Improving current recycling technology could help increase plastic recycling rates [1]. The aim of depolymerization is to produce monomers to make high-quality plastics again. However, the state-of-the-art for polypropylene (PP) is pyrolysis, which produces low-value product mixtures, due to the more than 500 °C required for thermal C–C bond cleavage [1]. The team of Vollmer investigates polymer conversion in a mechano-chemical[2] ball mill reactor (Figure 1A), which enables conversion below 60 °C [1]. Mechano-chemical homolytic bond scission is enabled by the forces acting on the polymer chains and produces radicals. This is combined with heterogeneous catalysis by catalytically functionalizing the surface of grinding spheres [3]. The activated surface of the grinding spheres can interact with the polymer radicals formed by the mechano-chemical action (Figure 1B). This promotes monomer formation over other reactions and is the reverse of controlled radical polymerization. Redox-active paramagnetic centers, such as Zr3+ and W5+ interact with mechano-radicals. The team also developed a strategy to measure radical formation rates and studied the kinetics of the depolymerization. In addition, a model was developed to predict the conversion rate based on ball milling parameters [4].
Figure 1. A) Milling container modified with gas in- and outlet to track gaseous products. B) Visualization of the mechano-chemical chain cleavage and density functional theory optimized structure of a radical interacting with the catalytic surface. C) Propene monomer flow and D) yield during milling of PP for 1 h using treated (S-ZrO2) and untreated spheres (ZrO2).

Polymers, typically known as main constituents in plastic materials, are mostly designed with a major focus on mechanical and chemical stabilities, but not on degradability. When reaching their end-of-life, some percentages of plastics are recycled mechanically or energetically, but still far too many end up in the environment degrading slowly in an uncontrolled way, and far too few are reused as depolymerized raw materials in the interests of an efficient circular economy. Chemical recycling of plastics still is an option for only a small number of polymers. For allowing triggerable (bio)degradabilities of polymers, certain chemical breakage points need to be implemented in the polymer chains. This would allow to degrade plastics in a defined manner to keep polymeric materials in a circular system with the highest utility value for as long as possible or to protect the environment.
This paper shows our approach to design thermoplastic and crosslinked polymers with tailored and controllable degradabilities based on internal breakage points. For this purpose, amino-acid based phosphoro(di/tri)amidates (APDA) have been selected as cleavable linkers and implemented for instance in polyurethanes and polysiloxanes. Phosphoramides and phosphoramidates have already been used by us for manufacturing 3D printed degradable hydrogels and resins [1-3].
The design and syntheses of these novel polymers and their monomers are shown, as well as their properties and degradabilities. Exemplary phosphorodiamidates, linked with glycine or valine, were used as a comonomer for thermoplastic polyurethane elastomers, containing 1,4 butanediol, pTHF 2000 and 4,4-MDI. Degradation was observed at pH 7 and pH 3.

Polymerization-Induced Self-Assembly (PISA) is a widely recognized and extensively studied technique for the creation of diverse self-assembled nanostructures. It has been successfully implemented with various monomer classes, particularly using Reversible Addition-Fragmentation Chain-Transfer (RAFT) polymerization. PISA leverages the in-situ polymerization of monomers to induce morphological transitions, leading to the formation of well-defined structures such as spheres, worms, and vesicles.
Recent advances in the field have demonstrated that depolymerization of RAFT polymers can be achieved under specific conditions (e.g., low temperatures) in a controlled manner. This controlled depolymerization is made possible by enhancing the deactivation of RAFT end groups, thereby enabling precise control over monomer release in block polymers. This discovery has opened new avenues for investigating the reverse of the PISA process, referred to as Depolymerization-Induced Structural Reassembly (DISA). By facilitating the controlled release of the second block monomer, DISA enables systematic monomer release, reversal of morphological transformations, and a deeper understanding of the fundamental mechanisms governing these transitions.
In this study, we employed controlled radical depolymerization techniques to systematically reverse the PISA process. By precisely tuning the depolymerization conditions, we achieved fine control over the rate of monomer release and the stepwise disassembly of complex morphologies. This approach allows for the generation of reverse phase diagrams, which map the morphological transitions in a backward sequence compared to conventional PISA. These reverse phase diagrams provide valuable insights into the relationships between molecular architecture, polymer chain behavior, and self-assembly dynamics.

