Ultrahigh molecular weight polyethylene: Catalysis, structure, properties, processing and applications

Abstract

Ultrahigh molecular weight polyethylene (UHMWPE) belongs to an emerging class of high-performance specialty polymers with a unique set of properties and applications. The field has witnessed many scientific and technological advances in recent years. However, synthesis of UHMWPE is not a trivial exercise and presents several challenges. This review addresses these fundamental challenges and provides an overview of recent developments in the field of UHMWPE. The nature of catalysts, reaction conditions that favor its formation, their physical properties, methods of processing them into products, and their applications are discussed. Recent developments in formation of UHMWPE in a disentangled state by use of appropriate catalysts and reaction conditions are also discussed. This has elicited considerable interest as a means of enabling melt processing of UHMWPE. This review provides a comprehensive source of information and understanding of the multifaceted aspects of UHMWPE with specific reference to chemistry, catalysis, processes for manufacturing, and an analysis of catalyst structure-polymer property relationships.

Introduction

Even after fifty years since its discovery, metal-catalyzed ethylene polymerization continues to draw significant academic and industrial attention. Though the exact genesis of the discovery can be argued, it is clear that Ziegler's systematic investigations of serendipitous results led to the development of “Mulheim atmospheric polyethylene process” [1]. The process is practiced worldwide and polyethylene (PE) is a household name today. Polyethylene is one of the largest volume polymers produced and our daily life is deeply impacted by this material. A review on polyethylene would be incomplete without connecting to the roots of this discovery and placing it in the right context.

In the early years of his independent research career, Ziegler was investigating the reactivity of organolithium compounds in C-C bond forming reactions. However, these reactions produced insoluble lithium hydride (LiH), which posed an additional challenge. To circumvent the solubility associated with LiH, researchers within the ‘Ziegler school', utilized LiAlH₄, which was more soluble in organic media. After detailed investigations, 'Ziegler school' was convinced that the metal responsible for forming C-C bond was aluminium, and not lithium. His-research focus then shifted to alkyl aluminium compounds and their reactivity in C-C bond forming reactions. Ziegler named the C-C bond-forming reaction as “Aufbau” reaction. In the Aufbau reaction, triethyl aluminium was exposed to ethylene to obtain higher carbon number alkyl aluminium compounds. Hydrolysis of such alkyl aluminium compounds produced linear long-chain macromolecules with molecular weight reaching 3000 Da. In one such “Aufbau” reaction, Ziegler's student noticed the formation of butene instead of the anticipated long-chain alkyl compounds. Ziegler did not ignore this observation and subsequent analysis revealed that nickel impurities were responsible for turning ethylene into butene. While the “Nickel Effect” opened up new opportunities to prepare butene, Ziegler and his coworkers investigated effect of various 3d metals on the Aufbau Reaction and stumbled upon the discovery of polyethylene. Thus, one of Ziegler's student, Heinz Martin, conducted the seminal experiment with titanium tetrachloride and triethyl aluminium in presence of ethylene to yield polyethylene. This led to the famous “Mulheim atmospheric polyethylene process”. Ziegler realized the potential of this process early on and submitted the first patent to German patent office in November 1953. The patent was licensed to many chemical manufacturers and led to the foundation of the polyolefin industry.

One of the licensees to the Ziegler patent was an Italian firm named “Montecatini” and Prof. Giulio Natta was a consultant to this company. Thus, Natta had access to Ziegler catalyst and recognized the potential of this system in stereoregular polymerization of α–olefins. Natta being a structural chemist conceived the idea that propylene is a prochiral molecule and might lead to stereoisomers in the resultant polymer. Subsequently, he discovered crystalline isotactic polypropylene (iPP) and demonstrated “Single Helix” structure of iPP. Due to their seminal contribution, these two names are indistinguishably used in the field of olefin polymerization catalysts. By the time Ziegler and Natta walked to the dais in Stockholm in 1963 to accept the Nobel Prize, polyethylene and polypropylene had already become commercial products.

