer strategy, © 2015 American Chemical Society

polymerizing two or more different macromonomers through living or controlled/living polymerization techniques.7−9,20−26 Considering the limited choice of independent living and controlled/living polymerization techniques for “grafting from” and coupling reactions for “grafting onto” strategies, respectively, the macromonomer strategy is considered as a better choice, especially in the case of synthesizing multiblock molecular cobrushes. Moreover, the macromonomer strategy promises 100% grafting efficiency. The only problem on this strategy is that the polymerization degree, in some cases, is limited by the huge steric hindrance around the active sites and by the low concentration of the macromonomer polymerizable entity. The versatility of this strategy was enhanced by using ring-opening metathesis polymerization (ROMP) of norbornylterminated macromonomers due to the good durability of the catalyst, the high reactivity of the norbornyl moiety driven by Received: April 7, 2015 Revised: May 16, 2015 Published: May 28, 2015 3556

DOI: 10.1021/acs.macromol.5b00713 Macromolecules 2015, 48, 3556−3562

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Macromolecules relief of the strong ring strain, and the low steric hindrance.20−23 Despite the fact that various well-defined molecular (co)brushes have been synthesized, there are only very few reports so far dealing with polyolefin-based molecular brushes.24−26 In the case of polyethylene (PE), one of the most important polymeric materials widely used in the modern society, the difficulty is attributed to the challenge on controlling molecular weight, distribution, and absence of side alkyl groups. (Alkyl)borane-initiated polymerization of ylides, mostly dimethylsulfoxonium methylide, discovered and coined as polyhomologation or C1 polymerization by Shea et al., is a powerful/efficient method to synthesize well-defined PEs.27−31 In this process, two successive steps are involved, i.e., (i) formation of an “ate” complex by the ylide (monomer) and alkylborane (initiator) and (ii) the subsequent intramolecular 1,2-migration reaction randomly inserting the methylidene group of the ylide into one of the three alkyl branches along with the elimination of dimethyl sulfoxide and meanwhile regenerating the alkylborane which participates in a new cycle. A boron-linked PE 3-arm star is generated by using excess ylide affording three linear hydroxyl-terminated PE (PE-OH) by oxidation/hydrolysis. By polyhomologation, a series of welldefined linear, cyclic, and star-like PEs have been synthesized as well as some PE-based copolymers by combining polyhomologation with other living or controlled/living polymerization techniques.32−36 Recently, by combining polyhomologation and ROMP, we have synthesized well-defined PE molecular brushes through the macromonomer strategy.26 In this paper we extend this macromonomer strategy to PE-based random and block co-, ter- and quarter molecular brushes as well as to bilayered molecular cobrushes.

Table 1. Molecular Characteristics of Norbornyl-Terminated PE, PCL, PS, and PEO Macromonomers sample

Mna (g/mol)

PDIb

functionalityc (%)

Tmd (°C)

PE700-Nor PE740-Nor PE1200-Nor PCL3030-Nor PCL5100-Nor PS2500-Nor PEO2100-Nor

700 740 1200 3030 5100 2500e 2100

1.05 1.06 1.05 1.07 1.14 1.06 1.10

99 99 94 96 99 94 95

78.5 85.0 100.2 46.9 52.8 70.0 (Tg) 49.1

ΔHmd (J/g) 149.6 157.8 152.8 66.9 69.8 127.3

a

Mn,macromonomer = Mn,precursor + 120 (molecular weight of the norbornene-carbonyl moiety); Mn,precursor was calculated from 1H NMR spectrum using the area ratio of protons in terminal CH2OH at δ = ∼3.5 ppm to the ones on the backbone. bDetermined by HT-GPC (1,2,4-trichlorobenzene, 150 °C, PS standards). cFunctionalities were determined by 1H NMR spectra (toluene-d8) and the conversion in the ROMP revealed by HT-GPC (RI signal). dMelting point (Tm) and fusion enthalpy (ΔHm) were determined by differential scanning calorimetry (DSC) under a nitrogen atmosphere (N2) (10 °C/min, second cycle). eDetermined by HT-GPC (PS standards).

