Catalytic Accelerated Polymerization of Benzoxazines and Their Mechanistic Considerations

Chapter 2

Catalytic Accelerated Polymerization of Benzoxazines and Their Mechanistic Considerations C. Liu1 and Q.-Y. Chen Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China 1

Corresponding author: e-mail: [email protected]

Chapter Outline 1 Introduction 2 Benzoxazine Polymerization Mechanism Consideration 3 Catalysts for Accelerated Benzoxazine Polymerization 3.1 Acidic Catalysts 3.2 Basic Catalysts

9 9 12 13 15

1 INTRODUCTION Benzoxazine resins have gained much interest and attention because of their many useful properties associated with conventional phenolics, such as excellent heat and chemical resistance, flame retardance, low dielectric constant, good dynamic mechanical properties, and relatively inexpensive cost. They also possess other desirable properties including near-zero shrinkage cure, low water absorption, good dimensional stability, excellent FST (fire, smoke, toxicity) properties, and super molecule-design flexibility [1]. These properties offer high potential to substitute epoxy, bismaleiimide, phenolics, or other thermosetting resins in many important applications. For example, commercial benzoxazines are good candidates for formulating prepregs used for aerospace compositions, and are currently utilized frequently in the aerospace industry. It is estimated that 30% weight savings may be obtained by replacing metal with benzoxazine composites, which results in reduced fuel consumption, and emissions. However, the high temperature (typically > 180°C) and relatively long reaction time (typically > 2 h) required for the complete polymerization of benzoxazines indicates their poor reactivity, which is one of the main drawbacks and has been a problem in their practical applications. Although some novel benzoxazine monomers with special functional groups exhibit good reactivity and can polymerize at lower temperatures, it is generally not practical and economical to use them for

4 Catalyst Effects on Polymerization Mechanism and Network Structures 4.1 Pure Benzoxazines 4.2 Benzoxazine-Epoxy Blends 5 Conclusion

16 16 19 20

industrial applications because of the high cost of preparing the benzoxazine monomer itself. As a result, use of an added catalyst to accelerate the polymerization of benzoxazines becomes a good alternative. Much effort has been devoted to developing convenient and efficient catalysts to accelerate the benzoxazine polymerization at lower temperatures and reduced reaction time, but has met with limited success. To develop a good catalyst, it is important to understand the benzoxazine polymerization mechanism. This chapter focuses on the understanding of benzoxazine polymerization mechanism and the catalysts reported for accelerating polymerization rate of benzoxazines. The influence of an added catalyst on the polymerization mechanism and the network structure of the resulting polymer is discussed as well.

2 BENZOXAZINE POLYMERIZATION MECHANISM CONSIDERATION The fundamental understanding of the benzoxazine polymerization mechanism is an important foundation to learn how a catalyst influences and may influence the progress and mechanism of benzoxazine polymerization, including the development of networks and the final structure units of polybenzoxazines. This only favors a good molecular understanding of the structure-property relationships because it is well known that the development of networks and the

Advanced and Emerging Polybenzoxazine Science and Technology. http://dx.doi.org/10.1016/B978-0-12-804170-3.00002-0 Copyright © 2017 Elsevier Inc. All rights reserved.

9

10

PART I Synthesis and Properties of Benzoxazine Resins

final structure units of polybenzoxazines to a great degree determine their various properties. This understanding can also guide us to develop good catalysts to efficiently accelerate the benzoxazine polymerization and tailor the desired structure for various practical applications. Although impressive progress has been achieved, the polymerization mechanism of benzoxazines remains elusive and is still under investigation. In 1965, Burke’s group reported the ring-opening reaction of benzoxazine with various phenols [2]. It was found that the aminoalkylation reaction occurred preferentially at the ortho position of phenols with both free ortho and para positions. McDongh and Smith in 1968 investigated the ring-chain tautomerism phenomenon of benzoxazines by nuclear magnetic resonance (NMR) analysis and proposed that the protonated benzoxazine prefers to produce iminium ions in strong acidic conditions [3]. Riess et al. proposed in 1985 two mechanistic pathways for the reaction of benzoxazine with various kinds of phenols [4]. They suggested that the formation of an intermolecular hydrogen bond of phenolic hydroxyl group with oxygen atoms of oxazine ring resulted in the ortho reaction. The para reaction resulted from the reaction of iminium species derived from the ring opening of benzoxazine via protonation by the phenol. Ishida and coworkers have devoted great effort to the study of polymerization mechanism of benzoxazines. They propose that the polymerization of benzoxazines proceeds through a cationic ring-opening mechanism based on the detailed study of various reaction catalysts [5]. The formation of the iminium ion intermediate by protonation of oxygen atom of benzoxazine with a strong acid followed by aromatic electrophilic substitution on another benzoxazine molecule may induce the ring-opening polymerization of benzoxazines [6]. Alternatively, both the oxygen and the nitrogen atom of benzoxazine can act as the potential initiation sites. Their coordination with a cationic catalyst results in the formation of a cyclic tertiary oxonium or nitronium ion. The subsequent insertion of the benzoxazine monomers through the reaction of the oxygen, nitrogen, or the unoccupied benzene ortho position of benzoxazine finishes the polymerization process, producing phenolic or phenoxy (N,O-acetal) structure repeat units depending on the polymerization conditions used [7] (Scheme 1). The phenoxy structure can rearrange into the corresponding phenolic structure upon further heating [8]. Ishida et al. recently investigated further the ring-opening polymerization mechanism by the detailed and systematic analysis of the crude products of various model benzoxazine dimerization reaction with a large number of pure-model compounds prepared based on the hypothesized initiation mechanism [9]. It was found that the protonation initiation can occur both on the oxygen and the nitrogen atom, but the corresponding nitrogen-protonated species is more stable,

