The kinetics of the polycondensation of 12-Hydroxystearic acid

The K&etics of the Polycondensation of 12-Hydroxystearic Acid C. E. H. BAWN and M. B. HUGLIN The polycondensation of 12-hydroxystearic acid, catalysed by p-toluenesulphonic acid, obeys simple second order kinetics for about 85 per cent of the reaction. Thereafter a tailing-off in degree of polymerization obtains. The constants K v and ~ in the Staudinger viscosity~molecular weight relationship are derived for poly 12-hydroxystearates.

THE mechanism of condensation polymerization between dibasic acids HOOC.[CH,],,.COOH and dihydric alcohols HO.[CH2],.OH as well as that of hydroxyacids HOOC.[CH2],.OH is well established, although a complete kinetic analysis of the latter type of reaction has not yet been reported. For all such reactions the degree of polymerization varies with time in accord with equation (1). 1/(1 -p)=~/CoKt+ 1

(1)

in which p denotes the extent of reaction, 1 / ( 1 - p ) is the degree of polymerization, K is the specific velocity constant, Co is the initial concentration of functional groups, 7 is the concentration of acid catalyst, and t is the time of reaction. For the reaction between adipic acid and diethylene glycol, Flory 1 has shown, that equation (1) is obeyed except for the initial stage of reaction. More recently Pope and Williams~ have corroborated this on the same system. The purposes of the present study were twofold: first, to reinvestigate the early stages of reaction on a new system and secondly, to utilize a branched hydroxy acid in order to observe any steric effect of the side chain on the kinetic behaviour. 12-Hydroxystearic acid was used as monomer and the reaction, which was performed in the melt to prevent any ring formation, can be represented thus nHO.CH(C~HI~).[CH~]~0.COOH > H--{---O.CH(CtHls).[CH2]~0.COO

},,---OH+ ( n - 1)H20

EXPERIMENTAL

Materials The monomer was crystallized five times from ethanol. Removal of stearic acid was effected by repeated refluxing with petroleum ether in which this impurity is soluble. Three crystallizations from ethylacetate yielded the monomer of m.pt 81 "0°C. The catalyst, p-toluenesulphonic acid of B.D.H. 'micro-analytieal reagent' grade, was used without further purification and was stored as a solution in AR chloroform. 257 P1

C. E. H. BAWN and M. B. HUGLIN APPARATUS

AND

METHOD

3 to 4 g of monomer were weighed out in pellet form for convenience of handling. The volume of catalyst solution necessary for the requisite value of 3' was pipetted into the round-bottomed reaction vessel and the solvent was pumped off. After addition of the monomer the vessel was placed in the thermostat and opened to a vacuum system (10-2mm). Boiling n-butanol, chlorobenzene, bromoform and cyclohexanol afforded temperatures in the thermostat of 116.6, 133-5, 152'5 and 160-5°C respectively. constant to + 0" 1 °. At convenient intervals of time the reaction vessel was isolated from the pumping line and dry air was admitted. The molten polymer was stirred, a sample withdrawn into a tared flask, and quickly chilled to quench the reaction. After the aliquots had attained room temperature in a desiccator, they were weighed, dissolved in chloroform and titrated against alcoholic sodium hydroxide solution, using phenolphthalein as indicator. Where necessary, neutralized ethanol was added to homogenize the solutions during titration. The degree of polymerization was calculated from ~ 1/(1 - p ) = ( w - 18t)/[(S+ 1907) t-Tw] (2) in which w is the weight of sample in grammes, t is the titre in moles of caustic soda, S is the molecular weight of one segment = 282, and 3' =

moles of p-toluenesulphonic acid monohydrate total moles of segments

Kinetics In Figure 1 the degree of polymerization is plotted against time for a fourfold range in 3, at 160"5°C. Equation (1) is obeyed from p = 0 to about p - 8 5 per cent reaction. Thereafter the chain length fails to increase at 11-

lib

-

Figure

/--Variation of degree of polymerization with time at 160-5° : (a) 7=0'02, (b) v=0'015, (c) ~,=0.01, (d) v=0"0075, (e) v=0-005

