Materials (e.g., particles, hydrogels) that provide extended release of one or more therapeutic agents are useful platforms for drug delivery. In part, the present invention relates to new triblock (ABC) bottlebrush copolymers which can be used in the formulation of particles and hydrogels for the extended release of therapeutic agents. In certain embodiments, the triblock bottlebrush copolymers, particles, and hydrogels described herein are thermally-responsive and gel at physiological temperature (e.g., upon administration to a subject), providing injectable and/or implantable gels which can be used for extended release drug delivery. The present invention also provides methods for extended release drug delivery, and methods of treating and/or preventing a disease or conditions in a subject, using the inventive copolymers, particles, and hydrogels. In addition, the present invention provides methods of preparing the triblock bottlebrush copolymers described herein.