Synthesis of Polyurethanes Using Organocatalysis: A Perspective

Perspective pubs.acs.org/Macromolecules

Synthesis of Polyurethanes Using Organocatalysis: A Perspective Haritz Sardon,*,† Ana Pascual,† David Mecerreyes,†,∥ Daniel Taton,‡ Henri Cramail,‡ and James L. Hedrick§ †

POLYMAT, University of the Basque Country (UPV/EHU), Joxe Mari Korta Center, Avda. Tolosa 72, 20018 Donostia-San Sebastian, Spain ‡ Laboratoire de Chimie des Polymères Organiques (LCPO), UMR 5629-CNRS-Université de Bordeaux − Institut National Polytechnique de Bordeaux, 16 Avenue Pey Berland, 33607 Pessac, France § IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, United States ∥ Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain ABSTRACT: Organocatalysis has become an invaluable tool for polymer synthesis, and its utility has been demonstrated in ring-opening, anionic, zwitterionic, and group-transfer polymerizations. Despite this, the use of organocatalysis in other polymerization reactions such as step-growth polymerizations remains underexplored, relative to more traditional metalbased polymerizations. Recently, the use of organic bases such as guanidines, amidines, N-heterocyclic carbenes, and organic “strong or super-strong” Brønsted acids to catalyze the synthesis of metal-free polyurethanes has shown to be competitive to commercially widely used dibutyltin dilaurate and dibutyltin diacetate catalysts. This Perspective article highlights recent advances in organocatalyst design for isocyanate-based polyurethane synthesis with the aim of comparing the activity and selectivity of each of the new catalytic reactions to each other and the traditional metal-based catalysts. The article also draws attention to new trends in isocyanate-free polyurethane synthesis and the key role that organocatalysis is playing in these innovative polymerization processes.

1. INTRODUCTION Polyurethanes (PUs) constitute one of the most important classes of polymeric materials with uses ranging from highperformance structural applications to foam padding.1 Because of their diverse utility and relatively low cost, these materials account for nearly 5 wt % of total worldwide polymer production and are expected to exceed 18 kilotons annually by 2016.2,3 Based on the original discovery by Bayer, linear PUs are usually prepared by a straightforward route involving the reaction of diols with diisocyanates in the presence of a catalyst.4,5 In the past decade, alternative and expectedly “greener” approaches have been developed for the synthesis of more sustainable PUs,6,7 including but not limited to (1) the utilization of less toxic isocyanate-free approaches, (2) advances on PU platforms based on vegetable8,9 and/or other renewable feedstocks such as CO2,10,11 and (3) the transition from organic solvent-based PUs to water-dispersed PUs to reduce the emission of volatile organic compounds to the atmosphere.12,13 The enormous importance of the catalysts in PU chemistry is well recognized.1,5 Organotin compounds have predominantly filled this catalytic role. They act as Lewis acids toward the polymerization with remarkable catalytic activity.14 Although their activation mechanism is still questioned, recent kinetic data suggest that the mechanism proposed by Bloodworth and Davies is the most likely.15 The catalytic cycle involves the Ncoordination of the isocyanate with the tin alkoxide formed by © XXXX American Chemical Society

alcoholysis of the starting tin compound, followed by transfer of the alkoxide anion onto the coordinated isocyanate to afford the N-stannylurethane, which then undergoes alcoholysis to generate the targeted urethane and tin alkoxide (Scheme 1). However, catalyst removal from PUs is often exceedingly difficult, and its cost is prohibitive, which is an important drawback for most applications.16,17 For instance, residual metal catalyst has been linked to adverse effects in dielectric materials, toxicity, and deleterious side reactions.18 It is important to note that, in many research articles, these drawbacks usually pass unnoticed, yet they should be carefully considered when designing PUs, especially for biomedical applications.19,20 Therefore, significant efforts have been dedicated to the development of tin-free catalysts for the synthesis of PUs that could avoid these adverse effects.6,12,21 This Perspective article intends to give alternative approaches to PUs that are environmentally favorable and avoid organotinbased catalysts. Key aspects related to organocatalyzed PU preparation are summarized using tin-based catalyst as the benchmark. This Perspective will have a particular focus on the open literature of organocatalyzed PUs with the objective of comparing the organic catalysts in terms of efficacy, ease of use, Received: February 23, 2015 Revised: April 17, 2015

A

DOI: 10.1021/acs.macromol.5b00384 Macromolecules XXXX, XXX, XXX−XXX

Perspective

Macromolecules

Scheme 1. Catalysis of the Isocyanate/Alcohol Reaction by Organotin-Based Catalyst According to Bloodworth and Davies (Adapted from Ref 5)

Figure 1. General mechanism of isocyanate/alcohol reaction (a) in the absence ofcatalyst, (b) in the presence of a base or nucleophilic activator, and (c) in the presence of an acid or electrophilic activator.

to the nitrogen forming the urethane bond (Figure 1a). Nevertheless, it is worth noting that, in order to achieve high molecular weight PUs, a catalyst is often required. The catalyst can operate in one of two following ways: activating the alcohol (nucleophilic activation) (Figure 1b) or the isocyanate (electrophilic activation) (Figure 1c). However, an electrophilic mechanism involving the isocyanate activation via hydrogen bonding to the O atom or to the N atom can also occur. Besides organotin catalysts, organic tertiary amines combined with organotin catalysts have been used for many years, in particular for preparing PU foams. However, the interest about

and selectivity along with tin-based catalysts to facilitate comparison. In addition, special attention is paid to the development of “greener” isocyanate-free approaches as a means to replace isocyanate-based PUs.

2. ORGANOCATALYZED STEP-GROWTH POLYADDITION OF DIOLS AND DIISOCYANATES Generally, most PUs are synthesized by a polyaddition reaction of diisocyanates with diols. Under catalyst-free conditions the nucleophilic center of the alcohol adds to the electrophilic carbon of the isocyanate group, followed by hydrogen shuffling B


Perspective

Macromolecules

Figure 2. Conventional organic acids, bases and tin catalysts employed in the isocyanate based PU synthesis. *It should be pointed out that organic bases can behave either as Lewis or Brønsted bases or as bifunctional catalyst, while organic acids can behave either as strong Brønsted acids or as bifunctional catalyst.

organocatalysis came only recently, when organic bases,22 such as N-heterocyclic carbenes (NHCs),23 amidines/guanidines, or organic acids24 or latent organic25 compounds have been reported to effectively catalyze isocyanate-based PU formation. The field of organic catalysis in PU synthesis is expected to grow and could provide an alternative tool to the more traditional metallo-organic catalysts in the near future. The goal of this Perspective article is to provide information about the emerging organocatalysts for the synthesis of PUs. There are many different ways to classify the organocatalysts.16,17,26 In this article we intend to classify them as either nucleophilic and electrophilic activators or organic bases and acids, respectively. Consequently, the activation mechanism will be different and will be discussed throughout the article. Figure 2 shows the most common catalysts used in PU synthesis classified into organic bases, organic acids, and organo-tin catalysts. It has been shown that this reaction depends on both the catalyst and the solvent characteristics, including polarity, hydrogen-bonding ability, and dielectric constant.5,27−29 These factors will not be taken into account here when comparing different organocatalysts, but they should be carefully considered when selecting the reaction conditions. 2.1. Use of Basic Catalysts. 2.1.1. Tertiary Amines. Tertiary amines have been extensively used in urethane forming reactions.1 Among amine catalysts, the most widespread industrial catalyst is 1,4-diazabicyclo[2.2.2]octane (DABCO) (Figure 2). PU foams were originally prepared in a two-step sequence involving prior synthesis of isocyanate-terminated urethane prepolymers followed by the addition of water. This process was then replaced by a one-step process using DABCO combined with tin catalyst that accelerates both the chain growth and carbon dioxide evolution.30 The gelling and