1

0

I 1

2

3

5

Time

h

258

KINETICS OF THE POLYCONDENSATIONOF 12-HYDROXYSTEARICACID the same uniform rate. Similar behaviour is exhibited in Figure 2 in which the course of the reaction at constant 3' of 0-01 is shown at different temperatures. For any particular constant time of reaction, a plot of the corresponding value of 1 / ( 1 - p ) versus 3' (equation 1) is linear and the direct dependence of rate on catalyst concentration is thus confirmed. In Figures 1 and 2 the commencement of curvature appears to be related to the extent of reaction solely. 3' and T are significant only insofar as they affect the time taken to reach this point. This phenomenon has already been noted by Davies 4 in an acid-catalysed esterification and by Flory ~

B/

11

..,,',/ Figure2--Variationofdegree~

of polymerizationwith time at ~,=0"01: (a) T=160-5°, (b) T=152"5°'

I

e / e / '/ / ~ b Regenerated reaction /(,B, ')-----/~ / ~,,f

(c) = l16T .----13 6 o 3 "5 ° '

1

0

/

/

~

1

2 3 Time

4

h

and Ivanoff 6 in polyesterifications, and was attributed to consumption of the catalyst. Assuming this to be a feasible explanation in this case a test was made by carrying out a reaction at 152-5°C with 7=0"01 until the start of curvature on the 1 / ( l - p ) versus t plot, when the reaction was quenched by chilling. After removal of unused catalyst (vide section on viscosity) and addition of fresh catalyst at the same initial concentration, the regenerated polycondensation proceeded linearly at a rather higher rate and its course was observed up to 95 per cent reaction, corresponding to 1 / (1 - p) of 20 (Figure 2 B). Tables 1 and 2 give the dependence of K 1 on catalyst and temperature. The mean value of K at 160-5" is 6"32 x 10 -3 m o l e - e P sec-L A n overall energy of activation of l l ' 4 5 k c a l m o l e -~ and a pre-exponential factor A = k T / h exp (S/R) of 3-65 x 10 -8 sec -~ are found. VISCOSITY OF P O L Y M E R S O L U T I O N S Polymers of molecular weights M~ were obtained by withdrawing samples at different intervals of time from a reaction at 160-5 °, with C,,. -- 0-0354 mole 1-1. They were dissolved in chloroform, shaken with ice-cold water to remove the catalyst and the chloroform layers were dried with magnesium 259

C . E. H . B A W N

a n d M . B. H U G L I N

Table/--Specific velocity constants K at 160"5°C for different catalyst concentrations C C&t. *

K t =KCoC~t.; C o = 1 0 0 0 / 2 8 2 m o l e l - t ; 102C~t. m o l e 1- t 1-77 2-65 3-54 5"31 7-08

C~t.=

1 0 0 0 ~,/282 m o l e 1 - 1

10aK 1 see-1

1 0 3 K m o l e --° I z s e c - Z

3-75 5-83 8"17 12"1 16"4

5"96 6"20 6"50 6"42 6-53

*Whilst it is convenient to have used 3' as defined in equation (2) for the calculation of 1/(l-p) trom experimentally measured quantities, the catalyst concentration Ccat is in units of mole l -a, where C..at" is siravly I 000-//282. C O and K will also involve these standard units. Flory's work on the negligible departure of the density of molten polyesters from unity when the chain length, and temperature (in this region) are altered justifies the use of the molarities instead of molalities, although the weights and not volumes are actually measured.

Table 2. S p e c i f i c v e l o c i t y c o n s t a n t s K a t c o n s t a n t c a t a l y s t c o n c e n t r a t i o n C o a t . = 0 " 0 3 5 4 m o l e 1- t f o r d i f f e r e n t t e m p e r a t u r e s K 1= K C o C ~ . = K × 1 0 0 0 / 2 8 2 × 0 " 0 3 5 4 104K t sec- t

1 "80

116.6 ° 133"5 152"5 160"5

3.28 6"17 ~17

T

1 0 a K m o l e - 2 12 $eC-Z

1 "44 2"61 4'91 6"50

sulphate. The solvent was removed by gentle heating under vacuum. Viscosities in the concentration range 0"1 to 1-0 g/100 ml solution were measured in chloroform at 25 ° ___0-02°C. The values of Ms were determined then by titration of these solutions against alcoholic sodium hydroxide. Mn=282 x 1/(1 - p ) + 18 For the relatively small range of Mr studied, the viscosities of the solutions, including that of the monomer, follow the relation ['7] = KvM~ + I,, in which o=1, K,=0.41 x 10-4dlg -1 and h = 0 " 0 3 2 d l g -1. These values of Kv and I~ are of the same order as those reported for other polyesters ~-9. NATURE

