Hydroxyl Functional Polybenzoxazine Precursor as a Versatile

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Hydroxyl Functional Polybenzoxazine Precursor as a Versatile Platform for Post-Polymer Modifications Betul Hanbeyoglu,† Baris Kiskan,† and Yusuf Yagci*,†,‡ †

Istanbul Technical University, Department of Chemistry, 34469, Maslak, Istanbul, Turkey Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: A new postmodification approach for the benzoxazine based thermosets is described. The approach involves attachment of hydroxyl functional groups to the mainchain polybenzoxazine precursors through monomer synthesis method. Hydroxyl functional polybenzoxazine precursor was successfully synthesized by using appropriate combinations of hexamethylene diamine with hydroxy functional diamine, namely 1,3-diaminopropan-2-ol in conjunction with bisphenol A and paraformaldehyde, and characterized. Modification reactions were performed by simple esterification reactions of hydroxyl groups with 2-bromopropanoyl chloride and methacryloyl chloride to yield ATRP macroinitiator and methacrylate containing polybenzoxazine precursors, respectively. ATRP macroinitiator was used to obtain polystyrene grafted polybenzoxazine precursors and photopolymerization of methacrylate groups was carried out successfully. The benzoxazine groups present in the all structures were shown to readily undergo thermally activated ring-opening polymerization in the absence of an added catalyst forming cross-linked networks. The thermal stability of the cured polymers was investigated and compared to that of a classical polybenzoxazine precursor.

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atures without using any catalyst or curative (Scheme 1).17,18 According to polymerization mechanism, polybenzoxazines can be considered as addition-cure phenolic systems.

INTRODUCTION The field of high-performance polymeric materials has expanded greatly as they have wide range of applications in various industries. The market demands for these materials, especially in aerospace, electronics and automotive industries, are increasing since many challenges in terms of high thermal stability, mechanical strength, and solvent resistance are overcome by their utility. Hence, various studies are focused on the development of new materials fulfilling property related industrial expectations. Recently, polybenzoxazines have been recognized as one of the valuable high-performance thermosetting polymers because they possess many characteristics of classical phenolic resins, such as heat resistance, mechanical strength and stable dielectric constants.1,2 In the meantime, these systems have additional superior properties compared to those conventional novolac, resole type phenolics and epoxy resins, such as low humidity uptake,3 high glass transition temperatures,4 high char yield,5−7 negligible volumetric shrinkage,8 and a small amount of byproduct formation during curing and noncatalytic polymerization.9−11 The features of polybenzoxazines are mainly consequence of the cyclic structure of the corresponding monomers and the formation of Mannich base bridges (−CH2−N(R)−CH2−) and the occurrence of hydrogen bonds between nitrogen and phenolic −OH groups after curing.12−16 The synthesis of polybenzoxazine proceeds through thermally activated ring-opening of oxazine at high temper© XXXX American Chemical Society

Scheme 1. Thermally Activated Polymerization of a Bis(benzoxazine) Monomer

Benzoxazine rings can also be opened at room temperature by Lewis acids19 and cationic photoinitiators.20 However, the structures of the polymers prepared by this way were complex and related to the ring-opening process of the protonated monomer either at the oxygen or nitrogen atoms. The high performance properties of benzoxazine-based polymers can further be improved or tuned by various strategies. The general strategies employed involve (i) controlling the end structure by synthesizing novel monomers Received: September 12, 2013 Revised: October 10, 2013

