In one aspect, a block copolymer described herein comprises a hydrophilic block including oxazoline monomer or oxazine monomer, and a cationic block comprising monomer including a linear or branched polyamine side chain. In another aspect, a polyion complex comprises a block copolymer comprising a hydrophilic block including oxazoline monomer or oxazine monomer, and a cationic block comprising monomer including a linear or branched cationic polyamine side chain, and a negatively charged biomolecular species associated with the block copolymer. The negatively charged biomolecular species can comprise one or more nucleic acids, such as RNA, DNA, and/or other oligonucleotides.