Scheme 1. Synthesis of PE-Based Random Co-, Ter-, and Quarter-Molecular Brushes by ROMP

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As examples, GPC traces of P(PE740)72-co-P(PCL3030)34 and P(PE740)144-co-P(PCL3030)22-co-P(PS2500)15 (entries 2 and 4 in Table 2) are shown in Figure 1. In the case of P(PE740)72-coP(PCL3030)34, it is clear (left part of Figure 1) that both PE-Nor and PCL-Nor were consumed during the ROMP, resulting in narrow-distributed molecular cobrush (conversion higher than 99%). Similar results were found in the synthesis of P(PE740)144-co-P(PCL3030)22-co-P(PS2500)15 (right part of Figure 1). The residual PE-Nor macromonomers, in some cases, were challenging to remove due to the similar behavior of PE-Nor and the corresponding molecular brush in different solvent/ nonsolvent pairs. Another proof of the successful synthesis comes from 1H NMR spectra. As shown in Figure 2, the characteristic chemical shift due to cyclic protons on the norbornyl group in PE-Nor and PCL-Nor at δ = 6.2−5.9 ppm (Ha) disappeared after ROMP, and new peaks at δ = 5.2−5.8 ppm of the internal double bonds (Ha′) appeared, an indication of the quantitative conversion of the macromonomers to brushes. Furthermore, both fingerprints for PE and PCL blocks were found in the 1H NMR spectrum of the final product having practically the same composition ratio to the macromonomer feed ratios. In the same way, a series of PE-based random co-, ter-, and quarter-molecular brushes were synthesized (Table 2 and Figures S7−S11). The number of the side branches was calculated from the weight-average molecular weights (Mw) determined by HT-GPC-LS (LS: light scattering) and the composition revealed by 1H NMR spectra based on the known molecular weight of the macromonomers. It should be noted that in the case of PEO-contained molecular cobrushes the successful syntheses were supported only by the 1H NMR

RESULTS AND DISCUSSION Several norbornyl-terminated macromonomers of polyethylene (PE-Nor), polycaprolactone (PCL-Nor), poly(ethylene oxide) (PEO-Nor), polystyrene (PS-Nor), and polyethylene-b-polycaprolactone (PE-b-PCL-Nor) were prepared by esterification of the hydroxyl-terminated precursors with 5-norbornene-2carboxylic acid in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) (Schemes S1−S3). Narrow distributions and high functionalities were revealed by high-temperature gel permeation chromatography (HT-GPC) and proton nuclear magnetic resonance spectroscopy (1H NMR), respectively (Table 1, Figures S1 and S2). Before copolymerization with PE-Nor, homopolymerizations of the different macromonomers were carried out in NMR tubes (toluene-d8 at 80 °C) in order to monitor in situ the kinetics. Depending on the macromonomer and molecular weight of the branch, complete conversion occurred between 30 min (PE-Nor) and 3 h (PEO-Nor) (Figures S3−S6). This information about the completeness of homopolymerization is very important in particular for the synthesis of block cobrushes with neat segments. Random PE-Based Random Co-, Ter-, and QuarterMolecular Brushes. The PE macromonomer was mixed with one (PS-Nor or PCL-Nor), two (PCL-Nor and PS-Nor, or PCL-Nor and PEO-Nor), or three (PCL-Nor, PS-Nor and PEO-Nor) other macromonomers and subjected to ROMP by using Grubbs first-generation catalyst as the initiator to synthesize random molecular cobrushes (Scheme 1). 3557


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Macromolecules Table 2. Molecular Characteristics of PE-Based Random Co-, Ter-, and Quarter-Molecular Brushes entry

samplea

Mtheorb (g/mol)

Mwc (g/mol)

PDIc

dn/dcd (mL/g)

conve (%)

1 2 3 4 5 6

P(PE740)115 P(PE740)72-co-P(PCL3030)34 P(PE740)128-co-P(PS2500)13 P(PE740)144-co-P(PCL3030)22-co-P(PS2500)15 P(PE)-co-P(PCL)-co-P(PEO) P(PE)-co-P(PCL)-co-P(PEO)-co-P(PS)