and the oxygen-protonated species is more reactive. In the final polybenzoxazine structure, the phenolic structure unit is less stable than the methylene bridge structure unit. Yagci et al. investigated the photoinitiated cationic polymerization of benzoxazines at room temperature and found that the structure of the polybenzoxazines was very complicated because of different ring-opening polymerization processes of the protonated oxygen or nitrogen species [10]. Ueda and colleagues proposed some possible polymerization pathways at the initial stage based on the study of acid-initiated model condensation reactions of benzoxazines with phenols [11]. On the basis of the literature mentioned previously and the detailed NMR study of ring-opening polymerization behavior of a model p-cresol-aniline-based benzoxazine, Sebastia´n, Marquet, and coworkers proposed an improved mechanistic scheme to explain various observed experimental results, as shown in Scheme 2 [12]. The polymerization mechanism of benzoxazines is considered to include three main steps: coordination ring-opening of oxazine ring, electrophilic attack, and rearrangement. First, the catalyst coordinates with an oxygen or nitrogen atom of benzoxazine, and ring opening occurs to generate three possible cationic intermediates: A, B, and C, by different heterolysis patterns. The following electrophilic reactions of each intermediate may involve O-attack, N-attack, and Aryl-attack at another benzoxazine molecule, resulting in the polymerization chain propagation. It is worth noticing that the Aryl-attack of the active intermediates may occur at various sites of benzene ring of benzoxazine including the arylamine ring of benzoxazine with varying degrees of reactivity, resulting in a more complex polymerization mechanism [13–17]. The inner structure of the polymer formed may contain various phenoxy structures and phenolic structures. Upon further heating at an elevated temperature or extended reaction time, phenoxy structures can rearrange to corresponding phenolic structures. Because the signals of CH2 units of the polybenzoxazine in 1H NMR spectra are most characteristic and easily recognized, the NMR analysis of various CH2 units in polybenzoxazine structure was performed to learn more about the polymerization mechanism of benzoxazines. It should be mentioned that using deuterated dimethyl sulfoxide (DMSO) instead of deuterated chloroform as the NMR solvent allows better resolution of signals in the 1H NMR spectra. During the ring-opening polymerization of benzoxazines, six different types of CH2 units can be formed: ArO-CH2-OAr (a), -(Ph)N-CH2-N(Ph)- (b), ArO-CH2-N (Ph)- (c), ArO-CH2-Ar (d), -(Ph)N-CH2-Ar (e) and Ar-CH2-Ar (f) (Scheme 2). CH2(a–d) and CH2(e–f) are phenoxy CH2 units and phenolic CH2 units, respectively. Although it is very difficult to exactly distinguish the contribution of every type of CH2 units on the signals in the 1H NMR spectrum, the possible chemical shift ranges of

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

11

(I) (II) O O

N

δ+

R Δ

Ring-opening M = Catalyst

O

N δ–

δ+ M R M δ+

O

R N δ–

δ+ δ+ R M δ– N O

N

(III)

O

R

n

Phenoxy structure Electrophilic substitution

δ+

N R

(I,II)

With reletively lower reactivity Active intermediates

OH (III)

N R n

Phenolic structure SCHEME 1 Proposed mechanism by Ishida and coworkers for benzoxazine ring-opening polymerization.

d

SCHEME 2 Improved mechanism for the ring-opening polymerization of benzoxazines. (Reproduced with permission from C. Liu, D. Shen, € R.M. Sebastia´n, J. Marquet, R. Schonfeld, Mechanistic studies on ring-opening polymerization of benzoxazines: A mechanistically based catalyst design, Macromolcules 44 (2011) 4616–4622. Copyright (2010) American Chemical Society)

various CH2 units are estimated and indicated in Scheme 2 by using the chemical shifts of similar CH2 units in the analogous known compound as the reference. The structural changes during benzoxazine polymerization presented in Scheme 2 are further supported by the 1 H NMR spectra of a thermally cured mixture of model p-cresol-aniline-based benzoxazine with 1 mol% lithium iodide (LiI) at 150°C or 200°C for different time in

deuterated DMSO (Fig. 1). As can be seen in Fig. 1b, 1H NMR spectrum of polymer P15min presents four main signals caused by different types of CH2 units at d 5.5– 5.3, 4.7–4.5 parts per million (ppm) (I); 4.7–4.4 ppm (II); 4.4–4.0 ppm (III); 4.0–3.5 ppm (IV). Signal I, apart from the peaks at 5.4 and 4.6 ppm caused by the starting p-cresol-aniline-based benzoxazine, may be ascribed to the residual oxazine rings in the structures formed by

12

PART I Synthesis and Properties of Benzoxazine Resins

FIG. 1 1H NMR spectra in deuterated DMSO of p-cresol-aniline-based benzoxazine monomer and various polymers obtained under different conditions. The polymer P15min, P0.5h, P1h and P5h were obtained by heating p-cresol-aniline-based benzoxazine in the presence of 1 mol% LiI at 150°C for 15 min, 0.5 h, 1 h, and 5 h, respectively. The polymer P200 was obtained by heating polymer P5h at 200°C for 2 h. Note: Signal I ¼ CH2 (residual oxazine rings of the structure formed by Aryl-attack of various intermediates, Scheme 2), Signal II ¼ CH2(a,b), Signal III ¼ CH2(c,d), Signal IV ¼ CH2(e,f). (Reproduced € with permission from C. Liu, D. Shen, R.M. Sebastia´n, J. Marquet, R. Schonfeld, Mechanistic studies on ring-opening polymerization of benzoxazines: A mechanistically based catalyst design, Macromolcules 44 (2011) 4616–4622. Copyright (2010) American Chemical Society.)

Aryl-attack of intermediates A, B, or C (Scheme 2). Signal II can be the result of phenoxy CH2 units CH2(a,b). Signal III probably indicates the presence of phenoxy CH2 units CH2 (c,d). Signal IV may be the result of phenolic CH2 units CH2 (e,f). The coordination ring-opening step is indicated by the disappearance of signal (I) (Fig. 1b and c). After complete ring opening and the following electrophilic attack steps, various CH2 units in the polybenzoxazine are formed, which is shown by signals (II, III, and IV) in Fig. 1c. The rearrangement of phenoxy CH2 units CH2(a,b) into phenoxy CH2 units CH2(d) and phenolic CH2 units CH2(e), respectively, and the transformation of phenoxy CH2 units CH2 (c,d) into phenolic CH2 units CH2(e,f) by rearrangement are demonstrated by the gradual disappearance of signals (II and III) in Fig. 1d, e, and f. These results suggest that coordination of ring-opening steps

occurs easily in benzoxazine polymerization process, and that rearrangement in labile phenoxy structure units to the final stable phenolic structure units is relatively difficult.