OF

POLYMER

The following evidence supports the conclusion that the reaction product consists entirely of open-chain polyesters and not ring compounds: (a) the viscosity exponent ~ = 1, which is typical for polyester solutions; (b) absence of any volatile product (e.g. lactone) other than water in the cold trap; (c) determination of the number of hydroxyl and carboxyl groups in a polymer sample by acetylation for the former and direct titration for the latter yielded 0.308 × 10-2 and 0-304 x 10-3 groups per gramme respectively. Further a reaction in the melt favours intermolecular reaction and not intramolecular condensation, which can occur in dilute solution. A lactone of the size required would, in any case, be most unstable. 260

KINETICS OF THE POLYCONDENSATION OF 12-HYDROXYSTEARIC ACID 0-20 "o 0"15

Figure 3--Dependence of

intrinsic viscosity on molecular weight for poly 12hydroxystearates in chloroform at 25°C

~'o.io

005

0

I

I

I

2

I

3

/4 10-3M. The polymers are very viscous colourless compounds, the amorphous nature of which is attributed to the n-hexyl group attached to each segment. T h e random presence of these groups on either side of the polyester backbone chain prevents parallel alignment for crystallinity. Comparable compounds without any branching, e.g. poly 11-hydroxyundecanoates even of low molecular weight have considerable crystallinity. DISCUSSIO,N It seems reasonable to assume that the failure of the kinetics to obey equation (1) in the later stages of reaction is caused by participation of the catalyst in the esterifieation. Since, to obtain convenient reaction times, the values of C~t. were some two to three times greater than those normally employed, the effective concentrations of --SO3H, --CO~H and ---OH become comparable towards the end of reaction. Hence, at this point, the p-toluenesulphonic acid is tantamount to a monofunctional impurity. More fundamentally, the accurate adherence of the kinetics to the equation 1 / ( 1 - p ) = K C o C ~ J + 1 over the early stages of reaction does not accord with the findings of Flory and others. His postulate, that the changes in kinetics are due to the pronounced changes in the characteristics of the medium in this region, is untenable in this particular reaction and cannot be universally true. It is of interest to compare the kinetic data with some published ones. All values of K have been interpolated to 100 °" In this work Kl00O, activation energy E, pre-exponential factor A=kT/hexp(S/R) and entropy of activation S are respectively 7-3 x 10 -4 mole -2 P sec -1, 11"45 keal mole-% 3 6 5 x 10-* sec -1 and - 4 2 - 8 cal mole -1 deg-L Corresponding values for the unbranched system adipic acid-diethylene glycol are 293 × 10-a, 11"15, 97.4 x 10-~ and - 3 6 ' 2 . E is therefore sensibly constant, the decrease in K~o0. for the branched system being reflected in A and may be attributed to the larger negative value of S which increases the free energy of activation.

Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool (Received February 1962) 261

C. E. H. BAWN and M. B. I'-IUGLIN REFERENCES 1 FLORY, P. J. 1. Amer. chem. Soc. 1939, 61, 3334 2 POPE, M. T. and WILLIAMS,R. J. P. J. chem. Soc. 1959, 3579 a FLOR¥, P. ~. J. Amer. chem. Soc. 1940, 62, 1057 DAVmS, M. M. Trans. Faraday Soc. 1937, 33, 331; 1938; 34, 410 FLORY, P. J. J. Amer. chem. Soc. 1940, 62, 2261 IVANOFF, N. Bull. Soc. chim. Fr. 1950, 347 7 FORDYCE, R. and HIBBERT,H. J. Amer. chem. Soc. 1939, 61, 1912 s KRAEMAR,E. O. and VAN NATTA,F. J. J. phys. Chem. 1932, 36, 3175 a BAKER, W. O., FULLER, C. S. and HEISS, J. H. J. Amer. chem. Soc. 1941~ 63, 2142

262