A curable composition comprising: an oligomer; at least one monomer; and an amount of a substantially non-reactive oligomeric additive having a structure or average structure according to formulae,,,, or; CAP 1 -RT'-OOC-NH-R 1 -NH-COO-RT'-CAP 1 R M -[OOC-NH-R A -NH-COO-RT'-CAP 1] n CAP 1 -RT'-OOC-NH-R 1 -NH-[COO-R 2 -OOC-NH-R 1 -NH] w -COO-RT'-CAP 1 CAP 2 -X-OCN-R 1 -NCO-X-CAP 2 R M -[OOC-NH-R A -NCO-X-CAP 2] n CAP 2 -X-OCN-R 1 -NH-[COO-R 2 -OOC-NH-R 1 -NH] w -CO-X-CAP 2 wherein CAP 1 is a capping moiety forming a non-reactive terminus, RT' is a derivative moiety having a reactive terminus that is capped by CAP 1, R 1 is a polyisocyanate core moiety free of urethane bonds, R A is R 1 or -R 1 -(NH-COO-R C -OOC-NH-R 1) t - where R C is a polyol-derived core moiety and t has an average value in the range of 0 to 4, R M is a core moiety having an average functionality of n, where n is greater than 2.2, R 2 includes either at least one crystallizable polyol-derived moiety or at least one cyclic rigid moiety, and w is greater than zero; CAP 2 -X- is a capping moiety forming a non-reactive terminus, X is S, O, or N, which average structure has less than 1.0 RT or isocyanate terminal groups, the RT group being a reactive terminal group which is not capped by a CAP 1 or CAP 2 moiety; which substantially non-reactive oligomeric additive is effective to yield a cured product having a fracture toughness value that is higher than the fracture toughness value of a cured product of an otherwise identical composition lacking the non-reactive oligomeric additive. an oligomer; at least one monomer; and an amount of a substantially non-reactive oligomeric additive having a structure or average structure according to formulae,,,, or; CAP 1 -RT'-OOC-NH-R 1 -NH-COO-RT'-CAP 1 R M -[OOC-NH-R A -NH-COO-RT'-CAP 1] n CAP 1 -RT'-OOC-NH-R 1 -NH-[COO-R 2 -OOC-NH-R 1 -NH] w -COO-RT'-CAP 1 CAP 2 -X-OCN-R 1 -NCO-X-CAP 2 R M -[OOC-NH-R A -NCO-X-CAP 2] n CAP 2 -X-OCN-R 1 -NH-[COO-R 2 -OOC-NH-R 1 -NH] w -CO-X-CAP 2 wherein CAP 1 is a capping moiety forming a non-reactive terminus, RT' is a derivative moiety having a reactive terminus that is capped by CAP 1, R 1 is a polyisocyanate core moiety free of urethane bonds, R A is R 1 or -R 1 -(NH-COO-R C -OOC-NH-R 1) t - where R C is a polyol-derived core moiety and t has an average value in the range of 0 to 4, R M is a core moiety having an average functionality of n, where n is greater than 2.2, R 2 includes either at least one crystallizable polyol-derived moiety or at least one cyclic rigid moiety, and w is greater than zero; CAP 2 -X- is a capping moiety forming a non-reactive terminus, X is S, O, or N, which average structure has less than 1.0 RT or isocyanate terminal groups, the RT group being a reactive terminal group which is not capped by a CAP 1 or CAP 2 moiety; which substantially non-reactive oligomeric additive is effective to yield a cured product having a fracture toughness value that is higher than the fracture toughness value of a cured product of an otherwise identical composition lacking the non-reactive oligomeric additive. CAP 1 is a capping moiety forming a non-reactive terminus, RT' is a derivative moiety having a reactive terminus that is capped by CAP 1, R 1 is a polyisocyanate core moiety free of urethane bonds, R A is R 1 or -R 1 -(NH-COO-R C -OOC-NH-R 1) t - where R C is a polyol-derived core moiety and t has an average value in the range of 0 to 4, R M is a core moiety having an average functionality of n, where n is greater than 2.2, R 2 includes either at least one crystallizable polyol-derived moiety or at least one cyclic rigid moiety, and w is greater than zero; CAP 2 -X- is a capping moiety forming a non-reactive terminus, X is S, O, or N, which average structure has less than 1.0 RT or isocyanate terminal groups, the RT group being a reactive terminal group which is not capped by a CAP 1 or CAP 2 moiety; which substantially non-reactive oligomeric additive is effective to yield a cured product having a fracture toughness value that is higher than the fracture toughness value of a cured product of an otherwise identical composition lacking the non-reactive oligomeric additive.