A

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with desired functionality,21−25 (ii) preparing composites or blends of polybenzoxazines with organic−inorganic polymers or particles,26,27 and (iii) synthesis of main-chain, side-chain, or end-chain polybenzoxazine precursors, which are generally combination of various thermoplastic polymers with benzoxazine units.1,28,29 Because of its simplicity and versatility, the monomer synthesis method has supported the development of various benzoxazines bearing functional groups. Using suitable phenols, primary amines and formaldehyde various designed monomers with propargyl,30−33 nitrile,34 coumaryl,35 allyl,4 and alcohol36 groups can readily be prepared. However, depending on the particular application involved, every strategy has its own advantages and/or disadvantages. For example, most of the benzoxazine monomers are powdery and difficult to prepare films from thereof. Cold flow and evaporation of the monomers can cause serious problems during curing. Polymeric benzoxazine precursor strategy can overcome many problems stemming from monomer usage. Because of the polymeric nature, polybenzoxazine precursors enable an easy preparation of films and processability of the products thus formed. Moreover, depending on the thermoplastic component used in polybenzoxazine precursors, toughness or thermal properties etc. could be controlled. Along these lines, various polybenzoxazine precursors were previously prepared. For example, benzoxazine containing polyesters,37 polyethers,38,39 polytriazoles,40,41 poly(methyl methacrylates),42 polystyrenes,43−45 poly(ε-caprolactone),46 polysiloxanes,47,48 and polyacetylenes49 were readily obtained through conventional and controlled/ living polymerization methods and click reactions. Furthermore, the use of difunctional components in classical monomer synthesis method facilitated formation of main-chain polybenzoxazines.28,50,51 Besides syntheses, the incorporation of additional functional groups is also vital for post polymer modification and will open another way to further improve and control the properties of the polybenzoxazine resins. In this article, side-chain hydroxyl functional polybenzoxazine precursors were synthesized through monomer synthesis using hydroxyl functional diamine in the process. In our design, the hydroxyl group was deliberately selected as the functional group to introduce additional hydrogen bonding sites as well as to enable incorporation of other functional groups. As will be shown below, suitable atom transfer radical polymerization (ATRP) initiator is successfully attached to the precursor by a simple esterification reaction. Subsequent ATRP of styrene provided the possibility of the incorporation of polystyrene chains to thermally curable polybenzoxazine precursors via grafting from approach. Moreover, methacrylate groups were also incorporated by a similar esterification process to afford curability by photo curability by photochemical means.

