Elsevier

Polymer

Volume 111, 24 February 2017, Pages 20-26
Polymer

Synthesis of high crystalline syndiotactic 1,2-polybutadienes and study on their reinforcing effect on cis-1,4 polybutadiene

https://doi.org/10.1016/j.polymer.2017.01.022Get rights and content

Highlights

  • Acetylphosphonate is explored for iron catalyst as additive for production of s-PBD.

  • The catalyst is high 1,2 syndiotactic specificity with high thermal stability.

  • s-PBD is proved to be a good reinforcer for cis-1,4 polybutadiene.

Abstract

A novel compound diethyl acetylphosphonate has been explored as efficient additive in iron based catalyst for stereo-polymerization of 1,3-butadiene to afford high crystalline 1,2-syndiotactic polybutadiene. Excellent 1,2 selectivity in range of 91.0–98.4% with controllable crystallinity has been achieved at high catalytic efficiency in wide ranges of cocatalyst and additive feeding as well as at large temperature scope. As reinforcing agent, two crystalline polymers differing in crystallinity 4.4% and 51.9% are blended and co-cured with cis-1,4 polybutadiene (PB) with 10 wt-%, providing samples PB1 and PB2, respectively. SEM from the sample fracturesurface shows the s-PBD particles are intimately mixed and homogeneously dispersed in PB matrix due to the strong interfacial interaction. The analysis of the mechanical properties of sample PB2 finds the tensile strength of increases by one-fold and the elongation at break increases by two-fold when head to head comparison to those of the CB filled PB, while the shear modulus G′ and loss factor (tan δ) are as good as those charged with CB. These results highlight the role of s-PBD as reinforcing agent of PB rubber, yielding high performance rubber materials with improved mechanical properties.

Introduction

The regio- and stereoselective polymerization of conjugated dienes promoted by transition metal complexes is an extensively studied topic and a challenging field in both academic and industrial environments for the relevance of these materials as synthetic rubbers and plastics. In particular, because of its industrial importance, many studies have been devoted to the cis-1,4-butadiene polymerization while the 1,2 polymerization has received rather limited attention. 1,2 Polymerization of 1,3-butadiene is able to produce three stereoisomers depending on the configuration of the vinyl group pending along the chain: atactic, syndio- and isotactic-1,2 polybutadiene. Syndiotactic 1,2-polybutadiene (s-PBD) with predominant 1,2 content (>95%) and syndiotacticity (pentads rrrr > 95%), is a thermoplastic resin characterized by high crystallinity and high modulus, therefore, it has many unique uses [1]. For example, films, fibers, and various molded articles can be made from syndiotactic 1,2-polybutadiene in the presence of stabilizer [2], [3], [4], [5], [6]. Of note is that it can also be well blended into and co-cured with natural or synthetic rubbers due to the close chemical structures with abundant double bonds as efficient crosslinking sites. Therefore, the interfacial interaction with rubber matrix is envisioned to be superior to those of typical fillers like silicates [7], [8], [9], [10], [11], graphene [12], [13], [14], [15], clay [16], [17], [18] and polypropylene [19], [20], [21], where chemical modification of their surfaces or the use of surfactants are necessary to improve the filler-rubber interaction because of the hydrophilic or inert properties of the filler surface.

The past decades have witnessed the significantly advances in controlling stereoselectivity of α-olefins catalyzed by the ligand orientated catalysts. In contrast, 1,2-stereoselectivity of 1,3-butadiene seems challenging and the efforts to control the 1,2-stereoselectivity in transition metal complexes catalyzed 1,3-butadiene polymerization by judicious choice of the ancillary ligand has met with limited success. Well-defined catalysts able to initiate 1,2 stereo polymerization are confined to chromium and cobalt compounds with a limited type of trialkyl (aryl) phosphine ligand [22], [23], [24], [25]. It may be considered that the driving forces for stereoregulation of s-PBD are different from those for α-olefins. In the coordination polymerization of olefins, the stereospecificity strongly depends on the symmetry of metallocene catalyst, notable for group 4 metal catalysts. Regiochemistry of 1-alkenes polymerization can occur via 1,2 (or primary) as well as via 2,1 (or secondary) insertion and lead to growing chains with a primary or a secondary C atom bonded to the metal atom, respectively, while syndiotactic polymers are generated by a regular alternation of insertions of re and si coordinated [23], [26], [27], [28], [29]. However, in the polymerization of conjugated dienes, both monomer coordination mode, e. g. cis-ƞ4 [30], [31], trans-ƞ4 [32], and trans-ƞ2 [23], [24], [33], [34], [35] and insertion (1,2- and 1,4) determine the resultant regio- and stereoselectivity. These are fundamentally affected by the nature of binding atom for additive or ligand as well as operational conditions such as cocatalyst, temperature, solvent. For example, some phosphine and oxygen incorporated additives or ligands have been proved to promote 1,2 selectivity for cobalt, chromium and vanadium catalysts [22], [27], [36], [37], while 1,4 enhancement effect of chloride is observed for titanium, iron, vanadium and nickel [38], [39]. Therefore, recent research is focused on exploration of novel ligand or additive consisting of favorable heteroatom that is able to tune the electronic and steric properties of active species. Considering additive is commercially available reagent that can avoid the tedious synthetic procedures and also allow some notable operational advantages, there is remarkable worldwide research activity and interest in the development of additive which has been among the most general tools to promote the 1,2 selectivity for butadiene polymerization. As such, controlling of 1,2-regio and stereo-specificity of 1,3-butadiene polymerization have been realized by elegantly modifying phosphine additive for both cobalt and chromium catalyzed polymerization [32], [40], [41].