3 CATALYSTS FOR ACCELERATED BENZOXAZINE POLYMERIZATION In light of this understanding of the benzoxazine polymerization mechanism, a catalyst accelerates the benzoxazine polymerization by inducing fast ring opening of benzoxazine monomer. The benzoxazine polymerization is an autocatalyzed reaction until vitrification is reached and diffusion begins to control the curing process afterwards [18–20]. A good catalyst should be able to effectively coordinate with oxygen or nitrogen atoms to produce active

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

δ+ A Acidic catalysts [A]

δ– O

δ+

R N

Cationic ring-opening

+

N

Polybenzoxazine

R

R O

Benxoazine monomer

OA

13

N δ– B Basic catalysts [B]

δ– O

–

R δ+ N

Nucleophilic ring-opening

O +

N

B

Benxoazine monomer

Polybenzoxazine

R SCHEME 3 Proposed two types of ring-opening patterns with different catalysts.

iminium cation intermediates and greatly promote the rearrangement step to obtain higher percentage of stable phenolic structure, especially Mannich phenolic structure. As shown in Scheme 3, most reported effective catalysts are acidic compounds and prefer to accelerate the benzoxazine polymerization by a typical cationic ring-opening pattern. However, basic compounds are also able to accelerate the benzoxazine polymerization by a nucleophilic ring-opening pattern, although they are not much studied. It should be mentioned that in many cases the two types of ring-opening patterns may exist simultaneously. In other words, the cationic part of a catalyst coordinates with oxygen or nitrogen atoms of benzoxazine to accelerate ring-opening of benzoxazine, and the anionic part may play a nucleophilic role meanwhile, more or less to facilitate the ring-opening step. It is expected that a highly effective catalyst would consist of a cationic part with a high affinity towards oxygen and/or nitrogen atoms and an anionic part with both strong nucleophilic ability and good leavinggroup properties.

3.1

Acidic Catalysts

The majority of effective catalysts reported for accelerating benzoxazine polymerization rate are acidic catalysts, probably because of the nature of cationic ring-opening polymerization. Some representative acidic catalysts presented in the literature are shown in Fig. 2. Ishida et al. investigated in detail the effect of many potential catalysts on benzoxazine polymerization. On the basis of comprehensive survey and studies of various cationic, anionic, and radical catalysts, they proposed a cationic ring-opening mechanism for benzoxazine polymerization [5]. In general, an acidic catalyst is more effective. Many Lewis acids, such as PCl5, PCl3, POCl3, TiCl4, AlCl3, and MeOTf were shown to be effective for the benzoxazine polymerization at moderate temperatures. Interestingly, it was found that the BF3 Et2O complex was not an effective catalyst for the polymerization reaction of benzoxazine in methylene chloride solution, while BF3 2H2O complex

exhibits good catalytic activity, which might be to the result of the formation of free protons derived from the reaction of BF3 with more basic water [21]. Recently, it was demonstrated by Ronda and coworkers that BF3 Et2O in combination with alcoholic solvents is an efficient catalyst system for the benzoxazine polymerization in solution under mild conditions [22]. It was proposed that the benzoxazine polymerization proceeds through an intermediate hemiaminal ether that leads mainly to the formation of diphenylmethane bridges, and not only the classic Mannich-type bridges. All these results suggest that exploring a wide range of metallic compounds would be a good way to develop highly efficient catalyst for accelerated rate benzoxazine ring-opening polymerization. Moreover, it is also advantageous that the catalytic activity of a metallic compound can be conveniently tuned by combination of center metals with appropriate ligands. Various phenolic structures derived from the ring-opening of the oxazine ring or dimers and higher oligomers in the benzoxazine monomers can actually act as catalysts for further ring-opening polymerization reactions, resulting in the autocatalyzed nature of benzoxazine polymerization [20]. Some phenols have been shown to exhibit a good catalytic effect on the benzoxazine polymerization (Fig. 2). Benzoxazine polymerization with a strong carboxylic acid such as trifluoroacetic acid as the catalyst was found to proceed with no lag between the ringopening and aromatic electrophilic substitution steps by an iminium cation intermediate. [6] By comparison, aminomethyl ester species are initially generated when a weak carboxylic acid, such as adipic acid, is used as catalyst for the benzoxazine polymerization. While the dielectic constant increases, they are transformed into the iminium ion form of the reactive intermediate and the following electrophilic attack reactions occur. In general, weak carboxylic acids show better performance on the benzoxazine polymerization (Fig. 2). Yagci’s group studied the photoinitiated ring-opening cationic polymerization of a monofunctional benzoxazine with onium salts such as diphenyliodonium hexafluorophosphate (Ph2I+PF6-) and triphenylsulfonium hexafluorophosphate

14

PART I Synthesis and Properties of Benzoxazine Resins

FIG. 2 Representative acidic catalysts used for accelerated benzoxazine polymerization.