concurrent blowing processes must be kept in proper balance in order to obtain the desired product.31,32 The activation mechanism of DABCO is still unclear.33 According to Schwetlick et al.,34 the urethane formation involves the protonation of the catalyst and the nucleophilic addition of the alcohol to the isocyanate, followed by the proton transfer to the complex (Figure 1b). Nevertheless, other authors have suggested that the complexation of isocyanate with the tertiary amine occurs first, which is followed by the nucleophilic attack of the alcohol.35,36 Recently, Hatanaka evidenced on the basis of density functional theory (DFT) calculations that, due to the strong basicity of DABCO, the mechanism postulated by Schwetlick and co-workers is the dominant pathway in the urethane formation.37 The use of DABCO has been popular because tertiary amines catalyze both isocyanate−hydroxyl and isocyanate−water reactions. Nevertheless, Cramail and co-workers found that, when using DABCO with linear aliphatic isocyanates such as isophorone diisocyanate, the conversion was only slightly enhanced compared to a noncatalyzed system.22 These data suggested that catalysts with increased basicity were required to enhance the catalytic activity of the isocyanate alcohol reaction. 2.1.2. Cyclic Guanidines and Amidines. The guanidines and amidines (1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, MeCN pKa TBDH+ = 26), N-methyl-1,5,7-triazabicyclododecene (MTBD, MeCN pK a MTBDH + = 25.5), and 1,8diazabicyclo[5.4.0]undec-7-ene (DBU, MeCN pKa DBUH+= 24.3)) can behave as Brønsted bases with comparable basicities.38,39 Although they have been extensively employed in ring-opening polymerization (ROP) processes,16,40 their use as catalysts in step-growth polymerizations is still underexploited. In a pioneering work, Landais and co-workers investigated the use of these guanidines in the synthesis of PU C


Perspective

Macromolecules

Scheme 2. Organocatalyzed Synthesis of PU from IPDI and PTMO-650 Using Cyclic (a) and Acyclic Guanidines (b) (Adapted from Ref 22)

According to this mechanism, the order of addition of the different reagents might influence the mechanistic outcome since adducts from the nucleophilic addition of guanidines onto isocyanates are quite stable. Cyclic guanidines thus appear as attractive alternatives to tin-based catalysts for PU synthesis with comparative or even superior reaction rates. Nevertheless, more detailed studies are required to establish the operative reaction mechanism. It may occur through a dual activation as postulated for cyclic guanidines in different polymerization processes.42−44 2.1.3. N-Heterocyclic Carbenes. N-Heterocyclic carbenes (NHCs) represent another class of organic catalysts that have emerged not only in organic chemistry but also, more recently, in metal-free polymer synthesis.45−47 They are neutral divalent species of carbon possessing only six electrons in their valence shell. NHCs exhibit σ-donor properties, leading to either a Brønsted basic (pKa ∼ 15−30 in H2O) or nucleophilic-type catalysis.45,48 As recently reviewed by Taton and co-workers, NHCs have been employed to catalyze chain-growth polymerization and some step-growth polymerizations,49 including PU synthesis from aliphatic diisocyanates and diols. Linear PUs could indeed be obtained under relatively mild conditions (30−50 °C) using a low catalyst loading (1 mol % 1,3-bis(ditertiobutyl)imidazol-2-ylidene), relative to the monomer.23 However, extensive cross-linking was observed when aryl diisocyanates were used due to interchain cyclodimer and cyclotrimer formation. The order of addition of reactants was crucial to achieve PUs. In particular, deactivation of the NHC occurred when the diisocyanate monomer was added before the diol, whereas rapid polymerization was observed by changing the addition order. These results suggested that PUs were generated following a hydrogen-bonding mechanism, where the alcohol was activated by the NHC, before its addition onto the diisocyanate monomer (Figure 4). 2.2. Use of Organic Acids. Alternatively to organic bases, strong and “super-strong” organic acids have been shown to readily catalyze the alcohol−isocyanate reaction.3,24,50 Preliminary investigations by Nordstrom et al.51 reported an organic acid-catalyzed reaction between isocyanates and alcohols at high temperatures (130 °C), but results were not as good as those using tin catalysts. In a recent study by Sardon et al., strong acids were employed to activate the reaction of alcohol with isocyanate, allowing the authors to reach high molecular weight PUs.3 In this work, polymerizations were carried out by dissolving equimolar amounts of hexamethylene diisocyanate (HDI) and poly(ethylene glycol) 1500 (PEG1500) in dichloromethane by addition of different acids. Monomer conversion was negligible after 6 h without any catalyst. In contrast, strong

from equimolar amounts of commercially available isophorone diisocyanate (IPDI) and dried poly(tetramethylene oxide) 650 (PTMO-650).22 The authors found that, under bulk polymerization conditions, the addition of diols to diisocyanates was efficiently catalyzed by cyclic guanidines (Scheme 2a), leading to PUs with a high molecular weight (Mw = 74 000 Da). The authors also investigated acyclic guanidines as catalysts (Scheme 2b). Activation through a general basic catalysis should imply that the reaction rate should increase with the basicity of the catalyst.41 Thus, comparing acyclic guanidine, naphthyl bis guanidine, and cyclic guanidine, N-methyl-1,5,7-triazabicyclododecene, which exhibit similar pKa values (25.4 and 25.43, respectively, in CH3CN),38,39 similar catalytic activities were expected. Nevertheless, cyclic guanidines were found more active than the acyclic analogous, suggesting that another mechanism might operate in the isocyanate−alcohol polymerization process. The so-called nucleophilic mechanism that is depicted in Figure 3 was thus postulated. Adducts issued from

Figure 3. Catalytic cycles for the formation of the urethane linkage using cyclic guanidines as organocatalysts. Adapted from ref 22.

the nucleophilic addition of 1 equiv of the guanidine onto 2 equiv of the isocyanate could be isolated. The latter adducts were found capable of further triggering the step-growth polymerization, thus behaving as latent catalytic systems (section 2.3.1). This mechanism, however, was found less prominent in the case of acyclic guanidines due to their low nucleophilicity, explaining their lower catalytic activity. D


Perspective

Macromolecules

catalyst, and the apparent rate constants, k′, were determined for each acid (where k′ = k[acid]; see Figure 5).3 A loose inverse correlation between the pKa value and the catalytic activity was evidenced: lower pKa values led to higher rate constants. Nevertheless, carboxylic acid-containing acids such as trifluoroacetic acid (TFA; H2O, pKa = 0.018) were found ineffective and gave conversions of 3% or less at 6 h, even though the pKa value was lower than that of diphenyl phosphate. In order to better understand this effect, a comprehensive computational study was carried out to determine possible mechanistic pathways of different acids (triflic acid, MSA pKa = −2, dimethyl phosphate, pKa = 1.1 H2O, and acetic acid, pKa = 4.818) as well as the basic catalyst DBU to facilitate comparison.53,54 DFT calculations indicated that the reaction followed a dual hydrogen-bonding mechanism involving electrophilic activation of the isocyanate via the isocyanate nitrogen, with simultaneous nucleophilic activation of the alcohol. Hence, both the strength of a given acid and the nucleophilicity of its conjugate base played a vital role in the bifunctional catalysis of urethane formation (Figure 6). A simultaneous activation was also reported for the ROP of esters and carbonates.18,55 Finally, the reaction could be performed in both solution or bulk polymerization conditions, which proved suitable for polymerizing sterically hindered secondary alcohols such as polypropylene glycol and isocyanates such as IPDI. 2.3. Additional Opportunities Offered by Organocatalysis in the Step-Growth Polyaddition of Diols and Diisocyanates. 2.3.1. Latent Catalysts. One important opportunity offered by organocatalysis is to develop latent catalysts for PU synthesis. In some applications such as reaction injection molding processes, catalysts which display a delayed action may be required to mediate the polyaddition and then cure the material on demand.56 Nonmetallic catalysts exhibiting a delayed action have thus been reported as a means to replace commonly used, but toxic, mercury derivatives.18 Back in 1997, Noomen published a comprehensive review about the possibilities of guanidinium based salts to solve the pot life− cure rate dilema. 57 These catalysts are generated by neutralization of DBU with an organic carboxylic acid (Figure 7a). The presumed deblocking of the catalyst occurs by decomposition of the salt upon heating at elevated temperatures (typically at 130 °C) with the assumption of reversibility at lower temperatures with a stable carboxylic acid such as benzoic acid. Initial experiments showed that blocked catalysts give a pot life of more than 2 weeks. The best performance was obtained with DBU/benzoic acid salt. Conversely when acids,

