Concerning the evaluation of the stereospecificity of propylene polymerization catalysts

CONCERNING THE EVALUATION OF THE STEREOSPECIFICITY OF PROPYLENE POLYMERIZATION CATALYSTS* (A note in connection with A. P. Firsov, B. G. Kashporov, Yu. V. Kissin and N. M. Chirkov's letter to the Editor) G. NATTA, P. PINO and G. MAZZANTI Instiluto for Industrial Chemistry, Milan Polytochnic Instituto Instituto for Industrial Organic Chomistry, Pisa Univorsity Potrochemicals Division, Montocatini. Scientific Rosoarch Laboratory, Milan (Received 3 February 1964)

IN TIIEI P~letter to the Editor of the Journal of Polymer Science, Firsov, Kashporov and Chirkov on the basis of certain experimental data assert t h a t the conclusion of Natta and his coworkers with respect to the dependence of the stercoregularity of polypropylene on the nature of the metal in the Me(C2Hs)3 compound used is mistaken [1]. Moreover, the authors assert t h a t N a t t a and his coworkers are mistaken in proposing on the basis of data about the solubility of polypropylene in n-hel)tane t h a t "the fractionation of this polymer is determined only by the content of stereoregular and atactic structures in it". With regard to the first assertion, an approximate assessment of the stereospe¢,ificity of a catalytic system in the polymerization of propylene m a y be made at least by two different methods: 1) from the total amount of the crystalline part in the polymer sample, without considering the spatial structure of each macromolecule as a whole (that is, without finding out whether it is isotactic or hlock-stereoregular); 2) from the concentration of isotactic macromolccules [2]. In the first case, the total degree of crystallinity of the polymer, determined by X-ray diffraction [3], m a y serve as a crude assessment of the stereospccificity of the catalyst system. In the second case for t)olypropylene having an intrinsic viscosity [t/] in tetralin at 135°C, exceeding 0.25, the stercospecificity of the catalyst is usually determined from the content of the fraction insoluble in boiling heptane. This method is used in practice in so far as it. is more sig,.fificant in comparison with other methods in assessing the properties of the catalyst. Up to the l)resent time, solvent extraction is in fact the only method of reliable quantitative separatio,l of an isotactic polymer of high molecular w(qght from a hlock-stere(~regular polymer [4]. *Vysokomol. soyed. 6: No. 8, 1534-1536, 1964. 1703

1704

G. NATTA et al.

In the case of polymers of propylene of very low molecular weight, it is impossible to use heptane extraction because the isotactic sample of very low molecular weight has a high solubility and a lower melting temperature [5, 6]. No matter how each criterion is used, it should be stated that the views of Natta and his coworkers about the differing stereospecificity of the catalyst, obtained on the basis of Be (C2Hs) 2 and Al (C2H5)a in fact evidently are not refuted by the experimental data of Chirkov et al. (Table 1). TABLE 1. FIRSOV et al. [1] DATA 0N TIIE FRACTIONATION OF I'OLYPROPYLENE SAMPLES SYNTHESIZED IN THE PRESENCE OF a-TiCI,-Be(C~H6)OR AND a-TICI,-A1 (C2Hs)3 CATALYSTS

F r a c t i o n , a n d its c o n c o n t r a t i o n in samplo, %

Mo(C~Hs)n I

Bo(C,Hs) ~ AI(C,Hs) 3 Ditto

,,

[ Insolublo

in

hoptano, 91-98 ;i D i t t o 7-80 ' Solublo in boiling, b u t insolublo ; in coht h o p t a n o , 8--12

i Dogroo o f Dogroo of ' e r y s t a l l i n i t y crystallinity from I from X-ray[ diffraction iniYarod photographs, spoctra, % I %

'. ',

Average dogre(~ of

erystallinity for e n t i r o [ sample, %

boiling

I. Solublo in eohl h o p t a n e ,

8-13

60.5-69.5 I 57--62

-54-57

61.6 46.7

55

54'8 5"5 2-6

i i

55 18-32

I

Moreover, we suggest that the comparison made by Chirkov et al. 011 the stereospecificity of the Zn(CzHs)2-TiCI3 catalyst with one of the other two catalysts systems is not fully satisfactory. In fact, in the experiments of Firsov et al, the polymerization conditions for the system TiCI3-Zn(C2Hs) 2 differ substantially from the conditions selected by them in the case of the systems TiC13AI(C2Hs) a and TiCI3-Be(C2Hs) 2 (Table 2). In conformity with the data published earlier by us [7] in assessing the stereospecificity of the system according to method 2, the catalyst system Zn(C2Hs) 2TAULE 2. POLYMEItlZATION CONDITIONS FOR PItOPYLENE IN TIlE PRESENCE OF a-TICIs, USED BY FIRSoY et al. [1] Organomotallic compound AI(C2Hs)3 Bo(C2H~)2 Zn(C2Hs)~

