Three-dimensional printing

A three-dimensional (3D) printing kit, comprising:

• a build material composition including an oxygen solution-treated semi-crystalline thermoplastic polymer having a surface energy density greater than 41 mN/m, wherein a surface of the oxygen solution-treated semi-crystalline thermoplastic polymer:
• i) is modified with an amine (—NH2) functional group and is prepared using ammonia;
• or
• ii) is modified with a urethane functional group and is prepared using a compound selected from the group consisting of urea and a carbamic acid derivative;
• or
• iii) is modified with an epoxy functional group and is prepared using a glycidyl methacrylate;
• and
• a fusing agent to be applied to at least a portion of the build material composition during 3D printing, the fusing agent including an energy absorber to absorb electromagnetic radiation to coalesce the oxygen solution-treated semi-crystalline thermoplastic polymer in the at least the portion.