(Ph3S+ PF6-) [10]. Irradiation of the catalyst results in the formation of protons. The subsequent protonation of benzoxazine can possibly occur either at oxygen or nitrogen atoms. The structure of the polymers formed is complex and suggests a complicated polymerization mechanism. Endo and coworkers developed several active catalyst systems for accelerating benzoxazine polymerization. (1) Electron-deficient hexafluoroacetylacetonato (F6-acac) complexes of manganese and iron were found to be highly active catalysts for benzoxazine polymerization, which are moisture tolerant and harmless to the thermal stability of the resulting polybenzoxazine [23]. (2) They first reported the use of metal free and neutral aromatic urethanes derived from the phenolic compounds such as resorcinol and phenyl isocyanate as efficient catalysts for the polymerization of benzoxazines in [24]; the possible catalytic mechanism is presented in Scheme 4. Thermal dissociation of these urethanes may result in the formation of real active species, resorcinol and phenyl isocyanate, which facilitate the rapid induction of the ring-opening polymerization of benzoxazine and finally are incorporated into the resulting polybenzoxazine. (3) Cyclohexyl p-toluenesulfonate (TsOCH) and neophenyl p-toluenesulfonate (TsONP) were estimated as metal free and highly efficient thermally latent catalysts for the benzoxazine ring-opening polymerization [25]. They do not catalyze the polymerization below 100°C while inducing the polymerization at elevated temperatures. The

possible mechanism may involve the thermal dissociation of these sulfonate to afford the real active species alkyl cations and/or p-toluene sulfonic acid (PTS). It is worth noticing that Ishida’s group reported in 1999 that TsOCH showed good catalytic activity on cationic ring-opening polymerization of benzoxazines [26]. (4) It was further found that more efficient ring-opening polymerization of benzoxazine was achieved by using a dual system composed of 2-ethyl-4-methylimidazole (EMI) and PTS [8], which can also be considered a latent catalyst system, with PTS providing active protons to coordinate with oxygen or nitrogen atoms of benzoxazine, and EMI as the nucleophile to facilitate the ring opening and the following rearrangement steps. (5) Recently, they demonstrated the ability of simple thiophenols as catalysts for the ring-opening polymerization of benzoxazines, which was designed based on the reversible nature of the reaction of benzoxazines with thiols (Scheme 5) [27]. It should be noted that Gorodisher and coworkers reported in 2011 the detailed study on the reactions of various thiols with benzoxazines [28]. Based on the NMR study on the ring-opening polymerization behavior of a model p-cresol-aniline-based benzoxazine under various conditions, Sebastia´n, Marquet, and coworkers further examined the mechanistic process of benzoxazine polymerization and designed and studied several bifunctional catalysts. Lil was found to be an extremely active and effective catalyst [12]. To our knowledge, it is one of the most effective catalysts yet reported for the

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

Ph

H N

O

H N

O

O

HO

O

OH

Ph

C +

Ph N

O

Me

Me

H

O N

O N

Me

Me

+

N

O

15

O

Me

C Ph N

–

O

–

+

N

O

OH

Me

Me Me –

OH

OH

O

N

O +

Ph

N Me HO Me

N Me Me

SCHEME 4 Plausible mechanism for the catalytic effect of urethanes on the benzoxazine polymerization.

SCHEME 5 Reversible reaction of benzoxazines with thiols.

–

O

N

Ph

H

+

O

RS Ph N

OH

+

R-SH

Me

ring-opening polymerization of benzoxazines. They further established the catalytic activities of a number of different catalysts and their effects on the structure of the final polymer by dynamic differential scanning calorimetry (DSC) and NMR analysis [29].

3.2

Basic Catalysts

Compared with numerous acidic catalysts reported for accelerating benzoxazine polymerization rate, basic catalysts have not drawn much attention, although some amines and imidazoles were demonstrated to show good catalytic effect onbenzoxazine polymerization in some Japanese patents [30,31]. We chose several typical basic catalysts including EMI, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 4dimethylaminopyridine (DMAP), triphenylphosphine (PPh3), and tributylphosphane (PBu3) to investigate and compare their catalytic activities on ring-opening polymerization of the model p-cresol-aniline-based benzoxazine by

OH N Me

Me

Me

– N RS Me

SR

Me

DSC analysis. The corresponding mixtures of p-cresolaniline-based benzoxazine with various basic catalysts were placed in a DSC and heated with a rate of 10°C/min. Fig. 3 shows the plots and data obtained by the DSC. Pure pcresol-aniline-based benzoxazine reacts with an onset temperature of 261°C and a maximum temperature of 268°C, showing its poor reactivity. All of the basic catalysts studied exhibit catalytic effects, more or less. In cases of DBU, PBu3, and DMAP, the heat-evolution profiles are significantly shifted toward a low temperature field, and the corresponding onset and maximum temperatures decrease by 61°C and 44°C, respectively, demonstrating their obvious catalytic effects. By comparison, EMI shows lower catalytic activity with onset and maximum temperatures at 221°C and 237°C, respectively. When PPh3 is used, the onset and maximum temperatures are just slightly lower than that observed with pure p-cresol-aniline-based benzoxazine, which might be the result of its lower nucleophilic ability. These results also suggest that basic catalysts are not as efficient as acidic catalysts. However, considering the different

16

PART I Synthesis and Properties of Benzoxazine Resins

6

Catalysts No catalyst EMI DBU DMAP PPh3 PBu3

Heat flow (W/g)

4

2

Tonset (°C)

Tmax (°C)

ΔH (KJ/mol)

261 221 200 212 257 209

268 237 226 224 265 227

345 321 355 353 301 372

Pure monomer

Exo Up

FIG. 3 DSC plots and data of pure p-cresolaniline-based benzoxazine and its mixtures with 5 mol% of various basic catalysts.

PPh3

DMAP

EMI

PBu3 DBU

0

–2

–4 0

50

100

150 Temperatures (°C)

200

catalytic patterns (Scheme 3), basic catalysts might have unique catalytic effects and result in special and valuable inner structures in the final polymer. Moreover, they may be used in combination with an acidic catalyst to form a dual catalyst system to facilitate the accelerated benzoxazine polymerization.

