Copolymerization of propylene and chlorotrifluoroethylene. Estimation of monomer reactivity ratios

European Polymer Jou."nal, 1970, Vol. 6. pp. 763-772. Pergamon Press. Printed in England.

COPOLYMERIZATION OF PROPYLENE AND CHLOROTRIFLUOROETHYLENE. ESTIMATION OF MONOMER REACTIVITY RATIOS M. RAGAZZINI, D. CARCANO, M . ~IODENA a n d G. C. SERBOLI Montecatini Edison S.p.A., Centro Ricerche di Bollate, Via S. Pietro, 50--20021 Bollate, Milano, Italy

(Receired 28 July, 1969) Abstract--The results of propylene--chlorotrifluoroethylene copolymerizations, carried out with a boron alkyl catalytic system at temperatures in the range - 4 0 ° to 40 °, are reported. The monomer reactivity ratios of both monomers have been evaluated by a simplified nonlinear least squares method. The probabilistic distribution of monomer units along the copolymer chain has been also calculated. The copolymers obtained show a nearly perfect alternating structure. INTRODUCTION COPOLYMERIZATION o f c h l o r o t r i f l u o r o e t h y l e n e w i t h e t h y l e n e by b o r o n - a l k y l s was d e s c r i b e d by us in p r e v i o u s p a p e r s ; tt) the influences o f p o l y m e r i z a t i o n t e m p e r a t u r e a n d m o n o m e r feed c o m p o s i t i o n o n the s t r u c t u r e a n d the c h a r a c t e r i s t i c s o f the p r o d u c t w e r e i n v e s t i g a t e d . A s a c o n t i n u a t i o n o f o u r studies on fluorine c o n t a i n i n g c o p o l y m e r s , we n o w r e p o r t results c o n c e r n i n g the c o p o l y m e r i z a t i o n o f c h l o r o t r i f l u o r o e t h y l e n e with propylene. Only some information on radiation-induced copolymerization of c h l o r o t r i f l u o r o e t h y l e n e with p r o p y l e n e has been r e p o r t e d in the literature. ~2~ EXPERIMENTAL

Materials. Propylene and chlorotrifluoroethylene were polymerization grade materials obtained from Phillips Petroleum Co. and Allied Chem. Corp. respectively. Propylene was 99" 5 per cent pure with a typical analysis of 1000 ppm ethane, 2000 ppm propane and 2000 ppm Lsobutane. Chlorotrifluoroethylene was 99- 9 per cent pure. Triethylboron (Callery) b.p. 94 ° at 760 mmHg and diethylether (C. Erba RP) were used without further purification. All other materials were reagent grade. Polymerization. The copolymerization runs were carried out in a 100 ml stainless steel autoclave equipped with a thermometer and a metering apparatus for the injection of the catalyst. The reaction vessel was carefully purged with purified oxygen-free nitrogen. Transfer of calculated amounts of propylene and chlorotrifluoroethylene from their storage steel cylinders to the autoclave was accomplished by cooling the autoclave with a dry-ice-acetone bath. The monomer mixture was stirred at least 5 hr at the reaction temperature and, after equilibrium had been reached, a sample was removed and analyzed. The appropriate amount of triethylboron complexed with diethyl ether and activated with oxygen was then injected into the reaction vessel. The polymerization was stopped at very low conversion (usually 5 per cent or less). The polymer, which is soluble in the monomer mixture, was recovered from the autoclave in the form of a clear film by venting off the excess of monomers. It was dissolved in diethyl ether and reprecipitated with methyl alcohol. This treatment was repeated twice and finally the product was dried under vacuum at room temperature for 48 hr. Determination of the molar ratio propylene-chlorotrifluoroethylene (Ma/Mi) in monomer mixture. A sample of liquid monomer mixture at the reaction temperature was removed from the autoclave by means of a stainless steel capillary, tube and analyzed at 50 ° by a Perkin-Elmer chromatograph. A 9 m column with i0 per cent "KeI-F oil No. 3" on "'Teflon 6" was used. Determination of the molar ratio propylene--chlorotrifluoroethylene (mr~m2) in the copolymer. The (mr/m2) ratio was evaluated by checking the carbon content in the copolymer with an F & M, C,H,N analyzer, Mod. 185. Infra-red analyses. A Perkin-Elmer 225 grating double beam i.r. spectrophotometer was used. 763

