You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Self-assembly of symmetric brush diblock copolymers. Self-assembled structures of brush block copolymers (BrBCPs) with polylactide (PLA) and polystyrene (PS) side chains were studied. The polynorbornene-backbone-based BrBCPs containing approximately equal volume fractions of each block self-assembled into highly ordered lamellae with domain spacing ranging from 20 to 240 nm by varying molecular weight of the backbone in the bulk state, as revealed by small-angle X-ray scattering (SAXS). The domain size increased approximately linearly with backbone length, which indicated an extended conformation of the backbone in the ordered state. In situ SAXS measurements suggested that the BrBCPs self-assemble with an extremely fast manner which could be attributed to a reduced number of entanglements between chains. The strong segregation theory and Monte Carlo simulation also confirmed this near-linear dependence of the domain spacing on backbone length, rationalizing experimental results.
{"Chemical": ["polystyrene", "polylactide", "polynorbornene", "PLA"]}