4 CATALYST EFFECTS ON POLYMERIZATION MECHANISM AND NETWORK STRUCTURES 4.1

Pure Benzoxazines

Benzoxazines, especially with high purity, have comparatively poor reactivity in the absence of catalysts, which may hinder their widespread applications. The catalyst has the greatest impact on the ring-opening step and induces a faster benzoxazine polymerization, but it does not affect the later processes associated with cross-linking to the same

250

300

degree [18–20]. In the middle to later stages, the catalystacceleration effect is decelerated significantly, which might be caused by decreased freedom in motion of the polymer in solid state in part caused by the development of a hydrogenbonding network. On the other hand, the inherently difficult transformation of a relatively labile phenoxy structure into a final stable phenolic structure by rearrangement may be another reason (Fig. 1) [8,12]. However, the nature of the catalyst has a profound influence on the progress and termination mechanism of the benzoxazine polymerization. As a result, different final network structures and cross-link density may be generated with different catalysts, which may allow, in principle, the tailoring of a particular structure and, therefore, the final properties of the polymer. A dual catalyst system composed of PTS and EMI was proposed for efficient catalysis of ring-opening polymerization of benzoxazines (Scheme 6) [8]. PTS is advantageous for C-O dissociation of benzoxazines, and EMI serves as a nucleophile to facilitate the C-O dissociation and to prevent

SCHEME 6 Proposed catalytic rearrangement model of the dual catalyst system comprising of PTS and EMI.

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

its recombination. This dual catalyst system is particularly efficient for successful main-chain rearrangement of phenoxy into phenolic structure, because it significantly suppresses the recombination of the iminium species into the original phenoxy structure. This process allows the iminium species time to move to the position for the Mannich reaction with the ortho position of the phenol moiety. LiI was designed and found to be a highly active difunctional catalyst for the ring-opening polymerization of benzoxazines (Scheme 7). Lithium cation effectively coordinates with oxygen or nitrogen atoms and accelerates ringopening of benzoxazines. Iodide ion is the nucleophilic anionic part with good leaving ability and can react with the ring-opened iminium intermediates to prevent its rapid recombination with the phenolate (Scheme 7) [12]. This synergistic effect is also highly efficient for the rearrangement of phenoxy into phenolic structures. The effect of various catalysts on the ring-opening polymerization and structure of a simple-model p-cresol-anilinebased benzoxazine was further investigated by qualitatively analyzing the shape of signal at 4.0–3.5 ppm in 1H NMR spectra caused by phenolic CH2 units of the final polymer [29]. Several typical catalysts were chosen to study how a catalyst can affect the structure of the final polymer structure. The polymerization of p-cresol-aniline-based benzoxazine were carried out in the presence of 1 mol% of various catalysts at 180°C for 2 h, and the structure of the phenolic polymer was compared by analyzing their 1H NMR spectra in DMSO-d6. As can been seen in Fig. 4, except for the Mannich phenolic CH2 units, more complex types of phenolic CH2 units such as arylamine Mannich phenolic CH2 units, general phenolic CH2 units, and arylamine general phenolic CH2 units, may exist with different catalysts. As a result, the amounts and type of phenolic CH2 units might be adjusted by using an appropriate catalyst. It was reported in 2012 that in the presence of FeCl3, ring-opening polymerization of benzoxazines proceeds effectively to generate the arylamine Mannich phenolic structure in the polybenzoxazine because the coordination of Fe3+ with the oxygen atom decreases reactivity of phenol ring and makes the cross-linking reactions occur on the paraposition of aniline [17]. This arylamine Mannich structure can delay degradation of the aniline moiety and improve the thermal properties of polybenzoxazine. On

the other hand, the coordination of Fe3+ with the oxygen and nitrogen atom in the polybenzoxazine may increase the cross-link density and anchor the potentially weak moieties, therefore improving the thermal properties of the polymer. Recently, the catalytic effect on the ring-opening polymerization of benzoxazines was compared in detail using two representative acidic catalysts, 3,30 -thiodiphenol (TDP) and 3,3’-thiodipropionic acid (TDA), at a variety of loadings [32]. The nature of the catalyst has an obvious influence on the progress and termination mechanism of the polymerization reaction and therefore the network architecture and final properties of the polybenzoxazines. It was found that TDA better reduces the onset of polymerization and improves cross-link density and thermal properties (Tg and thermal stability) of the polybenzoxazine. Because TDA (pKa ¼ 4.1) is a stronger acid than TDP (pKa ¼ 6.6), it is reasonable that TDA shows higher catalytic activity on the benzoxazine polymerization and induces a faster ring opening of benzoxazine. On the basis of vibrational spectroscopic analysis, the thermal polymerization and TDP-catalyzed polymerization of benzoxazines conform to a similar polymerization mechanism, and the resulting polymer network has similar cross-link density and glass transition temperatures. By comparison, the ring-opening polymerization of benzoxazine with TDA progresses via a different mechanism. After faster ring opening of more benzoxazine monomers, the chain propagation is more cluster-like (less linear) before cross-linking, leading to more reaction sites and a higher cross-link density and a subsequently higher Tg. In addition, we propose that the higher acidity of TDA might result in deactivation of the phenol ring of benzoxazine toward active cationic intermediates and, consequently, more reactions of the active intermediates on the arylamine ring of benzoxazine. This change of reaction path further increases the cross-link density and, therefore, the thermal properties of the polybenzoxazine. The effect of catalyst loading is initially really obvious but reaches on apparent plateau at 10–15 mol%, and the addition of >15 mol% of catalyst had a negative effect on the cross-link density, Tg and thermal stability of the polymer. Although the conclusion was made on the basis of the experimental results without any plausible explanation, we suppose that the catalyst added may act, in part,

Nucleophile

R O

N

LiI

δ+ Li δ– O Coordination

I δ+ R N

17

Benzoxazine monomer

OLi N

I

R Leaving group SCHEME 7 Possible mechanism for LiI-catalyzed ring-opening polymerization of benzoxazines.

