Bol. Soc. Chil. Quím., 47, 137-144 (2002)

FRANCO M. RABAGLIATI^*, CARLOS J. CARO, MONICA A. PEREZ

Grupo Polímeros, Departamento Ciencias Químicas, Facultad Química y Biología,
Universidad de Santiago de Chile. Casilla 40, Correo 33, Santiago, Chile.
(Received: November 22, 2001 - Accepted: February 28, 2002)

SUMMARY

The copolymerization of styrene with p-methylstyrene has been tested using combined diphenylzinc-additive initiator systems, including diphenylzinc, Ph₂Zn, a metallocene: cyclopentadienyltitanium trichloride, CpTiCl₃, and methylalumoxane, MAO, as well as the binary CpTiCl₃-MAO initiator system. Both combined initiator systems were able to induce S/p-MeS copolymerization and the corresponding homopolymerizations. Conversion to copolymer increases with an increase of p-MeS in the initial feed, while p-MeS homopolymerization led to greater conversion than that of S. From the copolymers obtained using these initiator systems only those produced from S/p-MeS = 95/5 (mol/mol) in the feed showed to be crystalline, with Tm values lower than that of pure s-PS, making them easier to process.

KEY WORDS: styrene, polymerization, copolymerization, diphenylzinc, metallocenes, catalysts, stereoregularity, tacticity.

RESUMEN

Se estudió la copolimerización de estireno con para-metilestireno usando sistemas combinados difenilcinc-aditivo como iniciadores de la copolimerización. Se emplearon los sistemas difenilcinc, Ph₂Zn, un metaloceno: ciclopentadienil titanio tricloruro, CpTiCl_3, junto con metilaluminoxano, MAO, y también el sistema binario: CpTiCl₃-MAO. Ambos sistemas resultaron ser activos iniciadores tanto de la copolimerización S/p-MeS, como de las respectivas homopolimerizaciones. Los resultados alcanzados indican que la conversión a copolímero aumenta con el aumento de la proporción de p-MeS en la carga inicial, mientras que la homopolimerización de p-MeS produjo mayores conversiones que la homopolimerización de S. De los copolímeros obtenidos mediante estos sistemas iniciadores sólo los producidos a partir de la carga inicial S/p-MeS = 95/5 resultaron ser cristalinos con valores de Tm menores que para el s-PS, lo que los hace mas facil de procesar.

PALABRAS CLAVES: estireno, polimerización, copolimerización, difenilcinc, metalocenos, estereoregularidad, tacticidad.

INTRODUCTION

In our studies of styrene copolymerization with a-olefins (1-3) and of styrene with p-substituted styrenes (1,3,4), we reported that the nature of the comonomer, the styrene/comonomer molar ratio in the initial feed, the type of metallocene employed, the presence or absence of diphenylzinc as a component of the initiator system, and the polarity of the reaction media had a great influence on the polymerization process.

Zambelli et al. (5), studying the homopolymerization of styrene and the homopolymerization of para-substituted styrene and its copolymerization with styrene in the presence of the syndiotactic specific catalytic system tetrabenzyltitanium-methylalumoxane, Bz₄Ti-MAO, reported that I⁺ substituents enhance the reactivity and, on the other hand, that stereospecifity is affected by the substituents of the aromatic ring even when they are in the para position.

Our results for S/p-t-Bu^tS copolymerization using the Ph₂Zn-CpTiCl₃-MAO initiator system (4) agree with Zambelli's assessment in the sense that conversion to copolymer increases as the amount of p-Bu^tS in the initial feed increases, and according to the ¹H- and ¹³C-NMR spectra the poly(styrene-co-para-tert-butylstyrene) copolymers obtained were syndiotactic in nature, even though they did not show any crystalline melting temperature.

To obtain additional information on the behavior of I⁺ para-substituted styrenes in terms of their copolymerization with styrene using the Ph₂Zn-CpTiCl₃-MAO initiator system, we have performed new experiments, this time studying the copolymerization of styrene with para-methylstyrene initiated by Ph₂Zn-CpTiCl₃-MAO and also by the binary CpTiCl₃-MAO initiator system. Experiments including solvents of increasing polarity were also performed for initial feed S/p-MeS = 50/50, using both Ph₂Zn-CpTiCl₃-MAO and CpTiCl₃-MAO initiator systems.