Figure 4. Catalytic mechanism for the formation of the urethane linkage using 1,3-bis(ditertiobutyl)imidazol-2-ylidene as organocatalyst. Adapted from ref 23.

acids such as triflic acid (pKa = −1318) or trifluoromethanesulfonimide (pKa = −1852) (Figure 5) were extremely active

Figure 5. Linear plot of 1/(1 − p), where p is the fraction of monomer converted to polymer, as a function of time for the acid-catalyzed polymerization. k′ = k[acid] (units of M−1 s−1) was determined from the slope. Adapted from ref 3.

exhibiting PU conversions exceeding 98% (Figure 5). Importantly, the use of acids gave better results compared to dibutyltin dilaurate and DBU (82% and 86%, respectively) after 6 h. Apart from sulfonic acids, phosphonic acid such as diphenylphosphate (DPP, pKa = 1.118), a relatively weak acid, afforded PUs after 48 h. To more precisely compare catalytic activities, Sardon et al. performed kinetic studies for each

Figure 6. Catalytic mechanism for the formation of the urethane linkage using organic acids as organocatalysts, where RC is the reactant complex, TS is the transition state, and PC is the product complex. Adapted from ref 3. E


Perspective

Macromolecules

Buchmeiser and Naumann have recently reviewed different classes of protected/masked NHCs used in metal-free polymerization reactions.59 Related adducts (or complexes) can release the free carbene catalyst upon increasing the temperature or by a simple solvent effect. Examples of these protected NHCs include imidazol(in)ium carboxylates (NHCCO2; Figure 7c).60,61 NHC-CO2 are of significant interest because they are not active during the premixing of ingredients and formation of the desired metal-free PU formation can be achieved with temperature. In this way, such protected carbenes are practical approaches to toxic mercury derivatives for the latent synthesis of PUs.56 2.3.2. Synthesis of Functional Polyurethanes: Organocatalyst Selection Guide. As far as monomers featuring functional groups are concerned, basic organocatalysts, such as DBU or acidic organocatalysts, such as triflic acid, must be used with caution. Indeed, some functional groups can poison the catalystthus deactivating itaffecting the molecular weight and properties of the resulting PU.62−65 For instance, a pentafluorophenyl ester-containing diol was recently developed for the synthesis of functionalized PUs. It is known that this functionality is sensitive to bases.17,18 It was shown that strong bases such as DBU could react with the pentafluorophenyl ester groups. However, polymerization could be carried out using triflic acid in dichloromethane (Scheme 3a), and after 24 h, high molecular weights were obtained (Mn ranging from 25 to 37 kDa and Đ =1.4). These values were in good agreement with the previously reported values for acidcatalyzed isocyanate−alcohol polymerization. Interestingly, this monomer allows a broad range of functionality to be incorporated into linear PUs using primary amines. This method opens new ways for efficient polymer−protein conjugation to design many therapeutic protein formulations for PUs.66 Xu et al.67 employed a tertiary amine-containing diol, namely, N-methyldiethanolamine, to be polymerized with a diisocyanate in the presence of DBU in dicloromethane (DCM) (Scheme 3b). After 24 h, a high molecular weight PU was obtained, Mn up to 22 kDa and Đ =1.7. These values were consistent with those previously reported for the base-catalyzed isocyanate alcohol polymerization in solution. This polymer was successfully coated onto silica particles and was tested as an antimicrobial polymeric compound, against both Gram-positive

Figure 7. (a) Reversible reaction between the benzoic acid/DBU salt. Adapted from ref 53. (b) Reversible guanidinium based delayed action catalyst. Adapted from ref 25. (c) NHC-based delayed action catalysts. Adapted from ref 56.

that decompose with temperature, such as cyanoacetic acid, was combined with DBU, this process became irreversible, and the protected catalyst had a single use. On the basis of DFT calculations, Fukushima et al. recently found that when benzoic acid is combined with DBU, the adduct that is formed exists as an ion pair of a conjugate acid (DBUH+) and a conjugate base (BA−). The conjugate base serves as the catalyst while the conjugate acid serves as cocatalyst, and the adduct does not decompose even at high temperatures.58 It is worth noting that this technology is implemented, and many DBU/acid salts are commercially available for PU synthesis with different characteristics. More recently, Cramail and co-workers prepared a new class of delayed-action catalysts for the synthesis of PUs (Figure 7b).25 These latent catalysts were prepared in a one-pot process through addition of 1 equiv of the guanidine to 2 equiv of a suitable isocyanate. Briefly, they found that MTBD was able to react with benzylisocyanate producing heterocyclic compound (Figure 7b). The structure was unambiguously assigned by Xray diffraction studies. No activity was noted at room temperature for several hours. Upon heating at 60 °C, polymerization took place, and the molar masses and dispersities achieved were in the same range to those obtained with tin-based catalyst.

Scheme 3. (a) General Synthesis of Functionalized PUs from a Precursor Polymer Bearing Pendant Pentafluorophenyl Esters (Adapted from Ref 24) and (b) Organocatalyzed Synthesis of PUs with Antimicrobial Behavior (Adapted from Ref 63)

F


Perspective

Macromolecules

Figure 8. Most employed isocyanate-free approaches for synthesizing PUs.

8.1).70 This method requires the synthesis of specialized monomers, followed by their reaction with diamines in a stepgrowth polymerization process. Alternatively, PUs could be achieved by means of activated linear dicarbonates and their subsequent reaction with diamines. Similarly, isocyanate-free PUs could be obtained from the reaction of activated linear carbamates with diols (Figure 8.3). Finally, the cationic ringopening polymerization (CROP) of cyclic carbamates can also lead to isocyanate-free PUs (Figure 8.4). Nevertheless, the high stability of the carbamate ring makes challenging the control of the polymerization. In all cases, resorting to organocatalysis is expected to enhance the polymerization kinetics.72 3.1. Step-Growth Polyaddition of Cyclic Dicarbonates and Diamines. The most popular synthetic alternative to synthesize isocyanate-free PUs is by the reaction between dicyclic carbonates and aliphatic diamines. Among cyclic carbonates, 5- and 6-membered dicyclic carbonates monomers have been the most studied.73 The ring-opening reaction with amines generates poly(hydroxyurethane)s (PHUs).7,71,74−78 This reaction is preferred because of its atom economy, though the reactivity is deemed slow and a catalyst is generally required. It should be mentioned that important differences exist in terms of reactivity between 5- and 6-membered cyclic carbonates: 6-membered are ≈3 times more reactive.74 According to the work of Tomita et al.,79,80 6-membered cyclic carbonates react quantitatively with hexylamine at 30 °C over a period of 24 h, in the presence of N-dimethylacetamide, whereas 5-membered cyclic carbonates are converted at much lower rates (e.g., 34%). Hence, model 6-membered ring dicyclic carbonates have been shown to be more appropriate for aminolysis reaction. However, Andrioletti and co-workers71 recently conducted a rational study of the aminolysis of 5-membered ring cyclic carbonates. The authors investigated different organocatalysts, including phosphines, bases, phosphazenes and thioureas together with some inorganic Lewis acids. Interestingly, TBD and the cyclohexylphenylthiourea (TU) were able to catalyze

and Gram-negative bacteria. The authors suggested that these polymer-coated particles hold great potential for a use in water and air purification to prevent bacterial infections.67 2.4. Pros and Cons of Organocatalyzed IsocyanateBased Polyurethane Synthesis. In summary, both organic basic and acidic catalysts have proven effective for the synthesis of PUs. Strong organic acids, including trifluoromethanesulfonimide, triflic acid, and p-toluenesulfonic acid (PTSA), appear more active than strong basic catalysts such as DBU, at room temperature in DCM. However, it is worth noting that strong organic acids may react with humidity traces that may reduce their catalytic activity and some strong bases can react with CO2 as recently shown by Smith et al.68 They studied the urethane formation in a supercritical CO2, and they found that the catalytic activity of DBU and MTBD in supercritical CO2 was reduced compared to reactions in conventional media because of the formation of guanidinium or amidinium alkylcarbonate salts. These results suggest that organocatalyzed polymerization should be performed under a dry and inert atmosphere.69 Importantly, not only the strength of a given acid/base matters in catalyzing urethane formation but also the nucleophilicity of its conjugate acid/base.