75500 147000 120 000 160000 150000 213000

85100 156700 127900 153000 g g

1.19 1.06 1.12 1.14 g g

−0.09 −0.080 −0.019 −0.045 g g

98 >99 90 89 >99 >99

Tmf (°C) 90.0 83.9, 83.0 82.0, 82.9, 76.2,

48.2 34.3 42.7 27.7

ΔHmf (J/g) 128.8 20.7, 34.3 24.6 15.5, 9.9 12.3, 19.8 11.0, 17.6

a Nomenclature: for example, in the case of P(PE740)72-co-P(PCL3030)34, 740 and 3030 refer to the molecular weight of PE and PCL macromonomers; 72 and 32 are the numbers of the PE and PCL branches in the resultant brush, respectively. bThe theoretical molecular weight (Mtheor) was calculated by feed ratios of the macromonomer to the Grubbs catalyst (initiator). cThe Mw and PDI were determined by HT-GPC-LS system (LS: light scattering). dDetermined by the HT-GPC system (refractive index detector). eThe conversion (conv) was measured by area ratio of the peaks for brush to the one for residual macromonomer in HT-GPC trace (refractive index signal). fMelting point (Tm) and the fusion enthalpy (ΔHm) were determined by DSC (N2, 10 °C/min, second cycle). gNot determined.

Figure 1. Monitoring the synthesis of P(PE740)72-co-P(PCL3030)34 and P(PE740)144-co-P(PCL3030)22-co-P(PS2500)15 (entries 2 and 4 in Table 2) by HT-GPC (TCB, 150 °C, all the peaks except PS-Nor are negative and shown as positive for better comparison).

Figure 3. DSC traces of PE-based random molecular cobrushes and macromonomers together with a PE homomolecular brush for comparison (N2, 10 °C/min, second cycle).

PS the Tm of PE is slightly lowered due to the lower regularity of PE block as a consequence of its disordered distribution along the molecular cobrushes. In the case of P(PE)-coP(PCL)-co-P(PEO) there are two Tmthe one corresponds to PE and other corresponds to the PEO + PCL mixture, since it is lower than both Tm of PEO and PCL. Similarly, P(PE)-coP(PCL)-co-P(PEO)-co-P(PS) possesses two Tmone corresponding to PE and the other to PEO + PCL (cocrystallinity). The Tg of PS is masked by the Tm of PE. The existence of PS suppresses the Tm of PEO + PCL even more than in the case of triblock (restriction of chain movement). On the other hand, in order to prove that the existence of 1−10% macromonomer in molecular brushes does not affect the thermal behavior, the thermal properties of P(PE740)72-co-P(PCL3030)34 in the presence of 10% PE740-Nor was determined (Figure S14). The Tm results were practically the same. The reduced crystallinity of PE block was also confirmed by the improved solubility of the molecular cobrushes. For example, in the case of P(PE740)72-co-P(PCL3030)34, a clear and transparent solution in toluene-d8 can be obtained at room temperature. The homogeneity of the solution was proved by the very similar 1H NMR spectra obtained at room temperature and 80 °C (Figure S15). Synthesis of PE-Based Block Molecular Cobrushes. The living behavior and good tolerance of Grubbs catalyst at high temperature give the opportunity to synthesize PE-based block molecular cobrushes through a sequential ROMP process (Scheme 2). P(PE1200)31-b-P(PS2500)9 (Table 3, entries 1 and 2), for example, was synthesized by sequential polymerization of PE-Nor (conversion: 94%) and PS-Nor (conversion: 92%) (Figure 4). The low distribution (PDIs ≤ 1.07) indicates good control of the final product molecular characteristics. The

Figure 2. Full 1H NMR spectrum (right) of P(PE740)72-coP(PCL3030)34 (entry 2, Table 2, fingerprints of PE and PCL blocks) and magnified spectrum in directions x and y (left) (disappearance of the norbornene cyclic protons and appearance of the internal double bond protons of the cobrush; spectra taken in toluene-d8 at 80 °C).