764

M. RAGAZZIN'I, D. C A R C A N O , M. M O D E N A and G. C. SERBOLI

X-ray analyses. X-ray spectra were recorded with a Philips diffractometer equipped with scintillation counter and pulse height analyzer. Ni-filtered CuKa radiation was used. Some analyses have been performed on a flat plate camera. Molecular weight determination. The number average molecular weight of the copolyrners ,~, was determined by means of a Mechrolab vapour pressure osmometer using chloroform solutions at 37 °.

RESULTS AND DISCUSSION The polymerization products are white powders softening in the range 60~-120 ° and easily soluble in ethers, ketones, and aromatic, aliphatic and halogenated hydrocarbons; their molecular weights (5°) are in the range 4000-10,000. Elemental analysis shows the presence of both monomers in the polymers. The i.r. spectra of our product containing 50 per cent of chlorotrifluoroethylene (a) of atactic polypropylene (b), and of polychlorotrifluoroethylene (c) are reported in Fig. 1: no bands characteristic of these two homopolymers are apparent in the spectrum of (a). Some evidence of crystallinity has been found by X-ray analysis only on stretched films. Copolymerization experiments, carried out at different temperatures, using as reaction medium the fO0~-,

~o~I

Y

°o

c I-

I

i

IGC - -

[

;

1

ao ~ 60-40--

20--

Cb)

f O 0 --

8c~

20~-

o~ "~C.C'O

! 3000

! ZOO0

i ~600 Wovenumber,

~ 1200 crn-'

I 800

! 400

220

FiG. 1. Infra-red spectra of films of: (a) a copolymer, containing 50 mole ~0 of chloromfluoroethylene. obtained at --40 ~, (b) atactic polypropylene, (c) polychlorotrifluoroethytene.

Copolymerization of Propylene and Chlorotrifluoroethylene

765

m o n o m e r mixture (in which the p o l y m e r is soluble up to a c o n c e n t r a t i o n o f a b o u t 5 per cent) are r e p o r t e d in Table 1. TABLE 1. N~OLE ~ / OF PROPYLENE IN MONOMER FEED AND 1~ COPOL~.'.'.'.'MER

T (°C)

-40

No.

M~

m~

T No. CC)

1 2 3 4 5 6 7 8

11.51 19.85 30.86 40-46 60.28 70.61 80.23 90.49

47-00 48.65 49.85 49-65 50.25 51.60 51.98 54-76

0

1 2 3 4 5 6 7 8

Nit

mt

T No. ~:C)

5.85 10.00 10.35 18.18 19.94 53.33 80.09 91.30

44.44 46.17 46.59 47-84 48.45 50.05 53.85 56-69

40

1 2 3 4 5 6 7 8

M~

m~

20-12 20.18 20.76 39.20 60.17 78.57 79.66 80.00

47.63 47.42 48.65 50.25 51-03 56-18 55.12 54.94

The reactivity ratios have been evaluated using the m e t h o d by F i n e m a n a n d Ross ;(s) plots for the - - 4 0 ° experiments are r e p o r t e d in Figs. 2 a n d 3. In o r d e r to o b t a i n a m o r e reliable estimate o f the c o p o l y m e r i z a t i o n p a r a m e t e r s and o f their error limits, the nonlinear least squares m e t h o d p r o p o s e d by Behnken (4) and by Tidwell and M o r t i m e r ~) has been used. A c c o r d i n g to this m e t h o d , the values o f r l a n d r2 which minimize S2(r), i.e. the sum o f the squares o f the differences between the observed a n d c o m p u t e d c o p o l y m e r c o m p o s i t i o n s , are selected as more p r o b a b l e estimates (F~, i2). The j o i n t confidence limits are defined by the set of values r l a n d r_, which satisfy the e q u a t i o n :