Iron catalyst is advantageous due to the earth abundance and low cost, as well as the biocompatibility and negligible environmental impact. Despite persistent efforts in almost 30 years, the development of these catalysts is limited to technological improvements rather than to search for new catalysts. Limited by effective additive, 1,2 stereoselective polymerization catalyzed by iron catalyst still represents a challenge as the system can be considered deficient in some regards, such as low catalytic activity, moderate 1,2-regularity and harsh operation condition. Recently, the discovery of additive dialkyl(aromatic)phosphites and triarylphoshate for iron catalyst, have made an impressive leap at control of it's stereoselectivity, and led to a versatile iron-based catalysts for synthesis of syndiotactic 1,2-polybutadiene (1,2-selectivity: 87–95%, rrrr: 81–90%), atactic 1,2-polybutadiene (as elastomer, 1,2-selectivity: 55–87%) as well as their block copolymer in our group [33], [42], [43], [44]. Of note is that the stereochemistry of active species from this catalyst is also significant at higher temperature. These promising breakthroughs, on one hand, accelarate our moumentus on study of mechanistic details of diene insertion, chain growth, and chain release processes at the molecular level to explore the factors responsible for both excellent activity and stereoselectivity, on the other hand, encourage us to explore the mechanical properties and study potential applications of the resultant polymers. Herein, we wish to report another novel compound and its ability as additive to regulate the stereoselective polymerization of 1,3-butadiene. Subsequently, the addition of the produced s-PBD as reinforcing agent for cis-1,4 polybutadiene is firstly described, and the mechanical, thermal and morphological properties of these novel blends have been investigated and discussed, which could be a clue for fortifying general rubber.

Section snippets

Materials

Triethylphosphite, Iron(III) acetylacetonate (Fe(acac)3), Acetyl chloride and tri-iso-butylaluminum (TIBA) (1.1 mol/L in toluene) were purchased from Energy chemical. Polymerization grade 1,3-butadiene was supplied from Valley Gas Co., Ltd., purified by n-butyllithium in liquid state prior to use. All solvents used were purified by the standard procedures. Cis-1,4-polybutadiene (BR, trade name BR9000, cis-1,4-content > 97%, Mn = 6.6 × 104 g/mol, Mw/Mn = 5.2, Mooney viscosity ML1+4 at

Catalyst components

The activity of this polymerization system follows as a function of cocatalyst feeding at [Bd]/[Fe] = 2000, reaching a plateau of 99,360 g/mol at 20 of [Al]/[Fe] feeding, more active than the reported iron catalysts [42], [43], [44]. Increasing the monomer feeding harvests a four-fold increment of activity at [Bd]/[Fe] = 8000 referring to that obtained at 2000. The 13C NMR analysis of the polybutadiene samples reveals a prevailingly 1,2 microstructure (93–98.2 mol-%, Table 1) in all polymers,

Conclusions

Diethyl acetylphosphonate has been demonstrated as efficient stereoregulating additive in iron catalyst for 1,2 syndiotactic polymerization. The mild polymerization conditions such as readily and cheap catalyst components, benigh polymerization conditions hexane as solvent at 50–80 °C, as well as high activity and high syndiotacticity selectivity demonstrate the ability of the polymerization process to yield s-PBD with easy accessibility. The presence of crystalline s-PBD produces a two-fold

Acknowledgements

This work is supported by the Natural Science Foundation of China (Grant No. 21304050), the Natural Science Foundation of Zhejiang (LY17E030002), the Natural Science Foundation of Ningbo (2016A610045), the Key Innovation Team of Zhejiang province (201150001-07) and Open Research Fund of Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (KLSR 2015001). This work is also sponsored by K. C. Wong Magna Fund in Ningbo University.

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