Reversible addition-fragmentation chain transfer (RAFT) depolymerization represents an attractive and low-temperature chemical recycling methodology enabling the near-quantitative regeneration of pristine monomer. Yet, several mechanistic aspects of the process remain elusive. Herein, we shine a light on the RAFT depolymerization mechanism by elucidating the effect of pendant side chain on the depolymerization kinetics. A systematic increase of the number of carbons, or the number of ethylene glycol units on the side chain, revealed in a significant rate acceleration with longer side-chain lengths. Notably, radical initiator addition during the depolymerization of poly(methyl methacrylate) and poly(hexyl methacrylate), resulted in rate equilibration. Moreover, incorporation of a low DP of hexyl monomer as the second block of poly(methyl methacrylate), led to comparable rates with poly(hexyl methacrylate), indicating that chain activation is the rate-determining step in RAFT depolymerization. These insights not only deepen our understanding of depolymerization but also pave the way for developing more efficient and customizable depolymerization systems.

Polymer foams are materials made by a solid polymeric matrix containing void cells, generated by the introduction of a blowing agent, generally supercritical CO₂. They find applications in a wide range of fields, thanks to the versatility of their properties that can be tuned in terms of shape, dimension and distribution of cells within the structure of the material.
Foams with graded morphology offer structural and functional properties not attainable with uniform materials, making them good candidates as substitute of layered products, difficult to recycle. Mono-material products should be a technological target attainable through the “polymer structuring” process. Among all, the interplay between polymer crystallization and bubbles formation during processing, plays an important role in tuning foam structure [1,2].
In the present contribution we present a novel indirect method to study this complex interplay in foaming of poly(lactic acid) (PLA), a renewable thermoplastic polyester. PLA samples with stratified crystal density were foamed in different conditions to obtain a time dependent foaming profile at constant temperature. The twofold role played by the crystals as a barrier for the gas sorption and as nuclei for crystal growth was exploited to develop peculiar foam morphologies, as exampled in Figure 1. These preliminary results pose the basis for to optimize processing for the production of 100% biodegradable PLA graded foams.

Acknowledgements.
The authors acknowledge financial support from the Italian Ministry of Research, PRIN 2022 PNRR P20229YNXX, financed by the European Union – Next Generation EU.

Today, our modern life is unimaginable without plastics due to their versatility and cost-effective production. Industrial plastic production follows a linear economic model in which customized polymers are designed and disposed of in landfills, incinerators or in the environment after single usage. Furthermore, 79.4% of all plastics in 2023 are made from fossil raw materials.¹ The problem of environmental pollution from plastic waste and the associated new approach to establishing a circular plastics economy is leading to a growing demand for environmentally friendly, bio-based alternatives. Polylactide (PLA) is one of the most promising bio-based, biodegradable bioplastics already being used as a packaging material.² In the industrial production of PLA, a toxic catalyst (tin(II) bis(2-ethylhexanoate), [Sn(oct)₂]) is currently used and remains in the polymer.³ The challenge is to replace Sn(oct)₂ with an environmentally friendly catalyst and incorporate it in the depolymerization of PLA to path the way towards a circular economy.⁴
We present a new highly active zinc hybrid guanidine catalyst for lactide and caprolactone polymerization. Particularly outstanding are the similar polymerization rates. Metal guanidine catalysts known from literature have so far shown a significantly higher activity in lactide polymerization.⁵ For the first time, a catalyst with activities of the same order of magnitude is presented here. The catalyst is also active in the chemical recycling of PLA, polycaprolactone (PCL) and polyethylene terephthalate (PET). Its reusability in recycling is particularly noteworthy (figure 1). The introduced catalyst combines polymerization and depolymerization and is therefore an interesting candidate for a circular plastics economy.