Synthesis And Self-assembly Of Amphiphilic Diblock Copolymer Based On Poly(ε-caprolactone)

Amphiphilic block copolymers demonstrate a series of attractive properties and can self-assemble to micelle formation by microphase separation in selective solvent, and have been successfully used for drug carriers, surgical suture, orthopedic fixation materials, and in tissue engineering. In the current work, amphiphilic block copolymers composed of hydrophobic Poly(ε-caprolactone) (PCL) and hydrophilic Polyvinyl pyrrolidone (PVP) or Polyethylene glycol (PEG) were synthesized by free radical polymerizetion (FRP) and then ring open polymerizetion (ROP). The molecular structures and characteristics of the copolymers and their micellar behaviors were investigated as follow:First, poly(N-vinylpyrrolidone) with a hydroxyl end group (PVP-OH) was successfully synthesized by free radical polymerization using mercaptoethanol (MCE) as chain transfer agent, AIBN as initiator, and DMF as solvent. The amphiphilic PVP-PCL was prepared by ring-opening polymerization of s-caprolactone (ε-CL) using PVP-OH as macroinitiator and Sn(Oct)2 as catalyst. The chemical structure of PVP-PCL was characterized by'HNMR and FT-IR. The number average molecular weight (Mn) and polydispersity index (PDI) was determined to be 6.1×103 and 1.57 by GPC separately. The value of critical micelle concentration (CMC) was measured to be 1.7 mg/L by fluorescence spectrum using pyrene as a probe, which suggest high stability of its self-assembled micelle formation. The dynamic light scattering (DLS) results indicated that the average micelle size was 100-200 nm with low polydispersity (<0.1). Besides, the morphology of the PVP-PCL micelle was mainly spherical determined by scanning electron microscope (SEM).Three kinds of PCL-PEG-PCL triblock copolymers were synthesized by ROP of×-CL in the presence of PEG with molar masses of 4000 and 6000, using low toxic Sn(Oct)2as catalyst. The Mn and chemical structure of the polymers were characterized by GPC,1HNMR and FT-IR. The results showed that Mn of the polymers were conformed with the theoretical values and all PDI were below 1.5. The mass fractions of PEG/PCL in the polymers were 50/50 and 33/67 determined by 1HNMR. The CMC value of PCL-PEG-PCL with different PCL length in aqueous medium were determined to be 0.95 mg/L and 0.38 mg/L for PCL4000 and PCL8000 by fluorescence probe technique. The CMC value decreased with increase of PCL length in PCL-PEG-PCL. The PCL-PEG-PCL could self-assemble into micelles in aqueous solution with an average size of 165-218 nm and PDI of 0.074-0.076 determined by DLS. The micellar size increased with the increase of PCL length for fixed PEG length and with increase of the molecular weight of PCL-PEG-PCL for fixed PEG/PCL ratio.Using pyrene as a fluorescence probe for a further study of the micellization of PVP-PCL diblock copolymer in aqueous solution. First, the partition of pyrene between the micellar phase and the water phase was studied and the partition equilibrium constant (Kv) was calculated to be 3.97×105 which is based on the relation between the partition equilibrium constant and the excition intensity ratio I336 nm/I333 nm of pyrene. It was indicated from the Kv value that the micellar core is highly hydrophbic. and then the microenvironment of the micillar core with fluorescence anisotropy was studied, as a result, the pyrene was soluted in the micellar core and the rotation of pyrene was hindered because of some premicelles formed in the solution, so the anisotropy value increased with increase of the concentration of PVP-PCL when the concentration was below the CMC value. But with the further increasing concentration, the fluorescence anisotropy value kept constant resulted from the unchange of the aggregation number in one micelle above the CMC value. The results of the steady-state fluorescence spectrum indicated that the emission intensity of pyrene in the micelle solution presented a linar decreasing during heating, and had a crossover point at 53℃which in agreement with the melt temperature of PCL, while the pyrene in the aqueous solution didn't have, combined with the results of DSC and DLS, it's concluded that the melting of the core of PVP-PCL changed the microenvironment that pyrene located during heating, which resulted in variety of the slope. The time-resolued fluorescence spectrum was then used to estimate the fluorescence decay of pyrene in micellar solutions of PVP-PCL and the results showed that there was only one lifetime corresponding to the micellar phase at all temperature, suggesting that pyrene is completely solubilized in the micellar phase.