3. ISOCYANATE-FREE ORGANOCATALYZED SYNTHESIS OF POLYURETHANES The emerging trend to perform PU synthesis using isocyanatefree routes stems from toxic isocyanate monomers such as methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) and their eventual ban.6,70 A catalyst is also generally required in these isocyanate-free polymerization routes to promote the attack of the nucleophilic part of the monomer (of alcohol or amine type) on the electrophile moiety of the comonomer (carbonate or carbamate).71,72 Figure 8 shows the most relevant isocyanate-free methods for the synthesis of PUs. Most of the works on isocyanate-free PU preparation utilize bifunctional cyclic carbonates as monomer substrates (Figure G


Perspective

Macromolecules

Figure 9. Alternative “greener” approaches to develop more sustainable PUs with the help of organocatalysis.

suggest that when 5-membered ring dicyclic carbonates are used, a strong organic base should be employed. Toward this goal, Henderson and co-workers72 investigated the effect of TBD in the aminolysis reaction of 5-membered ring dicyclic carbonates at room temperature. The authors found that using 10 mol % of TBD they were able to reach Mn up to 54 kDa (T = 30 °C). 3.2. Step-Growth Polycondensation of Linear Activated Dicarbonates and Diamines. Bifunctional linear carbonates are commercially available compounds that represent alternative candidates to access isocyanate-free PUs83 in the presence of organocatalyst. Recently, Sardon et al. reported an efficient and environmentally friendly method for synthesizing PUs in aqueous solution, using linear dicarbonates.84 Their rational design was based on the high reactivity of the pentafluorophenyl carbonate intermediate with amines. First, alcohols were converted into pentafluorophenol activated dicarbonates, followed by the polymerization of the dicarbonates with a diamine. This reaction was carried out in the presence of triethylamine (TEA), that was, a weak organic base facilitating the nucleophilic attack of the amine. The authors showed that high molecular weight PUs (Mn =15−16.5 kDa) could be obtained in aqueous media due to the high reactivity of the employed activated carbonates. The high reactivity allowed the urethane formation reaction before the carbonate decomposition in aqueous media. One drawback of this reaction, however, comes from the use of toxic pentafluorophenol that is produced as a side product; hence, polymers must be processed before the final application. One possibility to avoid the utilization of pentafluorophenol was reported by Pan et al.85 and by Koning and co-workers,86 who used dimethyl carbonate and diphenyl carbonate. In this case, they reacted primary amines with carbonate in the presence of TBD catalyst to achieve polyureas. 3.3. Step-Growth Polycondensation of Linear Activated Carbamates and Diols. Another isocyanate-free route to PUs involves the reaction between dicarbamates with diols. This reaction is less preferred because of the low reactivity of diols compared to amines, and a strong catalyst is generally

the reaction of sterically hindered amines at room temperature. Thus, while benzylamine reacted to a limited extent with 5membered ring carbonate in the absence of catalyst (conversion 28%), the conversion increased to 74% and 94%, in the presence of the TU and TBD, respectively. Therefore, in principle, a proper selection of the organocatalyst (cocatalysts) allows for an effective polymerization of both the 5- and 6membered ring as a route to poly(hydroxyurethane)s. Only a few investigations have dealt with the synthesis of isocyanate-free PUs utilizing 5- and 6-membered cyclic carbonates. For instance, Guillaume and co-workers81 reacted 5-membered ring dicyclic carbonates with 6-hexamethylenediamine, in DCM, for 5 days at 70 °C without any catalyst. The attained molecular weights were in the range of 68 kDa, although long reactions times were required. Recently, Cramail and co-workers found that, under bulk polymerization conditions, reaction times could be considerably reduced without affecting the molecular weights.74,82 The employment of less reactive diamines required higher temperatures (140 °C), and longer reaction times were needed to get high molecular weights (3−13 days). In order to solve these reactivity issues, Cramail and coworkers designed 6-membered ring carbonate dimers.74 They found that lower temperatures, 50 °C, and shorter reactions times, from 30 min to 8 h, were required to achieve PHUs of molecular weights (Mn) around 20 kDa. At high conversion, however, a chemical gel was formed, likely due to the high reactivity of 6-membered ring dicyclic carbonates, making possible the reaction between the carbonate and the alcohol and leading to cross-linking reactions. The stepwise polyaddition of bicyclic carbonates and polyamines thus represents a new synthetic method to PHUs, though more work is needed to get control of the reaction conditions to achieve high molecular weights. When 6membered ring dicyclic carbonates are used, the efficiency of the reaction is dependent on selectivity of the reaction with diamines than with alcohol. Higher selectivity might be achieved using a weak organic base that could enhance the reactivity of amines toward carbonates. These combined studies H


Perspective

Macromolecules

Scheme 4. Ring-Opening Polymerization of Cyclic Carbamates in the Presence of Methyl Trifluoromethanesulfonate Initiator (Adapted from Ref 87)

required. Meier and co-workers87 reported the synthesis of high molecular weight PUs by means of the polycondensation of castor oil derived dimethyl dicarbamates and aliphatic diols using TBD as catalyst. Under bulk conditions, low molecular weights PUs were obtained (1.5−4 kDa), which was explained by the early solidification of the reaction mixture and the limitations in the removal of methanol side product. Nevertheless, PUs with molecular weights up to 25 kDa could be obtained when the polymerization was carried out in semibatch conditions, with sequential addition of TBD and a simultaneous increase of temperature. Similarly, Koning and co-workers86 reported the synthesis of well-defined biobased polyureas using dicarbamates and diamines. They found that using 10 mol % of TBD catalyst, polymers with Mn ranging from 27 to 36 kDa and Đ =1.4−1.8 were obtained. The main limitation of this isocyanate-free approach is related to the synthesis of carbamates. In most of the cases, carbamates are obtained using amines and phosgene or one of its derivative, being the synthesis process as toxic as the described for isocyanate reagents, because of the utilization of phosgene-based components. Therefore, new synthetic developments to more sustainable PUs have thus been proposed in the past decade. Results are promising, but further optimization is required for the potential industrial development.6 3.4. Ring-Opening Polymerization of Cyclic Carbamates. Besides linear carbamates, PUs can also be obtained by ROP of 6- and 7-membered ring carbamates.6 Neffgen and coworkers studied the cationic ring-opening polymerization of trimethylene urethane (tetrahydro-2H-1,3-oxazin-2-one) in the melt.88,89 Use of methyl trifluoromethanesulfonate as the initiator generated poly(trimethylene urethane) in yields of ∼70% (Scheme 4). Unlike other catalyst/initiators, such as secbutyllithium or dibutylmagnesium, the ring-opening catalyzed with methyl triflate gave the targeted poly(trimethylene urethane) with a uniform microstructure. Polymerizations were carried out at high temperatures (100−120 °C). The suggested mechanism involves an active intermediate species with the structure of the 5,6-dihydro-4H-1,3-oxazinium trifluoromethanesulfonate. This assumption was supported by the fact that poly(trimethylene urethane)−TfOMe adduct was isolated and characterized in solution. In the same vein, the authors employed trifluoromethanesulfonic acid as catalyst and boron trifluoride diethyl etherate as initiator.89 In all cases, poly(trimethylene urethane) with a uniform microstructure was obtained. However, these polymerizations were not well-controlledexperimental Mn values differing from theoretical values. In contrast, performing the reaction with 7-membered ring cyclic carbamates, the polymerization showed some characteristics of a “living” process.90 In this case, indeed, the observed experimental Mn values and theoretical ones were similar, and the dispersity of the polymers was 1.9.

