Well-Defined Bilayered Molecular Cobrushes with Internal

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Well-Defined Bilayered Molecular Cobrushes with Internal Polyethylene Blocks and ω‑Hydroxyl-Functionalized Polyethylene Homobrushes Hefeng Zhang and Nikos Hadjichristidis* Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia S Supporting Information *

ABSTRACT: Novel well-defined bilayered molecular cobrushes with internal polyethylene blocks, P(PEcore-b-PScorona) (PE: polyethylene; PS: polystyrene), and ω-hydroxyl-functionalized polyethylene homobrushes, P(PE-OH), were synthesized through the macromonomer strategy. Two main steps were involved in the synthesis of the P(PEcore-b-PScorona) bilayered cobrushes: (i) formation of norbornyl-terminated macromonomer (Nor-PE-b-PS) by esterification of PS-b-PE-OH (combination of anionic polymerization, hydroboration, and polyhomologation) with 5-norbornene-2-carboxylic acid and (ii) ring-opening metathesis polymerization (ROMP) of Nor-PE-b-PS. The synthesis of P(PE-OH) was achieved by (i) hydroboration of tertbutyldimethylsilyl-protected allyl alcohol, followed by polyhomologation of dimethylsulfoxoniun methylide with the formed tri[3-(tert-butyldimethylsilyloxyl)propyl]borane initiator, oxidation/hydrolysis, and esterification of the TBDMS-O-PE-OH with 5-norbornene-2-carboxylic acid to afford the macromonomer TBDMS-O-PE-Nor, and (ii) ROMP of TBDMS-O-PE-Nor, followed by deprotection. Nuclear magnetic resonance spectroscopy (1H and 13C NMR) and high temperature gel permeation chromatography (HT-GPC) were used to characterize all macromonomers/molecular brushes and differential scanning calorimetry (DSC) to study the thermal properties. The molecular brush P(PE-b-PS) showed lower melting point (Tm) and better solubility in toluene than the corresponding macromonomer PS-b-PE-Nor. In the case of homobrushes, the thermal properties were strongly affected by the presence of the PE end-groups.

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INTRODUCTION

from bilayered molecular cobrushes synthesized by reversible addition−fragmentation chain transfer polymerization (RAFT).2f−i The resultant nanotubes have found applications in separation of nanoobjects.2g Nevertheless, it should be pointed out that there are very few reports dealing with polyolefin-based molecular brushes and much less dealing with layered structures due to the poor controllability of olefin polymerizations.3 Undoubtedly, polyethylene (PE) is one of the most important polymeric materials witnessed by various applications in our daily life and huge annual production. Good crystallinity gives this material high mechanistic performance in bulk as well as thermal responsibility in solution. To better evaluate the structure−properties relationship, considerable efforts have been made to synthesize well-defined linear

Layered structures provide different chemical environments in the core and periphery of polymers and thus make them potential candidates for nanoreactors, drug release systems, nanoobject templates, etc.1 As one of this kind of structure, layered molecular cobrush constructed by a backbone and block copolymer branches is an ideal choice for designing/ fabricating nonspherical nanoobjects such as nanotubes and nanorods.2 Up to now, some layered molecular cobrushes have been already synthesized by using appropriate living or controlled/living polymerization techniques and used for nanoobject fabrication. For example, a bilayered molecular cobrush P(PIcore-b-PAAcorona) (PI: polyisoprene; PAA: poly(acrylic acid)) has been synthesized by combining ring-opening metathesis polymerization (ROMP) and nitroxide-mediated polymerization (NMP). Cross-linking of the peripheral PAA blocks followed by degradation of the internal PI blocks afforded hollow nanomaterials.2e By using this strategy, welldefined organic nanotubes with tunable sizes were constructed © 2016 American Chemical Society

Received: December 7, 2015 Revised: January 29, 2016 Published: February 15, 2016 1590

