Boletín de la Sociedad Chilena de Química
versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000
POLYMERIZATION OF STYRENE BY DIPHENYLZINC-ADDITIVE
PART X. HOMO- AND COPOLYMERIZATION OF STYRENE USING
Ph2Zn - METALLOCENE - MAO SYSTEMS.
Grupo Polímeros, Departamento Ciencias Químicas, Facultad Química y Biología,
Universidad de Santiago de Chile. Casilla 40, Correo 33, Santiago, Chile.
#Departamento Ingeniería Química, Facultad Ciencias Físicas y Matemáticas,
Universidad de Chile. Casilla 2777, Santiago, Chile.
(Received: January 13, 2000 - Accepted: March 02, 2000)
In memoriam of Doctor Guido S. Canessa C.
Combination of diphenylzinc, Ph2Zn, cyclopentadienyl titanium trichloride, CpTiCl3, and methylaluminoxane, MAO, were used to initiate styrene, S, polymerization and its copolymerization with 1-hexadecene, 1-C16H32, and with p-tert-butylstyrene, p-ButS. For homopolymerization an almost pure syndiotatic polystyrene, s-PS, was obtained when using Ph2Zn-CpTiCl3-MAO combination, while other combinations of these components produced atactic polystyrene, a-PS, or almost no polymer at all. CpTiCl3-MAO and Ph2Zn-CpTiCl3-MAO were effective initiator systems for S/1-C16H32 and for S/p-ButS copolymerization. S/1-C16H32 copolymers showed crystalline melting temperature at range 235 - 255 °C while S/p-ButS copolymers did not show any melting temperature signal at DSC, but behave as stereoregular polymer.
KEY WORDS: styrene, polymerization, copolymerization, diphenylzinc, metallocenes, catalysts, stereoregularity, tacticity.
La combinación de difenilcinc, Ph2Zn, ciclopentadienil titanium tricloruro, CpTiCl3, y metilaluminoxano, MAO, se usaron como iniciador de la polimerización de estireno, S, y de su copolimerización con 1-hexadeceno, 1-C16H32, y con p-ter-butilestireno, p-ButS.
En la homopolimerización mediante Ph2Zn-CpTiCl3-MAO se obtuvo poliestireno sindiotáctico, s-PS, casi puro, mientras que las otras combinaciones de estos componentes producen poliestireno atáctico, a-PS, o no polimerizan al estireno. Los sistemas CpTiCl3-MAO y Ph2Zn-CpTiCl3-MAO resultaron ser iniciadores efectivos de la copolimerización de S/1-C16H32 y de S/ p-ButS. Los copolímeros S/1-C16H32 mostraron, señal de temperatura de fusión cristalina en el intervalo 235 - 255 °C, mientras que los copolímeros S/p-ButS no mostraron Tm, pero se comportan como polímeros estereoregulares.
PALABRAS CLAVES: estireno, polimerización, copolimerización, difenilcinc, metalocenos, estereoregularidad, tacticidad.
In our studies on styrene polymerization we have used various additives together with diphenylzinc: H2O (1), butanols (2), butanone (3), zinc chloride (4) and copper chloride (4). Conversion to polymer was most influenced by the nature of the additive employed and its molar ratio to Ph2Zn. At that time our results indicate that polymerization was also influenced by the nature of the reaction media suggesting an ionic pattern for the propagation step (1,2, 4). For butanone we considered a sort of competition between styrene and butanone for coordination to the zinc metal atom (3).
Our works on styrene polymerization continued on a second stage using combined systems including Ph2Zn, a metallocene, both zirconocene or titanocene, and methylaluminoxane, MAO, as initiator. Zirconocenes were: bis(indenyl)zirconium dichloride, Ind2ZrCl2, (5,6) isopropyl (cyclopentadienyl)(1-fluorenyl)zirconium dichloride, i-Pr(Cp)(Flu)ZrCl2, and ethenyl-bis(indenyl)zirconium dichloride, Et(Ind)2ZrCl2; (5) and others (7). The titanocenes explored were: biscyclopentadienyl titanium dichloride, Cp2TiCl2, (8) and bis(n-butylcyclopentadienyl) titanium dichloride (n-BuCp)2TiCl2, (9).
Using Ph2Zn, titanocene [either Cp2TiCl2, or (n-BuCp)2TiCl2] and MAO systems s-PS was obtained while the zirconocene produced atactic polystyrene, a-PS, with a certain amount, less than 20%, of s-PS. For all the systems employed, the one including titanocene and that including zirconocene, there was a critical conversion dependence on the molar ratio metallocene/Ph2Zn used. (6-9) Furthermore, our results were also indicative about the polarity of the polymerization medium. The polymerization process became more effective with polar solvents, in agreement with an ionic pathway for polymer propagation. (7) This situation was also previously observed when working with the two-component initiator systems. (1,2,4)
The present paper reports on new experimental results on styrene polymerization using Ph2Zn-metallocene-MAO initiator systems as well as on styrene copolymerization with a-olefins and with p-tert-butylstyrene.
