From Plastics to Rings and Back Again: Catalytic Recycling of Waste Oxygenated Plastics

尽管在过去的一个世纪里，塑料的使用量呈指数级增长，但人们很少关注如何处理寿命结束时留下的大量塑料。迫切需要一个解决日益增长的塑料垃圾的方法。大多数塑料来自石油化工产品，虽然机械回收很发达，但它通常与高能源成本和材料降解有关。化学回收有可能创造一种循环塑料经济，因为原始原料可以在温和的条件下回收，使它们能够重新结合到新的塑料中，而不需要新的原材料。塑料是由称为聚合物的长链状分子组成的。而这些又是由一系列单体组成的。单体的连接顺序、连接方式和每个单体的单独结构决定了聚合物的性质以及它们用于哪种类型的塑料。化学回收将塑料中的聚合物分解成它们的起始单体，然后这些单体可以重复使用。化学回收面临许多挑战。许多商业塑料由于其化学成分与化学回收不相容，因此需要可以化学回收的替代塑料来取代这些塑料。含有氧键的聚合物，如聚碳酸酯，特别有希望，因为它们更容易被分解。聚碳酸酯可以由二氧化碳和环氧化物结合而成。环氧化物是含有一个氧原子的三元环。改变所用的环氧化物会影响所得聚碳酸酯的性能。虽然以前的研究表明，各种聚碳酸酯可以被分解以回收起始环氧化物，但对这些起始环氧化物的选择性是一个主要问题，因为通常会形成环状碳酸酯产物。催化剂是克服这一挑战的关键工具，因为它们有助于环氧化物的形成以及加速分解反应。虽然已经成功地证明了几种不同的聚碳酸酯被催化分解为它们的起始环氧化合物，但要想在工业规模上采用聚碳酸酯，它们的性能必须与目前的商业聚合物相竞争。聚环己烯碳酸酯(PCHC)和聚碳酸亚丙酯(PPC)是目前最有潜力取代现有商业聚合物的两种聚碳酸酯。不幸的是，事实证明，由于选择性问题，这些聚碳酸酯的化学回收变得更加困难。Williams小组最近的工作已经开发出一种新的、高效的催化剂来将PCHC化学循环到其环氧化物1，但PPC的化学循环仍然是一个挑战。该项目旨在开发将PPC化学循环回到其起始环氧化物的催化剂。以PCHC为例，本项目将研究聚碳酸酯催化分解背后的机理，以及改变聚碳酸酯的形状和结构如何影响其分解速度。这些信息将被用来指导未来的催化剂和聚合物设计。此外，还将探索其他类型含氧聚合物的化学回收，特别是聚(酯-丙基醚)的化学回收。最终目标是开发有效的化学回收聚合物的方法，以实现可持续的循环塑料经济。该项目属于EPSCR‘制造未来’研究领域。

Despite the exponential increase in plastic usage in the past century, little attention has been paid to dealing with the huge amount of plastic left at the end of life. A solution to the growing plastic waste is desperately needed. The majority of plastics are derived from petrochemicals and whilst mechanical recycling is well developed, it is typically associated with high energy costs and material degradation. Chemical recycling has the potential to create a circular plastic economy as the original starting materials can be recovered under mild conditions, allowing for their reincorporation into new plastics and avoiding the need for new raw materials.Plastics are made up from long, chain-like molecules called polymers. These in turn are built up from a series of monomers. The order in which the monomers are connected, how they are connected and the individual structure of each monomer determines the properties of the polymer and which types of plastic they are used for. Chemical recycling breaks down the polymers in the plastic to their starting monomers which can then be reused.Chemical recycling faces many challenges. Many commercial plastics are incompatible with chemical recycling due to their chemical make-up and therefore alternative plastics which can be chemically recycled are needed to replace these. Polymers which contain oxygen linkers, such as polycarbonates, are particularly promising as they can be broken down much more readily. Polycarbonates can be produced by combining carbon dioxide with an epoxide. Epoxides are three-membered rings which contain an oxygen atom. Altering the epoxide used affects the properties of the resulting polycarbonate. Whilst previous research has shown that various polycarbonates can be broken down to recover the starting epoxide, the selectivity towards these starting epoxides is a major issue as a cyclic carbonate product is often formed instead. Catalysts are key tools in overcoming this challenge as they can help to favour the epoxide formation as well as speed up the decomposition reaction. Although the catalysed decomposition of several different polycarbonates to their starting epoxides has been successfully demonstrated, for polycarbonates to be adopted on an industrial scale their properties must be competitive with current commercial polymers. Poly(cyclohexene carbonate) (PCHC) and poly(propylene carbonate) (PPC) are two types of polycarbonate which have the greatest potential to replace current commercial polymers. Unfortunately the chemical recycling of these polycarbonates has proven more difficult due to selectivity issues. Recent work by the Williams group has developed a new and highly effective catalyst to chemically recycle PCHC to its epoxide1 but the chemical recycling of PPC remains a challenge.This project aims to develop catalysts for the chemical recycling of PPC back to its starting epoxide. Using PCHC as a case study, this project will investigate the mechanism behind the catalysed decomposition of polycarbonates and how changing the shape and structure of the polycarbonate affect its rate of decomposition. This information will be used to inform future catalyst and polymer designs. In addition, the chemical recycling of other types of oxygenated polymers, specifically poly(esters-alt-ethers), will be explored.The ultimate goal is to develop effective methods of chemically recycling polymers to enable a sustainable circular plastic economy. This project falls within the EPSCR 'Manufacturing the Future' research area.