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Synthesis of a Hydroxyl Functional Polybenzoxazine Precursor, (Poly(2-propanolbenzoxazine-co-hexylbenzoxazine) (P(IPB-co-HB)). Polybenzoxazine precursors were prepared according to the reported procedure.52 In a 250 mL round-bottomed flask with a magnetic stirrer, 1,3-diaminopropan-2-ol (0.495 g, 5.37 mmol), hexamethylenediamine (0.66 g, 5.67 mmol), bisphenol A (2.511 g, 11 mmol), and a mixture of 25 mL of ethanol and 50 mL of toluene was added. The mixture was refluxed for 16 h. After cooling the flask, the reaction mixture was concentrated under vacuum and then poured into 200 mL of methanol. The resulting precipitate dried under vacuum. A yellowish powder was obtained with a 65% yield. Synthesis of Poly(isopropyl-2-bromopropanoate benzoxazine-co-hexylbenzoxazine) P(IPBBr-co-HB). Poly(IPB-co-HB) (0.3 g) was dissolved in 60 mL of chloroform and 6 mL of triethylamine mixture. The mixture was cooled in an ice bath. Then, to this solution 2-bromopropanoyl chloride (0.9 mL, 8.59 mmol) was added dropwise. After addition the mixture was stirrer at room temperature for 6 h and finally refluxed for 2 h. The reaction mixture was washed with K2CO3 solution and distilled water. The chloroform phase was dried using anhydrous Na2SO4. After the salt was filtered off, the solvent was concentrated by rotary evaporator and poured into 150 mL of methanol. The precipitate was collected and dried under vacuum to afford P(IPBBr-co-HB). Synthesis of Poly(isopropyl methacrylatebenzoxazine-cohexylbenzoxazine) (P(IPMaB-co-HB)). Poly(IPB-co-HB) (0.3 g) was dissolved in 60 mL of chloroform and 6 mL of triethylamine mixture. The mixture was cooled in an ice bath. Then, to this solution, methacryloyl chloride (0.9 mL, 9.3 mmol) was added dropwise. After addition the mixture was stirrer at room temperature for 6 h and finally refluxed for 2 h. The reaction mixture was washed with K2CO3 solution and distilled water. The chloroform phase dried using anhydrous Na2SO4. After the salt was filtered off, the solvent was concentrated by rotary evaporator and poured into 150 mL of methanol. The precipitate was collected and dried under vacuum to afford P(IPMaB-co-HB) Synthesis of Poly(isopropylpropionatebenzoxazine-co-hexylbenzoxazine-graf t-polystyrene) (P(IPPB-co-HB-g-PSt)). A Schlenck tube with a magnetic stirrer was evacuated and backfilled with dry nitrogen several times. The catalyst CuBr (5 mg), PMDETA as ligand (8.3 μL), P(IPBBr-co-HB) (50 mg), DMF (1 mL), and styrene (1 mL) were filled in a tube under inert atmosphere. The tube was degassed three times using the freeze and thaw method and then placed in an oil bath at 90 °C and stirred at that temperature for 4 h, after which the reaction was stopped and the mixture was diluted with THF, passed through a silica gel column, and finally poured into a 10fold excess of methanol. The precipitated polymer was collected and dried under vacuum (conversion of styrene: 15%). Characterization. 1H NMR spectra of all polymers were recorded in chloroform, using a Bruker AC250 instrument at a proton frequency of 250 MHz. The FTIR spectra were recorded at Perkin-Elmer Spectrum One with ATR accessory (ZnSe, Pike Miracle Accessory) and cadmium telluride (MCT) detector. Resolution was 4 cm−1 and 32 scans with 0.2 cm s−1 scan speed. DSC measurements were performed on Perkin-Elmer Diamond DSC with a heating rate of 10 °C min−1 under nitrogen flow (20 mL min−1). TGA measurements were performed on a Perkin-Elmer Diamond TA/TGA with a heating rate of 10 °C min−1 under nitrogen flow (200 mL min−1). Photocalorimetry (Photo-DSC). Photocuring behavior of the acrylate groups was investigated by photo-DCS experiments. The photodifferential scanning calorimetry (Photo-DSC) measurements were carried out by means of a modified Perkin-Elmer Diamond DSC equipped with a high pressure mercury arc lamp. A uniform UV light intensity is delivered across the DSC cell to the sample and reference pans. The intensity of the light was measured as 76 mW cm−2 by a UV radiometer covering broad UV range. A typical experiment was performed as follows: P(IPMaB-co-HB) (5 mg), DMF (0.3 mL), bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) as photoinitiator (2 mg) was mixed to prepare a thick solution. This solution was placed in an aluminum pan and exposed to 350−400 nm light for 2 min at 30 °C under a nitrogen flow of 20 mL min−1. The reaction

EXPERIMENTAL SECTION

Materials. Bisphenol A (Acros Organics, 97%), 1,3-diaminopropan-2-ol (Fluka, 98%), hexamethylenediamine (Acros Organics, 99.5%), paraformaldehyde (Aldrich, 99.5%), 2-bromopropionyl chloride (Aldrich), N,N-dimethylformamide (DMF) (Sigma-Aldrich, 99.8%), tetrahydrofuran (THF) (Sigma-Aldrich, ≥99.9%), chloroform (VWR, 99.2%), ethanol (Sigma-Aldrich, ≥99.8%), toluene (Merck, 99.9%), diethyl ether (Sigma-Aldrich, 99.7%), methanol (Merck, 99.9), methacryloyl chloride (Fluka, 97%), and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO or Irgacure 819) (Ciba Specialty Chemicals) were used as received. Triethylamine (Sigma-Aldrich, ≥99%) was dried using NaOH pellets. B

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heat liberated in the polymerization was directly proportional to the number of acrylate double bonds reacted in the system. By integrating the area under the exothermic peak, the conversion of the methacrylate groups (C) or the extent of the reaction was determined according to eq 1:

