Forming a two-dimensional polymeric sheet includes translating a portion of a flexible substrate through a first liquid precursor to coat the portion of the flexible substrate with the first liquid precursor, thereby yielding a precursor-coated portion of the flexible substrate. The precursor-coated portion of the flexible substrate is translated through an interface between the first liquid precursor and a second liquid precursor, thereby reacting the first liquid precursor on the precursor-coated portion of the flexible substrate with the second liquid precursor to yield a polymer-coated portion of the flexible substrate.
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