Polymerizations were carried out under argon atmosphere in a 100 cm3 Schlenk tube equipped with a magnetic stirrer. Solvent toluene (ca., 35 cm3), MAO solution, Ph2Zn, and metallocene toluene-solution, were sequentially charged by syringe under argon pressure. Polymerization was initiated by injecting the required amount of styrene or simultaneosly styrene and comonomer. The reactions were kept with stirring at 60C for the required length of time.
Polymerization was finished by adding a mixture of hydrochloric acid and methanol. The polymers, coagulated in acidified methanol, were recovered by filtration after washing several times with methanol, and dried in vacuum at 60C.
The polystyrene samples were fractionated by exhaustive extraction with boiling butanone. The insoluble fraction in this solvent was also established for the different samples and considered as syndiotactic polystyrene.
Intrinsic viscosities, [h], of the amorphous polymer and of the butanone-soluble fraction, were measured at 25C in chloroform. Viscosity-average molecular weights, Mv, were calculated according to equation (10): [h] = 1.12x10-4 Mv0.73 valid for the molecular weight range 7-150x104.
For the butanone-insoluble polymer, intrinsic viscosity was measured in 1,2-dichlorobenzene at 135C and determined by the one point method. (11)
DSC analyses were performed by using Rheometrics Scientific DSC apparatus in a nitrogen atmosphere at a heating rate of 10°C/min, and reheated at the same rate. Sample of 3 to 4 mg were used. The reported melting points were obtained in the second scan.
Table I compares results shown in previous papers (7-9) with new experimental results for styrene polymerization initiated by Ph2Zn-metallocene-MAO and related systems. It can be noted that Ph2Zn by itself produces a very low conversion to PS but the obtained polymer presents a very high molecular weight. Combination of Ph2Zn-MAO, increases notoriously the conversion to PS but the observed molecular weight are more than 20 times lower than those obtained using Ph2Zn alone, emphasizing the chain transfer agent capacity of MAO (12). Combined Ph2Zn-Cp2TiCl2-MAO system increases the conversion to polymer and even more important, the contents of s-PS are much higher in this case. Actually this was the first of our polymer showing crystalline melting signal. A similar situation, regarding conversion and nature of obtained PS, can be appreciated for Ph2Zn-(n-BuCp)2TiCl2 -MAO initiator system, where (n-BuCp)2TiCl2 resulted more effective than Cp2TiCl2 and also produce a crude polymer which is almost pure s-PS. In contrast to these combinations, the participation of zirconocene, Ind2ZrCl2, resulted in a yield comparable to the one when using Cp2TiCl2 but the obtained polymer was an almost amorphous PS. Anyhow, the crude polymer contains a low fraction of boiling butanone-insoluble PS suggesting that this initiator system also produced a certain amount of s-PS. In front of these results CpTiCl3, together with MAO, was used as styrene polymerization initiator systems, both with and without Ph2Zn. CpTiCl3-MAO and Ph2Zn-CpTiCl3-MAO systems resulted to be much more effective initiators than the previous ones. Conversion of 30.5% and 45.7% were obtained in a much shorter polymerization period. Only after 6 hours against 48 hour reactions for the other systems. Furthermore the crude polymer was practically pure s-PS.
Table I: Styrene polymerization by Ph2Zn-Additive systems in toluene after 48 hours at 60°C.a
|CpTiCl3-MAO (6 hrs)||30.500||00.25*||¾||Nd||250.8||255.0||99.2|
|Ph2Zn-CpTiCl3-MAO (6 hrs)||45.700||00.23*||¾||101.1||259.0||263.0||99.8|
a) Polymerization conditions: Total volume = 60 mL, [S] = 2,1 mol/L, [MAO] = 0,33 mol/L,
[Ph2Zn] = [Metallocene] = 2.0E-04 mol/L.
b) Based on initial styrene
c) Measured in chloroform at 25°C, * Measured in o-dichlorobenzene at 135°C.
d) Boiling butanone-insoluble PS.
e) Taken from ref. 7
f) Taken from ref. 8
g) Taken from ref. 9
nd = not determined
In front of these results we think that our Ph2Zn-metallocene-MAO produces PS which atactic or syndiotactic nature depends on the metallocene employed. Titanocene produced mainly or only s-PS while zirconocenes gave mainly a-PS with a certain content of s-PS. Furthermore, both the amount of obtained polymer and its crystalline content are much related to the nature of titanocene, from the values reported CpTiCl3 is the most efficient, suggesting to produce the largest amount of Ti+3 active species, which have been attributed the role of generation of s-PS. ( 13,14 ).
