A non-radiation curable reinforcing agent for optical fiber coatings and coating compositions. The reinforcing agent includes structurally flexible soft block segments and structurally rigid hard block segments. The soft block segments and hard block segments include urethane or urea linkages and act as strengthening additives in optical fiber coatings. Strength reinforcement occurs through interactions of the reinforcing agent with the polymeric network formed from curable components of the coating composition. Interactions include physical entanglements and hydrogen bonding. Soft block segments include block units that may include high molecular weight polyol linkages and soft block segments include block units that may include low molecular weight alkylene linkages. Coatings that include the reinforcing agents exhibit low Young's modulus, high tensile strength, and low glass transition temperatures and are suitable for use as primary coatings in optical fibers.

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