C=

ΔHt ΔH0theory

(1)

is the Here ΔHt is the reaction heat evolved at time t and ΔHtheory 0 = 86 kJ mol−1 for an theoretical heat for complete conversion. ΔHtheory 0 acrylic double bond.53 The rate of polymerization (Rp) is directly related to the heat flow (dH/dt) by eq 2:

Rp =

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⎛ dH ⎞ dC theory ⎟ / ΔH =⎜ 0 ⎝ dt ⎠ dt

(2)

RESULTS AND DISCUSSIONS As stated in the Introduction, although benzoxazines have many superior properties over conventional phenolic resins, these properties should still be improved in order to broaden the application fields of benzoxazines. It occurred to us that hydroxyl functional groups as repeating units in the main chain of polybenzoxazine precursor would facilitate various possibilities to modify the structures and essentially final properties of the cured benzoxazine thermosets through simple organic reactions. Accordingly, two different diamines one of which contain a hydroxyl group in its structure and the other has a longer aliphatic chain for flexibility of the backbone and bisphenol A as diphenol source were used to construct the precursor. The overall synthetic strategy is presented in Scheme 2.

Figure 1. 1H NMR spectrum of P(IPB-co-HB).

products in benzoxazine synthesis was previously reported.36,54 The reversibility of the reaction was further fortified by Endo et al.55 In this connection, it should be pointed out that such alcohol functional benzoxazines have relatively lower ringopening reaction temperature compare to classical benzoxazine monomers due to the activation effect of above-mentioned reversible reaction in which phenolic OH groups play an important role as catalyst. Moreover, the FT-IR spectrum of P(IPB-co-HB) further evidence the expected structure. As can be seen from Figure 2a, in addition to the band corresponding to the C−O−C oxazine ring at 1228 cm−1 and aromatic C−H stretching vibration at 3020 cm−1, the O−H and C−O (primary alcohol) stretching bands at 3402 cm−1 and 1118, and 1059 cm−1, respectively, were noted. Furthermore, the band at 931 cm−1 corresponds to the oxazine ring attached benzene ring.56 The hydroxyl functional P(IPB-co-HB) precursor was used in further modification reactions. First, ATRP initiator functionality was introduced by a simple esterificaiton process. Thus, P(IPB-co-HB) was reacted with 2-bromopropanoyl chloride using excess triethylamine to capture the formed hydrochloric acid, since under acidic conditions it is known that 1,3benzoxazines undergoes ring-opening reaction. Consequently, poly(isopropyl-2-bromopropanoate benzoxazine-co-hexylbenzoxazine) (P(IPBrPB-co-HB)) was obtained as macro ATRP initiator (See Scheme 4). In Figure 3a, 1H NMR spectrum of P(IPBrPB-co-HB) is shown. The protons resonating at 4.80 (O−CH2-N) and 3.92 ppm (Ar−CH2−N) suggest that under esterification conditions oxazine rings are remained. Moreover, the peak at 4.39 ppm (-CH-Br) gives evidence incorporation of 2-bromopropionate groups. In addition, stretching vibration band of ester carbonyl was noted at 1750 cm−1 (Figure 2b). The bands belonging to oxazine ring were also visible at 1223 and 1117 cm−1 as C−O− C oxazine ring mode and C−O (ether) stretching bands, respectively. The stretching vibration band of C−N group at 1498 cm−1 was also recorded. The ATRP of styrene using P(IPBrPB-co-HB) as macroinitiator was successfully conducted and the benzoxazine ring structure was preserved in the final precursor during the process (Scheme 4). Graft copolymer structure and composition were investigated using 1H NMR and FT-IR spectroscopy. In Figure 3b, the peaks at 4.59 and 4.01 ppm indicate the

Scheme 2. Synthesis of Hydroxyl Functional Polybenzoxazine Precursor (P(IPB-co-HB))

