PolymerChemistry

COMMUNICATION

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View Article OnlineView Journal | View Issue

aCNRS/Univ. Bordeaux, ISM, UMR 525

y.landais@ism.u-bordeaux1.fr; Tel: +33 5 4bCNRS/Univ. Bordeaux, LCPO, UMR 5629, F

ipb.fr; Tel: +33 5 40 00 62 54

† Electronic supplementary information905749 and 905750. For ESI and crystalloformat see DOI: 10.1039/c2py21006a

Cite this: Polym. Chem., 2013, 4, 904

Received 20th November 2012Accepted 20th December 2012

DOI: 10.1039/c2py21006a

www.rsc.org/polymers

904 | Polym. Chem., 2013, 4, 904–90

Latent catalysts based on guanidine templates forpolyurethane synthesis†

Jerome Alsarraf,a Frederic Robert,a Henri Cramail*b and Yannick Landais*a

Guanidines react with 2 equivalents of isocyanates, providing bench

stable heterocycles, which exhibit novel delayed-action catalysis for

the synthesis of polyurethanes from both alkyl- and

aryldiisocyanates.

Polyurethanes (PUs) constitute an important class of polymerswith a wide range of applications.1 Such polymers are accessiblefollowing different routes, but the most developed andstraightforward one involves the condensation of poly-isocyanates and polyols, a reaction discovered more than sixtyyears ago by Bayer.2 The addition of an alcohol onto an isocy-anate takes place at room temperature but is generally a slowprocess. Various catalysts are known to accelerate this reaction,including organometallic reagents such as dibutyltin dilaurate(DBTDL),3 but also “greener reagents”, including tertiaryamines such as DABCO and N-alkyl morpholines or amidines(DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene).4 For most applica-tions including preparation of coatings, lms, adhesives orelastomers, and in the case of reaction injection moldingprocess, catalysts which display a delayed action are usuallyrequired to mediate the polyaddition and then cure the mate-rial.5 In this context, phenylmercury neodecanoate (PMND) andanalogues have demonstrated unique ability to increase the“pot life” of the polyol and polyisocyanate mixture and allowdelayed polymerization.6 The use of these latent catalysts hashowever been restricted due to their known propensity toproduce hazardous degradation products, including inorganicand elemental mercury. Efforts have therefore been concen-trated recently on the development of new non-metallic cata-lysts or less toxic metal catalysts and co-catalysts that could

5, F-33400, Talence, France. E-mail:

0 00 22 89

-33600, Pessac, France. E-mail: cramail@

(ESI) available. CCDC 905747, 905748,graphic data in CIF or other electronic

7

replace mercury derivatives.7,8 In this context, we have recentlyintroduced cyclic guanidines as efficient catalysts for the poly-addition of alcohols onto isocyanates.9 The latter are particu-larly powerful so that polymerization reaches completion withina few minutes. During these studies, we discovered thatguanidines react with mono-isocyanates, forming complexheterocyclic systems, which upon thermal activation, regen-erate the guanidine precursor, thus restoring the catalyticactivity.9 We report here the complete studies on the delayedaction of such catalysts during PU synthesis, along with thedistinct behavior of related amidines such as DBN (1,5-dia-zabicyclo[4.3.0]non-5-ene) and DBU.

During mechanistic studies on organocatalyzed synthesis ofurethanes using monofunctional alcohols and isocyanates,guanidine 1a (MTBD) was reacted with benzylisocyanate,producing heterocyclic compound 2a, in which two equivalentsof isocyanate had been incorporated (Scheme 1). The structureof 2a was unambiguously assigned by X-ray diffraction studies.A series of similar heterocycles were next prepared using thesame protocol, leading to 2b,c in high yields. The reaction wasalso efficient with acyclic guanidines10 as shown with thepreparation of 2d from benzyltetramethyl guanidine

Scheme 1 Heterocycles 2a–d and 4a,b.

This journal is ª The Royal Society of Chemistry 2013

Table 1 Synthesis of PU in bulk from stoichiometric amounts of IPDI Ia andPTMO-650 IIa in the presence of organocatalysts 1a, 2a–d, and 4a,b

Entry Catalysta Conv.b (%) Mwc,d,e Mn

c,d,e D ¼ Mw/Mne

1 DBTDL >98 132 84.6 1.562 MTBD 1a >98 69.2 43.7 1.583 2a >98 57.6 36.4 1.584 2b >98 53.6 33.4 1.615 2c >98 61.5 38.3 1.606 2af 98 68.0 43.5 1.567 1af >98 67.6 43.6 1.558g 2a >98 119 77.0 1.559 2d 87 — — —10 4a 70 — — —11 4b 58 — — —12 No Cat. 64 — — —

a 1 mol% of catalyst was used. b Estimated by measuring, using FT-IR,the disappearance of the isocyanate N]C]O band (aer 18 h ofstirring). c Expressed in kg mol�1. d Aliquots of PU were taken aerquenching the reaction mixture with MeOH (aer 18 h of stirring).e Estimated through SEC analysis; DMF as an eluent with PSstandards. f Performed using 0.1 mol% of catalyst. g Performed using1.1 equiv. of IPDI.

