The invention provides elastomeric polymer networks and semi-interpenetrating networks in which a linear PEA, PEUR or PEU polymer is crosslinked by ester or alpha-amino-acid containing cross-linkers that polymerize upon exposure to active species. Bioabsorbable elastomeric internal fixation devices fabricated using such polymer networks and semi-interpenetrating networks are useful for in vivo implant and delivery of a variety of different types of molecules in a time release fashion. Alpha-amino-acid containing ester amide cross-linkers are also provided by the invention.