Polybenzoxazine

18

PART I Synthesis and Properties of Benzoxazine Resins

OH

OH

I N

Me

Me

OH

IV

II

I : Mannich phenolic CH2 units II: Arylamine Mannich phenolic CH2 units

N

III: General phenolic CH2 units IV: Arylamine general phenolic CH2 units

OH Me

III

Me

OH Me

CH2(I,II) (a)

Standarda

(b) PTS (c) EMI CH2(III,IV) (d) LiI CH2(III,IV) (e) LiBr CH2(III,IV) (f) LiOTf CH2(III,IV) (g) Al(OTf)3 (h) NH4I (i) ZnCl2 9

8

7

6

5

4

3

2

PPM

FIG. 4 1H NMR spectra in DMSO-d6 of thermally cured mixtures of p-cresol-aniline-based benzoxazine with 1 mol% various catalysts at 180°C for 2 h. a Polymer obtained by thermal polymerization of p-cresol-aniline-based benzoxazine at 20°C for 4 h. (Reproduced with permission from C. Liu, D. Shen, R.M. Sebastia´n, J. Marquet, R. Schonfeld, € Catalyst effects on the ring-opening polymerization of 1,3-benzoxazine and on the polymer structure, Polymer 54 (2013) 2873–2878.

as a terminator by its own reaction with the active intermediates. When its loading is too high (>15 mol%) in the system, the chain propagation process may be disturbed or even prevented, which decreases the cross-link density and consequently reduces the Tg and thermal stability of the resulting polymer. Moreover, too much loading amount of the catalyst may result in the polymer degradation issue. Decreasing the benzoxazine polymerization temperature is highly desirable and, by using various catalysts, different degrees of success have been achieved. However, in view of the previous discussion, polymerization temperature is still an important factor for obtaining polybenzoxazine with good properties. Although benzoxazines can polymerize under lower temperatures with extended reaction time by using a catalyst, the corresponding polymers may possess

different inner structure and poor properties. As shown in Scheme 8, (1) the ring-opening polymerization of benzoxazines performed at lower temperatures (typically < 160°C) under a catalyst provides polybenzoxazine with more complex inner structures because at lower temperatures and in the presence of a catalyst, more ring-opening patterns seem to be available, resulting in three main intermediates: A, B and C. The following aromatic electrophilic substitution reactions of these active intermediates operate by O-attack, N-attack, and Aryl-attack patterns with low selectivity, affording various phenoxy or phenolic structures. The polybenzoxazine obtained commonly possesses relatively poor properties because of less Mannich phenolic structure units generated, which is thought to account for many unusual properties of polybenzoxazine by forming the very

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

19

SCHEME 8 Proposed polymerization temperature and catalyst effect on benzoxazine polymerization mechanism and inner structure of polybenzoxazine.

stable six-membered hydrogen bond [33,34]. More importantly, the chain propagation process possibly faces more difficulty at lower temperatures because of the competing intramolecular six-membered ring hydrogen bonding formation [35,36]. Monofunctional benzoxazines cannot provide a large molecular-weight linear polybenzoxazine. (2) However, when the benzoxazine polymerization is carried out at higher temperatures (typically > 180°C), O-CH2 bonds of the benzoxazine monomers may be in complete or partial dissociation state, and ring opening of the benzoxazines selectively generates the thermodynamically more stable intermediate A. The following aromatic electrophilic substitution reactions of intermediate A mainly are Oattack or Aryl-attack type electrophilic reactions, providing only Mannich phenoxy and Mannich phenolic structures. The subsequent fast rearrangement of Mannich phenoxy structure into Mannich phenolic structure proceeds at higher temperatures. Moreover, the chain propagation process goes smoothly at higher temperatures as well. As a result, the final polybenzoxazine has larger amounts of Mannich phenolic structure with better network development and therefore might possess better properties. Notably, too high polymerization temperature may destroy the inner hydrogen-bonding interaction and lead to polymer degradation. In brief, when conducting the polymerization of benzoxazines in practical applications, we should appropriately choose the polymerization temperature, polymerization time, and catalyst to achieve the best cost/performance ratio.

4.2

Benzoxazine-Epoxy Blends

Despite their excellent properties, polybenzoxazines possess surprisingly low chemical cross-link density in comparison with the ordinary thermosetting resins, and the molecular origin of their unusual properties originate from large amounts of complex hydrogen bonds [33,34] that act as

physical cross-link points. To further improve the processing and properties of polybenzoxazine, it has been reported that copolymerization of benzoxazines with epoxies may improve the cross-link density and inner structure of polybenzoxazines by the reaction of the phenolic hydroxyl groups from the ring opening of benzoxazine monomers with epoxy groups, and therefore enhance thermal and mechanical properties [37]. Investigation of the polymerization behavior of a benzoxazine-epoxy blend suggests that the copolymerization of benzoxazine and epoxy resins occurs at high temperatures after the polymerization reaction of benzoxazines [38]. Thus it is valuable for practical applications to use catalysts to accelerate the copolymerization reaction rate at lower temperatures and reduce the copolymerization time. Notably, the catalysts effective for the benzoxazine polymerization, especially acidic catalysts, commonly show weakened catalytic effects on the benzoxazine-epoxy blend, because they may be deactivated because of their coordination with epoxy groups. During the polymerization of a benzoxazine-epoxy blend in the presence of a catalyst, three types of reactions may occur including benzoxazine homopolymerization, epoxy homopolymerization ,and benzoxazine-epoxy copolymerization (Scheme 9). Different catalysts can be used to selectively polymerize the blend components, to control the polymerization kinetics of the individual components in the benzoxazine-epoxy blend to a degree by choosing the appropriate catalyst. Depending on the catalyst and the blend composition, the inner structure of the resulting polymer can be varied, and a predominantly interpenetrating network of homopolymers or grafted copolymer structure may be generated, which allows the final properties to be tailored as desired [39]. The copolymerization of a bisphenol A-based benzoxazine-epoxy blend in the presence of a thermolatent catalyst (the salts of p-toluenesulfonic acid and amines) was

20

PART I Synthesis and Properties of Benzoxazine Resins

R O

SCHEME 9 Three types of polymerization reactions in a benzoxazine-epoxy blend in the presence of a catalyst.