DOI: 10.1021/acs.macromol.5b02652 Macromolecules 2016, 49, 1590−1596

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Macromolecules polyethylene (controllable molecular weight, low polydispersity index, and absence of branches). A few methodologies have been developed for this purpose, such as anionic4 and coordination (chain transfer) polymerization5 of ethylene, hydrogenation of anionically synthesized polybutadiene,6 or of poly(cyclic olefin) obtained by ROMP.7 Still, the resultant PE blocks can only partly meet the requirements of a “welldefined structure”. The synthesis of well-defined PE especially with moderate/high molecular weight, up to now, is still a challenge. (Alkyl)borane-initiated polymerization of ylide, coined as polyhomologation by Shea, has been emerged as an efficient/ powerful tool for the synthesis of well-defined linear PE.8 In this method, two repeatedly continuous steps are involved: (i) formation of an “ate” complex between ylide (monomer) and borane (initiator) and (ii) subsequent intramolecular 1,2migration reaction which randomly inserts the −CH2− moiety from the ylide into one of the three alkyl branches of borane with simultaneous release of dimethyl sulfoxide and regeneration of an alkylborane species ready for the next cycle. The procedure results in a boron-link PE 3-arm star which, after oxidation/hydrolysis, gives three hydroxyl-terminated linear PE (PE-OH).8,9 By using this method, some well-defined PEs have been designed/synthesized8−10 as well as PE-based copolymers by combination with living and controlled/living polymerization techniques, such as nitroxide-mediated polymerization (NMP),11 ring-opening polymerization (ROP),12 anionic polymerization,13 and atom transfer radical polymerization (ATRP),14 etc.8a,b,13b For example, in our recent work, anionic polymerization was combined with polyhomologation to afford PE-based block copolymers. BF3·OEt2 was used to quench the living homo/copolymeric anion and form a 3-arm star with boron junction point which served as a macroinitiator for the polyhomologation of dimethylsulfoxoniun methylide, resulting in the corresponding block copolymers, such as PS-b-PE (PS: polystyrene), PB-b-PE (PB: polybutadiene), and PS-b-PI-b-PE (PI: polyisoprene), etc.13 Furthermore, a series of PE-based homo- and random/block, as well as layered molecular cobrushes with external PE blocks, P(PCLcore-PEcorona) (PCL: polycaprolactone), have been synthesized through the macromonomer strategy.3a,b In this strategy, norbornyl-terminated PE macromonomer was synthesized by polyhomologation, oxidation/hydrolysis, and esterification of the produced hydroxylterminated homo/copolymer with norbornene-2-carboxylic acid, followed by ROMP homopolymerization or copolymerization with other macromonomers. The macromonomer strategy is the best method for producing well-defined molecular brushes since (i) both backbone and branches are well-controllable and (ii) the grafting efficiency is practically 100%.15 Encouraged by the successful synthesis of the well-defined PE-based random, block, and bilayered cobrushes (PE: external block), in the present work we report two new strategies toward the synthesis of well-defined molecular cobrushes with internal PE blocks P(PEcore-b-PScorona) and PE-based ωhydroxyl-functionalized molecular homobrushes P(PE-OH).

Scheme 1. General Reactions for the Synthesis of P(PEcore-bPScorona) by Combining Polyhomologation with Anionic Polymerization and ROMP