3.5. Pros and Cons of Organocatalyzed IsocyanateFree Polyurethanes Routes. Miscellaneous isocyanate-free synthetic approaches to PUs (or PHUs) can be employed via an organocatalytic pathway. The conventional method of producing linear and network PUs is based on reaction of polyols and toxic isocyanates. Isocyanate-free PUs are mainly obtained by a synthetic route involving diols, polyols, carbon dioxide, and diamines, which are more environmentally friendly reagents than isocyanates. Taking into account the depletion of petrochemical resources and consequently the presumable increase of petroleum-based products, isocyanate-free PUs may be a cost-effective sustainable route for PU production. In our opinion, the alternative that uses 5-membered ring dicyclic carbonates and their step-growth polymerization with diamines is the most promising to compete the high demand of PUs.72,74 These materials could be synthesized by the chemical insertion of carbon dioxide into naturally abundant epoxides.91,92 The production of useful chemicals from CO2 is not only economically beneficial but also it is beneficial for the global environment.7 We would like to point out that not only it is important to synthesize polyurethanes using renewable sources, but also to synthesize polymers using sustainable polymerization processes. In this sense, the utilization of water as reaction media is an area of high scientific and industrial interest. In this particular case, the low reactivity of 5membered ring dicyclic carbonates might provide a positive impact in the polymerization process and will allow its polymerization in water. The key factor consists to design a catalyst that allows the reaction between the dicyclic carbonate and the diamine in high yields before the carbonate decomposition in aqueous media. Nevertheless, still much needs to be done to improve the lack of reactivity of these systems comparing to isocyanate-based PUs. The design of cheap and sustainable processes for the molecular structure of cyclic carbonates and their oligomers is essential for that development, and organocatalysis will play a key role in this development and industrial implementation.

4. SUMMARY AND OUTLOOK In this Perspective, the use of organocatalysis in the synthesis of PUs was highlighted. Organocatalysis has been demonstrated in the conventional synthesis of PUs using diols and diisocyanates as well in new isocyanate-free synthetic routes. It was shown that both organic acids and bases promoted the step-growth polyaddition of diols and diisocyanates with comparable reaction times to metal-based catalysts. In addition, the use of organocatalysis in the synthesis of delayed PU formulations as well as in the synthesis of PUs with different functionalities has been demonstrated. Finally, the importance of the organocatalysis to develop more sustainable isocyanate-free PUs was reviewed. I


Perspective

Macromolecules Critically, important highlights for the future of a sustainable polyurethane platform where organocatalysis will be an important pillar include (1) the move from toxic isocyanates to isocyanate free approaches, (2) to replace petroleum-based reagents with biobased or other CO2 based reagents, and/or (3) to make the transition from organic solvent-based to waterbased PUs.

■

and polyurethanes. During her PhD she spent 6 months at IBM Almaden Research Center in San Jose, CA. She joined the group of Prof. Daniel Taton at the Laboratoire de Chimie des Polymères organiques (LCPO) at the University of Bordeaux. Currently, she is working in the design of innovative N-heterocyclic carbenes for diverse polymerization reactions.

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest. Biographies

Prof. David Mecerreyes graduated from the University of Basque Country in 1994 and received his PhD in polymer chemistry in 1998 from the University of Liege. After a 2 year postdoctoral stay at IBMAlmaden Research Center, USA, he joined CIDETEC-IK4 in 2002 where he became the head of the nanotechnology research unit. He was one of the pioneers in the synthesis and utilization of polymeric ionic liquids (PILs) in different applications including electrochemical devices, gas separation membranes, or polymer electrolytes. In January 2011, he became Ikerbasque Research Professor at POLYMATUniversity of the Basque Country. He is currently leading the Innovative Polymers Group. His research concerns the development of innovative materials for energy and environment applications and the utilization of sustainable pathways for the synthesis of polymers using organocatalysis.

Dr. Haritz Sardon received both his B.S. and Ph.D. from the University of Basque Country in 2005 and 2011, respectively. During his PhD he was developing waterborne polyurethanes with superior mechanical and thermal properties. Afterward, he was funded by the Basque Government to carry out a 2 year postdoc at IBM Almaden Research Center in San Jose, CA. He is currently on a Juan De La Cierva funded position with Prof. David Mecerreyes (POLYMAT-University of Basque Country). His research is focused investigating new application of organocatalysis in polymer synthesis (isocyanate- and nonisocyanate-based polyurethanes) and in the design of polyurethanes containing cationic units for CO2 separation applications.

Prof. Daniel Taton received his PhD in polymer chemistry in 1994 at the University Pierre et Marie Curie (Paris VI). He develops his research activities at the Laboratoire de Chimie des Polymères Organiques (LCPO) at the University of Bordeaux. His main contributions are in the field of macromolecular engineering, through the design of various macromolecular architectures, including star-like polymers, dendrimer-like polymers, nanogels, and block copolymers featuring specific blocks (oligosaccharides, polypeptidic, or polymeric ionic liquids). In recent years, he has been focusing on the use of Nheterocyclic carbenes (NHCs) as organocatalysts for polymerization reactions, including group transfer and ring-opening polymerizations as well as step-growth polymerization for the synthesis of polybenzoins or polyurethanes. In view of better handling and recycling NHCs, he

Dr. Ana Pascual earned both the B.S. and the PhD from the University of Basque Country in 2006 and in 2014, respectively. After her B.S. she moved to Indulita transformacion termoplastica y termoestable S.L company where she became the responsable of the injection molding process. In 2009, she came back to the academia and joined the Innovative Polymer Group at POLYMAT-University of Basque Country. During her PhD she was working in the new opportunities for organocatalysis in the synthesis of aliphatic polyesters, polyesters, J


Macromolecules also develops masked catalysts and/or polymer-supported versions that can in situ deliver the catalytic active form.

Perspective

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors gratefully acknowledge European Commission through project SUSPOL-EJD 642671 for financial support, and Haritz Sardon gratefully acknowledges financial support from MINECO through project FDI 16507.

