Influence of Temperature Ageing of Ziegler-Natta Catalyst on 1, 3 ...

ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS

Cis-1,4-polybutadiene Polymerization Ziegler-Natta catalyst Catalyst temperature ageing Neodymium For the investigations a catalyst constituted by di-isobutylaluminium hydride (DIBAH), neodymium versatate and tbutyl chloride was used. The aim of this work is to evaluate the effect of the catalyst ageing temperature on the molecular polymer characteristics (microstructure and molecular weight) and the catalyst activity. The catalysts were aged at 10, 25 and 40 8C for 24 h. The polybutadienes were characterized by size exclusion chromatography (SEC) to determine the molecular weight distribution and by FT-IR to determine the microstructure.

Einfluß der Wa ¨rmealterung eines Ziegler-Natta Katalysators auf die Polymerisation von 1,3Butadien Cis-1,4-Polybutadien Polymerisation Ziegler-Natta Katalysator Katalysatoralterung Neodym Fu ¨r die Untersuchung wurde ein Katalysator eingesetzt, der aus Di-isobutylaluminium hydrid (DIBAH), Neodym versatate und tert-butylchlorid hergestellt wurde. Das Ziel dieser Arbeit ist, den Effekt der Wa ¨rmealterung des Katalysators auf die molekularen Eigenschaften des Polymeren (Mikrostruktur und Molekulargewichtsverteilung) sowie die Katalysatoraktivita ¨t zu untersuchen. Die Katalysatoren wurden bei 10, 25 und 40 8C u ¨ber 24 h gealtert. Die erhaltenen Poly-butadiene wurden hinsichtlich ihrer Molekulargewichtsverteilung durch Gelpermeationschromatographie (GPC) und ihrer Mikrostruktur durch FT-IR charakterisiert.

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KGK Januar/Februar 2006

Influence of Temperature Ageing of Ziegler-Natta Catalyst on 1,3-Butadiene Polymerization

Ziegler-Natta catalysts have been used extensively for many years for the production of polybutadienes, particularly where high 1,4-cis contents are required. More recently, the use of coordination catalysts based on lanthanides has permitted higher degrees of stereoregulation to be achieved. For example, neodymium based systems have been shown to be particularly effective in producing polybutadienes with 1,4-cis contents of about 98 % and more [1 – 8]. High-cis polybutadiene produced by neodymium catalyst presents properties such as high abrasion resistance, good crack resistance; high resilience, low hysteresis and heat build up, among others. The properties level is in general superior to the polymers produced by other catalyst systems. Due to the mentioned properties, the tire industry is the main field of application for these polymers. Other uses of high-cis polybutadiene are in golf balls, hoses and shoe soles [9 – 11]. The catalyst systems based on lanthanides can be classified into three groups: Group I includes lanthanide (Ln) halides or a complex compound of the halide LnHal3.3L, where L is an electron-donating organic ligand, such as esters of phosphoric acid, sulfoxides, alcohols and cyclic ethers. These compounds in combination with an organo-aluminium compound produce active and stereoselective catalysts for cis-1,4 butadiene polymerization. Group II includes catalysts in which the initial lanthanide compound LnX3 does not contain a halogen. The catalysts of this type include carboxylates Ln(OCOR)3, alkylphosphates Ln(OPOR)3 or alcoxides Ln(OR)3. In addition to LnX3 and AlR3, these catalytic systems necessarily include halogen-containing components. The role of these components is to halogenate the lanthanide by exchange reactions. Group III includes catalysts in which the initial halogen-containing compound of lanthanide has a Ln-C bond. A change in

the catalyst system components and the reaction parameters may affect the polybutadiene characteristics [12 – 15]. The catalyst system used in this work belongs to Group II and consists of neodymium versatate (catalyst), diisobutylaluminium hydride (cocatalyst) and tert-butyl chloride (neodymium chlorinating agent). The scope of the present work was to evaluate the effect of the catalyst ageing temperature on the polymer molecular characteristics such as the microstructure and molecular weight distribution as well as the catalyst activity.

Experimental Catalyst synthesis Catalysts were prepared in oven-dried nitrogen-purged bottles fitted with a rubber septum. The order of components addition was diisobutylaluminium hydride DIBAH (Al), neodymium versatate (Nd) and tert-butyl chloride (Cl). The reaction temperature employed was 10 8C and the molar ratio of catalyst components was Al:Nd:Cl ¼ 11:1:3. The catalysts were aged at 25 and 40 8C for 24 hours and after that they were aged for more 1, 15 and 40 days at 10 8C (Table 1).