results (Figures S10 and S11) since the products could not be analyzed by the HT-GPC (PEO block is absorbed by the columns as shown in Figure S12). The random molecular cobrush structure plays an important role on the thermal properties. For comparison, a PE homomolecular brush was synthesized and characterized by DSC (entry 1, Table 2 and Figure S13). As seen in Figure 3, a melting point of 90.3 °C was found in PE homomolecular brush which is higher than the one of 85.0 °C given by macromonomer of PE-Nor. The increased Tm agrees well with our previous results which are attributed to the double (span) molecular weight in the brush structure.26 However, in all cases of PE-based cobrushes, the Tm of PE block is lower than the corresponding macromonomer. For example, two melting points of 48.2 and 83.9 °C in P(PE740)72-co-P(PCL3030)34 were found corresponding to the PE and PCL blocks, respectively. By introducing one more block of PEO or/and 3558


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Macromolecules Scheme 2. Synthesis of PE-Based Block Molecular Cobrushes by Sequential ROMP

Table 3. Molecular Characteristics of PE-Based Block Molecular Cobrushes entry

sample

Mtheora (g/mol)

Mwb (g/mol)

PDIb

dn/dcc (mL/g)

convd (%)

Tme (°C)

ΔHme (J/g)

1 2 3 4 5 6

P(PE1200)31 P(PE1200)31-b-P(PS2500)9 P(PCL5100)35 P(PCL5100)35-b-P(PE700)55 P(PCL5100)14 P(PCL5100)14-b-P(PE700)306

30000 65000 150000 183000 82000 250000

36600 58600 178000 216400 73100 287100

1.07 1.06 1.11 1.20 1.24 1.40

−0.090 −0.030 −0.078 −0.081 −0.075 −0.079

94 92 99 94 92 98

104.1 103.2 51.9 51.2, 86.7 51.0 51.7, 87.4

108.7 75.2 56.2 29.7, 32.6 56.1 17.2, 56.9

a The theoretical molecular weight was calculated by the feed ratio of macromonomers to Grubbs catalysts (initiator). bThe Mw and PDI were determined by the HT-GPC-LS system. cDetermined by the HT-GPC system (refractive index detector). dThe conversion (conv) was determined by area ratio of the peaks for brush to the one for the unreacted macromonomer in HT-GPC trace (refractive index signal). eMelting point (Tm) and the fusion enthalpy (ΔHm) were determined by DSC (N2, 10 °C/min, second cycle).

conversions of PE-Nor and PS-Nor. Moreover, fingerprints for both PE and PS blocks were found in the 1H NMR spectrum of the final product. It should be mentioned that the macroinitiator formed by ROMP of the first macromonomer is very sensitive and can be deactivated during the sampling and addition of the second macromonomer. For example, in the case of P(PE)-b-P(PCL), a shoulder was found in the GPC trace which was attributed to the deactivation of the living chains of P(PE)[Ru] (Figure S16). The removal of the P(PE) from the product P(PE)-bP(PCL) is challenging because of the high crystallinity of PE block. In an attempt to overcome this difficulty, we inversed the sequence of the macromonomer addition (from PCL-Nor to PE-Nor) since the deactivated P(PCL) could be easily removed by fractionation. For example, in the synthesis of P(PCL5100)35-b-P(PE700)55 (entry 4, Table 3) after sequential ROMP of PCL5100-Nor and PE700-Nor, the final block molecular cobrush is contaminated with deactivated P(PCL5100)[Ru] (Figure 6). To remove this P(PCL5100) residue, the crude product was dissolved in hot toluene, and the turbid solution formed, after cooling down to the room temperature, was centrifuged. From the two phases generated, a gel-like solid on the bottom (target molecular cobrush of P(PCL5100)35-b-P(PE700)55) and a clear solution on the top (unreacted P(PCL5100)35), the copolymer brush could be easily isolated. Another product of P(PCL5100)14-b-P(PE700)306 was also successfully synthesized and fractionated using a similar centrifugation technique. The HT-GPC traces indicated high purities of the two final products (Figure 6 and Figure S17). Subsequently, the successful synthesis was also confirmed by the 1H NMR results (Figures S18 and S19). The thermal properties of the resulting block molecular cobrushes were measured by DSC (Figure 7, Figures S20 and S21). In the case of P(PCL5100)35-b-P(PE700)55, both the melting points of PCL and PE blocks can be found at 51 and 87 °C, respectively. It should be noticed that similar to the PE homomolecular brush, the PE side branches in the block molecular cobrushes also shows higher T m than the corresponding macromonomer of PE-Nor. Similar phenomena