EXPERIMENTAL

Polymerizations were carried out under argon in a 100 cm³ Schlenk tube equipped with a magnetic stirrer. Solvent (to complete 25 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 60 C for the required length of time.

Polymerization was terminated 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 60 C.

The crude polymer samples were fractionated by exhaustive extraction with boiling butanone. The fraction insoluble in this solvent was also established for the different samples and considered to be a stereoregular polymer.

Intrinsic viscosities, [h], of the amorphous polymer and of the butanone-soluble fraction, were measured at 25 C in chloroform. For the butanone-insoluble polymer, intrinsic viscosity was measured in 1,2-dichlorobenzene at 135 C and determined by the one point method (6).

DSC analyses were performed in a nitrogen atmosphere using a Rheometrics Scientific DSC apparatus at a heating rate of 10°C/min, and reheated at the same rate. Samples of 3 to 4 mg were used. The reported melting points were obtained in the second scan.

The NMR spectra of the samples were recorded on a Bruker AMX-300 spectrometer in deuterated benzene at 70 C. Chemical shifts were calibrated with tetramethylsilane (TMS) used as an internal reference.

The NMR spectra of samples soluble at room temperature were recorded on a Bruker Avance DRX-300 spectrometer in deuterated chloroform, with TMS as an internal reference.

RESULTS AND DISCUSSION

In previous papers (1, 3, 4) we have reported on the copolymerization of styrene with p-tert-butylstyrene using metallocene-MAO and Ph₂Zn-metallocene-MAO initiator systems. Our results showed that both kinds of systems were effective initiators of S/p-Bu^tS copolymerization, as well as of the homopolymerization of S and p-Bu^tS. The resulting P(S/p-Bu^tS) copolymers were syndiotactic in nature when the initiator system included a titanocene such as (n-BuCp)₂TiCl₂ or CpTiCl₃, but atactic when using the zirconocene Ind₂ZrCl₂ (4).

In this paper we report on the copolymerization of S with p-MeS using CpTiCl₃-MAO and Ph₂Zn-CpTiCl₃-MAO initiator systems. Table I shows the results of S/p-MeS copolymerization at various S/p-MeS molar ratios in the initial feed, initiated by the CpTiCl₃-MAO system. These results show that there was an increase in conversion with an increase of p-MeS in the feed, accounting for the inductive effect of a methyl group in the para position of the benzene ring of p-MeS, improving the nucleophilic capacity of -CH=CH₂ group making p-MeS a more reactive monomer than styrene. As already established the S polymerization using Ph₂Zn-metallocene-MAO proceed through coordination of monomer to the active species followed by its insertion at the growing chain-active species link. Then polymerization propagates through a cationic pathway. (4)

From the results in Table I it is also seen that the intrinsic viscosity of the copolymers decreases as the amount of p-MeS component increases in the initial S/p-MeS mixture, and that Tg increases as the amount of p-MeS increases in the initial S/p-MeS molar ratio.

The DSC thermograms of the copolymers obtained are shown in Figure 1. It is seen that a melting temperature was observed only for the homopolymerization of styrene and for the copolymer resulting from an initial S/p-MeS molar ratio of 95/5.

Table II, shows the results for S/p-MeS copolymerization when using the Ph₂Zn-CpTiCl₃-MAO initiator system. As in the case of the CpTiCl₃-MAO initiator system, conversion increases with an increase of p-MeS in the initial feed in accordance with previous results (3). From the results in Table II is seen that the inclusion of Ph₂Zn in the initiator system produced an increase in conversion to copolymer.

The other results reported in Table II are similar to those of Table I regarding the nature of the copolymers obtained. Copolymer composition was calculated from the ¹H-NMR spectra, by integration of the main chain CH and CH₂ and the side chain CH₃ proton signal. It was found, as for S/p-Bu^tS copolymerization (4), that the copolymers obtained with these initiator systems are richer in p-MeS compared with the initial feed from which they originated.

Figure 2 shows the ¹H-NMR spectra of the copolymers obtained using Ph₂Zn- CpTiCl₃-MAO and various initial feed S/p-MeS ratios, including the corresponding to the homopolymerization of p-MeS.