Autoren I.L. Mello, F.M.B. Coutinho, B.G. Soares and D.S.S. Nunes, Rio de Janeiro (Brazil) Corresponding author F.M.B. Coutinho Universidade do Estado do Rio de Janeiro Instituto de Quimica Rua Sao Francisco Xavier 524 Pavilho Haroldo Lisboa da Cunha Rio de Janeiro, RJ, 20559-900, Brazil

1 Catalyst ageing conditions & Ti

(a)

ti

(b)

10 8C

25 8C

40 8C

t10 8C

(c)

1 15 40 24 h

1 15 40 1 15 40

(a) Ti – Initial ageing temperature (b) ti – Initial ageing time at Ti (c) t10 8C – Ageing time at 10 8C

Polymerization All polymerizations were carried out in a 1L stainless steel Parr reactor. The reaction system was inertized through purge with extra dry nitrogen, additionally treated in drying columns packed with activated alumina. To the Parr reactor was added a dry solution of butadiene in hexane (35 %wt/wt) followed by the addition of dry hexane. The reaction medium was heated up to the stated reaction temperature and then the catalyst was injected by syringe. The reaction temperature employed was 70 8C. Polymerizations were terminated after 2 h by adding a hexane solution of 3,5-di-tert-butyl-4-hydroxytoluene (BHT – 50 %wt/wt) and stabilized by adding a hexane solution of trinonylphe-

1 &

nylphosphite (TNPP – 10 %wt/wt). The polymer was coagulated under stirring in distilled water at 80 8C for 1h. Finally, it was dried in an oven at 65 8C until constant weight. Reaction conversion was calculated relating the polymer mass obtained to the monomer mass fed to the reaction medium.

Polymer characterization The polymer microstructure was characterized by infrared spectroscopy using a Perkin-Elmer Spectrometer (model Spectrum One). Polymer films formed on KBr cells were prepared from 2 % wt/v chloroform solutions. The content of isomeric units were obtained from the absorbances at 725 cm1 (cis-1,4), 910 cm1 (1,2-vinyl) and 965 cm1 (trans-1,4). The polymer molecular weight and weight distribution were evaluated by size exclusion chromatography (SEC) using a Waters 150-C Plus SEC equippment, fitted with a RI detector at 30 8C. As mobile phase THF was used at 1 mL/min flow rate. An universal calibration curve was constructed by using monodisperse polystyrene standards. Polymer solutions (0,1 % wt/wt) were filtered using filters of 0.45 lm pore diameter before injecting in the chromatograph.

Results and discussion The catalysts initially had been aged the 10, 25 and 40 8C for 24 hours with the objective

of evaluating the influence of the catalyst ageing temperature. After that time, such catalysts were kept at 10 8C by 1, 15 and 40 days to evaluate the influence of catalyst ageing time on polymerization reaction and molecular weight and molecular weight distribution of poly-butadiene (Figures 1 and 2). The catalysts aged at 25 8C showed a higher activity than the other systems, independently of the posterior ageing time (Figure 1). Probably, that result is due to the fact that the alkylation reaction is faster at 25 8C than at higher temperatures [16]. This assumption was confirmed by the lower conversions obtained by the catalysts aged at 40 8C. The r Nd-C bonds formed in the alkylation stage seems to be more stable at lower temperatures [17]. Even though, an increase in temperature favors the alkylation reaction, as the bonds formed are unstable. At higher temperatures these bonds break quickly, decreasing the number of active sites and thus, decreasing the catalyst activity. In Figure 2, a trend of increasing of molecular weight is noticed as a function of both the catalyst ageing temperature and ageing time. This behavior suggests a transformation of the catalyst system with the ageing. In accordance with the literature [16, 18], there are several active sites with different sensibilities in relation to ageing. The sites

1 Influence of catalyst ageing & conditions on polymerization conversion (a) initial ageing at 10 8C for 24 h; (b) initial ageing at 25 8C for 24 h and (c) initial ageing at 40 8C for 24 h


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ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS

2 &

2 Influence of ageing conditions & on high-cis polybutadiene number-average molecular weight (Mn) and weightaverage molecular weight (Mw)

2 Influence of catalyst ageing conditions on polybutadiene microstructure & Ti

(a)

ti

(b)

10 8C

25 8C

24 h

40 8C

t10 8C

(c)

cis-1,4

trans-1,4

vinyl

0 15 40

98.2 98.3 98.0

1.2 1.1 1.3

0.6 0.6 0.7

0 15 40

98.0 98.2 98.4

1.4 1.1 1.0

0.6 0.7 0.6

0 15 40

98.2 98.2 98.3

1.2 1.2 1.0

0.6 0.6 0.7

(a) Ti – Initial ageing temperature (b) ti – Initial ageing time at Ti (c) t10 8C – Ageing time at 10 8C

that remained active after the catalyst ageing are the responsible ones for the formation of high molecular weight of the polymer. In agreement with the mechanism proposed by Kwag [19], the more unstable active sites are those formed from the most external neodymium atoms of the neodymium compound, hence more susceptible to the deactivation and consequently, responsible for lower molecular weights. That could explain the trend of molecular weight increasing found in this work. The microstructure of high-cis polybutadiene was practically not affected by the variation of the catalysts ageing conditions

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(Table 2). The cis-1,4 units content remained between 98.0 and 98.4 %, and the trans-1,4 units between 1.0 and 1.4 %. The content of vinyl units did also not present significant variation. Thus, we can conclude that stereoselectivity of the active sites was not affected by the ageing conditions tested in this work.

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