Neodymium versatate catalyst for the 1,3-butadiene polymerization – Effects of reaction parameters
Graphical abstract
Introduction
New elastomers and vulcanization compounds with wider ranges of properties have been continuously sought, given the more demanding consumer market and the necessity to enhance rubber manufacture processes and to produce sustainable materials, such as a green tires [1].
The annual consumption of synthetic rubbers was close to 15 million metric tons in 2016 [1]. Particularly, polybutadiene (BR) is the second most important synthetic elastomer, after the SBR rubber, poly(styrene-co-butadiene) rubber [2]. As a matter of fact, synthetic rubbers are demanded in several areas, including the automotive, construction and materials industries, among others. However, the most important segment for elastomers is the tire industry, which consumed about 60% of all BR produced in 2014 [3]. It is estimated that the global demand for elastomers, and BR in particular, will increase approximately 2.4-3.0% per year until 2022 [3].
The most studied and developed organometallic catalysts that are used for production of polybutadienes are based on cobalt, nickel, titanium and neodymium. These catalysts can produce elastomers with different characteristics, depending on the metal that is used. For example, cobalt produces branched chain polybutadienes, whereas neodymium produces essentially linear polymers [4], [5], [6], [7]. Neodymium catalysts are used commonly for the manufacture of butadiene elastomers because they provide very high catalytic activities and polymer materials with very high 1,4-cis contents, that present many comparative advantages, such as better abrasion resistance, higher resilience and improved hysteresis characteristics [4], [7], [8], [9], [10].
It is known from previous studies that many reaction parameters, including the reaction temperature and the catalyst ageing time, can affect the macrostructure of the produced polybutadienes. For example, average molecular weights of the final product are expected to increase when the catalyst is aged [6], [11], [12], [13], [14]. However, simultaneous variations of reaction temperatures and catalyst ageing times for a given catalyst system have not been performed, so that synergistic variable interaction effects have not been characterized and analyzed in previously published reports. For this reason, the main objective of the present work is to evaluate the combined effects of reaction time, reaction temperature and catalyst ageing on the course of 1,3-butadiene polymerization reactions performed with the neodymium versatate catalyst. It is shown for the first time that the isomers content of the final resin is sensitive to ageing, but not to the reaction temperature in the analyzed experimental range. Besides, it is also shown for the first time that chain crosslinking takes place when reaction is allowed to continue after reaching monomer conversions close to 100%.
Section snippets
Materials
1,3-Butadiene (highly pure polymer degree), hexane (highly pure polymer degree) and the antioxidant Irganox 1520 L/BASF were supplied by Arlanxeo (Brazil). Neodymium versatate (NdV) was provided in solutions of hexane (50% w/w) by Rhodia/Solvay (USA). Terc-butyl chlroride (TBC, with minimum purity of 99 wt%) was provided by Sigma Adrich (Brazil) and mixed with hexane to produce solutions containing 30 wt% of TBC. Di-isobutylaluminium hydrate (DIBAH) was provided as solutions in hexane (1 M) by
UV characterization of the catalyst
Long neodymium acid carboxylic chains are used as catalysts, when exchanged with rare earth or transition metals, which are insoluble in water and soluble in polar organic compounds [23]. The color of trivalent neodymium (Nd+3 (4f3)) is associated to low intense bands of visible spectrum of 4G5/2, 4G7/2⟵ 4I9/2 at 575 nm and transition duplet of 4G7/2 ⟵ 4I9/2 at 521 nm and 2K13/2,4G9/2 ⟵4I9/2 at 508 nm. The absorption band at 575 nm is attributed to Nd+3 (blue-pink color) while absorption of
Conclusions
Neodymium versatate catalysts were synthesized, aged at 5 °C for 0–60 days and tested in solution 1,3-butadiene polymerization. The reactions were performed in the temperature range from 60 to 80 °C for 40–120 min. It was shown that the isomers content of the final resin is sensitive to ageing and temperature. There are synergetic effects that make aged catalysts more sensitive to temperature variations than fresh catalysts. Besides, aged catalysts allowed for production of polybutadienes with
Acknowledgments
The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil) and Arlanxeo (Brazil) for providing scholarships and financial support.
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