The structure of hydroxyl functional precursor namely poly(2-propanolbenzoxazine-co-hexylbenzoxazine) (P(IPB-coHB)) was characterized by spectral analysis. In Figure 1, 1H NMR spectrum of P(IPB-co-HB) is shown. The protons resonating at 4.79 (O−CH2−N) and 3.91 ppm (Ar−CH2−N) are clear evidence of the formation of oxazine rings on the precursor. Moreover, the incorporation of 2-hydroxypropyl group in the backbone is evident by the peaks at 3.74 (−CH− OH) and 2.70 (−N−CH2−) and the protons of hexyl group are detectable between 1 and 2 ppm. In addition to benzoxazine formation, the NMR spectrum of P(IPB-co-HB) also reveals signals of N-substituted oxazolidine ring. The slight peaks at 9.78, 4.32, and 3.62 ppm can be attributed to protons of (Ph-OH), (−N−CH2−O−) and (−NCH2−O−CH2−) of oxazolidine. The reversible formation of oxazolidine structure is depicted in Scheme 3. This reaction takes place over zwitter ionic form of ring opened oxazine and the attack of hydroxyl group resulting in ring closure reaction to yield oxazolidine. Similar oxazolidine formation as side C

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Scheme 3. Formation of N-Substituted Oxazolidine on the Backbone of the Precursor

Figure 2. FT-IR spectra of P(IPB-co-HB) (a), P(IPBBr-co-HB) (b), and P(IPPB-co-HB-g-PSt) (c).

Scheme 4. Synthesis of P(IPBrPB-co-HB) and P(IPPB-coHB-g-PSt)

Figure 3. 1H NMR spectra of P(IPBrPB-co-HB) (a), and P(IPPB-coHB-g-PSt) (b).

ring mode, at 1121 cm−1 as C−O (ether) and at 1497 cm−1 C− N as stretching vibration bands. Moreover, aromatic C−H stretching vibration and aromatic overtones of styrene residue emerges at 3053 and 1807−1950 cm−1, respectively. Further modification reaction of the P(IPB-co-HB) precursor was also performed in order to obtain photocurable polybenzoxazine precursor. The synthesis involves esterification reaction of the hydroxyls of P(IPB-co-HB) with methacryloyl chloride and in the presence triethylamine under inert atmosphere (Scheme 5). The obtained precursor namely poly(isopropyl methacrylatebenzoxazine-co-hexylbenzoxazine) (P(IPMaB-co-HB)) possesses both methacrylate and thermally curable benzoxazine functionalities, which can be activated sequentially. The structure of the P(IPMaB-co-HB) was confirmed by spectral analysis. The 1H NMR spectrum of the P(IPMaB-coHB) (Figure S1) exhibits typical oxazine peaks at 4.81 and 3.93 ppm corresponding to N−CH2−O and N−CH2−Ar indicating the retention of the oxazine ring during modification reaction which is important for the subsequent thermal treatment.

presence of aromatic protons belonging to the oxazine ring (O−CH2−N) and (N−CH2−Ar), respectively. Additional aromatic and aliphatic protons of polystyrene confirm successful grafting process. Moreover, in the FT-IR spectrum presented in Figure 2c, the stretching vibration band of ester carbonyl could be seen at 1738 cm−1. The bands belong to oxazine ring are also visible at 1384 cm−1 as C−O−C oxazine D

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of the repeating units in the case of main chain benzoxazines. Figure 4, and Table 2 illustrate DSC results obtained with the

Scheme 5. Modification of P(IPB-co-HB) with Methacryloyl Chloride

Notably, alkene protons of the methacrylate group appear between 6.30 to 5.75 ppm. In the FT-IR spectrum of P(IPMaBco-HB) (Figure S2), the stretching vibration of O−H is at 3410 cm−1 is significantly reduced after the esterification reaction. Carbon−nitrogen stretching vibrations of oxazine at 1497 cm−1 and stretching vibrations of ester carbonyl of acrylate monomer at 1751 cm−1 are also detectable. Molecular weights of the polymeric precursors obtained at various stages as measured by gel permeation chromatography (GPC) are tabulated in Table 1. Because of the fact that several

Figure 4. DSC thermograms of P(IPB-co-HB)(a), P(IPBrPB-co-HB) (b), and P(IPPB-co-HB-g-PSt) (c).