OH

N Δ or Catalyst

N R

(1)

n

Benzoxazine homopolymerization O (2)

Catalyst CH2R Epoxy homopolymerization R

O

N Δ or Catalyst

(3)

O

n

H H

CH2R

O

OH

m

CH2R

O Epoxy

N

R Benzoxazine-epoxy copolymerization

n

Δ or Catalyst

found to occur more rapidly and led to a resulting polymer with superior mechanical properties than those from benzoxazine-epoxy blend without a catalyst. This was the result of a higher cross-link density [40]. Nevertheless, study of the copolymerization of benzoxazine-epoxy blend in the presence of EMI suggests that EMI-catalyzed homopolymerization of the epoxy occurs at lower temperatures, and the copolymerization of the residual epoxy with benzoxazine occurs at elevated temperatures. On the other hand, during the polymerization process of benzoxazineepoxy blend, the three types of polymerization reactions show different sensitivity to the polymerization temperatures. Their sequences can be changed by different initial polymerization temperatures, which affects the inner structures and final properties of the resulting polymer [41].

5 CONCLUSION Benzoxazines have a slow rate of polymerization compared with other thermosetting materials such as epoxy resins; for many applications, a faster rate is desirable. The effort to develop a variety of catalysts, including acidic and basic catalysts, to efficiently accelerate benzoxazine polymerization rate at lower temperatures and reduced reaction time has been the focus of many studies with different degrees of success. Understanding benzoxazine polymerization mechanism is helpful to know how catalysts influence the development of networks and inner structures of polybenzoxazines. This knowledge can guide the search for efficient catalysts for accelerated benzoxazine polymerization. The catalyst accelerates the benzoxazine polymerization by smoothly initiating a fast ring opening of benzoxazines at the initial stage, and does not affect the later process to the same degree. However, the nature of the catalyst is important in the progress and mechanism of the benzoxazine polymerization, and affects the network architecture and therefore the final inner structure and properties of the polybenzoxazines. There is still further territory to explore in understanding the polymerization mechanism

N R

n

of benzoxazines and how a catalyst influences the development of networks and inner structure of the resulting polymer. This understanding will help to find desired catalysts that can provide polybenzoxazines with good properties at lower temperatures and reduced reaction time.

REFERENCES [1] H. Ishida, T. Agag (Eds.), Handbook of Polybenzoxazine Resins, Elsevier, New York, 2011. [2] W.J. Burke, J.L. Bishop, E.L. Mortensen Glennie, W.N. Bauer Jr., A new aminoalkylation reaction. condensation of phenols with dihydro-1,3-aroxazines, J. Org. Chem. 30 (1965) 3423–3427. [3] A.F. McDonagh, H.E. Smith, Ring-chain tautomerism of derivatives of o-hydroxybenzylamine with aldehydes and ketones. Nuclearmagnetic resonance spectra of immonium ions, J. Org. Chem. 33 (1968) 8–12. [4] G. Riess, M. Schwob, G. Guth, M. Roche, B. Lande, B.M. Culbertson, J.E. McGrath (Eds.), Advances in Polymer Synthesis, Plenum, New York, 1985. [5] Y.X. Wang, H. Ishida, Cationic ring-opening polymerization of benzoxazines, Polymer 40 (1999) 4563–4570. [6] J. Dunkers, H. Ishida, Reaction of benzoxazine-based phenolic resins with strong and weak carboxylic acids and phenols as catalysts, J. Polym. Sci. A Polym. Chem. 37 (1999) 1913–1921. [7] Y.X. Wang, H. Ishida, Synthesis and properties of new thermoplastic polymers from substituted 3,4-dihydro-2H-1,3-benzoxazines, Macromolecules 33 (2000) 2839–2847. [8] A. Sudo, R. Kudoh, H. Nakayama, K. Arima, T. Endo, Selective formation of poly(N, O-acetal) by polymerization of 1,3-benzoxazine and its main chain rearrangement, Macromolecules 41 (2008) 9030–9034. [9] P. Chutayothin, H. Ishida, Cationic ring-opening polymerization of 1,3-benzoxazines: mechanistic study using model compounds, Macromolecules 43 (2010) 4562–4572. [10] F. Kasapoglu, I. Cianga, Y. Yagci, T. Takeichi, Photoinitiated cationic polymerization of monofunctional benzoxazine, J. Polym. Sci., Part A: Polym. Chem 41 (2003). [11] T. Hayakawa, Y. Osanai, K. Niizeki, O. Haba, M. Ueda, The curing reaction of 3-aryl substituted benzoxazine, High Perform. Polym. 12 (2000) 237–246.