and quenching with allyl bromide. DPE reduces the reactivity of PSLi, and thus the lithium−halogen exchange reaction is avoided. The good agreement in molecular weight of PS-allyl, determined by high temperature gel permeation chromatography (HT-GPC) (Mn = 1800 g/mol, PS standards) and proton nuclear magnetic resonance spectroscopy (1H NMR) (Mn = 1900 g/mol) calculated from comparison of the allyl group hydrogens with the PS backbone hydrogens, indicates the high functionality of the PS-allyl (∼95%). The PS-allyl was then hydroborated with BH3·THF complex to generate a 3-arm PS star with boron junction point, PS3B, which served as a macroinitiator for the polyhomologation of dimethylsulfoxoniun methylide. The formed 3-arm star block copolymer, (PS-b-PE)3B, was oxidized/hydrolyzed to afford hydroxylterminated block copolymer, PS-b-PE-OH. It should be noticed that for the hydroboration reaction less than the stoichiometric amount of BH3·THF complex was used to avoid the presence of PS2BH and PSBH2 which are polyhomologation initiators leading to PE-OH, which cannot be easily removed from the PS-b-PE-OH. Moreover, the sensitivity of BH3·THF complex, PS2BH, PSBH2, and PS3B to the air results in a lower concentration of macroinitiator, PS3B, than that calculated from the ratio of the monomer to BH3·THF. Therefore, in order to control the molecular weight of PE block, the real concentration of PS3B should be determined (see Experimental Section and Figures S1 and S2 in the Supporting Information). By using the real concentration of the PS3B, well-defined controllable PS-b-PE-OH were synthesized and of high purity after fractionation (Experimental Section and Figure S3 in the Supporting Information).13 The norbornyl macromonomer, PS-b-PE-Nor, was obtained by esterification of PS-b-PE-OH with 5-norbornene-2-carboxylic acid in the presence of DCC/DMAP following procedures given in our recent papers (DCC: N,N′-dicyclohexylcarbodiimide; DMAP: 4-(dimethylamino)pyridine).3a,b The macromonomer was polymerized by ROMP in toluene at 80 °C using the Grubbs first-generation catalyst (initiator) to afford the bilayered molecular cobrush, P(PEcore-b-PScorona). The synthesis of the bilayered cobrushes was followed step by step by HT-GPC (Figure 1). The peak corresponding to the PS-b-PE-OH precursor is not shown in Figure 1 since it is practically the same as the one shown in Figure 1 for

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RESULTS AND DISCUSSION The general reactions for the synthesis of molecular cobrushes with internal PE blocks are given in Scheme 1. The initial step involved the synthesis of allyl-terminated polystyrene (PS-allyl) by anionic polymerization of styrene followed by end-capping of polystyryllithium (PSLi) with 1,1-diphenylethylene (DPE) 1591

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chemical shift at δ = 1.3 ppm corresponding to the −CH2− groups (Figure S4 in Supporting Information). After esterification the terminal hydroxyl group of PS-b-PE-OH at δ = 3.4 ppm disappeared and two new peaks appeared: one at δ = 3.9−4.1 ppm indicating the formation of −CH2−O−C(O)− and another at δ = 5.9−6.1 ppm corresponding to the double bond of nobornyl group (−CHCH−). The nearly quantitative esterification reaction efficiency led to a high functionality (98%) (PS-b-PE-Nor). After ROMP, the −CH CH− peaks between 5.9 and 6.1 ppm (cyclic vinylic protons) quantitatively disappeared, and new peaks appeared between 5.3 and 5.7 ppm (internal vinylic protons), indicating the production of the polynorbornyl chain. The thermal properties of P(PEcore-b-PScorona), as well as of macromonomer PS-b-PE-Nor and its precursors, PS-b-PE-OH and PS-allyl, were studied by differential scanning calorimetry (DSC) (Figure 3 and Table 1). PS-allyl showed a low glass

Figure 1. Following the synthesis of bilayered molecular cobrush with internal PE blocks P(PEcore-b-PScorona) by HT-GPC (1,2,4-trichlorobenzene, 150 °C). Negative peaks are shown in positive style for better comparison.

macromonomer. Clearly, the corresponding peaks for the macromonomer and molecular cobrush moved to higher molecular weight range. Consequently, the GPC results indicate the successful synthesis of the bilayered molecular brushes. The residual macromonomer peak in the chromatogram of the cobrush is attributed to the nonquantitative esterification efficiency. The GPC results are supported by the 1H NMR spectra shown in Figure 2 and Figure S4 (Supporting Information). In

Figure 3. DSC thermograms of the processors, PS-allyl, PS-b-PE-OH, PS-b-PE-Nor, and the bilayered molecular cobrushes with internal polyethylene blocks P(PEcore-b-PScorona) (N2, 10 °C/min, the second heat cycle).

