In situ cyclization modification in polymerization of butadiene by rare earth coordination catalyst
Introduction
The cyclized rubbers are produced by the cyclization of the corresponding rubbers or the cyclopolymerization of the corresponding conjugated dienes. They are often used in the formulation of adhesives, inks, paints, photoresists and tyres. Much work was reported on the cationic cyclization of polyisoprene, NR and cis-1,4-polybutadiene (BR) before 1980s [1]. Recently, there still are some new cyclized rubbers being synthesized, new cyclization methods and applications coming forth [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and a lot of patents being issued for which the cyclized rubbers as important components in the formulations [12].
The structure of cyclized rubbers is still not well understood. Kossler et al. [13] investigated the structure of completely cyclized BR by IR spectroscopy and iodometric analysis. They concluded that long sequences (up to 20 units) of orthocondensed cyclohexane rings were formed after long reaction times. However, Priola et al. [14] used IR and 13C NMR spectra to study the structure of partly cyclized BR and they realized that the average length of the polycyclic sequences was between 2.5 and 4, depending on the cyclization extent.
The cyclization of styrene–butadiene rubber (SBR) with Lewis acid, AlCl3 or TiCl4, and the cocatalyst (some organic halides systems) was reported, but it was not very successful, for example, the intrinsic viscosity ([η]) of cyclized SBR was higher than that of untreated SBR, because Lewis acid used is not really suitable for the cyclization [2]. Cationic catalyst systems based on alkyl aluminum chloride and organic halides are excellent for the cyclization of BR, so the cyclizations of SBR and neoprene can also be achieved by them [4], [5]. It is well known that there is a little cyclic structure in neoprene due to cyclization, but the further cationic cyclization of neoprene has been seldom reported until now [5]. Lal [3] synthesized a new class of amorphous thermoplastic elastomers by selectively cyclizing polyisoprene blocks in polyisoprene–polybutadiene–polyisoprene block copolymers with different cationic catalysts.
The cationic cyclopolymerization of conjugated dienes, such as isoprene and butadiene, is also an old research field, which Gaylord, Kossler and their cooperators published a series of papers about [15]. Peng et al. [16], [17], [18], [19] reported much work on the cyclopolymerization of pentadiene. The cyclized rubbers obtained by this way have the similar structure and properties like those obtained by the direct cyclization of the rubbers [15]. The catalysts for the cyclopolymerization of conjugated dienes are usually Lewis acid, AlCl3 or TiCl4, and the cocatalyst, diethylaluminum chloride-TiCl4, and so on like for the cyclization of rubbers. The greatest shortcoming of the cyclopolymerization of conjugated dienes is the very low yield of products. By the way, the radical cyclopolymerization of unconjugated dienes, which gives rise to highly cyclized polymers, has being paid more and more attention [20]. But it is a little different from the cyclopolymerization of conjugated dienes.
We have done some work in the cationic cyclization of rubbers by using alkyl aluminum chloride and organic halide systems [4], [5], [6], [7], [8], [9]. In this study, we aimed to obtain the cyclized polybutadiene by in situ cyclization modification in polymerization of butadiene by rare earth coordination catalyst. Butadiene was polymerized to a certain extent in the presence of a rare earth coordination catalyst of neodymium chloride and i-propyl alcohol, and triethyl aluminum (NdCl3·3i-PrOH–AlEt3) in toluene and the allyl chloride was then added to the reactive solution with AlEt3 to in situ cyclize the formed polybutadiene and cyclopolymerize the unreacted butadiene monomers.
Section snippets
Materials
Butadiene (Beijing Yansan Petrochemical Co. Ltd., China) was purified prior to use by passing it through the columns containing sodium hydroxide and active silica gel pellets, respectively. Neodymium oxide (Nd2O3, Shanghai Yuelong Plant, 99.95%) and triethyl aluminum (AlEt3, Carl Roth Co., Germany) were used as purchased. Allyl chloride was dried over 4A molecular sieve and then distilled. Toluene was dehydrated by usual method. Other pure-grade reagents, such as ethanol, methanol and
Effect of molar ratio of allyl chloride to AlEt3 (Cl/Al)
Ziegler-Natta catalysts based on rare earth metals have attracted more and more attention, since they were firstly employed in the homopolymerization of butadiene [22]. Rare earth coordination catalysts have been developed as effective catalysts for the polymerizations of conjugated dienes, alkynes, cyclic monomers and so forth [23], [24]. In this work, the polymerization of butadiene is carried out with a rare earth coordination catalyst NdCl3·3i-PrOH–AlEt3 in toluene to some extent, and then
Acknowledgment
The financial support to this work by the National Natural Science Foundation of Guangdong Province (No. 031380) is gratefully acknowledged.
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