How to make opposites compatible: Polyethylene (PE) and isotactic polypropylene ( i PP) are the two most widely used commodity plastics and thus make up a large fraction of the waste stream. However, the two plastics will not mix together, which limits options for dealing with mixed waste and decreases the value of recycled products. Eagan et al. report the synthesis of multiblock copolymers of i PP and PE by using a selective polymer initiator (see the Perspective by Creton). The high-molecular-weig ht blocks could be used to reinforce the interface between i PP and PE and allow blending of the two polymers. Science , this issue p. 814 ; see also p. 797

Properly addressing the global issue of unsustainable plastic waste generation and accumulation will require a confluence of technological breakthroughs on various fronts. Mechanical recycling of plastic waste into polymer blends is one method expected to contribute to a solution. Due to phase separation of individual components, mechanical recycling of mixed polymer waste streams generally results in an unsuitable material with substantially reduced performance. However, when an appropriately designed compatibilizer is used, the recycled blend can have competitive properties to virgin materials. In its current state, polymer blend compatibilization is usually not cost-effective compared to traditional waste management, but further technical development and optimization will be essential for driving future cost competitiveness. Historically, effective compatibilizers have been diblock copolymers or in situ generated graft copolymers, but recent progress shows there is great potential for multiblock copolymer compatibilizers. In this perspective, we lay out recent advances in synthesis and understanding for two types of multiblock copolymers currently being developed as blend compatibilizers: linear and graft. Importantly, studies of appropriately designed copolymers have shown them to efficiently compatibilize model binary blends at concentrations as low as ∼0.2 wt %. These investigations pave the way for studies on more complex (ternary or higher) mixed waste streams that will require novel compatibilizer architectures. Given the progress outlined here, we believe that multiblock copolymers offer a practical and promising solution to help close the loop on plastic waste. While a complete discussion of the implementation of this technology would entail infrastructural, policy, and social developments, they are outside the scope of this perspective which instead focuses on material design considerations and the technical advancements of block copolymer compatibilizers.

Montmorillonite‐based organoclay was incorporated into rigid polyurethane foams. Clay was dispersed first in the isocyanate component ultrasonically. The dispersion was investigated using small angle X‐ray scattering and rheology. It was found that using toluene as common solvent enhanced dispersion significantly. The addition of 1 wt% clay resulted in reduced cell size from 0.40 to 0.28 mm and increased cell number density on 300‐index foams. Gas chromatography showed that diffusion of blowing agent out of the closed cells of the foam decreased by more than 82% with clay loading. The reduction in permeability is caused by the combined effect of smaller cell size and dispersed nanoclay acting as diffusion barrier. At 1% clay no mechanical strength improvements were observed. Higher levels of clay loading caused significant viscosity increase and a yield stress that made mixing the reactive components difficult.