A “graft onto” method was combined with an epoxidation reaction and living anionic polymerization to successfully synthesize a series of SIS-g-PB copolymers with defined branch numbers and branch lengths. These copolymers were utilized to formulate various hot-melt pressure-sensitive adhesives (HMPSAs). Their molecular structure and bulk properties were characterized by 1H-nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and rheometry. The adhesion performances were characterized in terms of holding power and 180° peel strength. The epoxidation reaction alone would negatively influence the rheological properties of the parent SIS copolymers, particularly for low-temperature applications. Controlled addition of the low-Tg PB blocks can significantly improve the low-temperature properties of the SIS copolymers. Both η* and G′ increased in the lower shear frequency regime (<101 rad s−1) but decreased in the higher shear frequency regime (>101 rad s−1) with branch number and branch length, in which branch length had a greater effect than the branch number. As a result, the 180° peel strength of the SIS-g-PB based HMPSAs displayed reached 0.23 kN m−1, which is more than twice the value for SIS-based HMPSAs.

A monomer-activated anionic polymerization approach was utilized to synthesize poly(styrene-b-isoprene-b-styrene-b-ethylene oxide) tetrablock terpolymers (SISO), which were melt-mixed with tackifiers and plasticizer to develop polar SISO-based hot-melt pressure-sensitive adhesives (HMPSAs) for transdermal delivery of hydrophilic drugs. Their hydrophilic performance was characterized using contact angle analysis. Their adhesive performances were measured in terms of 180° peel strength and holding power. In vitro drug release experiments were carried out using a modified Franz type horizontal diffusion cell, in which geniposide was chosen as a hydrophilic model drug. The results show that poly(ethylene oxide) (PEG) blocks exhibit substantial effects on adhesive performance and the release behavior of the model drug. The shorter PEG molecular chains enhance adhesive performance and the cumulative release rate of the model drug in the SISO-based HMPSAs. The longer PEG molecular chains tend to crystallize. Their crystallization structures have negative effects on adhesive performance and limit the dissolution and diffusion of drugs in the SISO-based HMPSAs. Therefore, appropriate PEG molecular chains are required to fabricate SISO-based HMPSAs with excellent adhesive performance for transdermal delivery of hydrophilic drugs.