Indian Journal of Chemical Technology Vol. 9, July 2002, pp. 330-333

Articles

Catalytic activity of DBTDL in polyurethane formation

Sobhan Niyogi, Sanjay Sarkar & Basudam Adhikari"

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

Received 30 May 2001 ; revised received 13 February 2002; accepted 14 March 2002

Polyurethane has been prepared from neopentyl glycol (NPG) and toluene diisocyanate (TDI) using dibutyItin dilaurate (DBTDL) catalyst. The rates of both the uncatalyzed and catalyzed polyurethane formation reactions were investigated. In case of catalyzed reaction the sequence of addition of catalyst was also investigated and it was observed that the activity of the catalyst is dependent on the sequence of addition of DBTDL catalyst. Titrimetric method was used for the determination of residual isocyanate content. Highest reaction rate was observed when the catalyst was added after the addition of TDI to diol and lowest rate was observed when no catalyst was used. It was observed that when the catalyst was added to NPG followed by TDI addition the rate of reaction was slower compared to that when catalyst was added to the mixture of diol and TDI. IH NMR and IR study of the complex formation between the tin catalyst and diol was evaluated to account for the lower reaction rate.

The kinetics of urethane fOimation reaction between isocyanate and alcohol has been extensively studiedl.{i. However, comparatively little information has been reported on the polyurethane reaction kinetics7

. 1' . Various amine and metal salt catalysts have been used for the synthesis of polyurethanes8.9.11.14. Smith8 and later on Frisch et af. 15 have shown that the metal catalyst [Sn2+] can form complex with the hydroxyl groups of diol. The formation of a reactive ternary intermediate complex among catalysts, diol and isocyanate in the polyurethane formation reaction was also reported8. Luo et af. 12 have discussed about the better functioning of DBTDL catalyst over the other catalysts in polyurethane formation. With the help of NMR they showed the formation of a ternary complex among catalyst, alcohol and isocyanate. But none has ever investigated the role of sequence of addition of catalyst to the diol followed by diisocyanate addition or the addition of catalyst to a solution containing diol and isocyanate. This paper deals with the role of sequence of addition of DBTDL catalyst to a diol (NPG) then TDI and to a solution containing NPG and TDI. The formation of complex between DBTDL and NPG was also verified separately by IR and I H NMR spectroscopy.

Experimental Procedure

Materials NPG (2,2 dimethyl 1,3 propane diol), dibutyltin

dilaurate (DBTDL), 1,3,5-trichlorobenzene, dibutylamine

' For correspondence: (E-mail : ba@ matsc.iitkgp.ernet.in ; Fax: 091 -03222·55303, 82700)

(Fluka, Switzerland), 2,4-toluene diisocyanate (TDI) (E. Merck, Germany), hydrochloric acid, methanol (E. Merck, India) were used as received and were of AR grade. Tetrahydrofuran (THF) (E. Merck, India) was dried before use following a standard procedure l6

.

IR study IR studies of the DBTDL-NPG complex (prepared

by mixing 1: 1 mole ratio of the two chemicals at room temperature) on a KBr disc were carried out using a Perkin-Elmer model 883 spectrophotometer. For IR study a drop of DBTDL and NPG solution in dry THF was placed on a KBr disc followed by drying the solvent.

NMRstudy For IH NMR study of the complex between

DBTDL-NPG (1: 1 mole ratio, was prepared by dissolving NPG and DBTDL in CDCh at room temperature) and NPG were carried out using a Bruker AC 200 MHz NMR instrument. CDCI) and TMS were used as solvent and internal standard respectively.

Kinetic study Diol and diisocyanate reaction was carried out in

THF solution. At first, NPG (8.5567 mmol) was dissolved in dry THF in a three neck round bottom flask fitted with dry nitrogen purging system and the flask was placed on a magnetic stirrer. Two DBTDL solutions of different concentrations (0.00] 6 and 0.1 moIlL) were prepared in dry THF. For kinetic study

Niyogi et al. : Catalytic activity of DBTDL in polyurethane formation

0.1 mL of catalyst solution was used in each experiment depending on the total weight of the reactants.

Four different sets of kinetic studies were carried out. In all the four sets NPG to TO! molar ratio was 1 :2. In the first set, which is uncatalyzed, TDI was added to the NPG solution in THF, and after a regular time interval percentage of TOI reacted was monitored by titrimetric method 9. 17.

In the second study, 0.1 mL OBTOL solution of 0.0016 mollL concentration was added to the NPG and TOI mixture. Percentage TOI reacted was monitored by the previous method. In the third study a higher catalyst concentration (0.1 mL of 0.1 mollL) was added to the solution of NPG and TDI followed by isocyanate estimation as before. In the fourth kinetic study, 0.1 mL of 0.1 mollL OBTOL catalyst was added to the NPG solution in THF and after 2-3 min of stirring TDI was added. Here also the TOI reacted was determined as before.