(1) Chattopadhyay, D. K.; Raju, K. V. S. N. Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 2007, 32 (3), 352−418. (2) Engels, H.-W.; Pirkl, H.-G.; Albers, R.; Albach, R. W.; Krause, J.; Hoffmann, A.; Casselmann, H.; Dormish, J. Polyurethanes: Versatile materials and sustainable problem solvers for today’s challenges. Angew. Chem., Int. Ed. 2013, 52 (36), 9422−9441. (3) Sardon, H.; Engler, A. C.; Chan, J. M. W.; García, J. M.; Coady, D. J.; Pascual, A.; Mecerreyes, D.; Jones, G. O.; Rice, J. E.; Horn, H. W.; Hedrick, J. L. Organic acid-catalyzed polyurethane formation via a dual-activated mechanism: Unexpected preference of N-activation over O-activation of isocyanates. J. Am. Chem. Soc. 2013, 135 (43), 16235− 16241. (4) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34 (10), 1068−1133. (5) Delebecq, E.; Pascault, J.-P.; Boutevin, B.; Ganachaud, F. On the versatility of urethane/urea bonds: Reversibility, blocked isocyanate, and non-isocyanate polyurethane. Chem. Rev. 2012, 113 (1), 80−118. (6) Kreye, O.; Mutlu, H.; Meier, M. A. R. Sustainable routes to polyurethane precursors. Green Chem. 2013, 15 (6), 1431−1455. (7) Nohra, B.; Candy, L.; Blanco, J.-F.; Guerin, C.; Raoul, Y.; Mouloungui, Z. From petrochemical polyurethanes to biobased polyhydroxyurethanes. Macromolecules 2013, 46 (10), 3771−3792. (8) Lebarbé, T.; More, A. S.; Sane, P. S.; Grau, E.; Alfos, C.; Cramail, H. Bio-based aliphatic polyurethanes through ADMET polymerization in bulk and green solvent. Macromol. Rapid Commun. 2014, 35 (4), 479−483. (9) More, A. S.; Lebarbé, T.; Maisonneuve, L.; Gadenne, B.; Alfos, C.; Cramail, H. Novel fatty acid based di-isocyanates towards the synthesis of thermoplastic polyurethanes. Eur. Polym. J. 2013, 49 (4), 823−833. (10) von der Assen, N.; Bardow, A. Life cycle assessment of polyols for polyurethane production using CO2 as feedstock: insights from an industrial case study. Green Chem. 2014, 16 (6), 3272−3280. (11) Langanke, J.; Wolf, A.; Hofmann, J.; Bohm, K.; Subhani, M. A.; Muller, T. E.; Leitner, W.; Gurtler, C. Carbon dioxide (CO2) as sustainable feedstock for polyurethane production. Green Chem. 2014, 16 (4), 1865−1870. (12) Sardon, H.; Irusta, L.; Fernández-Berridi, M. J. Synthesis of isophorone diisocyanate (IPDI) based waterborne polyurethanes: Comparison between zirconium and tin catalysts in the polymerization process. Prog. Org. Coat. 2009, 66 (3), 291−295. (13) Sardon, H.; Irusta, L.; Fernández-Berridi, M. J.; Lansalot, M.; Bourgeat-Lami, E. Synthesis of room temperature self-curable waterborne hybrid polyurethanes functionalized with (3aminopropyl)triethoxysilane (APTES). Polymer 2010, 51 (22), 5051−5057. (14) Devendra, R.; Edmonds, N. R.; Söhnel, T. Computational and experimental investigations of the urethane formation mechanism in the presence of organotin(IV) carboxylate catalysts. J. Mol. Catal. A: Chem. 2013, 366 (0), 126−139. (15) Bloodworth, A. J.; Davies, A. G. 975. Organometallic reactions. Part I. The addition of tin alkoxides to isocyanates. J. Chem. Soc. 1965, No. 0, 5238−5244. (16) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M. Organocatalysis: Opportunities and challenges for polymer synthesis. Macromolecules 2010, 43 (5), 2093−2107. (17) Dove, A. P. Organic catalysis for ring-opening polymerization. ACS Macro Lett. 2012, 1 (12), 1409−1412.

Prof. Henri Cramail received his engineering degree from Polytechnic Institute of Bordeaux in 1987 and obtained his PhD from the University of Bordeaux 1 in 1990. After a postdoctoral stay at the University of Durham, U.K., he became an Assistant Professor of Polymer Chemistry at the University of Bordeaux 1. In 1999, he was appointed Professor at the same University. Prof. Cramail is currently the director of the Laboratory of Chemistry of Organic Polymers (LCPO) and he is leading the “Biopolymers and Biobased Polymers” team within LCPO. His research concerns the development of green pathways to biobased polymers from renewable resources. Indeed, he has been working in the development of sustainable polymerization processes for the synthesis of polyurethanes, including the synthesis of polyurethanes using isocyanate-free routes, water-based polymerization processes, or the use of metal-free catalyst methods.

James L. Hedrick received his Ph.D. from James McGrath at Virginia Tech in Material Science and Engineering. He joined IBM Research in 1985 in the Advance Organic Materials Group. James has focused on the synthesis and basic structure−property relationships on synthetic polymers for advance microelectronic and biomedicinal related applications. Areas of emphasis include organocatalytic methods to biocompatible/degradable polymers, functional oligomers, copolymers, and complex architectures. He is the recipient of the ACS, Division of Polymer Chemistry, Carl S. Marvel Award 2003, ACS, Division of Polymer Chemistry, Industrial Sponsors Award 2006, Belgian Polymer Chemistry Award 2008, 2009 Cooperative Research Award in polymer science and engineering with Robert Waymouth of Stanford (ACS PMSE Division), and ACS Fellow in Polymer Division. He has coauthored more than 390 papers and has more than 100 patents issued. K


Perspective

Macromolecules (18) Coady, D. J.; Horn, H. W.; Jones, G. O.; Sardon, H.; Engler, A. C.; Waymouth, R. M.; Rice, J. E.; Yang, Y. Y.; Hedrick, J. L. Polymerizing base sensitive cyclic carbonates using acid catalysis. ACS Macro Lett. 2013, 2 (4), 306−312. (19) Cooke, G. M.; Forsyth, D. S.; Bondy, G. S.; Tachon, R.; Tague, B.; Coady, L. Organotin speciation and tissue distribution in rat dams, fetuses, and neonates following oral administration of tributyltin chloride. J. Toxicol. Environ. Health, Part A 2008, 71 (6), 384−395. (20) Nachtergael, A.; Coulembier, O.; Dubois, P.; Helvenstein, M.; Duez, P.; Blankert, B.; Mespouille, L. Organocatalysis paradigm revisited: Are metal-free catalysts really harmless? Biomacromolecules 2014, 16 (2), 507−514. (21) Blank, W. J.; He, Z. A.; Hessell, E. T. Catalysis of the isocyanatehydroxyl reaction by non-tin catalysts. Prog. Org. Coat. 1999, 35 (1− 4), 19−29. (22) Alsarraf, J.; Ammar, Y. A.; Robert, F.; Cloutet, E.; Cramail, H.; Landais, Y. Cyclic guanidines as efficient organocatalysts for the synthesis of polyurethanes. Macromolecules 2012, 45 (5), 2249−2256. (23) Coutelier, O.; El Ezzi, M.; Destarac, M.; Bonnette, F.; Kato, T.; Baceiredo, A.; Sivasankarapillai, G.; Gnanou, Y.; Taton, D. NHeterocyclic carbene-catalysed synthesis of polyurethanes. Polym. Chem. 2012, 3 (3), 605−608. (24) Sardon, H.; Chan, J. M. W.; Ono, R. J.; Mecerreyes, D.; Hedrick, J. L. Highly tunable polyurethanes: organocatalyzed polyaddition and subsequent post-polymerization modification of pentafluorophenyl ester sidechains. Polym. Chem. 2014, 5 (11), 3547−3550. (25) Alsarraf, J.; Robert, F.; Cramail, H.; Landais, Y. Latent catalysts based on guanidine templates for polyurethane synthesis. Polym. Chem. 2013, 4 (4), 904−907. (26) Fèvre, M.; Vignolle, J.; Gnanou, Y.; Taton, D. 4.06 Organocatalyzed ring-opening polymerizations. In Polymer Science: A Comprehensive Reference; Möller, K. M., Ed.; Elsevier: Amsterdam, 2012; pp 67−115. (27) Chang, M.-C.; Chen, S.-A. Kinetics and mechanism of urethane reactions: Phenyl isocyanate−alcohol systems. J. Polym. Sci., Part A: Polym. Chem. 1987, 25 (9), 2543−2559. (28) Han, Y.; Yang, P.; Li, J.; Qiao, C.; Li, T. The reaction of ohydroxybenzyl alcohol with phenyl isocyanate in polar solvents. React. Kinet., Mech. Catal. 2010, 101 (1), 41−48. (29) Vandenabeele-Trambouze, O.; Mion, L.; Garrelly, L.; Commeyras, A. Reactivity of organic isocyanates with nucleophilic compounds: amines; alcohols; thiols; oximes; and phenols in dilute organic solutions. Adv. Environ. Res. 2001, 6 (1), 45−55. (30) Sonnenschein, M. F.; Wendt, B. L. Design and formulation of soybean oil derived flexible polyurethane foams and their underlying polymer structure/property relationships. Polymer 2013, 54 (10), 2511−2520. (31) Cinelli, P.; Anguillesi, I.; Lazzeri, A. Green synthesis of flexible polyurethane foams from liquefied lignin. Eur. Polym. J. 2013, 49 (6), 1174−1184. (32) Dworakowska, S.; Bogdał, D.; Zaccheria, F.; Ravasio, N. The role of catalysis in the synthesis of polyurethane foams based on renewable raw materials. Catal. Today 2014, 223 (0), 148−156. (33) Silva, A. L.; Bordado, J. C. Recent developments in polyurethane catalysis: Catalytic mechanisms review. Catal. Rev. 2004, 46 (1), 31− 51. (34) Schwetlick, K.; Noack, R.; Stebner, F. Three fundamental mechanisms of base-catalysed reactions of isocyanates with hydrogenacidic compounds. J. Chem. Soc., Perkin Trans. 2. 1994, No. 3, 599− 608. (35) Farkas, A.; Strohm, P. F. Mechanism of amine-catalyzed reaction of isocyanates with hydroxyl compounds. Ind. Eng. Chem. Fundam. 1965, 4 (1), 32−38. (36) Frisch, K. C.; Klempner, D. Advances in Urethane Science and Technology, 12th ed.; Technomic Publishing Company, Inc.: Lancaster, PA, 1992; Vol. 12. (37) Hatanaka, M. DFT analysis of catalytic urethanation. Bull. Chem. Soc. Jpn. 2011, 84 (9), 933−935.