S'-(r) --- S2(i) - - 2s 2 ,F(.,..~=2)

(1)

1.8

1.5

ro

x 0.5

0'i

/

-0.2 0

~0

I

i

40

50

FIG. 2. The least squares straight line and the values of (F/f)" (f-- I) vs. data at -40°,

80

F2/ffrom the experimental

766

M. RAGAZZhNI, D. CARCANO, M. MODENA and G. C. SERBOLI

0

I

O

t~ A_

-05

I

20

40

55

Fro. 3. The least squares straight line and the values of ( f - 1)/Fvs.lTF 2 from the experimental data at - - 4 0 ° . where s 2 is an estimate of the experimental error variance that can be obtained from the residual sum of squares, i.e. : s 2 -- $2(:) N--2

(2)

N is the number of experiments and ,F~2. N-2~ is the value of the tabulated F distribution ~° with 2 and N-2 degrees of freedom at the 100.(I --a) probability level, (usually 95 per cent). Substituting Eqn. (2) in Eqrl. (I) gives:

S2(r) =

• (I + 2

(3) N--2

]

In Table 2 values of I + 2 ,F~2. N_2JN-2) are tabulated; it is evident that a set of at least six copolymerizations is needed in order to take advantage of the experimental accuracy. We have devised a simplification of the methods of Behnken and Tidwell in order to perform the calculations with a desk-top computer. The rt and r 2 values for which S2(r) is minimized may be found rather easily by trial and error, using as initial estimates the parameters evaluated from the Fineman and Ross plots. Since the computation of enough values of S2(r) to define the ellipsoidal contour bounding the confidence region in the rlr2 plane is rather tedious, we have chosen to calculate only the four points corresponding to the intersection of the contour with the straight lines parallel to the coordinate axes passing through the minimum value (Fig. 4). The r, and r, values obtained in this way are assumed to be a first estimate of the confidence limits; the rectangular confidence area so defined is compared in Fig. 5

Copolymerization of Propylene and Chlorotrifluoroethylene

OOl8--

767

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F

068 1-70 I'BI

0"76

176

063

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=

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0.02t

r z

FiG. 4. Tabulation of the 10'L S2(r) values computed in order to find the more probable estimates of r~ and r2 and of their approximate 95 per cent confidence limits. Four-decimal values of rt and r2 have not been included in this calculation because they are practically meaningless. T A B L E 2 . VALUES OF 1 "7- 2 :tF~]. t¢-2) FOR a = 0 ' 0 5 AS A FUNCTION OF N

N 3 4 5 6 7 8 9 10 12 15 20 120

1+

2 ~t~U'(2. N--21

N-2 4~ 20 7.3 4.5 3-3 2.7 2.3 2.1 1.8 1.6 1.4 1"2 1"05 1

with the 95 per cent ellipsoidal confidence region. We think that at least the order of magnitude of the error limits is obtained more reliably in this way than by applying the least squares method to the copolymerization equation linearized according to Fineman and Ross. The proposed simplification is increasingly accurate the more the experiments are planned according to the "two point design", because in this case the axes of the ellipsoidal confidence limits will tend to be parallel to the co-ordinate axes. In an ideal

768

M. R A G A Z Z I N I , D. C A R C A N O , M. M O D E N A and G. C. S E R B O L I

0-018

I

r:

O-O22

0027

O'Oll

!

'.

:

!

0.016 ,"z

i

'.

t

0.021

FiG. 5. Ninety-five per cent confidence region based on sum of squares contour (Eqn. 3). This is an almost ideal situation; in other cases, part of the ellipse may lie outside the rectangle. case, the rectangle circumscribing the ellipse will be obtained. The copolymerization parameters and their error limits computed with the Fineman and Ross and the simplified nonlinear least squares method are collected in Table 3. TABLE 3. EVALUATIONo r MONOMER REACTIVITY RATIOS BY SIMPLIFIED NONLIq','EARLEAST SQUARES AND FINEMAN AND ROSS METHODS*

T (°C)