Catalytic Accelerated Polymerization of Benzoxazines Chapter 2

[12] C. Liu, D. Shen, R.M. Sebastia´n, J. Marquet, R. Sch€onfeld, Mechanistic studies on ring-opening polymerization of benzoxazines: A mechanistically based catalyst design, Macromolcules 44 (2011) 4616–4622. [13] H. Ishida, D.P. Sanders, Regioselectivity and network structure of difunctional alkyl-Substituted aromatic amine-based polybenzoxazines, Macromolcules 33 (2000) 8149–8157. [14] H. Ishida, D.P. Sanders, Regioselectivity of the ring-opening polymerization of monofunctional alkyl-substituted aromatic amine-based benzoxazines, Polymer 42 (2001) 3115–3125. [15] A. Van, K. Chiou, H. Ishida, Use of renewable resource vanillin for the preparation of benzoxazine resin and reactive monomeric surfactant containing oxazine ring, Polymer 55 (2014) 1443–1451. [16] M.W. Wang, R.J. Jeng, C.H. Lin, Study on the ring-opening polymerization of benzoxazine through multisubstituted polybenzoxazine precursors, Macromolecules 48 (2015) 530–535. [17] Q.C. Ran, D.X. Zhang, R.Q. Zhu, Y. Gu, The structural transformation during polymerization of benzoxazine/FeCl3 and the effect on the thermal stability, Polymer 53 (2012) 4119–4127. [18] H. Ishida, Y. Rodriguez, Curing kinetics of a new benzoxazine-based phenolic resin by differential calorimetry, Polymer 36 (1995) 3151–3158. [19] J. Jang, S. Shin, Cure studies of a benzoxazine-based phenolic resin by isothermal experiment, Polymer J. 27 (1995) 601–606. [20] H. Ishida, Y. Rodriguez, Catalyzing the curing reaction of a new benzoxazine-based phenolic resin, J. Appl. Polym. Sci. 58 (1995) 1751–1760. [21] J.A. Cid, Y.X. Wang, H. Ishida, Cationic polymerization of benzoxazine monomers by boron trifluoride complex, Polym. Polym. Comp. 7 (1999) 409–420. [22] R. Andreu, M. Gàli, V. Ca´diz, G. Lligadas, J.A. Reina, J.C. Ronda, BF3 OEt2 in alcoholic media, an efficient initiator in the cationic polymerization of phenyl-1,3-benzoxazines, J. Polym. Sci. A Polym. Chem. 51 (2013) 5075–5084. [23] A. Sudo, S. Hirayama, T. Endo, Highly efficient catalystsacetylacetonato cmplexes of tansition metals in the 4th period for ring-opening polymerization of 1,3-benzoxazine, J. Polym. Sci. A Polym. Chem. 48 (2010) 479–484. [24] A. Sudo, A. Mori, T. Endo, Promoting effects of urethane derivatives of phenols on the ring-opening polymerization of 1,3-benzoxazines, J. Polym. Sci. A Polym. Chem. 49 (2011) 2183–2190. [25] A. Sudo, H. Yamashita, T. Endo, Ring-opening polymerization of 1,3benzoxazines by p-toluenesulfonates as thermally latent initiators, J. Polym. Sci. A Polym. Chem. 49 (2011) 3631–3636. [26] Y. Wang, H. Ishida, Methyl p-toluenesulfonate-initiated cationic polymerization of benzoxazine resin, Polym. Mat. Sci. Eng., ACS 81 (1999) 114. [27] A.W. Kawaguchi, A. Sudo, T. Endo, Promoting effect of thiophenols on the ring-opeing polymerization of 1,3-benzoxazine, J. Polym. Sci. A Polym. Chem. 52 (2014) 2523–2527. [28] I. Gorodisher, R.J. DeVoe, R.J. Webb, Catalytic opening of lateral benzoxazine rings by thiols, in: H. Ishida, T. Agag (Eds.), Handbook of Benzoxazine Resins, Elsevier, Amsterdam, 2011, pp. 211–234.

21

[29] C. Liu, D. Shen, R.M. Sebastia´n, J. Marquet, R. Sch€onfeld, Catalyst effects on the ring-opening polymerization of 1,3-benzoxazine and on the polymer structure, Polymer 54 (2013) 2873–2878. [30] S. Miura, N. Kano, (Shikoku Chemicals Corp.), Thermosetting resin composition, Jpn. Kokai Tokkyo Koho Patent JP2000178332A, Jun. 27, 2000. [31] Sumitomo Bakelite Co., Ltd., Thermosetting resin composition, Jpn. Kokai Tokkyo Koho Patent JP200086863A, Mar. 28, 2000. [32] I. Hamerton, L.T. McNamara, B.J. Howlin, P.A. Smith, P. Cross, Examining the initiation of the polymerization mechanism and network development in aromatic polybenzoxazines, Macromolecules 46 (2013) 5117–5132. [33] S. Wirasate, S. Dhumrongvaraporn, D.J. Allen, H. Ishida, Molecular origin of unusual physical and mechanical properties in novel phenolic materials based on benzoxazine chemistry, J. Appl. Polym. Sci. 70 (1998) 1299–1306. [34] H.D. Kim, H. Ishida, A study on hydrogen-bonded network structure of polybenzoxazines, J. Phys. Chem. A 106 (2002) 3271–3280. [35] A. Laobuthee, S. Chirachanchai, H. Ishida, K. Tashiro, Asymmetric mono-oxazine: an inevitable product from mannich reaction of benzoxazine dimers, J. Am. Chem. Soc. 123 (2001) 9947–9955. [36] S. Chirachanchai, A. Laobuthee, S. Phongtamrug, Self termination of ring opening reaction of p-substituted phenol-based benzoxazines: an obstructive effect via intramolecular hydrogen bond, J. Heterocycl. Chem. 46 (2009) 714–721. [37] H. Ishida, D.J. Allen, Mechanical characterization of copolymers based on benzoxazine and epoxy, Polymer 37 (1996) 4487–4495. [38] C. Jubsilp, K. Punson, T. Takeichi, S. Rimdusit, Curing kinetics of benzoxazine-epoxy copolymer investigated by non-isothermal differential scanning calorimetry, Polym. Degrad. Stabil 95 (2010) 918–924. [39] T. Endo, A. Sudo, H. Yamashita, J. Nishida, T. Huver, R. Sch€onfeld et al., Copolymerization method, WO2009115488A1, Sep. 24, 2009. [40] H. Kimura, A. Matsumoto, K. Ohtsuka, Studies on new type of phenolic resin—curing reaction of bisphenol-A-based benzoxazine with epoxy resin using latent curing agent and the properties of the cured resin, J. Appl. Polym. Sci. 109 (2008) 1248–1256. [41] H. Wang, P. Zhao, H. Ling, Q. Ran, Y. Gu, The effect of curing cycles on curing reactions and properties of a ternary system based on benzoxazine, epoxy resin, and imidazole, J. App. Polym. Sic. 127 (2013) 2169–2175. [42] M.A. Espinosa, V.C.M. Galia´, Synthesis and characterization of benzoxazine-based phenolic resins: crosslinking study, J. Appl. Polym. Sci. 90 (2003) 470–481. [43] A.A. Gallo, Method for preparing polybenzoxazine, US 6,376,080 B1, Apr. 23, 2002. [44] S. Miura, (Shikoku Chemicals Corp.), Thermosetting resin composition, Jpn. Kokai Tokkyo Koho Patent JP2003082099A, March 19, 2003. [45] I. Gorodisher, (3 M Inovative Properties Company), Polybenzoxazine composition, WO 2011/025652 A1, Mar. 3, 2011. [46] I. Hamerton, B.J. Howlin, P. Mhlanga, W.A.W. Hassan, Using QSPR techniques to predict char yield arising from the thermal degradation of polybenzoxazines, Polym. Degrad. Stab. 98 (2013) 446–452.