Table 2. DSCa Results of P(IPB-co-HB), P(IPBrPB-co-HB), P(IPMaPB-co-HB), and P(IPPB-co-HB-g-PSt) polymer P(IPB-co-HB) P(IPBrPB-coHB) P(IPMaPB-coHB) P(IPB-co-HBg-PSt)

Table 1. Molecular Weight Characteristics of Polybenzoxazine Precursors

a

polymer

Mna (g mol−1)

Mw/Mna

P(IPB-co-HB) P(IPBrPB-co-HB) P(IPMaPB-co-HB) P(IPPB-co-HB-g-PSt)

1820 2610 2590 3460

1.67 1.36 1.33 1.24

maximum curing temperature (°C)

on-set (°C)

end-set (°C)

curing ΔH (J/g)

238 229

164 181

280 252

−258 −191

180

155

215

−193

242

212

283

−6

DSC thermograms were obtained under N2 flow at a heating rate of 10 °C·min−1 a

Measured by GPC according to polystyrene standards.

precursors, P(IPB-co-HB) (a), P(IPBrPB-co-HB) (b), and P(IPPB-co-HB-g-PSt) (c). Added polystyrene mass and due to the Cu catalyst used in ATRP system, the amount of exotherm for P(IPPB-co-HB-g-PSt) decreased drastically, since it is well-known that Cu catalyst as a Lewis acid can facilitate the ring-opening of benzoxazines. However, polystyrene grafted system is still curable and becomes insoluble after thermal treatment. In the case of P(IPMaPB-co-HB), DSC profile has two exothermic major peaks, the first one has a maximum at 122 °C, which corresponds to thermal polymerization of methacrylic double bonds and the second exothermic peak emerging with a maximum at 180 °C belongs to ring-opening reaction of benzoxazines (see Figure S4). The on-set, end-set, and exotherm values for P(IPMaPB-co-HB) are also presented in Table 2. Thermogravimetric analysis (TGA) was employed in order to determine the decomposition characteristics of the polymers. The TGA profiles of the samples are shown in Figure 5, and the results are tabulated in Table 3. Expectedly, cured P(IPPB-coHB-g-PSt) exhibited higher thermal stability. The T5%, T%10, and char yields are higher than those for the P(IPB-co-HB) and its other derivatives. Moreover, all the samples were also compared to a classical main chain polybenzoxazine derived from bisphenol A and 1,6-diamonohexane (P(PB-co-HB)). It could be seen that thermal degradation behaviors of P(IPB-coHB) and P(PB-co-HB) are close to each other. However, the alcohol functionality in P(IPB-co-HB) is destroyed during degradation and the overall char yield is slightly lower than that of P(PB-co-HB). Moreover, both P(IPBrPB-co-HB) and P(IPMaPB-co-HB) have lower char yield compared to that of

successive reactions, namely condensation polymerization, and ATRP are involved in the overall process, the polymers have different chain lengths and molecular weight distributions. Expectedly, after ATRP, the polydispersity of the final polymer decreased down to 1.24, which is a very good value for a polymer composed of a backbone synthesized by a condensation polymerization technique. It should also be pointed out that all the obtained precursors form free-standing films by solvent casting. The photocuring of P(IPMaPB-co-HB) with bis(2,4,6trimethylbenzoyl)phenylphosphine oxide as photoinitiator was also studied. The heat released during the photocuring of the formulation was followed by photo-DSC. An estimated conversion−time plot referring to the photoinitiated free radical polymerization under light emitting between λ = 315−450 nm is shown in Figure S3. The shape of the curves indicates the existence of two stages: a rapid first stage followed by a slow stage. At the second stage, gelation and vitrification of the polymerizing methacrylate most likely render the diffusion of the components more difficult. The photocured P(IPMaPBco-HB) becomes insoluble in common solvents. And a subsequent thermal curing generates extra cross-linking, which presumably increase the stiffness of the polymer. Further analyses of the polymers were performed using DSC and TGA. It is known that thermally activated ring-opening polymerization of 1,3-benzoxazines is an exothermic reaction which possess an exothermic peak generally between 180 and 270 °C depending on the structure and concentration or nature E

