1. Background
Polyolefins were first commercialized in the 1930s by Imperial Chemical Industries (ICI) and were applied as underwater cable coatings. After almost a century of development, polyolefins have become one of the most widely used synthetic polymers, accounting for almost half of all global plastics production [
1]. Due to their many superior properties, they have found many everyday applications, but their annual production and widespread usage in single-use packaging have made them a major component of post-consumer plastic waste [
2]. Polyolefins are indispensable in the modern chemical industry and society, and their global market size is expected to grow rapidly in the foreseeable future. However, the current linear production and consumption of polyolefins is unsustainable and has created a serious environmental burden. Traditional recycling of polyolefins is performed via either mechanical recycling or pyrolysis. Mechanical recycling has been the dominant polyolefin recycling strategy, but side reactions including degradation and crosslinking seriously damage material properties, leading to products with lower quality compared with the original materials. Pyrolysis also faces many issues such as low energy efficiency, poor product selectivity, and environmental pollution. Due to the limitations of these two strategies, novel recycling strategies or the redesigning of polyolefins are highly desirable.
A large amount of polyolefins has been produced and accumulated/disposed of, and the situation will only worsen in the future. Therefore, innovative pathways are needed to deal with the existing and ever-increasing post-consumer polyolefin waste, which may be considered a valuable resource/feedstock instead of waste or pollution. The overall concept and microstructure of polyolefins should be reconsidered and redesigned to enable their more circular and sustainable development. Within this perspective, many research efforts have led to the development of various strategies to alleviate these challenges. Herein, some of the most notable advances will be summarized and discussed in two categories from the authors’ perspective: ① mechanical or catalytic upcycling of existing polyolefin waste into valuable resources or feedstocks to products with higher quality, and ② redesigning polyolefins for efficient production and property enhancement, as well as degradation and sustainable development.
2. Upcycling of polyolefins
2.1. Mechanical upcycling
Traditional mechanical recycling of polyolefins always generates products with poorer qualities and therefore less utility. This is due to reactions such as chain-scission or crosslinking during extrusion, or commingling with other types of plastics in waste streams. Many strategies have been explored to address the first issue. For example, Milliken & Company developed a process to couple recycled polypropylene (PP) with PP impact copolymers to modify the melt-flow and impact properties of post-consumer plastic waste.
One of the biggest challenges in recycling waste plastics is separating polymer mixtures. Due to the interfacial tension between different polymer phases, direct mechanical recycling produces products with very poor material properties. Compatibilization by adding amphiphilic additives efficiently restores the performance and value of recycled mixed plastics [
3]. For example, Coates et al. [
4] added the multi-block copolymer, polyethylene (PE)-
block-isotactic polypropylene (
iPP) (i.e., PE-
b-
iPP) to enhance the mechanical properties of PE/
iPP blends, as well as the adhesion between PE/
iPP interfaces, as shown in
Fig. 1(a). We recently explored several synthetic routes to prepare various linear multi-block copolymer (
lMBCP)-containing PE blocks and polar polymeric blocks (e.g., PE-
b-polybutylene terephthalate (PBT), PE-
b-polyethylene glycol (PEG), PE-
b-poly(ethylene succinate) (PES), PE-
t-polycaprolactone (PCL), PE-
t-polylactic acid (PLA), and PE-
b-polycarbonate (PC)) [
5]. The resulting
lMBCPs in
Fig. 1(b) were used as additives for the compatibilization/upcycling of mixtures of polar polymer and polyolefins.
We have also developed several stapler strategies for the direct blending of mixed plastics using a reactive compatibilization strategy and amphiphilic block copolymer strategy [
6]. In this strategy, various pre-formed functionalized graft copolymers are reacted with a very small amount of stapler molecules to synergistically enhance the interfacial adhesion of mixed polymers. These stapler strategies were used for a variety of two-component and three-component polymer systems by adding an extremely small amount of graft copolymer (as low as 0.1 weight percent (wt%)) and stapler molecules (as low as 0.01 wt%). This approach dramatically increased the mechanical properties of the mixed plastics, providing exciting opportunities and practical solutions for the direct upcycling of mixed plastics.