(38) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: Unification of different basicity scales. J. Org. Chem. 2005, 70 (3), 1019−1028. (39) Kaljurand, I.; Rodima, T.; Leito, I.; Koppel, I. A.; Schwesinger, R. Self-consistent spectrophotometric basicity scale in acetonitrile covering the range between pyridine and DBU. J. Org. Chem. 2000, 65 (19), 6202−6208. (40) Mespouille, L.; Coulembier, O.; Kawalec, M.; Dove, A. P.; Dubois, P. Implementation of metal-free ring-opening polymerization in the preparation of aliphatic polycarbonate materials. Prog. Polym. Sci. 2014, 39 (6), 1144−1164. (41) Bacaloglu, R.; Cotarcâ, L.; Marcu, N.; Tölgyi, S. Kinetics and mechanism of isocyanate reactions. II. Reactions of aryl isocyanates with alcohols in the presence of tertiary amines. J. Prakt. Chem. 1988, 330 (4), 530−540. (42) Fu, X.; Tan, C.-H. Mechanistic considerations of guanidinecatalyzed reactions. Chem. Commun. 2011, 47 (29), 8210−8222. (43) Hammar, P.; Ghobril, C.; Antheaume, C.; Wagner, A.; Baati, R.; Himo, F. Theoretical mechanistic study of the TBD-catalyzed intramolecular Aldol reaction of ketoaldehydes. J. Org. Chem. 2010, 75 (14), 4728−4736. (44) Kiesewetter, M. K.; Scholten, M. D.; Kirn, N.; Weber, R. L.; Hedrick, J. L.; Waymouth, R. M. Cyclic guanidine organic catalysts: What is magic about triazabicyclodecene? J. Org. Chem. 2009, 74 (24), 9490−9496. (45) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510 (7506), 485− 496. (46) Enders, D.; Niemeier, O.; Henseler, A. Organocatalysis by Nheterocyclic carbenes. Chem. Rev. 2007, 107 (12), 5606−5655. (47) Naumann, S.; Dove, A. P. N-Heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers. Polym. Chem. 2015, in press. (48) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable carbenes. Chem. Rev. 1999, 100 (1), 39−92. (49) Fevre, M.; Pinaud, J.; Gnanou, Y.; Vignolle, J.; Taton, D. NHeterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis. Chem. Soc. Rev. 2013, 42 (5), 2142−2172. (50) Subrayan, R. P.; Zhang, S.; Jones, F. N.; Swarup, V.; Yezrielev, A. I. Reactions of phenolic ester alcohol with aliphatic isocyanates transcarbamoylation of phenolic to aliphatic urethane: A 13C-NMR study. J. Appl. Polym. Sci. 2000, 77 (10), 2212−2228. (51) Nordstrom, D. J.; Barsotti, R. J.; Stolarski, V. L. In Acid Catalysis of Two Component Urethane Clearcoats for Automotive Applications, Waterborne, High-Solids, and Powder Coatings Symposium New Orleans, LA Feb 5−7, 1997; New Orleans, LA, 1997. (52) Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I. Equilibrium Acidities of Superacids. J. Org. Chem. 2010, 76 (2), 391−395. (53) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-consistent molecular-orbital methods. IX. An extended Gaussian-type basis for molecular-orbital studies of organic molecules. J. Chem. Phys. 1971, 54 (2), 724−728. (54) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-consistent molecular orbital methods. XII. Further extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules. J. Chem. Phys. 1972, 56 (5), 2257−2261. (55) Susperregui, N.; Delcroix, D.; Martin-Vaca, B.; Bourissou, D.; Maron, L. Ring-opening polymerization of ε-caprolactone catalyzed by sulfonic acids: Computational evidence for bifunctional activation. J. Org. Chem. 2010, 75 (19), 6581−6587. (56) Naumann, S.; Buchmeiser, M. R. Latent and delayed action polymerization systems. Macromol. Rapid Commun. 2014, 35 (7), 682−701. L


