Synthesis and application of polyethylene-based functionalized hyperbranched polymers
Introduction
Highly branched polymers have been studied extensively in recent years. Compared to the corresponding linear polymer, highly branched polymers, such as dendrimer and hyperbranched polymers, possess unique physical properties stemming from their different architectures. Dendrimers are well-defined, highly ordered molecules with three-dimensional structures. By employing suitable synthetic strategies, functional groups in dendrimers are located at predesigned sites, resulting in adjustable properties. The unique structure of dendrimers leads to some exceptional properties promising for particular applications, such as drug delivery [1], gene carriers [2], templates for mesoporous materials [3], molecular recognition [4], [5], signal amplification [6], [7], catalysis [8], [9], [10] and pollution removal [11]. However, preparation of dendrimers, which usually contains multistep synthesis and purification, is tedious and costly. It is desired to develop new efficient methodologies to produce dendrimers or replacements.
Conventional synthesis of dendrimers is inconvenient and time-consuming using a chain of iterative growth and activation steps. Although some revised strategies have been developed to simplify the synthesis, the design and preparation of monomers are still challenging for commercial production. Despite their imperfection in symmetric topology, hyperbranched polymers, featuring a multitude of branches and three-dimensional fractal structure with cores, branches and peripheries, still retain exceptional chemical and physical properties similar to dendrimers. The preparation of hyperbranched polymers greatly reduces synthetic cost and time compared with dendrimers. They are often easily produced in one-pot and one-step, suitable for industrial application in a large scale without stepwise sophisticated reactions and costly purifications. Consequently, less costly and easily obtained hyperbranched macromolecules are desired to replace dendrimers in some particular applications.
Large scale polyethylene production is often performed in the present of early transition metal catalysts. Because of their high oxophilicity, early transition metal catalysts tend to be poisoned by polar comonomers. Therefore, it is difficult to functionalize polyethylene through direct copolymerization, desirble for industrial production. Conversely, late transition metal catalysts have lower oxophilicity tolerant toward polar comonomers. However, before 1995, due to β-hydride elimination, only dimers or oligomers could be produced with late transition metal catalysts under mild conditions [12], [13], [14], [15]. Introducing bulky, aryl substituents in α-diimine Ni or Pd, enable the synthesis of high molecular weight polyethylene with such catalysts [16]. In addition, Pd α-diimine catalysts enable copolymeriziation of ethylene with polar monomers [17]. The functionalized polyethylenes possess a high branching degree, with the incorporated polar groups dominantly located at the end of the branches. Based on the discovery of these new catalysts, Guan showed that the topology of polyethylenes produced by using Pd α-diimine catalysts may be controlled by adjusting the polymerization conditions [18] characterizing the control mechanism as a “chain-walking polymerization” (CWP). At a relatively low pressure of feeding ethylene, chain walking leads to the formation of branch-on-branch structures, namely, a hyperbranched topology. As revealed by light scattering and 13CNMR, the topology of polyethylenes changes with polymerization pressure from linear to hyperbranched to dendritic [19]. Chen et al. reported that polar ethylene copolymers initiated by Pd α-diimine catalysts can also be topologically controlled by changing the ethylene pressure [20].
Polyethylene-based functionalized hyperbranched polymers are promising materials for replacing dendrimers in certain suitable areas, offering significant reduction in production time and cost. Tunable topology and functionality of this new class of polyethylenes has attracted attentions from industry and academia. However, limited work has focused on the application of this class of polymer. Although hyperbranched polyethylene (HBPE) has been reviewed, the functionalization and applications of this new polymer has not been introduced fully and completely [21], [22]. The aim of this article is to shed light on different strategies and applications of polyethylene-based functionalized hyperbranched polymers.
Section snippets
Incorporation of functionality into polyethylene-based functionalized hyperbranched polymers
Owing to competing β-hydride elimination, late transition metal catalysts are more prone to oligomerize or dimerize ethylene compared to early transition metal catalysts. In 1995, Brookhart M. et al. reported a new class of Ni- and Pd-based catalysts 1–5 (Fig. 1) for polymerization of ethylene or α-olefin [16].
The incorporation of bulky α-diimine ligand onto the metal center can weaken β-hydride elimination, so that polyethylenes with average molecular weight ranging from 1.8 × 103 to 1.6 × 106
Applications of polyethylene-based functionalized hyperbranched polymers
Cationic Pd α-diimine-prepared functionalized HBPE is a potential material to replace well-defined dendrimers, because the one-pot synthesis of these polymers involves simplified preparation steps and monomer design. Peripherally located polar sites on functionalized HBPE give further opportunity to modify such materials for numerous applications, such as nanocarriers, bioconjugates, catalysis and live-cell imaging etc. Examples of the published applications of polyethylene-based functionalized
Conclusions and perspectives
The properties of polymers depend not only on the types of monomers used to prepare them, but also the architecturally complex structure—composition, topology and functionality. Materials made from the same monomers with varying molecular structures have diverse properties and applications. For example, polyethylene with different branching degree leads towards the diversity in their properties. Low density polyethylene is flexible and light, mostly used as packing films. But high density
Acknowledgment
Financial supports from the Science and Technology Program of Zhejiang Province (2008C14089) and Science and Technology Innovation Team of Ningbo (2011B82002) are gratefully acknowledged.
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