versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
Styrene/1-alkene copolymerization by CpTiCl3-additive initiator systems.
R.A. CANCINO*, F.J. RODRÍGUEZ, M.A. PÉREZ, F.M. RABAGLIATI.
Grupo de Polímeros, Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile. email@example.com; Fax: (56)-2-6812108
Initiator systems resulting from the combination of cyclopentadienyltitanium trichloride (CpTiCl3), ethylaluminium dichloride (EtAlCl2) with and without methylaluminoxane (MAO), were used to initiate styrene (S) homopolymerization and S/1-alkene copolymerization. The CpTiCl3-EtAlCl2 system turned out to be an effective initiator of styrene homopolymerization as well as of styrene/1-decene (S/1-C10H20) and styrene/1-hexadecene (S/1-C16H32) copolymerization. Both S/1-C10H20 and S/1-C16H32 copolymers obtained from various S/1-alkene molar ratios in the initial feed contained variable amounts of boiling-butanone-insoluble product which was a S/1-alkene copolymer according to NMR and DSC analyses. The copolymers obtained showed Tg values which decrease as the proportion of 1-alkene in the initial feed increases.
Keywords: styrene, polymerization, copolymerization.
One of the most important achievements in the field of polymer chemistry during the past 50 years has been the discovery of the Ziegler-Natta catalyst systems in the early 1950's, when the synthesis and characterization of several crystalline polyhydrocarbons was published, reporting studies about stereospecific polymerization of olefins (1,2). In most cases, polymerization of vinyl monomers lead to isotactic polymers, such as isotactic polystyrene (i-PS) (3), which was produced by Natta in 1955. Syndiotactic polymers were relatively rare until recently; Ishihara et al. succeeded in the synthesis of syndiotactic polystyrene (s-PS) in 1985 (4), which was the first known case of syndiospecific polymerization of styrene. The s-PS obtained had a high degree of cristallinity. About the same time, the obtention of s-PS was also reported in 1987 by Pellechia et al., who claimed to have succeeded in the preparation of a highly syndiotactic polystyrene in the presence of catalytic systems consisting of a metallocene and methylaluminoxane (5).
Ten years later, Oliva et al reported on zirconocene-based catalysts for ethylene-styrene copolymerization using the rac-Et(Ind)2ZrCl2 and iPr[Cp][Flu]ZrCl2 metallocenes activated with MAO (6). They found that the 13C-NMR spectra of two copolymers obtained with similar compositions in the initial feed showed different compositions and structure in the final product, depending on the metallocene symmetry. On the other hand Maciejewski et. al. (7) reported the influence of trialkylaluminium reagents on propylene polymerization with bridged and unbridged 2-arylindene metallocene polymerization catalysts. They showed that the alkylaluminium content had a greater effect on the activity of the initiator and on the physical properties of the products.
Trialkylaluminium reagents have a number of effects on homogeneous metallocene catalysts. Such species can alkylate catalyst precursors (8-11), scavenge trace impurities present in the monomer feed (12), and act as chain transfer agents (13). In our studies on styrene polymerization using combined systems including a Zr or Ti metallocene and MAO as initiator, systems based on the metallocenes Cp2TiCl2 and (n-BuCp)2TiCl2 produced syndiotactic polystyrene, while the zirconocenes Ind2ZrCl2 and Et(Ind)2ZrCl2 produced atactic polystyrene (a-PS), with a low content (less than 20%) of s-PS (14,15). In a previous paper (16) we reported on styrene polymerization using Ph2Zn-metallocene-MAO initiator systems as well as on styrene copolymerization with 1-alkene such as 1-hexadecene using the initiator systems CpTiCl3-MAO and Ph2Zn-CpTiCl3-MAO. In both cases a large amount of polymer was obtained with a high percentage of boiling-butanone-insoluble polymer, suggesting that the products were mainly syndiotactic polystyrene. In the present work we report recent results on the homo and copolymerization of styrene with 1-decene and 1-hexadecene using initiator systems resulting from the combination of CpTiCl3 and MAO, and also by using CpTiCl3-alkylaluminium and CpTiCl3-MAO/EtAlCl2 combinations.
Homo and copolymerization reactions were carried out in an inert atmosphere in a 100-ml Schlenk tube with a magnetic stirrer. Solvent toluene, MAO, alkylaluminium and metallocene toluene solutions were sequentially charged by syringe under positive argon pressure. Polymerization was initiated by injecting the required amount of styrene or styrene and comonomer simultaneously. The reactions were kept at 60 C stirring for the required length of time.