Figure 4. Monitoring the synthesis of P(PE1200)31-b-P(PS2500)9 (entries 1 and 2, Table 3) by sequential ROMP of PE and PS macromonomers by HT-GPC (TCB, 150 °C, athe peaks are negative and shown as positive for better comparison).

successful synthesis was also proved by the 1H NMR results (Figure 5). The quantitative disappearance of protons on the terminal norbornyl group at δ = 6.2−5.9 ppm reveals high

Figure 5. Full 1H NMR spectrum (right) of P(PE1200)31-b-P(PS2500)9 (entry 2, Table 3) (fingerprints of PE and PS blocks) and magnified spectrum in directions x and y (left) (disappearance of the norbornene cyclic protons and appearance of the internal double bond protons of the cobrush; spectra taken in toluene-d8 at 80 °C). 3559


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Figure 6. Synthesis of P(PCL5100)35-b-P(PE700)55 (entry 4, Table 3) by sequential ROMP of PCL-Nor and PE-Nor, as well as fractionation by centrifugation method monitored by HT-GPC (TCB, 150 °C; all the peaks are negative and shown as positive for better comparison).

Figure 7. DSC traces of PE-based block molecular cobrushes of P(PCL5100)35-b-P(PE700)55 (entries 3 and 4, Table 3) (N2, 10 °C/min, second cycle).

Scheme 3. Synthesis of Bilayered Molecular Cobrushes with Block Side Branches

were also found in the case of P(PE1200)31-b-P(PS2500)9 and P(PCL5100)14-b-P(PE700)306 (Figures S20 and S21). The existence of macromonomer does not change the thermal properties of the brushes, as has been proved by mixing the molecular brushes with the corresponding macromonomers (Figure S22). The Tm results are practically similar. On the other hand, both PCL and PE blocks show sharper melting peaks than the macromonomers, which are attributed to the increased regularity in the molecular brushes. Synthesis of PE-Based Bilayered Molecular Cobrushes. Layered structures provide different chemical environments in the core and periphery; thus layered molecular cobrushes can lead to nonspherical functional nanomaterials, such as organo-nanotube, micelle, etc.15,16 The PE-based bilayered molecular cobrushes were synthesized according to Scheme 3. PE870-OH prepared by polyhomologation was used as a macroinitiator to polymerize ε-caprolactone in the presence of phosphazene base P2 as catalyst. The resulting PE870-b-PCL1550 was then functionalized with a norbornyl group by esterification with 5-norbornene-2-carboxylic acid, affording the corresponding norbornyl-terminated macromonomer of PE870-b-PCL1550-Nor with 95% functionality (Figure S23). After ROMP of the block macromonomer, the bilayered molecular cobrushes of P(PE870-b-PCL1550)43 and P(PE870-bPCL1550)197 were synthesized. As revealed by the HT-GPC traces, both products showed narrow/symmetric peaks and high conversion (≥94%) (Figure 8). The residual in the GPC trace was attributed to

unfunctionalized PE-b-PCL. The success of synthesis was also proved by the 1H NMR results (Figure 9). After ROMP the norbornyl group was quantitatively consumed, resulting in the formation of internal double bonds indicating the successful synthesis of the expected brushes. The crystallinities of the two bilayered molecular cobrushes of P(PE-b-PCL) were revealed by the DSC as well as those of the initial macromonomers (Figure 10). The crystallinity of the PCL block was decreased after transformation of terminal hydroxyl group to norbornyl moieties as indicated by a lower Tm and melting enthalpy (ΔHm) (entries 2 and 3, Table 4). After polymerization, the lack of chain movements prevent the PCL block to crystallize as indicated by the presence of only one melting point corresponding to PE block in the