Fig. 1 Polymerization of Ia and IIa with latent catalysts 2a–d and 4a,b.

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(BnTMG) 3. Heterocycles 4a,b were also obtained from thecorresponding amidines DBN and DBU respectively.11

Compounds 2a–c as well as 4a,b were obtained as crystallinecompounds, which proved to be stable for months on thebench, and thus, can be manipulated without particular care.Hence, they are much easier to handle than their parentguanidines, which are air-sensitive oily compounds andsomehow difficult to purify.

Preliminary investigations were then carried out to get someinsights into the reactivity of these tricyclic systems. Forinstance, addition of phenethyl alcohol 5 onto 2a in reuxingTHF led to urethane 6 and returned MTBD 1a quantitatively (inthe absence of any other side-products) aer 40 h, according tothe 1H NMR spectrum of the crude mixture, which indicatesthat the formation of 2a is reversible (Scheme 2). In addition,and although the reaction proved to be quite slow, 1 mol% of 2awas shown to catalyze the reaction between BnNCO and alcohol5, emphasizing its potent role as a pre-catalyst for the formationof the urethane linkage.7,12

Extension of these experiments to the synthesis of PU wasperformed by heating an equimolar amount of IPDI (iso-phorone diisocyanate) Ia and PTMO-650 (polytetramethyleneoxide,Mn¼ 650 gmol�1) IIa (Scheme 3) at 60 �C, in the presenceof 1 mol% of 2a. Although the reaction was again slower thanthe reaction performed with pure 1a (ESI†), PU with similarmolar masses and dispersity was obtained (Table 1, entries 2and 3). This series of experiments clearly indicated thatheterocycle 2a could behave as a latent catalyst. Based on theseresults, the ability of 2a–d and 4a,b (at 1 mol%) to catalyze thepolyaddition reaction of IPDI Ia and PTMO-650 IIa was studiedby varying the temperature. Polymerization of an equimolaramount of Ia and IIa in the presence of 1 mol% of the catalystwas carried out at room temperature (20 �C) for 30 minutes,then the reaction mixture was placed in an oil bath with aconstant temperature of 60 �C. Results are gathered in Fig. 1(zooms of the region between 0 and 60 min are provided in theESI†).

Scheme 2 Regeneration of MTBD 1a from 2a in the presence of 5.

Scheme 3 Structures of diisocyanate and diol monomers.

This journal is ª The Royal Society of Chemistry 2013

DBTDL and MTBD 1a were used as a reference. From theseresults, it is clear that both catalysts do not exhibit delayedaction, catalyzing the process even at room temperature (20 �C),the conversion of the isocyanate being complete aer 1.5 h. Incontrast, 2a–c were shown to display delayed behavior, with nocatalysis at 20 �C and a rapid rate increase at 60 �C. Lowering theamount of latent catalyst 2a (0.1%) led, as expected, to lowerconversion aer 3 h. Heterocycle 2d derived from BnTMGshowed a low catalytic activity which reects that of the freeBnTMG evaluated previously for the same reaction.9 Finally,4a,b were shown to be poor catalysts, even at 60 �C. PU char-acteristics with respect to molar mass and molar mass distri-bution obtained from SEC analysis are given in Table 1. Themolar mass values provided by SEC should not be taken asabsolute values as the SEC calibration was carried out usingpolystyrene standards. Nevertheless, these data indicate thatpolymerization in the presence of guanidines and latent cata-lysts provides polyurethanes with reasonable molar masses. Thedispersity values (Mw/Mn), in the range 1.55–1.60, attest to theabsence of any side reactions such as cross-linking via theformation of allophanates and isocyanurates. It is worth

Polym. Chem., 2013, 4, 904–907 | 905

Scheme 4 C–N bond lengths in latent catalysts 2a and 4a,b and X-ray structuresof 2a and 4a (hydrogens have been omitted for clarity).

Fig. 2 Delayed catalysis during PU synthesis from Ia–c and IIa,b.