transition temperature (Tg) of 62.0 °C due to its low molecular weight (Mn,HT‑GPC = 1800 g/mol). In the case of PS-b-PE-OH, a melting peak for the PE block at 114.6 °C was found with a crystallinity degree (Xc) of 49.4%. In the case of PS-b-PE-OH (Figure 3), the small peak at higher temperature could be due to PE block with different crystal/mesophase structure and not in free PE since it disappears in the other thermograms. After functionalization with the norbornyl group, the melting point (Tm) was slightly decreased to 112.6 °C and the Xc to 46.0% due to the reduced flexibility since both ends are fixed. After ROMP of the PS-b-PE-Nor, both the Tm and Xc were further decreased to 110.1 °C and 30.8% in the molecular brush, respectively. The further decrease of Tm of PE block from the macromonomer to molecular cobrush may be a consequence of the existence of the peripheral PS block, which further suppresses the movement of the PE chains. This is supported by the fact that the PE homo molecular brushes always showed higher Tm than that of corresponding PE macromonomer.3a,b The Tg of PS block is present in all copolymers thermograms with a slight decrease. The layered structure and the internal position of PE improve the solubility of the molecular brushes. As shown in

Figure 2. Following the synthesis of bilayered molecular cobrush with PE internal blocks P(PEcore-b-PScorona) by 1H NMR spectroscopy (toluene-d8, 90 °C).

the spectrum of PS-allyl, the chemical shifts at δ = 5.2, 4.7, and 4.5 ppm are attributed to the vinyl protons (−CHCH2) of the terminal allyl group. After hydroboration and polyhomologation, the peaks of allyl group disappeared in the spectrum of PS-b-PE-OH, and a new peak attributed to −CH2−OH appeared at δ = 3.4 ppm. The quantitative disappearance of the allyl group indicates high purity of PS-b-PE-OH. The formation of PE block was also confirmed by the appearance of the new 1592

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Table 1. Molecular Characteristics and Thermal Properties of the Bilayered Molecular Cobrushes with Internal Polyethylene Blocks sample

Mtheor (g/mol)

Mn,HT‑GPCa (g/mol)

PDIa

Mn,NMRb (g/mol)

Tgc (°C)

Tmc (°C)

Xcc (%)

PS-allyl PS-b-PE-OH PS-b-PE-Nor P(PE-b-PS)

1720 5400 5500 300000

1800 6300 6450 323000d

1.10 1.16 1.15 1.36d

1900 5650 5750

62.0 56.0 52.5 57.5

114.6 112.6 110.1

49.4 46.0 30.8

Determined by the HT-GPC-RI system calibrated with PS standards (RI: refractive index) (1,2,4-trichlorobenzene, 150 °C). bCalculated from 1H NMR spectra by using the area ratio of the backbone to the end-group (toluene-d8, 90 °C). cDetermined by DSC, Xc was calculated by Xc = ΔHm/ (ΔHm+ × wPE), where a ΔHm is the melting enthalpy of the sample, ΔHm+ = 288 kJ/kg is the fusion enthalpy of 100% crystalline PE,16 and wPE is the weight ratio of PE block calculated from 1H NMR spectra. dMw and PDI were determined by HT-GPC-LS system (LS: light scattering) (1,2,4trichlorobenzene, 150 °C). a

ω-Hydroxyl-functionalized homo PE molecular brushes are useful intermediates for constructing bilayered molecular brushes by “grafting from” strategy employing appropriate living and controlled/living polymerization techniques or “grafting onto” strategy using highly efficient coupling reaction, such as “click” chemistry. Furthermore, they can be converted to other functional groups by appropriate modification. Herein, we developed a new strategy for the synthesis of ω-hydroxyl functionalized PE molecular brushes based on the macromonomer strategy (Scheme 2). In this strategy, tri[3-(tert-butyldimethylsilyloxyl)propyl]borane was synthesized by reaction of allyl alcohol with tertbutyldimethylchlorosilane followed by hydroboration with the BH3·THF complex. This borane compound then served as a functional initiator for the polyhomologation of dimethylsulfoxonium methylide (Figures S5−S7 in Supporting Information). After oxidation/hydrolysis, the resulting αhydroxyl-ω-(tert-butyldimethylsilyloxyl) PE (TBDMS-O-PEOH) was transformed to the norbornyl-macromonomer TBDMS-O-PE-Nor by reacting with 5-norbornene-2-carboxylic acid, followed by ROMP with Grubbs catalyst first generation as initiator. The resulting TBDMS-functionalized molecular brushes, P(PE-O-TBDMS), after deprotection of TBDMS group with tetra-n-butylammonium fluoride led to ω-hydroxylfunctionalized PE molecular brushes, P(PE-OH). The synthesis was monitored by HT-GPC and 1H NMR spectroscopy (Figures 5 and 6). The molecular characteristics and thermal properties of hydroxyl-functionalized PE molecular brush and its precursors are shown in Table 2.