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industrial importance. For example, by using suitably selected diisocynates or diacidchlorides in conjunction with hydroxyl functional precursor, thermally curable polyurethane or polyester networks with improved properties can be obtained. Another alternative involves ring-opening polymerization of lactones with the precursor by using tinoctaate as catalyst may lead to the formation of biocompatible thermosets. Further studies in these lines are now in progress.

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ASSOCIATED CONTENT

S Supporting Information *

NMR, FT-IR, conversion−time, and DSC plots. This material is available free of charge via the Internet at http://pubs.acs.org

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Figure 5. TGA profiles of cured polymers P(IPB-co-HB), P(PB-coHB), P(IPBrPB-co-HB), P(IPMaPB-co-HB), and P(IPPB-co-HB-gPSt).

Corresponding Author

Table 3. TGAa,b Profiles of P(IPB-co-HB), P(PB-co-HB), P(IPBrPB-co-HB), P(IPMaPB-co-HB), and P(IPPB-co-HBg-PSt) polymer

T5% (°C)c

T10% (°C)d

Tmax (°C)e

Yc (%)f

P(IPB-co-HB) P(PB-co-HB) P(IPBBr-co-HB) P(IPMaPB-co-HB) P(IPB-co-HB-g-PSt)

246 247 249 236 354

264 256 279 261 380

258, 397 248, 394 335 420 426

18 21 16 16 37

AUTHOR INFORMATION

*(Y.Y.) Telephone: +90 212 285 3241. Fax: +90 212 285 6386. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Istanbul Technical University Research Fund.

TGA thermograms were obtained under N2 flow at a heating rate of 10 °C·min−1 bAll the samples were cured before analysis and curing was performed in TGA at 230 °C for 15 min under N2 stream (200 mL/min.). cT5%: The temperature for which the weight loss is 5% d T10%: The temperature for which the weight loss is 10% eTmax: The temperature for maximum weight loss. fYc: Char yields at 800 °C under nitrogen atmosphere. a

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P(IPB-co-HB), since converting hydroxyls of P(IPB-co-HB) to ester groups makes the end-structures hydrolyze susceptible systems. As seen from TGA traces, the incorporation of polystyrene to benzoxazines has a positive effect on thermal stability. Similar synergism was previously observed for polystyrene samples possessing benzoxazine side groups incorporated by click chemistry.43

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CONCLUSION In conclusion, we have demonstrated that the main chain polybenzoxazine precursors with hydroxyl side groups can readily be prepared via monomer synthesis approach. Accordingly, Mannich type polymerization of two different diamines, one of which contain a hydroxyl group in its structure and the other has a longer aliphatic chain, and bisphenol A as diphenol source result in the desired product. Typical modification examples were demonstrated for the incorporation of ATRP initiator and acrylate functionalities through a simple esterification process. In the former case, the successful ATRP process yielded main-chain polybenzoxazine precursors with polystyrene grafts. Through the acrylate functionalization dual curable precursors were obtained. All the prepared benzoxazine precursors were shown to readily undergo thermally activated ring-opening reaction in the absence of added catalyst and formed cross-linked networks. The polymers cured in this way exhibited comparable thermal stability to classical polybenzoxazine precursors. The approach described here is modular and can be extended through many other systems which may open new pathways for the preparation of advanced materials of F

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dx.doi.org/10.1021/ma401888g | Macromolecules XXXX, XXX, XXX−XXX