Novel second generation dendrimer with terminal thiol groups and its thiol–ene photopolymerization with unsaturated monomers

Abstract

A facile and effective method to prepare a second generation dendrimer (G2SH) and the use of this molecule as monomer for thiol–ene photopolymerizations, are described. The first step involves the “Click” thiol–ene photoaddition of 3-allyloxy-1,2-propanediol onto pentaerythritol tetrakis mercaptopropionate, producing an intermediate octaol. In the second step, this intermediate is esterified with 3-mercaptopropionic acid, achieving a first generation dendrimer with eight thiol groups (G1SH), and finally, the same sequence of reactions is then performed with this compound to produce the second generation dendrimer with 16 groups (G2SH). This dendrimer was used as a monomer for thiol–ene photopolymerizations in conjunction with 1,6-hexanedioldiacrylate (HDDA), pentaerytritol triacrylate (TAC) and triallyl triazine (CN). The kinetics of photopolymerization were determined by RT-FTIR and optical pyrometry, finding the following order of reactivity: HDDA>TAC>CN. Homopolymerization and step-growth polymerization occurred simultaneously in the cases of the acrylates while the CN reacted almost exclusively with G2SH. The reactivity of G2SH was compared with that of precursors G1SH and PETKMP. The results showed that G2SH was slightly more reactive than the other two thiols. The viscoelastic properties of the obtained flexible materials were determined by DMA which showed that the polymers displayed decreasing Tg's with increasing size of the thiol.