Table 4. Molecular Characteristics of PE-Based Bilayered Molecular Cobrushes entry

sample

Mtheora (g/mol)

Mobsb (g/mol)

PDIb

1 2 3 4 5

PE870-OH PE870-b-PCL1550 PE870-b-PCL1550-Nor P(PE870-b-PCL1550)43 P(PE870-b-PCL1550)197

790 2360 2500 104000 387000

870 2420 2580 112100 508100

1.07 1.07 1.07 1.14 1.38

dn/dcc (mL/g)

conv (%)

Tme (°C)

ΔHme (J/g)

−0.076 −0.076

100 100 95 94d 95d

91.4 90.9, 40.8 86.9, 27.9 86.1 85.0

201.5 74.6, 36.1 66.9, 21.5 61.7 57.4

a The theoretical molecular weight (Mtheor.) was calculated by feed ratio of the macromomomer to the Grubbs catalyst (initiator). bThe Mn and PDI of the macromonomer and precursors (entries 1−3) were determined by 1H NMR and HT-GPC (RI signal, PS standards), respectively, and the Mw and PDI of the molecular cobrushes (entries 4 and 5) were determined by the HT-GPC-LS system. cDetermined by the HT-GPC system (refractive index detector). dThe conversion (conv) was measured by area ratio of the peaks for molecular cobrush to the one for unreacted macromonomer in HT-GPC trace (RI signal). eMelting point (Tm) and the fusion enthalpy (ΔHm) were determined by DSC (N2, 10 °C/min, second cycle).

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Atomic force microscopy (AFM) has emerged as a powerful tool to visualize the shape of giant molecules such as molecular brushes2,39,38 and dendrimer-like polymers.39 However, it is more difficult for PE-based molecular brush visualization because of the difficulty in sample preparation caused by high crystallizability of PE block; for example, high temperature is required in the whole process (hot solvent and substrate). In the primary study, the single molecules of P(PE870-bPCL1550)197 was visualized by AFM on a silica wafer (Figure S24). The sample was prepared by dropping hot dilute solution in toluene onto a hot silica wafer (∼80 °C). The wormlike molecular brush was observed together with some aggregates and small spheres. The length of 50−150 nm agrees well with the theoretical value of 100 nm. Small spheres were supposed to be the broken molecular brushes caused by mechanical stress during the sample preparation.40

Figure 8. Monitoring the synthesis of bilayered molecular cobrushes of P(PE-b-PCL) (entries 4 and 5, Table 4) by HT-GPC (TCB, 150 °C; all the peaks are negative and shown as positive for better comparison).

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CONCLUSIONS By using the macromonomer strategy, a series of PE-based random, block, and bilayered molecular cobrushes were successfully synthesized. The well-defined structures have been confirmed by the HT-GPC and 1H NMR measurements. Thermal properties of the resultant products were revealed by DSC. The bilayered molecular cobrushes adopt a wormlike morphology on silica wafer substrate. This general strategy allows the synthesis of a rich variety of other PE-based molecular cobrushes. The influence of the molecular brush structure on the properties, such as selfassembly and phase separation, is under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section and HT-GPC, DSC, and 1H NMR results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00713.

Figure 9. Monitoring the synthesis of bilayered molecular cobrush of P(PE870-b-PCL1550)43 (entries 1−4, Table 4) by 1H NMR spectra (toluene-d8, 80 °C).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.H.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 10. DSC traces of bilayered cobrushes of P(PE870-b-PCL1550)43 and P(PE870-b-PCL1550)197 cobrushes (Table 3) (N2, 10 °C/min, second cycle).

temperature range −80 to 160 °C. Since PE chains are in the periphery, both the flexibility and regularity are reduced; as a result, melting points of PE block are slightly decreased. 3561


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