2.2. Catalytic upcycling
In contrast to the traditional pyrolysis process, catalytic upcycling breaks a portion of the C-C bonds in the polyolefin backbone and selectively produces high-value products under mild conditions [
7] such as olefin monomers, telechelic products, high-quality fuels, and other small-molecule products. For example, Hartwig et al. [
8] transformed PE and PP into propylene or a mixture of isobutylene/propylene using base-metal catalysts comprising tungsten oxide on silica (WO
3/SiO
2) and sodium metal on gamma-phase alumina (Na/γ-Al
2O
3) as shown in
Fig. 1(c).
Post- polymerization modification (reactive extrusion) of polyolefins using mechanical extrusion is a widely practiced industrial approach to improve their surface properties and broaden their applications. For example, maleic anhydride-grafted PE can be used to compatibilize PE with polar additives such as sawdust. Silane-grafted polyolefin elastomers (POEs) can be used to improve the adhesion of traditional polyolefin to the surface of glass or to upcycle post-consumer polyolefin waste. However, side reactions, including chain-scission/crosslinking, lead to property deterioration and discoloration, similar to the issues encountered during mechanical recycling. Recently, many catalytic C-H functionalization strategies have been applied to install various polar groups on polyolefin backbone in a mild and selective fashion [
9]. The resulting modified polyolefin materials showed greatly improved mechanical, thermal, paintable, and adhesive properties. The C-H functionalization of post-consumer polyolefins to install various polar groups has recently emerged as a powerful practical method to generate materials with diverse properties for enhanced waste utilization.
3. Redesigning of polyolefins
3.1. Redesigning for more efficient production and property enhancement
The redesigning of traditional polyolefin materials from synthetic and microstructural perspectives can potentially reduce the energy input and carbon footprint of the entire production process. This can also enhance polyolefin material properties by modulating their microstructures to prolong their service life and reduce material usage. However, polyolefins are currently produced in an extremely energy and cost-efficient process after almost a century of extensive academic and industrial research. For example, the capacity of PE or PP production can reach 500 000 t per year for a single production line, leaving very little room for any improvement in current systems. However, completely redesigning the microstructures and synthetic routes of traditional polyolefins may provide new opportunities to break this deadlock. Traditionally, the copolymerization of ethylene with α-olefin comonomers represents an important strategy to produce diverse polyolefin products/grades such as linear low-density polyethylene (LLDPE) or POE as depicted in
Fig. 2(a). However, α-olefin comonomers such as 1-hexene and 1-octene are mainly produced via ethylene oligomerization, which is a non-selective process that generates a distribution of different α-olefins, which requires energy-intensive separation steps. Recently, ethylene-based polyolefin elastomers (EPOEs) have emerged as a novel class of polyolefins prepared using ethylene as the only feedstock, bearing similar even better elastic and mechanical properties compared with classic POEs [
10]. This is achieved via an
in-situ branching process during the chain-walking polymerization of ethylene mediated by late-transition-metal catalysts (nickel or palladium).
As mentioned in the catalytic upcycling section, the incorporation of polar comonomers can generate a variety of polar, crosslinkable, self-healing, and photoresponsive novel functional polyolefin materials. Compared with the above-mentioned catalytic C-H functionalization strategy, catalytic copolymerization of olefins with polar functionalized monomers represents the most direct and economic strategy to obtain polar-functionalized polyolefins. However, this is a major challenge due to a series of side reactions induced by polar groups, such as the poisoning of the catalytic metal center. Recently, significant research has been directed to solve this issue, leading to numerous high-performance homogeneous and heterogeneous catalysts, efficient copolymerization strategies, and systematic studies on material properties. It is believed that the transition-metal-catalyzed coordination copolymerization of olefins with polar functionalized comonomers is close to realizing practical applications [
11].