It was not only the catalyst that drove the commercialization of Ziegler-Natta process, but the fact that the produced polymer had unique properties and found applications, which could not be met by other materials. Over the years, the process has been refined and various generations of Ziegler-Natta catalyst have been developed [2]. The first-generation titanium tetrachloride-alkyl aluminium catalytic system has been improved either by supporting titanium on various supports, or by adding various donors or by changing activators [3]. Our understanding of the mechanism of this reaction has been refined over the years and the monometallic Cossee-Arlman mechanism (Fig. 1) is now widely accepted as the mechanism for coordination-insertion polymerization of olefins.

Ziegler catalyst is typically used to manufacture linear low- and high-density polyethylene (LLDPE and HDPE). Suitably modified Ziegler catalysts are capable of producing ultrahigh molecular weight polyethylene (UHMWPE) and this is discussed in Section 3.5.1.

The discovery of homogeneous metallocene catalyst for polymerization of olefins in the presence of methyl aluminoxane (MAO) with exceptionally high activities was a landmark event in the history of metal catalysed olefin polymerization [4], [5], [6]. A discussion on metallocene catalysts and their ability to produce UHMWPE is presented in Section 3.5.2.

Subsequently, much attention has been focused on a new generation of catalytic systems called “post-metallocene's” which include both early and late transition metals as well as various ligand frameworks [7], [8], [9]. A discussion on post-metallocene catalysts and their suitability to produce UHMWPE will be examined in Section 3.5.3.

One of the most significant landmarks of twentieth-century chemistry has been the discovery of transition metal-catalyzed polymerization of ethylene and higher α-olefins. This event caused a flood of publications as well as an avalanche of patents resulting in a wide variety of products entering the market. Today, over two hundred million tons of polyolefins are manufactured globally, which is roughly half the total amount of polymers produced [10]. The uniqueness of polyolefin chemistry and the associated processes for their manufacturing lies in the ability to create a wide range of materials by judicious and appropriate choice of the catalysts, activators, ligands, monomers, type of reactors and reaction conditions [11]. Amongst various classes of polyolefins, polyethylene is one of the most widely used polymer with the simplest repeating unit, namely, methylene group (-CH₂-). The global demand for PE reached one hundred million metric tons in 2018 and the PE market size is projected to reach US $143 billion by 2026 [12]. A significant portion of this demand is met by the low-pressure metal-catalyzed polymerization process.

UHMWPE belong to an emerging class of high-performance specialty polymers with a unique set of properties and applications. The synthesis of UHMWPE is not trivial and processing of this material is even more challenging. These include, i) designing appropriate metal catalysts and co-catalysts, ii) identification of suitable reaction conditions which tend to enhance the rate of propagation and retard the rate of chain-transfer, iii) moderation of the Lewis acidity of the metal catalyst to reduce β-hydride elimination and, iv) manipulating the steric and electronic properties of the metal center to minimize transfer and termination reactions. Despite these difficulties, several catalytic systems capable of producing UHMWPE have been designed, new processing techniques have been developed and niche applications have emerged in the recent past. The academic and industrial interest in the chemistry and processing of such materials continues to be high as evidenced by the significant number of published papers (>2000) and patents (> 400) (Figs. 2 and 3). However, the available literature on UHMWPE is scattered and there is no comprehensive source where one can access information on chemistry, catalysis, processing, and catalyst structure-polymer property relationships.

The purpose of this review is to provide an overview of recent developments in UHMWPE and present an empirical correlation between the structural features of the catalysts and the nature of polymers produced. Our emphasis is on the principles of catalyst design and reaction conditions that enable the formation of UHMWPE, their physical properties, methods of processing, and key areas of applications. More recently, the preparation of UHMWPE in a disentangled state by the appropriate choice of catalysts and reaction conditions has elicited considerable interest. A UHMWPE in disentangled state might enable melt processing of such high molecular weight polymers and may circumvent the conventional solution processing that is being currently practiced by the industry.

The review is limited to recent advances in UHMWPE and excludes updates on commodity PE (LDPE, HDPE etc.) [1], [2], [3], [4], [5], [6], [7], [8], [9]. Further, post-modification of UHMWPE, UHMWPE composites and UHMWPE degradation has had been summarized elsewhere [13], [14], [15], [16], [17], [18], [19] and are beyond the scope of the present review.