Polymerization was finished by adding a mixture of hydrochloric acid and cool methanol, the precipitated polymers were recovered by filtration after washing several times with methanol, and dried in vacuum at 60 C. The homo- and copolymer styrene samples were fractionated by exhaustive extraction with boiling butanone. The viscosity-average molecular weight (Mv) of atactic polystyrene was determined through the equation [h] = 1.12x10-4 Mv0.73 (17), valid for the molecular weight range of 715x10-4; the corresponding intrinsic viscosities, ([h]), were measured at 25 C in chloroform. For the butanone-insoluble polymer, intrinsic viscosities were measured in 1,2-dichlorobenzene at 135 C and were determined by the one point method. (18)
DSC analyses were performed on Rheometric Scientific DSC equipment at a heating rate of 10 C/min with 3- to 4-mg samples. The reported melting points were recovered in the second scan. Copolymers were analyzed by 1H-NMR in a Bruker DRX-300 spectrometer at 300 MHz in CDCl3 at 60 C or in C6D6 at 70 C, depending on the stereoregular polystyrene content, using TMS as reference.RESULTS AND DISCUSSION
We have previously reported on styrene/1-alkene copolymerization with 1-hexene, 1-decene and 1-hexadecene using Ph2Zn-metallocene-MAO initiator systems with the metallocenes Cp2TiCl2, (n-BuCp)2TiCl2 and Ind2ZrCl2 (16). We found that for the Ph2Zn-metallocene-MAO systems, titanocenes gave mainly syndiotactic polystyrene regardless of the styrene/1-alkene proportion in the initial feed. On the other hand, zirconocene based systems were able to copolymerize styrene/1-alkene with a lower proportion of 1-alkene in the copolymer than there was in the initial feed (15,16,19). In this work we have included in the metallocene-MAO system an alkylaluminium such as Et3Al or EtAlCl2 for styrene/1-alkene copolymerization with 1-C10H20 and 1-C16H32. Our results show that the addition of alkylaluminium in the initiator systems has a large effect on the conversion as well as on the characteristics and properties of the material obtained. The nature of the alkylaluminium reagent has an important effect on the catalytic systems. It was observed that titanium based initiator systems in the presence of a strong Lewis acid like Et3Al lower the activity compared to systems activated only with MAO. The difference in the activities may be related to the reductor power of Et3Al, as it is known that methylaluminoxane is a good activator of homogeneous catalytic systems, producing the active reduced alkylated species of Ti+3. In the case of Et3Al the titanocene undergoes a complete reduction to the +2, +1 or 0 states, which are not active species for styrene polymerization.
Table 1 and 2 show the results obtained in the styrene/1-decene and styrene/1-hexadecene copolymerization respectively, using CpTiCl3-MAO initiator systems, which confirm the ability of such initiator systems to produce stereoregular polymers. The products obtained show Tm values in the region of s-PS, indicating that the products are mainly syndiotactic polystyrene. CpTiCl3-MAO systems were not effective initiators for either S/1-alkene copolymerization or 1-alkene homopolymerization with 1-C10H20 and/or 1-C16H32. The polymer obtained showed Tg values of ca. 100 C.
The 1H-NMR spectra shown in Figure 1, together with the Tm values in Table 1 and 2, confirm that the product obtained for S/1-C16H32 and S/1-C10H20 are mainly syndiotactic polystyrene when using the CpTiCl3-MAO initiator system.
On the other hand, Table 3 shows the behaviour of styrene/1-decene copolymerization using CpTiCl3-EtAlCl2 initiator systems. For such systems, the activities with EtAlCl2 were higher than those reached when using CpTiCl3-MAO initiator systems, as can be seen in Table 1. Regardless of the styrene/1-alkene ratio in the initial feed, CpTiCl3-EtAlCl2 systems were found to be able to induce styrene/1-hexadecene copolymerization as well as styrene homopolymerization. The decrease in Tg values is the first indication that the 1-alkene was incorporated into the polystyrene backbone, and on the other hand, the fact that there is only one Tg value confirms the presence of a single product, a copolymer.
Results obtained for the styrene/1-hexadecene copolymerization with CpTiCl3-EtAlCl2 initiator system are shown in table 4. They indicate a similar behavior that S/1-decene copolymerization including a marked decrease in Tg values as molar proportion of 1-alkene content increase in the initial feed. Conversion to polymer also follow a similar behavior when compared S/1-C10H20 and S/1-C16H32; that is to say, the yield decrease as the content of 1-alkene increase in the initial copolymerization mixture. In front of intrinsic viscosity values, very low values were noted in both cases confirming the high chain transfer effect of EtAlCl2. Any how it is clear that the inclusion of EtAlCl2 in the initiator system increase notoriously the conversion to polymer for styrene homopolymerization as well as of all other S/1-alkene proportions.
The use of the CpTiCl3-EtAlCl2 system produced a mixture of a-PS and poly(S-co-C16H32) copolymer, the presence of a true copolymer is confirmed by the 1H-NMR spectra in Figure 2, which show the characteristic CH3 group signal around 0.9 ppm, together with signals corresponding to the aromatic protons of the phenyl-group of styrene.
Table 5 shows the results of styrene/1-hexadecene copolymerization using the CpTiCl3-EtAlCl2/MAO initiator system and varying the EtAlCl2 content. The influence of the EtAlCl2/MAO ratio on conversion to copolymer as well as on the characteristics of the product parallel the decreasing of Tg values with increasing EtAlCl2 content in the initiator system, indicating an increase in olefin content in the product composition. All these copolymers showed only one Tg signal, in accordance with a single product.
Figure 3, shows the 1H-NMR spectra for the styrene/1-hexadecene copolymerization products, obtained using the CpTiCl3-EtAlCl2/MAO initiator system, where we can observe that the increase in the alkylaluminium/MAO ratio produce that the content of 1-alkene in the main chain also increase as it can be noted by the methyl group signal near to 0.9 ppm.
From these results we can conclude that, CpTiCl3-EtAlCl2 initiator systems induce the homo- and copolymerization of styrene with 1-hexadecene and 1-decene, producing a low content of stereoregular polystyrene. The EtAlCl2 content in the initiator systems determines the characteristics and properties of the product obtained. The incorporation of 1-alkene units increases with the EtAlCl2/MAO ratio.ACKNOWLEDGEMENTS
Financial support by the Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Santiago de Chile, DICYT-USACH and by the Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Grant 101-0036, 2000054 and 2010027, is gratefully acknowledged. R. A. Cancino thanks CONICYT for a Doctoral Fellowship. Authors also thank M. P. Cerda for carrying out the DSC measurements.REFERENCES
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