3.2. Redesigning for more degradation and sustainable development
Future sustainable development will require the development of efficient catalytic systems to prepare new polyolefin-like materials that are degradable and recyclable while possessing material properties comparable with those of traditional polyolefin materials. One efficient strategy to achieve this goal is the installation of a small number of predetermined breaking points (ester, siloxane, etc.) in the polyolefin main chains during chain-growth or step-growth synthesis. For example, Mecking et al. [
12] reported the generation of PCs or polyesters from long-chain C
18 dimethylester and C
18 diol, as displayed in
Fig. 2(b). The long methylene chain served as a crystalline unit to obtain PE-like material properties, while the ester or carbonate functional groups acted as breakable units that enabled these novel materials to be chemically recycled. Recently, Tang et al. [
13] reported the production of a series of AB-type telechelic PEs end-capped with -OH and -CO
2Et groups via coordinative chain-transfer polymerization (CCTP) of ethylene and 1-octene in the presence of a functionalized chain-transfer agent. The subsequent condensation polymerization reactions generated a variety of ester-linked LLDPE-, POE-, and olefin block copolymer (OBC)-like materials with properties comparable to those of commercial polyolefins, along with degradability/recyclability. We recently designed a cyclic-acyclic monomer metathesis polymerization (CAMMP) strategy to copolymerize cyclic monomers with different acyclic diene monomers containing chemically degradable functional groups such as siloxanes, carbonates, and esters, which could be deconstructed by treatment with fluoride, acids, or bases [
14]. By choosing different monomers, degradable thermosets, thermoplastics, and elastomeric polyolefins with great physical properties can be efficiently produced, as shown in
Fig. 2(c). We also designed a tandem olefin metathesis polymerization (TOMP) strategy that employed the ring-opening metathesis copolymerization (ROMCP) of commercial cyclic monomers with cyclic olefin monomers bearing breakable units generated
in-situ from the olefin ring-closing metathesis (RCM) of diene monomers [
15]. This route overcame the intrinsic disadvantage of diene-related chain-transfer reactions during CAMMP and enabled the efficient synthesis of degradable polyolefins with much higher polymer molecular weights and better physical properties.
4. Conclusions
With their low cost, huge annual production, and great chemical/physical properties, polyolefins have become indispensable in many modern products. However, they pose a severe environmental burden because they are often used and then discarded, such as for single-use polyolefins used in food packaging, delivery packaging, and agricultural films. Due to their intrinsic limitations, the currently employed recycling strategies, including mechanical recycling and pyrolysis, cannot address this waste, creating a need for more efficient and innovative strategies. Mechanical upcycling that can change post-consumer polyolefins into materials with properties comparable to or even better than the original virgin resin could address this issue. Catalytic upcycling that can selectively transform polyolefin wastes into high-value products under mild conditions is also highly desired. For either strategy, two critical issues have to be addressed: ① the isolation/purification of the target material from mixtures of plastics and/or contaminants and additives; ② the restoration of material properties from post-consumption-related deterioration to the original (or even better) state versus the virgin resins. This must be carried out in an energy-efficient and economical fashion to consider post-consumer polyolefins as valuable resources/starting materials instead of waste or pollution.
In the long run, strategies that could redesign the whole polyolefin system are fascinating and worthy of exploration. The production of polyolefins via more energy- and material-efficient routes and the synthesis of polyolefins with enhanced properties could help reduce their pollution and carbon footprint. The development of high-performance catalysts, novel synthetic strategies and processes, and new application areas could also contribute to achieving this objective. Finally, the design and low-cost production of novel polyolefin-like materials that are recyclable and degradable while possessing material properties comparable to those of traditional polyolefins could provide the ultimate solution for their true sustainable development.
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