Perspective

Macromolecules (57) Noomen, A. Applications of Michael addition chemistry in coatings technology. Prog. Org. Coat. 1997, 32 (1−4), 137−142. (58) Fukushima, K.; Coady, D. J.; Jones, G. O.; Almegren, H. A.; Alabdulrahman, A. M.; Alsewailem, F. D.; Horn, H. W.; Rice, J. E.; Hedrick, J. L. Unexpected efficiency of cyclic amidine catalysts in depolymerizing poly(ethylene terephthalate). J. Polym. Sci., Part A: Polym. Chem. 2013, 51 (7), 1606−1611. (59) Naumann, S.; Buchmeiser, M. R. Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo- and polymerization catalysis. Catal. Sci. Technol. 2014, 4 (8), 2466−2479. (60) Bantu, B.; Manohar Pawar, G.; Wurst, K.; Decker, U.; Schmidt, A. M.; Buchmeiser, M. R. CO2, magnesium, aluminum, and zinc adducts of N-heterocyclic carbenes as (latent) catalysts for polyurethane synthesis. Eur. J. Inorg. Chem. 2009, 2009 (13), 1970−1976. (61) Bantu, B.; Pawar, G. M.; Decker, U.; Wurst, K.; Schmidt, A. M.; Buchmeiser, M. R. CO2 and SnII adducts of N-heterocyclic carbenes as delayed-action catalysts for polyurethane synthesis. Chem.Eur. J. 2009, 15 (13), 3103−3109. (62) Engler, A. C.; Chan, J. M. W.; Fukushima, K.; Coady, D. J.; Yang, Y. Y.; Hedrick, J. L. Polycarbonate-based brush polymers with detachable disulfide-linked side chains. ACS Macro Lett. 2013, 2 (4), 332−336. (63) Chan, J. M. W.; Sardon, H.; Engler, A. C.; García, J. M.; Hedrick, J. L. Tetra-n-butylammonium fluoride as an efficient transesterification catalyst for functionalizing cyclic carbonates and aliphatic polycarbonates. ACS Macro Lett. 2013, 2 (10), 860−864. (64) Engler, A. C.; Chan, J. M. W.; Coady, D. J.; O’Brien, J. M.; Sardon, H.; Nelson, A.; Sanders, D. P.; Yang, Y. Y.; Hedrick, J. L. Accessing new materials through polymerization and modification of a polycarbonate with a pendant activated ester. Macromolecules 2013, 46 (4), 1283−1290. (65) Sanders, D. P.; Coady, D. J.; Yasumoto, M.; Fujiwara, M.; Sardon, H.; Hedrick, J. L. Synthesis of functionalized cyclic carbonate monomers using a versatile pentafluorophenyl carbonate intermediate. Polym. Chem. 2014, 5 (2), 327−329. (66) Vanparijs, N.; Maji, S.; Louage, B.; Voorhaar, L.; Laplace, D.; Zhang, Q.; Shi, Y.; Hennink, W. E.; Hoogenboom, R.; De Geest, B. G. Polymer-protein conjugation via a ‘grafting to’ approach - a comparative study of the performance of protein-reactive RAFT chain transfer agents. Polym. Chem. 2015, in press. (67) Xu, Q.; Sardon, H.; Chan, J. M. W.; Hedrick, J. L.; Yang, Y. Y. Polyurethane-coated silica particles with broad-spectrum antibacterial properties. Polym. Chem. 2015, in press. (68) Smith, C. A.; Cramail, H.; Tassaing, T. Insights into the organocatalyzed synthesis of urethanes in supercritical carbon dioxide: An in situ FTIR spectroscopic kinetic study. ChemCatChem 2014, 6 (5), 1380−1391. (69) Kaupmees, K.; Kaljurand, I.; Leito, I. Influence of water content on the acidities in acetonitrile. Quantifying charge delocalization in anions. J. Phys. Chem. A 2010, 114 (43), 11788−11793. (70) Kathalewar, M. S.; Joshi, P. B.; Sabnis, A. S.; Malshe, V. C. Nonisocyanate polyurethanes: From chemistry to applications. RSC Adv. 2013, 3 (13), 4110−4129. (71) Blain, M.; Jean-Gerard, L.; Auvergne, R.; Benazet, D.; Caillol, S.; Andrioletti, B. Rational investigations in the ring opening of cyclic carbonates by amines. Green Chem. 2014, 16 (9), 4286−4291. (72) Lambeth, R. H.; Henderson, T. J. Organocatalytic synthesis of (poly)hydroxyurethanes from cyclic carbonates and amines. Polymer 2013, 54 (21), 5568−5573. (73) Camara, F.; Benyahya, S.; Besse, V.; Boutevin, G.; Auvergne, R.; Boutevin, B.; Caillol, S. Reactivity of secondary amines for the synthesis of non-isocyanate polyurethanes. Eur. Polym. J. 2014, 55 (0), 17−26. (74) Maisonneuve, L.; More, A. S.; Foltran, S.; Alfos, C.; Robert, F.; Landais, Y.; Tassaing, T.; Grau, E.; Cramail, H. Novel green fatty acidbased bis-cyclic carbonates for the synthesis of isocyanate-free poly(hydroxyurethane amide)s. RSC Adv. 2014, 4 (49), 25795−25803.

(75) Maisonneuve, L.; Wirotius, A.-L.; Alfos, C.; Grau, E.; Cramail, H. Fatty acid-based (bis) 6-membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors. Polym. Chem. 2014, 5 (21), 6142−6147. (76) Ochiai, B.; Satoh, Y.; Endo, T. Nucleophilic polyaddition in water based on chemo-selective reaction of cyclic carbonate with amine. Green Chem. 2005, 7 (11), 765−767. (77) Besse, V.; Auvergne, R.; Carlotti, S.; Boutevin, G.; Otazaghine, B.; Caillol, S.; Pascault, J.-P.; Boutevin, B. Synthesis of isosorbide based polyurethanes: An isocyanate free method. React. Funct. Polym. 2013, 73 (3), 588−594. (78) Besse, V.; Foyer, G.; Auvergne, R.; Caillol, S.; Boutevin, B. Access to nonisocyanate poly(thio)urethanes: A comparative study. J. Polym. Sci., Part A: Polym. Chem. 2013, 51 (15), 3284−3296. (79) Tomita, H.; Sanda, F.; Endo, T. Reactivity comparison of fiveand six-membered cyclic carbonates with amines: Basic evaluation for synthesis of poly(hydroxyurethane). J. Polym. Sci., Part A: Polym. Chem. 2001, 39 (1), 162−168. (80) Tomita, H.; Sanda, F.; Endo, T. Polyaddition behavior of bis(five- and six-membered cyclic carbonate)s with diamine. J. Polym. Sci., Part A: Polym. Chem. 2001, 39 (6), 860−867. (81) Helou, M.; Carpentier, J.-F.; Guillaume, S. M. Poly(carbonateurethane): an isocyanate-free procedure from [small alpha],[small omega]-di(cyclic carbonate) telechelic poly(trimethylene carbonate)s. Green Chem. 2011, 13 (2), 266−271. (82) Carre, C.; Bonnet, L.; Averous, L. Original biobased nonisocyanate polyurethanes: solvent- and catalyst-free synthesis, thermal properties and rheological behaviour. RSC Adv. 2014, 4 (96), 54018−54025. (83) Deepa, P.; Jayakannan, M. Solvent-free and nonisocyanate melt transurethane reaction for aliphatic polyurethanes and mechanistic aspects. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (7), 2445−2458. (84) Sardon, H.; Engler, A. C.; Chan, J. M. W.; Coady, D. J.; O’Brien, J. M.; Mecerreyes, D.; Yang, Y. Y.; Hedrick, J. L. Homogeneous isocyanate- and catalyst-free synthesis of polyurethanes in aqueous media. Green Chem. 2013, 15 (5), 1121−1126. (85) Pan, W. C.; Lin, C.-H.; Dai, S. A. High-performance segmented polyurea by transesterification of diphenyl carbonates with aliphatic diamines. J. Polym. Sci., Part A: Polym. Chem. 2014, 52 (19), 2781− 2790. (86) Tang, D.; Mulder, D.-J.; Noordover, B. A. J.; Koning, C. E. Welldefined biobased segmented polyureas synthesis via a TBD-catalyzed isocyanate-free route. Macromol. Rapid Commun. 2011, 32 (17), 1379−1385. (87) Unverferth, M.; Kreye, O.; Prohammer, A.; Meier, M. A. R. Renewable non-isocyanate based thermoplastic polyurethanes via polycondensation of dimethyl carbamate monomers with diols. Macromol. Rapid Commun. 2013, 34 (19), 1569−1574. (88) Neffgen, S.; Keul, H.; Höcker, H. Ring-opening polymerization of cyclic urethanes and ring-closing depolymerization of the respective polyurethanes. Macromol. Rapid Commun. 1996, 17 (6), 373−382. (89) Neffgen, S.; Keul, H.; Höcker, H. Cationic ring-opening polymerization of trimethylene urethane: A mechanistic study. Macromolecules 1997, 30 (5), 1289−1297. (90) Kušan, J.; Keul, H.; Hö cker, H. Cationic ring-opening polymerization of tetramethylene urethane. Macromolecules 2001, 34 (3), 389−395. (91) North, M.; Pasquale, R.; Young, C. Synthesis of cyclic carbonates from epoxides and CO2. Green Chem. 2010, 12 (9), 1514−1539. (92) Blattmann, H.; Fleischer, M.; Baehr, M.; Muelhaupt, R. Isocyanate- and phosgene-free routes to polyfunctional cyclic carbonates and green polyurethanes by fixation of carbon dioxide. Macromol. Rapid Commun. 2014, 35 (14), 1238−1254.

M


Synthesis of Polyurethanes Using Organocatalysis: A Perspective

Recommend Documents