Nonlinear least squares

Fineman and Ross

--40

rt r,

0-022 + 0-005 0-016 _+ 0-005

rt r2

0-0223 4- 0.0007 0"01 4- 0"02

0

rt r2

0.03 + 0.01 0-017 + 0-005

rl r2

--0"030 0-001

40

rt r2

0"06 4- 0"02 0"028 ± 0-015

rt rz

0"06 0"03

Fineman and Ross

rt r,

0-03 4- 0"02 0"017 4- 0"001

4- 0"002 4- 0"07

rt r2

--0"03 4- 0"09 0"016 4- 0"001

4- 0"01 + 0"09

rt rz

0"06 4- 0"05 0"028 4- 0"006

*The error limits of the Fineman and Ross estimates are at 95 percent probability level; they have been obtained by multiplying the standard errors from the linear least squares procedure by the appropriate Student's t value. (t~)

M o n o m e r - p o l y m e r composition curves calculated using the reactivity ratios values estimated by the nonlinear method show that, as in the case of ethylene and chlorotrifluoroethylene, a copolymer very close to I/I molar composition is obtained over a wide range of monomer feeds (Fig. 6). The values of the product rtrz suggest that, on increasing the temperature, the tendency towards alternation decreases, as shown in Fig. 7, where the molar fractions

Copolymerization of Propylene and Chlorotrifluoroethylene

to I -

769

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,,

°

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,

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0

/

/

(c)

i

02

~

04

Mole f r a c t i o n M l ,

!

0.6

l

08

:0

monomer mixture

FIG. 6. Mole fraction of propylene (mx) in copolymer as a function of mole fraction of propylene (M,) in monomer mixture at various temperatures: (a) 40"; (b) 0°; (c) -40°.

of alternating sequences C7) of length n (n = 2, 5, 10) in the copolymer are reported for different compositions of the monomer mixture. The existence of a nearly perfect alternation of monomeric units is confirmed by i.r. spectroscopy. Figure 8 shows the i.r. spectra recorded between 3100 and 2800 cm-1 of some copolymers with varying percentage of propylene, prepared at 0 °, and of an atactic polypropylene. The spectrum of a copolymer containing 45.5 mole ~ of propylene (Fig. 8a) displays, in the above-mentioned spectral region, four bands at 3001, 2897, 2955 and 2855 cm-1,

770

M. RAGAZZINI, D. CARCANO, M. MODENA and G. C. SERBOLI

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/

°:!/ 0.4

ol

~

0-8

.]

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0"4i1~'

o~-/-

[ (c

0

02

04

0"6

t L

r.o Mole fraction M1, monomer mixture FIG. 7. Mole fraction of alternating sequences of various length in copolymer as a function of mole fraction of propylene (M,) in monomer mixture at various temperatures: (a) 40=; Co) 0°; (c) - 4 0 °. 08

(n = 2 m e a n s s u m o f the A B a n d B A a l t e r n a t i o n s ; n = 3, s u m o f the ABA + BAB a l t e r n a t i o n , etc.)

which can be assigned to the antisymmetric and symmetric stretching vibrations respectively o f the methyl and methylene groups in structures of the - - C F 2 - - C F C I - C H 2 - - C H ( C H 3 ) - - - C F z - - C F C I - - (I) type. These bands are shifted to higher frequencies with respect to the corresponding absorptions in polypropylene because of the presence o f a highly electronegative chlorofluorinated or fluorinated structures near the methylene and methyl groups.

Copoiymerization of Propylene and Chlorotrifluoroethylene

771

/ o

1 3,CC

2800

FIG. 8. Infra-red spectra (CCl.,-solutiorts) of: (a) a copolymer containing 45.5 mole ~o of propylene (b) a copolymer containing 55-4 mole % of propylene (c) a copolymer containing 50 mole % of propylene (d) atactic polypropylene. The spectrum of a copolymer containing 55.4 mole 70 of propylene is shown in Fig. 8b. In addition to the above-mentioned bands at 3001, 2897 and 2855 cm -I, two new bands are found at 2960 and 2878 cm -~. Since the absorptions at 2960 and 2878 cm-I do not correspond to those observed in polypropylene (2956, 2915, 2869 and 2839 cm-I, Fig. 8d), they must be due to propylene units linked together, as in this

