Synthesis of oligomers in arrays

A method of placing reagents in reaction compartments of an array of reaction compartments comprising:

• dispensing a first reagent to one or more porous islands in an array of porous islands, wherein each porous island is in fluid communication with at least one reaction compartment disposed adjacent to said porous island, and wherein each porous island is configured to restrict said first reagent from flowing laterally beyond each of said one or more porous islands, wherein said porous island defines an upper layer of said reaction compartment, and wherein a retainer layer is disposed adjacent to said reaction compartment and said retainer layer defines a lower layer of said reaction compartment, wherein the reaction compartment is configured as a channel between said porous island and said retainer layer, wherein the channel walls comprise a reaction surface, and wherein said retainer layer is separable from said reaction compartment;
• dispensing a second reagent to said one or more porous islands;
• and
• removing said first and second reagents from said reaction compartment.