The reaction rate constants were calculated9 for a second order reaction using Eq. l

K = 11 (a tv, ) ... (1)

where, K is the reaction rate constant (Llmol-s); a is the concentration of NPG, where the initial concentrations are the same (moIlL) and tv, is the half life in seconds when 50 % of TOI has reacted.

Method for the determination of unreacted isocyanate

From the reaction flask a specified amount of the reaction mixture was taken out at regular time intervals and immediately quenched by adding solution of trichlorobenzene and dibutylamine. This solution was prepared by mixing freshly distilled 12.9 g dibutylamine and 87.1 g trichlorobenzene and was kept in the dark before use. Unreacted amine was back titrated by a standard methanolic hydrochloric acid using bromocresol green as an indicator.

Results and Discussion The results of kinetic studies obtained for the

OBTOL catalyzed reactions as well as uncatalyzed reactions were plotted as % TOI reacted versus time in Fig. 1. It is observed that when the concentration of catalyst is maximum (0.1 mollL) the rate of reaction is highest (curve A in Fig. 1) while the rate is minimum (curve 0 in Fig. 1), when no catalyst is used. One most interesting observation is that when the catalyst concentration is 0.1 mollL, but the

Articles

6o.-____________________________ ~60

NPG + TDI + Catalyst 50 ••• _ •••..•• __ •..•. ~ A

40 40

1 '" 30

~ :!!

20

10

o 20 40 60 Time (min)

80 100

20

10

120

Fig. 1- Influence of catalyst addition sequence on the rate of isocyanate consumption

catalyst is added to NPG solution in THF before the addition of TOI the rate of reaction becomes lower (curve B in Fig. 1). The reason behind this decrease in reaction rate lies in the formation of a complex between NPG and OBOTL in absence of TOI. It is known that OBTOL forms complex with alcoholic -OH group 8.14. 18.19 and the structure may be written as (Structure I):

The formation of this type of complex in solution between NPG and OBTOL at equimolar ratio was confirmed by IR and NMR study. Fig.2 shows the comparative IR spectrum of NPG (curve A in Fig. 2) and NPG-OBTOL complex (curve B). From the spectra it was observed that the -OH peak position in a 1: 1 NPG-OBTOL complex is shifted to higher regiop (3422 cm· l

) from that of the pure NPG (3352 cm· I). Another reason for such shift may be ascribed to the decrease of hydrogen bonding in the NPG due to formation of NPG OBTOL complex. Smith8 and Pestemer et al. 20 also observed the shift of absorption bands after formation of reactant-catalyst complex. While the IH-NMR (Fig.3) study of the NPG-OBTOL (1: 1) complex shows the chemical shift of methylene protons of NPG to downfield (8= 3.490) compared to that of pure NPG methylene protons (8=3.434). This

f4H9 C ?,O" /Q-H2C'c ......... CH3

IlH23C, /~n, J 'CH ° I HG-H2C 3

C4H9 Structure I

331

Articles

A

I

~'\ , , ..... ,

~ 1\ ,

B,J \ , , , ,. , : \ r' II ( ,

"'" ' ',,' " , I

! "i .-.. I '" I - : 'I' '2 , , , . ' ::J , ,'1 I " .ci

, !" \ ... , : ' ,

<: I ~ ,

'-' , I 'I ,

Co I ~ n ' , , " , I ' iii , " "

, h1

.: I " " ,

] , " "

, " " .' ~~ I " "

," :V~m2 '"

" I

'" , ,

~ I~' loj ,

I , , , "

, I II f I, I

, II

I " I l , ~ I \I I " I '" I , , , I

I I, I , 'I

" " I"

II 'I ' ~ I'; : I

3422 ,I ,

" , , ,

"

, I, "

, i' i ~,1 , ' I ~' I' I'

" IJ I

4000 2000 1400 800 400 Wavenumber (em" )

Fig . 2- IR spectra of (A) NPG and (B) NPG-DBTDL Complex

shifting to downfield is due to the more deshielding of methylene protons in a 1: 1 NPG and DBDTL complex. It happens because the hydroxyl oxygen of NPG is involved in coordinate complex formation with the tin in DBTDL by donation of its lone pair of electrons and as a result, the electron density on methylene proton decreases. Therefore, it may be concluded from the IR and NMR studies that when DBTDL is added to NPG solution, a DBTDL-NPG complex is initially formed . But when after few minutes TDI is added this DBTDL-NPG complex breaks and a new complex among DBTDL, TDI and NPG is formed . This complex of DBTDL, TDI and NPG actually initiates the polymerization reaction. Such complex and polyurethane formation has been described in Scheme 1. Prior to the complex

332

Indian J. Chern. Techno!.. July 2002

3.490

(a)

3.434

(b) ~\.