772

M. RAGAZZINI, D. CARCANO, M. MODENA and G. C. SERBOLI

type o f structure: - - C F z - - C F C I - - C H _ , - - C H ( C H 3 ) - - - C H _ , - - C H ( C H 3 ) . . . . . . CFz - - C F C I - - (II); they are therefore less affected by the n e i g h b o u r i n g c h l o r o f l u o r i n a t e d or fluorinated g o u p than the c o r r e s p o n d i n g b a n d s for the c o p o l y m e r in Fig. 8a. The spectrum o f a c o p o l y m e r c o n t a i n i n g 50 mole O//oo f p r o p y l e n e (Fig. 8c) is very similar to that shown in Fig. 8a a n d suggests that in this c o p o l y m e r there is a nearly regular a l t e r n a t i o n o f - - C H _ , - - C H ( C H 3 ) - - - a n d - - C F _ , - - C F C 1 - - units. In conclusion, the results indicate that the propylene--chlorotrittuoroethylene c o p o l y m e r , like the ethylene--chlorotrifluoroethylene c o p o l y m e r previously studied, shows a m a r k e d tendency t o w a r d s a l t e r n a t i o n o f the c o m o n o m e r units along the chain; but the low tendency to crystallize indicates a high stereochemical irregularity o f the m a c r o m o l e c u l a r chain. Acknowledgements--We wish to acknowledge the contribution of Dr. L. Mariani for the carbon analyses and of Mr. G. Longo for the experimental work. REFERENCES (1) (2) (3) (4) (5) (6)

M. Ragazz.ini et al., Europ. Polym. J. 3, 129, 137 (1967). P. J. Man.no, Nucleonics 22, 64 (1964). M. Fin.eman and S. D. Ross, J. Polym. Sci. 5, 259 (1950). D. W. Behnken, J. Polym. Sci. A2, 645 (1964). P. W. Tidwell and G. A. Mortimer, J. Polym. Sci. A3, 369 (1965). H. Cram~r, The Elements of Probability Theory and Some of its Application. Almqvist & Wiksell, Stockholm (1954). (7) F. D. Price, J. chem. Phys. 36, 209 (1962). R~sumg----On rapporte les r~sultats de copolym~risations propyl~:n.e--chlorotrifluoro&hyl~ne provoqu~es par un. systerne catalytique de bore alcoyl~ dans le domaine de temperature - 4 0 ° ~. 40 °. Les rapports de r~activit6 des deux monomeres ont ~t6 ~valu~s par une m~thode simplifi~, non lin~aire, des moindres carr~s. On a 6galement calcul6 la distribution la plus probable des motifs monom~res le long de la chaine du copolyrn~re. Les copolym~res obtenus pr~sentent une structure alt~rn~e pratiquemerit parfaite. Sommario----Sono riportati i risultati di prove di copolimerizzazione propilen.e--clorotrifluoroetilene eseguite a diverse temperature nell'intervallo --40 ° a 40 ° in presenza di cataliz.zatori a base di boro alchile. I rapporti di reattivit~, dei due monomeri sono stati valutati median.te un calcolo di minimizzazione non lin.eare semplificato. E' stata pure calcolata la distribuzione probabilistica delle unitb. monomeriche lungo la catena del copolimero. I copolimeri prodotti presentano un.a struttura quasi perferttamente alternata. Zusammenfassung--Es wird fiber die Ergenbnisse von Propylen-Chlortrifluor~.thylen Copolymerisationen, durchgef'tihrt mit einem Boralkyl Katalysatorsystem im Temperaturbereich --40 ° zu 40", berichtet. Das Verh~.ltnis der Monomerreaktivit~ten beider Monomerer wurde durch eine vereinfachte nichtlineare Methode der kleinsten Quadrate bestimmt. Die Wahrscheinlichkeiten der Verteilung von Monomereirtheiten entlang der Copolymerkette wurden ebenfalls berechnet. Die erhaltenen Copolymeren zeigen eine nahezu vollkommen alternierende Struktur.