TomporaR*

Prossuro

t.tlro~

°C 3 200-600 mm 3 200-600 mm 5"4X 10 -2 I 4 - 9 a t m I [

30-70 30-70 50-70

i

"11 -- mohlr r a tio of the o r g a n o m e t a l l i e c o m p o u n d a n d tile compound o f the t r a n s i t i o n valency nmtal.

Propylcnc polymcriz~tion catalysts

1705

TiCla should be considerably less stercospccific than the catalysts AI(C2Hs)aTiC13 and Bc(C2Hs).,-TiCI a. It is true t h a t we are in agrecme~lt with the fact t h a t the use of the second method of assessment for polymers obtained with the Zn(C.zHs)., catalysts, does not givc entirely satisfactory results because of the very low molecular weight of the samplcs. To obtain more accurate quantitative data about the stereospecificity of the catalyst system TiCIs-Zn(C2Hs)2, it is necessary to carry out furth(,r investigations, in particular because of thc very low molecular weight of th(, i)olymcrs obtained. The latter circumstance was explained [8] as a result of the high rate of the chain transfer reaction on to alkyl groups of the zincalkvl in (~omparison with the corresponding reac'tion in the TiCI3-A1R s system. As regards the second remark of Firsov et al, Natta and his coworkers have already stated t h a t fractionation of polypropylene is detcrmined not only by the content in it of thc stercorcgular and ata('tic structures. We have l)Ut forward the suggestion that the spatial configuration of the ma(~romolc(,ulcs exerts a much greater influence on solubility than does molecular weight, at least in t he molecular weight range from l 04-1()6 [2]. Moreover. the influcnc(~ (,f moh,(.ular weight on the solubility of isotactic pol)mlcrs has already been emphasized in other articles as well, where tt crystalline polypropylcne soluble in (,thor was descrihed [9]. In conclusion it should be stated that the article of Firsov et al. do(,s not refut(~ the existence of a relationshi t) bctwccn the stereospe('ificity of thc catalyst and the nature, both of the compound of the m(,tal of transistion valenc.y, and of the organometallic' component uscd in the preparation of the (.at~dyst. and this suggestion, judging from current scientifi(' literature, is at the present time scar(.ely questionable. In addition, the question of factors dctcrnfining the unique stcreospeciiicity of catalysts on the basis of diffcrent organometallic compounds remains, in our vi(,w, open. Tr(lnslated by G. M()l)l,l.:.~ REFERENCES 1. P . A . FIRSOV, 4: 1124, 1962; 2. G. NATTA, P. 3. G. NATTA, P.

B. G. KASHPOROV, Yu. V. KISSIN and N. M. CHIRKOV, Vysokomol. ~oyed. J. Polymer Sci. 62: 104, 1962 PINO and C. MAZZANTI, Gazz. chim. ital. 87: 328, 1957 CORRADINI and M. CESARI. Rend. Accad. Naz. Lintel: (8), 22: l l . 1957 4. G. CRESPI and F. RANALLI, The Plastics [nsl. Trans. 27: 68, 1959; F. RANALLI and G. CRESPI, Matcrie plast. 3 4 : 181, 1959 5. P. LONGI, G. NAZZANTI, A. ROGGERO and A. M. LACHI, Makromol. Ch~m. 61: 63,

1963 6. G. NATTA, I. PASQUON, A. ZAMBELLI and G. GATTI, J. Polymor Sci. 51: 3s7, 1961 7. G. NATTA, J. Polymer Sci. 34: 21, 1959 8. G. NATTA, E. GIACHETTI, I. PASQUON and G. PAJARO, Chimica c indu~tria 42: 109, 1960

9. G. NATTA, I. PASQUON, A. ZAMBELLI and G. GATTI, Makromol. Ch~m. (in press)