Observation of many tangles supports a loop mechanism of entanglement in macroscopic systems. The untangling of an unknotted tangled cord gives curves resembling the behavior under constant stress of an uncrosslinked elastomer. Reinforcement as a function of filler loading has been calculated for a model system consisting of uniform spherical particles in an unknotted tangled matrix. These calculated curves show the maxima found in real systems. Reinforcement also increases with decreasing particle size. The effect on the entanglement of porous particles and particles forming surface attachments with the polymer have been considered. The maximum in the calculated reinforcement curve shifts toward higher filler loading with increasing surface activity. This shift is independent of particle size and suggests a method for assessing the relative effectiveness of different coupling agents.