Polymer Chemistry Communication

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noticing that molar masses obtained using MTBD 1a (entry 2)and the corresponding delayed action catalysts 2a–c (entries3–5) were closely related.

The catalytic activity of 2a was then tested in the synthesis offour different polyurethanes (Scheme 3 and Fig. 2). Catalyst 2ashowed as above efficient delayed-action catalysis, with nopolymerization at 20 �C for Ia,b, and a rate acceleration at 60 �C,including a short induction period as indicated by the smallinexion of the curve as soon as the temperature reached 60 �C.More reactive aromatic isocyanate Ic (TDI: 2,4-toluene diiso-cyanate) was shown to react slowly at 20 �C, complete conver-sion being attained in less than 1 h at 60 �C with only 0.1 mol%of 2a. In the other cases, the conversion of the isocyanatefunctions was complete aer 4 to 6 h. These experimentsdemonstrate that these latent catalysts exhibit a general reac-tivity, whatever the nature of the selected monomers. They arealso selective as shown by the synthesis of PU free of cross-linking (1H NMR analysis). PU characteristics, including molarmasses and molar mass distribution obtained from SEC anal-ysis, are given in Table 2.

Although the mechanism of the PU synthesis catalyzed byguanidines 1a and latent catalysts 2a–d has not yet beenestablished, our studies provide useful indications concerningthe delayed action of 2a–d. For instance, the almost quantitativeyield of urethane 6 and recovered MTBD 1a obtained duringreactions between 2a and alcohol 5 (in a 1 : 2 molar ratio)

Table 2 Polyaddition of Ia–c and IIa,b using organocatalyst 2a

Entry Monomers Catalysta Conv.b (%) Mwc,d,e Mn

c,d,e D ¼ Mw/Mne

1 Ia + IIb 2a >98 84.4 52.2 1.622f Ia + IIb 2ag >98 223 143 1.563 Ia + IIb No cat. 76 — — —4 Ib + IIb 2a >98 31.4 17.3 1.825 Ib + IIb No cat. 90 — — —6 Ib + IIa 2a >98 54.1 35.8 1.517 Ib + IIa No cat. 39 — — —8f Ic + IIa 2ag >98 63.0 34.7 1.819 Ic + IIa No cat. 96 52.8 36.7 1.44

a 1 mol% of catalyst was used. b Estimated by measuring, using FT-IR,the disappearance of the isocyanate N]C]O band (aer 18 h ofstirring). c Expressed in kg mol�1. d Aliquots of PU were taken aerquenching the reaction mixture with MeOH (aer 18 h of stirring).e Estimated through SEC analysis; DMF as an eluent with PSstandards. f Performed using 0.1 mol% of catalyst. g Performed using1.05 equiv. of IPDI.

906 | Polym. Chem., 2013, 4, 904–907

indicates that alcohol functions react with the isocyanuratefunctional group of 2a, to form urethanes, regenerating theguanidine precursor. One can clearly see in Fig. 1 and 2 aninduction period for catalysts 2a–c, indicating that such areaction might also occur with polyols used in PU synthesis.However, the curves of the latent catalysts 2a,b do not matchthat of the free guanidine 1a. This lower rate, as compared tothe case where 1a is used directly, may be due to a lowerconcentration in free guanidine 1a due to a slow release of thelatter in the medium.

Heterocycles 4a,b prepared from DBU and DBN behave aspoor catalytic systems, while DBU alone is as efficient as MTBD1a for PU synthesis.4,9 Such compounds do not decomposewhen heated in the presence of phenethyl alcohol 5, supportingthe hypothesis that the actual catalysts are not heterocycles2a–d but the parent guanidines. Finally, a careful examinationof the X-ray data of 2a revealed that the four C–N bonds at thequaternary center have very different lengths (Scheme 4). Thelongest and thus weakest bond is that between the quaternarycenter and nitrogen noted 4, suggesting the easy C–N4 bondbreaking and the formation of a zwitterionic species A in polarmedium.

The shortest bond lengths are those between the quaternarycenter and nitrogens 1 and 2, guring out the p-system of thisintermediate. The C–N4 bond cleavage likely initiates thedecomposition of 2a during reactions with alcohols. Incomparison, the same C–N4 bond in 4a,b is signicantly shorterand thus stronger, explaining the stability of these compoundsunder similar reaction conditions and consequently their poorcatalytic activities.

