Sustainable Polymers as System-Dependent Technologies: A Critical Engineering Assessment of LCA Evidence, Circular Compatibility, and Scalability Constraints

The transition toward sustainable polymer systems is frequently framed as a material substitution challenge. However, the environmental performance of bio-based, biodegradable, and recyclable polymers is not an intrinsic property of feedstock origin but a system-dependent outcome shaped by energy inputs, infrastructure compatibility, economic feasibility, and governance alignment. This review provides a critical engineering assessment of sustainable polymer technologies based on systematic evidence screening (n = 193), bibliometric analysis, life cycle assessment (LCA) synthesis, techno-economic indicators, end-of-life scenario interpretation, and a multi-criteria evidence synthesis based on source-linked quantitative indicators and categorical evidence. To avoid treating these components as parallel but disconnected analyses, the synthesis explicitly links bibliometric clusters with LCA, TEA, and infrastructure evidence categories through an evidence-gap mapping matrix and confidence grading scheme. The findings indicate that bio-based polymers, such as PLA, PHA, and PBS, may achieve reduced greenhouse gas emissions under optimized production conditions; however, these advantages are highly sensitive to electricity carbon intensity, land-use assumptions, and end-of-life infrastructure. Quantitative LCA evidence indicates that polylactic acid (PLA) may achieve greenhouse gas emission reductions in the range of approximately 25-55% compared to conventional PET under low-carbon energy scenarios (1.3-3.0 vs. 2.0-3.5 kg CO₂e/kg), while polyhydroxyalkanoates (PHA) show reductions of 10-40%, depending on fermentation efficiency and feedstock selection. However, these benefits can diminish or reverse under fossil-intensive energy mixes or when land-use change impacts are included, reinforcing the system-dependent nature of sustainability performance. Recyclability within established material streams frequently provides more stable long-term performance than biodegradable alternatives deployed without controlled composting systems. Evidence-strength analysis reveals moderate confidence in carbon footprint comparisons, weak-to-moderate confidence in techno-economic scalability claims, and conditional confidence in infrastructure-dependent outcomes. The results demonstrate that sustainable polymers are system-dependent technologies rather than inherently sustainable materials. Future engineering research must prioritize integrated LCA-TEA modeling, infrastructure-aligned material design, scenario-based end-of-life assessment, and harmonized sustainability reporting standards to enable scalable and verifiable circular polymer systems.