From these spectra it is seen that the signals of aromatic-H (d = 6.5 - 7.2 ppm), CH₃-Ar (d = 1.6 ppm), -CH₂- (d = 1.4 ppm), and -CH-Ar (d = 2.2 ppm), agree with the incorporation of both comonomers in the copolymer. The abundance of each comonomer in the polymer was calculated from the corresponding integrals, as mentioned above. The results indicate that the copolymers obtained were richer in p-MeS than the corresponding mixture in the initial feed (Table III).

a) Obtained by integration of the main chain CH and CH₂and the side chain CH₃ proton signal.

Figure 3 shows the DSC thermograms of S/p-MeS copolymers obtained using the Ph₂Zn-CpTiCl₃-MAO initiator systems. From them it can be seen that Tg values increase as the amount of p-MeS increases in the initial feed.

A further analysis of the effectiveness of both initiator systems and their relation to the S/p-MeS initial ratio is shown in Tables I and II as the activity ratio, referred to the activity of S homopolymerization. The values increase as the amount of p-MeS in the feed increases, and comparing the figures for both homopolymerizations it was found that for p-MeS the CpTiCl₃-MAO and Ph₂Zn-CpTiCl₃-MAO initiator systems are 2.1 times and 2.4 times more reactive, respectively, than for styrene homopolymerization. These higher reactivities can be attributed to the I⁺ effect of the methyl group in the para-position in the benzene ring of p-MeS. Zambelli, when comparing the homopolymerization of p-MeS with that of S for the Bz₄Ti-MAO initiator system, found that p-MeS polymerization is 1.8 times more effective than S polymerization (5).

Regarding the Tm values for S/p-MeS = 95/5 copolymerization initiated by the CpTiCl₃-MAO and Ph₂Zn-CpTiCl₃-MAO systems, the only ones that showed a melting temperature in agreement with their DSC thermograms, they can account for corresponding drops of 45 C and 38 C when compared with theTm of pure syndiotactic PS. That is to say that a low content of p-MeS in the copolymer produces a copolymer with a lower Tm, making it a polymer that retains the high resistance of s-PS, but at the same time is a material having better processing conditions. Kaminsky found that for the copolymerization of S/p-Bu^tS and S/p-MeS using the CpTiF₃-MAO initiator system, the styrene-rich copolymers had melting temperatures lower than s-PS, emphasizing that only small amounts of p-Bu^tS are needed to reduce the melting temperature by up to 40 C, whereas 13% of p-MeS in the feed (20% in the polymer) acts to decrease the melting point by up to 60 C (7).

Experiments were performed for each of these initiator systems using various solvents with increasing polarity, as shown in Tables IV and V. Toluene, Tol; chlorobenzene, CB; dichlorobenzene, DCB; and their 1/1 (v/v) mixture were employed. As was the case for S/p-Bu^tS copolymerization, in these copolymerizations conversion increases with the polarity of the reaction media: Tol < Tol-CB < CB < Tol-DCB < CB-DCB < DCB, confirming our previous findings in the sense that the polymerization process goes through an ionic-propagation stage.

On the other hand, the results indicate that the polarity of the reaction media leads to a decrease in the stereoregularity of the copolymer obtained, as seen from the decrease in the boiling-butanone-insoluble fraction content.

We can conclude that both the Ph₂Zn-CpTiCl₃-MAO and CpTiCl₃-MAO initiator systems are capable of inducing styrene/p-methylstyrene copolymerization. Conversion to copolymer increases with the amount of p-MeS in the initial feed in agreement with an I⁺ inductive effect of the CH₃-group in the para-position of the aromatic ring. Furthermore, conversion to copolymer also increases with the polarity of the reaction media. These findings agree with our previous conclusions in the sense that copolymerization propagates through a cationic pathway.
Further work is under way from which more conclusive results are expected.

ACKNOWLEDGEMENTS

Financial support from the Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Santiago de Chile, DICYT-USACH, Grant 05-9741-RC, and from the Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Grant 101-0036, are gratefully acknowledged. We also thank Miss Ana M. Cavieres for DSC measurements

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^* To whom correspondence should be addressed.