Figure 1, shows the DSC thermogram of PS obtained using Ph2Zn-metallocene-MAO systems. As metallocene: Ind2ZrCl2, (n-BuCp)2TiCl2 and CpTiCl3 were used. Thermograms clearly indicate the presence of crystalline material for the titanocenes while those obtained using Ind2ZrCl2, did not show any signal of crystalline melting. This polymer dissolved both in toluene and in chloroform and left a minor amount of boiling butanone-insoluble polymer corroborating the assesment that zirconocene mainly produces a-PS when using in Ph2Zn-metallocene-MAO initiator systems.
|Fig.1 DSC curves for crude PS obtained using Ph2Zn-metallocene-MAO systems, in toluene at 60°C. For metallocenes Ind2ZrCl2 and (n-BuCp)2TiCl2 48 hours polymerization, for CpTiCl3 6 hours polymerization. Polymerization conditions, as indicated in Table I. Second heating at 10° min-1.|
The best initiator systems, CpTiCl3-MAO and Ph2Zn-CpTiCl3-MAO, regarding homopolimerization of styrene were used to copolymerize styrene with 1-hexadecene and with p-tert-butylstyrene. Table II, shows the experimental results for copolymerization styrene/1-hexadecene using CpTiCl3-MAO initiator systems. The corresponding data for the homopolymerization of both comonomers were also included for comparison. Conversion to polymer is affected by the proportion of comonomers at the initial feed and, on the other hand, with exception of the product of homopolymerization of 1-hexadecene, all the products showed a crystalline melting temperature in the s-PS region. Finally, only one glass transition temperature was detected which varies accordingly to the comonomer initial feed, decreasing as the proportion of 1-hexadecene increases.
Table II. Styrene/1-hexadecene copolymerization by CpTiCl3-MAO initiator systems, in toluene after 6 hours at 60°C a
Table III. shows the results for styrene/1-hexadecene copolymerization using initiator system Ph2Zn-CpTiCl3-MAO. These results are similar with those obtained for CpTiCl3-MAO but with better conversions. This is indicative that the presence of Ph2Zn improves the activity of the initiator system.
|Table III. Styrene/1-hexadecene copolymerization initiated by Ph2Zn-CpTiCl3-MAO in toluene after 6 hours at 60°C.a|
Table IV shows the results of styrene/p-tert-butylstyrene copolymerization using CpTiCl3-MAO initiator system. It can be appreciated that conversion and Tg increases with the increase of p-tert-butylstyrene in the initial feed. Regarding crystallinity, only the styrene homopolymerization product shows Tm signals in the DSC thermogram.
|Table IV. Styrene/p-tert-butylstyrene copolymerization initiated by CpTiCl3-MAO in toluene after 6 hours at 60°C.a|
Table V shows the results obtained for styrene/p-tert-butylstyrene copolymerization using Ph2Zn-CpTiCl3-MAO. Again, it can be noted that conversion to polymer and Tg increase with the amount of p-tert-butylstyrene in the initial feed. As for CpTiCl3-MAO initiator system, no melting temperature signal was observed, despite their insolubility in common solvents. Anyhow, exploratory 1H-NMR and 13C-NMR studies, indicate a syndiotactic nature of the products and a copolymer composition in agreement with the initial feed, but slightly enriched in p-ButS . (15)
|Table V. Styrene/p-tert-butylstyrene copolymerization initiated by Ph2Zn-CpTiCl3-MAO in toluene after 6 hours at 60°C.a|
Figure 2 shows thermograms for copolymers S/1-C16H32 and S/p-ButS obtained using Ph2Zn-CpTiCl3-MAO initiator systems. Is clearly noted that for poly(styrene-co-p-tert-butylstyrene) the Tg values increase as the ButS proportion in the initial feed increases. This result is important in the sense of materials behavior as the copolymer products are moving to the thermal behavior field of an engineering plastic.
|Fig.2 DSC curves for crude copolymers styrene/1-hexadecene and styrene/p-tert-butylstyrene at various initial feed molar ratio, obtained using Ph2Zn-CpTiCl3-MAO systems, in toluene at 60°C during 6 hours. Polymerization conditions as indicated in Table III and Table V. Second heating at 10° min-1.|
Figure 3, shows the DSC thermogram of a sample of S/1-C16H32 copolymer obtained using Ph2Zn-(n-BuCp)2TiCl2 -MAO systems measured from subambient temperature. There is an endotherm signal at 11.2 °C suggesting a crystalline melting due to an ordered accomodation of pendant chains of 1-hexadecene incorporated to the copolymer.
|Fig. 3 DSC curves of crude copolymer styrene/1-hexadecene obtained using Ph2Zn-(n-BuCp)2TiCl2 -MAO system and measured from subambient temperature. Second heating at 10° min-1. Performed at Universidad Simón Bolivar, Caracas, Venezuela. Prof. Alejandro J. Müller Laboratory.|
We can conclude that Ph2Zn-CpTiCl3-MAO systems are capable to induce styrene homopolymerization, S/1-hexadecene and S/p-tert-butylstyrene copolymerization with better conversion and in shorter period of reaction time than for other titanocenes. Furthermore, the polystyrene obtained are almost pure syndiotactic polystyrene and the copolymers styrene-co-para-tert-butylstyrene have a higher service temperature than PS including s-PS.
Further works are now in progress from which conclusive results are expected.
Financial support from Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Santiago de Chile, DICYT-USACH, Grant 05-9741-RC, and from Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Grant 198-1135 are gratefully acknowledged. We thank Witco, Polymerchemikalien & Kunstharze, Bergkamen, Germany, for donation of methylaluminoxane. We also thank Miss Ana M. Cavieres for DSC measurements and her skillful assistance in experimental work.
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