Steering polymer growth by molding nanochannels: 1,5-hexadiene polymerization in high silica mordenite

Highlights

• Intruded hexadiene molecules in pure silica mordenite polymerize into a continuous 1D chain.

• Zeolite channels act as a mold shaping the polymer structure among many otherwise possible.

• The polymer consists of cyclic units intercalated by short linear side chains.

• The polymerization reaction involves all zeolite channels although initiated by a few acid sites.

Abstract

Zeolites are known as scaffolds for the assembly of molecules via non-covalent interactions yielding organized supramolecular materials. Yet their potential in driving the growth of low-dimensional systems requiring covalent bond formation is still unexplored. We incorporated 1,5-hexadiene in the channels of a high-silica mordenite and analyzed the material by infrared spectroscopy, X-ray powder diffraction, thermogravimetry and modeling techniques. Due to the few zeolite acid sites, 1,5-hexadiene experiences a slow conversion to a polymer mainly formed by cyclopentane units and featuring short side chains that are able to fit the channels. The shape-directing abilities of zeolite frameworks play a two-fold role, involving first the organization of the monomers inside the void-space and then the linear growth of the chain dictated by the channel geometry. These findings highlight the molding action of zeolites in directing transformations of covalent bonds under ambient conditions and may provide insights for obtaining confined polymers with intriguing prospective applications.

Introduction

The preparation and exploitation of low-dimensional materials is a fundamental issue in nanoscience and nanotechnology. Microporous materials such as zeolites are of great interest in this context, because of their unique pore topologies [[1], [2], [3]]. The “open space” of zeolites can be filled by suitable encapsulated guest molecules and cations that can diffuse through the pores and form organized low-dimensional aggregates tailored by the zeolite channels [[4], [5], [6], [7]]. Moreover, the fine tuning of their pore architecture and of their hydrophilic/hydrophobic character can improve their performance for technological or industrial applications [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. Different kinds of reactions can be promoted and accomplished in zeolite pores. For example, the formation of mixtures of long-chain polymers and small oligomers of pyrrole and thiophene have been reported to occur inside transition-metal substituted zeolites, such as Cu- or Ni-mordenites [19,20]. Also, the presence of Brønsted acid sites [[21], [22], [23], [24], [25]] may strongly affect the formation of a polymer inside the zeolite cavities: in particular, acidic zeolites themselves can induce a polymerization reaction [26].

The preparation - via a chemical reaction - of a continuous organic conductive nanowire embedded in zeolite micropores would therefore be of fundamental relevance in advanced materials fabrication. In fact, preparing isolated, self-standing, densely packed conducting polymers is extremely challenging when conventional protocols are adopted, since the aggregation and bending of chains prevent the production of real 1D systems [26]. Conversely, by imposing pressure (a few GPa) on zeolite systems, it is possible to induce a rearrangement of the chemical bonds among guest species by simple mechanical tuning of the intermolecular/interatomic distances [27]. Following this procedure, linear chains of polyethylene [28], polyacetylene [29] and polycarbonyl [30] can be synthesized inside zeolite channels and, very recently, phenylacetylene has been oligomerized at high pressure inside the zeolite Mordenite [31].

The aim of our study, which is a part of a wider project aimed at the preparation of conductive polymers embedded in a protective matrix, is to exploit the template effectiveness of zeolite in inducing a polymerization along preferential directions that would not occur in bulk. This paper is the first step towards the preparation of a hybrid material made from a linear polymer identified as suitable for pyrolysis to obtain a conductive polymer protected inside a hydrophobic matrix. The resulting innovative material would have the potential to be exploited in several technological applications including, for example, gas sensing. In this field, in fact, the ability to fully exploit the properties of such a hybrid (i.e. its hydrophobicity, huge surface area and confinement of the host species) would yield the combined advantages of low cross-sensitivity to humidity and high sensitivity. In addition, an effective host-guest interaction is expected to increase the stability of the guest material. Clearly, these are all desirable characteristics for a gas sensor but, despite several years of investigation using different approaches, their development is yet to be fulfilled.

As a zeolite scaffold, we chose a high silica mordenite (HS-MOR) (Fig. 1a), while as the molecule to polymerize we selected 1,5-hexadiene (hexa) (Fig. 1b). HS-MOR was selected for the following reasons: i) the framework must have a unidimensional channel system to promote the synthesis of isolated 1D polymer chains; ii) the dimensions of the pores must allow the penetration of hexa; iii) a residual porosity should be maintained after polymerization to allow the gas transit for exploitation in gas sensing. The unidimensional system of parallel channels in MOR (see the section “Materials” for a detailed description of the structure) perfectly fits with these requirements since hexa can enter (and consequently may be polymerized) in the 12 MR channels and the gas to be sensed can circulate in the smaller channels (8 MR) and be in contact with the conductive wire through side pockets. Hexa was chosen since its polymerization can result in an almost linear chain and the basic idea is to make the chain conductive after a selective pyrolysis reaction (not treated here).

It is worth noting that, in the absence of confinement, this molecule undergoes a series of complex reactions (e.g. Cope rearrangement [32], dimerization [33], polymerization [34]) characterized by intricate paths, leading to a mixture of products. A regular polymer, using 1,5-hexadiene as a monomer, can be obtained only via a homogeneous Ziegler-Natta catalyst, leading to poly (methylene-1,3-cyclopentane) (PMCP) [34].