A comprehensive theoretical and experimental study of the bromo effect on Diels−Alder (DA) and retro-Diels−Alder (rDA) reactions between the anthracene-containing dienes and maleimide dienophile has been conducted to improve thermal stability and poling efficiency of electro-optic (EO) polymers. Calculations with density functional theory (DFT) reveal that the bromo substitution on anthracene-based dienes leads to significantly increased cycloaddition exothermicities (9−12 kcal/mol) but has a minor effect on their activation barriers (0.6−3 kcal/mol) when reacted with N-phenylmaleimide dienophile. These calculated values correlate very well with the experimental results obtained from a series of model compounds. The DA/rDA cross-linking protocols based on these compounds can be applied to several nonlinear optical (NLO) polymers and result in large EO coefficients (r33) (as high as 69 pm/V) and greatly improved thermal stability (up to 250 °C). This demonstrates that controlled DA/rDA reactions can be use...