Figure 4, both P(PEcore-b-PScorona) and macromonomer PS-bPE-Nor were soluble in hot toluene (∼80 °C) and gave clear

Figure 4. Solutions of PS-b-PE-Nor (bottle A) and P(PEcore-b-PScorona) (bottle B) in toluene (2 mg/mL) at ∼80 °C (left image) and at room temperature (right image).

and transparent solutions (2 mg/mL). After being cooled to room temperature, the PS-b-PE-Nor solution underwent an obvious phase separation leading to a cloudy solution as a consequence of a strong aggregation of PE block. This cloudy solution is stable for a few days due to the stabilization effect of peripheral PS block. In contrast, a semitransparent solution was found in the case of P(PEcore-b-PScorona) after being cooled to room temperature, stable for at least a few weeks. This semitransparent solution was supposed to be a consequence of the compact structure of molecular brush and the existence of PS block in the periphery preventing further aggregation.

Scheme 2. General Reactions for the Synthesis of ω-Hydroxyl-Functionalized PE Molecular Brushes

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Table 2. Molecular Characteristics and Thermal Properties of Hydroxyl-Functionalized PE Molecular Brush and Its Precursors sample TBDMS-OPE-OH TBDMS-OPE-Nor P(PE-OTBDMS) P(PE-OH)

Mn,GPCa (g/mol)

PDIa

Mn,NMRb (g/mol)

Tmc (°C)

Xc c (%)

1650

2940

1.12

1780

116.5

55.0

1770

2900

1.14

1960

112.4

51.4

160000

175000d

1.19d

110.2

44.0

150000

162300d

1.22d

119.9

35.8

Mtheor (g/mol)

a

Determined by HT-GPC-RI system calibrated with PS standards (RI: refractive index) (1,2,4-trichlorobenzene, 150 °C). bCalculated from 1 H NMR spectra by using the area ratios of the backbone to the endgroup (toluene-d8, 90 °C). cDetermined by DSC, the Xc was calculated by Xc = ΔHm/(ΔHm+ × wPE), where a ΔHm is the melting enthalpy of the sample, ΔHm+ = 288 kJ/kg is the fusion enthalpy of 100% crystalline PE,16 and wPE is the weight ratio of PE block calculated from 1H NMR. dMw and PDI were determined by HT-GPC-LS system (LS: light scattering) (1,2,4-trichlorobenzene, 150 °C). Figure 5. Following the synthesis of ω-hydroxyl-functionalized PE molecular brush, P(PE-b-OH) (1,2,4-trichlorobenzene, 150 °C; all the peaks are negative and are shown in positive style for better comparison).

