The field of coarse-grained simulations of biopolymers and membranes has grown rapidly in recent years. Industrial groups manufacture and use polymers in fields as diverse as chemicals processing and personal care products, while academic researchers are interested in uncovering fundamental relations between molecular structure and macroscopic material properties. Biological membranes such as the cellular plasma membrane are of great interest to life scientists because of their role in cellular function. Experimental systems are usually polydisperse, and the cellular plasma membrane contains hundreds of distinct molecule types. Many coarse-grained simulation techniques have been used to explore amphiphilic membrane material properties and dynamics, but they typically contain only one or two species of molecule. They also require the precise configuration of the molecular components of a simulation to be specified in advance by the user to avoid the time-consuming stage of aggregate self-assembly. We describe here how a planar amphiphilic membrane is created by synthesizing each of its constituent molecules in situ according to user-defined growth rules that set the composition and molecular polydispersity, and subsequently simulated using dissipative particle dynamics. We explore the effects of polydispersity on the membrane material properties. The ability to synthesize and simulate polydisperse molecular aggregates may provide a simpler path to relating simulated and natural amphiphilic aggregates.