Conclusions

A new class of delayed-action catalysts for the synthesis ofpolyurethanes is described. These compounds are easilyprepared from readily available guanidines in a one-pot processthrough addition of the latter to two equivalents of a suitableisocyanate. 2a–c are crystalline compounds that are stable formonths at room temperature, and easier to handle than theparent guanidines. Such catalysts advantageously replace tin

This journal is ª The Royal Society of Chemistry 2013

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catalysts, which are potentially hazardous and exhibit nodelayed-action activity. As environmental concerns seriouslyrestrict the use of heavy metals such as tin and mercury, thepresent heterocycles hold a special interest in specic applica-tions where “life pot” of the mixture polyisocyanate/polyol hasto be increased signicantly. Such catalysts displayed no activityat room temperature for several hours. The polymer materialobtained with these catalysts shows molar masses and dis-persities in the same range than those obtained with tin,guanidines or other amine catalysts. Heterocycles 2a–c displaycatalytic activities with loading as low as 0.1 mol%. They alsoexhibit excellent selectivities with both aliphatic and aromaticisocyanates, producing PU without traces of cyclodimers andcyclotrimers, a limitation reported with other organo-catalysts,13,14 including N-heterocyclic carbenes (NHC), alsoused as latent catalysts.7,8 Finally, preliminary mechanisticalstudies indicate that this latent catalysis proceeds through thedecomposition of the heterocycles, with regeneration of theparent guanidine, also shown to be a very efficient catalyst forPU synthesis.

We are grateful to the “Agence Nationale de la Recherche”(ANR-09-CP2D-15) for generous support.

Notes and references

1 (a) G. Oertel, Polyurethane Handbook, Hanser Publishers,Munich, 1985; (b) Z. Wirpska, Poly(urethane)s: Chemistry,Technology, and Application, Ellis Horwood, London, 1993;(c) PlasticsEurope, Plastics – the Facts 2010. An analysis ofEuropean plastics production, demand and recovery for2009, 2010.

2 (a) O. Bayer, Angew. Chem., 1947, 59, 257; (b) O. Bayer andE. Muller, Angew. Chem., 1960, 72, 934.

3 (a) S.-G. Luo, H.-M. Tan, J.-G. Zhang, Y.-J. Wu, F.-K. Pei andX.-H. Meng, J. Appl. Polym. Sci., 1997, 65, 1217; (b)

This journal is ª The Royal Society of Chemistry 2013

K. K. Majumdar, A. Kundu, I. Das and S. Roy, Appl.Organomet. Chem., 2000, 14, 79.

4 A. L. Silva and J. C. Bordado, Catal. Rev. Sci. Eng., 2004, 46,31.

5 (a) I. S. Bechara, P. J. Zaluska and R. L. Mascioli, US 4086213A 19780425, 1978; (b) S. R. Burks, PCT Int. Appl., WO2011094244 A1 20110804, 2011; (c) J. W. Rosthauser,H. Nefzger, R. L. Cline and G. C. Erhart, US 6140381 A20001031, 2000.

6 (a) F. W. Abbate and H. Ulrich, J. Appl. Polym. Sci., 1969, 13,1929; (b) J. Robins, US 3583945 A 19710608, 1971.

7 For latent catalysts from N-heterocyclic carbene species inPU synthesis, see: (a) B. Bantu, G. M. Pawar, U. Decker,K. Wurst, A. M. Schmidt and M. R. Buchmeiser, Chem.–Eur. J., 2009, 15, 3103; (b) B. Bantu, G. Manohar Pawar,K. Wurst, U. Decker, A. M. Schmidt and M. R. Buchmeiser,Eur. J. Inorg. Chem., 2009, 1970.

8 O. Coutelier, M. El Ezzi, M. Destarac, F. Bonnette, T. Kato,A. Baceireido, G. Sivasankarapillai, Y. Gnanou andD. Taton, Polym. Chem., 2012, 3, 605.

9 J. Alsarraf, Y. A. Ammar, F. Robert, E. Cloutet, H. Cramail andY. Landais, Macromolecules, 2012, 45, 2249.

10 R. Richter, Tetrahedron Lett., 1968, 9, 5037.11 V. P. Arya and S. P. Shenoy, Indian J. Chem., Sect. B: Org.

Chem. Incl. Med. Chem., 1976, 14, 763.12 For related latent catalysts from N-heterocyclic carbenes in

ring-opening-polymerization of lactones, see: W. Jeong,J. L. Hedrick and R. M. Waymouth, J. Am. Chem. Soc., 2007,129, 8414.

13 H. A. Duong, M. J. Crossand and J. Louie, Org. Lett., 2004, 6,4679.

14 In the presence of NHC, arylisocyanates lead to a largeamount of cyclodimer and cyclotrimer, while only traces ofthese compounds are detected when alkylisocyanates areused (ref. 8).

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