(TBDMS-O-CH2-PE-OH, Ha) and to the ω-terminal hydroxyl methylene group (TBDMS-O-PE-CH2OH, Hb), respectively. After esterification the chemical shifts at δ = 3.4 ppm quantitatively moved to δ = 4.0 ppm (Hb′), indicating the formation of macromonomer, TBDMS-O-PE-Nor. The new peaks appearing at δ = 6.0 ppm were attributed to the cyclic vinylic protons of the norbornyl group (−CHCH−, Hc) which, after ROMP, moved to δ = 5.6 ppm (internal vinylic protons, Hc′) as a consequence of the ring stress releasing of the terminal norbornyl group. The resultant protected molecular brush P(PE-O-TBDMS) was deprotected leading to the hydroxyl-functionalized PE molecular brush, P(PE-OH), by removing the TBDMS group in the presence of tetrabutylammonium fluoride. The complete movement of the chemical shift at 3.6 to 3.4 ppm (from Ha to Ha′) indicates a quantitative efficiency of this deprotection reaction. It should be mentioned that the difference in chemical shifts of the protons of the two methylene groups (Ha and Hb) was used to determine the efficiency of oxidation/hydrolysis of 3arm star (TBDMS-O-PE)3B to the OH-functionalized linear. The area ratio of the Ha to Hb (1:0.95) indicates a mixture of TBDMS-O-PE-OH (95%) and TBDMS-O-PE (5%). Consequently, only TBDMS-O-PE-OH was transformed to TBDMS-O-PE-Nor and participated in polymerization; TBDMS-O-PE corresponds to residual peak in the HT-GPC chromatogram (Figure 5). These results are very close to those obtained by HT-GPC (92%) and 1H NMR (98%) for the OHfunctionalized molecular cobrush. Similarly, in the synthesis of bilayered molecular cobrush of P(PE-b-PS), the 98% of the hydroxyl group functionalization and 98% of the esterification reaction agree well with the conversion determined by HTGPC (95%) and 1H NMR (100%). The effect of molecular brush architecture and the existence of different functional groups in the periphery on the thermal properties of molecular brushes were revealed by the DSC measurements (Figure 7). In the case of TBDMS-O-PE-OH, a melting peak at 116.5 °C was attributed to the PE block, which decreased to 112.4 °C after introduction of the terminal norbornyl group. Further, after ROMP, a lower melting point of 110.2 °C was found in the molecular brush of P(PE-OTBDMS). The TBDMS group showed a similar effect with the PS block as mentioned in the case of cobrushes by limiting the

Figure 6. Following the synthesis of ω-hydroxyl-functionalized PE molecular brush, P(PE-OH), by 1H NMR spectroscopy (toluene-d8, 90 °C).

As shown in Figure 5, the HT-GPC chromatogram of the norbornyl-macromonomer TBDMS-O-PE-Nor possesses a narrow peak which moved to higher molecular range after ROMP of the macromonomer, indicating the formation of P(PE-O-TBDMS) (conversion: 92%). After deprotection, the peak corresponding to P(PE-OH) is practically the same as that corresponding to the protected one, which is logical since practically the molecular weight and PDI are the same. The existence of a residual peak at low molecular weight range was supposed to be a consequence of the nonquantitative norbornylization of the PE block and consequently unreactive during ROMP. These results agree with the 1H NMR results (vide inf ra). Another solid proof for the successful synthesis of P(PE-OH) comes from 1H NMR spectra (Figure 6). As shown in the spectrum of TBDMS-O-PE-OH, the chemical shifts at δ = 3.6 and 3.4 ppm were attributed to the α-terminal methylene 1594

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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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Figure 7. DSC thermograms of the precursors TBDMS-O-PE-OH, TBDMS-O-PE-Nor, and P(PE-O-TBTMS) and the final ω-hydroxylfunctionalized PE molecular brush P(PE-OH) (N2, 10 °C/min, the second heat cycle).

flexibility of the PE branch and thus decreased the Tm of the PE. After removing the TBDMS group, the Tm in the molecular brush of P(PE-OH) increased to 119.9 °C, which was supposed to be a consequence of decreased solubility and possible formation of hydrogen bonds between inter- or intramolecular hydroxyl groups.

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CONCLUSIONS Two novel strategies were developed for the synthesis of welldefined bilayered molecular cobrushes with internal PE blocks and OH-functionalized PE-based homobrushes. The first strategy is based on anionic polymerization, polyhomologation, and ROMP, while the second is based on polyhomologation using an OH-protected allyl compound and ROMP. The molecular brush P(PE-b-PS) showed lower melting point (Tm) and better solubility in toluene than the corresponding macromonomer PS-b-PE-Nor. In the case of homobrushes, the thermal properties were strongly affected by the presence of the PE end-groups. It should be pointed out that both strategies are general and open new horizons for the synthesis of bi/tri/ multilayered polymers with PE-internal blocks and ω-functionalized PE-based homobrushes. The new materials can be considered as organic nanocrystals and may find applications in developing new smart polymers, serving as PE crystallization modifiers, exploring novel crystallization-driven self-assembly system, etc.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02652. Experimental Section and more HT-GPC and NMR results (PDF) 1595

DOI: 10.1021/acs.macromol.5b02652 Macromolecules 2016, 49, 1590−1596

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DOI: 10.1021/acs.macromol.5b02652 Macromolecules 2016, 49, 1590−1596