Tuning Crystallization Pathways via Quantitative Defect Engineering in Polymers

Defect engineering has emerged as a powerful strategy in inorganic crystalline materials, yet its systematic application to polymer crystals remains challenging due to inherent structural complexities such as chain folding and broad molecular weight distributions. Here, we introduce a model conjugated polymer system — poly(cyclopentenylene vinylene) (PCPV) bearing fluorene units with two (Di, D) or one (Mono, M) neo-hexyl side chains — that bypasses chain folding entirely, enabling precise, quantitative defect control. Five series of random pDM copolymers were synthesized with a fixed degree of polymerization (DP = 20) and systematically varied D/M ratios. Despite increasing defect content, all copolymers crystallized into well-defined 2D nanosheets. Strikingly, higher defect fractions selectively promoted crystal growth along the (100) direction, compressing the d₂₀₀ lattice spacing from 16.1 to 15.6 Å. DFT calculations revealed that defect incorporation alters the electrostatic surface potential (ESP), modulating intermolecular interactions and packing order. This work establishes a quantitative, synthesis-driven framework linking defect density to crystallization pathway selection, offering a predictive design rule for engineering highly ordered conjugated polymer assemblies.