J I

4.0 3.5 3.0 2.5 ppm

Fig. 3 - 'H NMR spectra of (a) NPG- and DBTDL complex and (b) Pure NPG

formation between DBTDL and NPG one carboxylate ion from DBTDL and one H+ ion from NPG are dissociated due to their interactive influence. Dissociation of carboxylate ion from DBTDL was confirmed through NMR analysis by Luo et al. '2 . So when the DBTDL is added to NPG before the TDI addition, all the catalyst molecules remain engaged by forming complex with NPG and thus fail to form complex with TDI immediately and polyurethane formation rate becomes slower. But when catalyst is added after the addition of TDI to NPG solution, DBTDL, TDI and NPG complex is immediately formed and the polymerization proceeds at higher rate. Therefore, the breaking of this NPG-DBTDL complex and formation of complex among NPG, DBTDL and TDI takes some

Niyogi et al. : Catalytic activity of DBTDL in polyurethane formation Articles

Table I-Rate constant of copolyurethane formation reaction at 25°C SI.No Sequence of reactant addition Catalyst conc.x . OH : NCO ty,(s) K X 103

106 (mollL) molar ratio (Umol. s) 1.83 3.41 8.49

20.80

1 2 3 4

NPG + TDI o 1:2 518.7 278.5 111.9 45.6

NPG + TDI + Catalyst NPG + Catalyst + TDI NPG + TDI + Catalyst

3.00 1:2 18.75 1:2 18.75 1:2

CIIH23COO>Sn<C4H9 + HOH2C ....... C ....... CH3 CIIH23COO C4H9 HOH2C""'" ....... CH3

ll-CllH23COO-,-HT

C4H9

~0'-.1...ATH2C'-r."""'CH3 CII H23 ........ Sn, .../'-- ....... CH I HO-H2C 3

Cj:N~~:J

Scheme 1

time which is the reason behind the slower rate of reaction.

The reaction rate constants of all the four reactions were calculated for a second order reaction from Eq. 1 and the results are given in Table 1. It is observed that the rate constants (K) and the half-life (t/l2, at 50 % of isocyanate reaction) are dependent on the DBTDL concentration and the sequence of catalyst addition. The rate constant is highest and t /12 is lowest in case of reaction 4, where DBTDL was added after the addition of TDI to NPG solution. But in case of

reaction 3 though the same concentration of DBTDL catalyst was used the reaction, rate constant was decreased and half-life (t/l2) was increased from those of the reaction 4 as the sequence of catalyst addition was changed. These results also support the formation of complex between DBTDL and NPG as discussed earlier.

Conclusion It has been observed from this investigation that the

sequence of addition of reactants influences the rate of polyurethane formation. If the catalyst is added before the addition of TDI, the whole of catalysts added will not be available for reaction.

References 1 Entelis S G & Nesterov 0 V, Russ Chem Rev, 35 (1966) 917. 2 Frisch K C & Rumao L P, J Macromol Sci Macromol Rev.

C5 (1970) 103. 3 Van der Weji F W, J Polym Sci Polym Chem. 19 (1981) 381. 4 Ono H, Jones F N & Pappas S P, J Polym Sci Polym Chem.

23 (1985) 509. 5 Wong S W & Frisch K C, J Polym Sci Polym Chem. 24

(1986) 2867. 6 Chang M C & Chen S A, J Polym Sci Polym Chem, 25

(1987) 2543. 7 Dyer E, Taylor H A, Mason S J & Sampson J, J Am Chem

Soc, 71 (1949) 4106. 8 Smith H A, J Appl Polym Sci, 7 (1963) 85. 9 Wissman H G, Rand L & Frisch K C, J Appl Polym Sci. 8

(1964) 2971. 10 Lodygina V P, Stoubun E V & Baturin S M, Vysokomol

Soedin Ser A, 27 (1985) 921. 11 Aranguren M I & Wiliams Roberto J J, Polymer, 27 (1986)

425. 12 Luo S G, Tan H M, Zhang J G, Wu Y J, Pei F K & Meng X

H, J Appl Polym Sci, 65 (1997) 1217. 13 Ligabue R A, Monteiro A L, de Souza R F & de Souza M A,

J Molecular Catalysis A: Chemical, 157 (2000) 73. 14 Ligabue R A, Monteiro A L, de Souza R F & de Souza M A,

J Molecular Catalysis A: Chemical, 130 (1998) 101. 15 Frisch K C, Reegan S L & Floutz W V, J Polym Sci Part-Ai,

5 (1967) 35. 16 Perrin D D, Arrnarego W L F & Perrin D R, Purification of

Laboratory Chemicals, 2nd ed. (Pergamon, Oxford), 1980. 17 Hepbem C, Polurethane Elastomkers, (Applied Pub.

Science, London), 1981, 281. 18 Houghton R P & Mulvaney A W, J Organomat Chem, 578

(1996) 21. 19 Hosteltter F & Cox E F,lnd Eng Chem, 52 (1960) 609. 20 Pestemer M & Lauerer D, Angew Chem, 72 (1960) 612.

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