Chemoenzymatic Synthesis Of Amphiphilic Diblock Copolymer And Study Of Self-assembly Behavior

In the past decades, enzymatic polymerization and atom transfer radical polymerization (ATRP) have attracted considerable attentions due to their respective unique properties. However, chemoenzymatic synthesis based on the combination of enzymatic polymerization and ATRP was employed in the preparation of functional materials in recent years. This strategy has motivated more researchers to study chemoenzyme-catalyzed routes to obtain the novel polymeric materials. Futhermore, researchers found that many kinds of amphiphilic block copolymers have the potential applications as the biomedical materials. So, more and more researches focus on the crystallization behavior and the self-assembly behavior in solvents of block copolymers.The aim of the research in this thesis is to investigate and study the chemoenzymatic synthesis of functional diblock copolymer by combining enzymatic condensation polymerization and ATRP. And the resulting polymers were characterized and investigated their structures and properties. By employing a novel bifunctional initiator we got the macroinitiator using in the subsequence ATRP reaction in the enzymatic condensation polymerization and successfully obtained the functional diblock copolymer. This novel one-step method allowed enzymatic condensation polymerization and ATRP to be conducted sequentially without any intermediated steps to get the initiator of ATRP. At the same time, the structures, crystallization behavior of the resulting polymers and self-assembly behavior of the diblock copolymers in solvents were investigated in detail. In Chapter 1, at first, the research advances, characteristic and impact factors of the enzymatic polymerization taken in organic media were summarized in brief. And we enumerated the advantages and disadvantages of the enzymatic polymerization in organic media. At the same time, the related chemical polymerizations appeared in chemoenzymatic synthesis of this thesis (ATRP) were summarized in brief. Also, the previous reports of chemoenzymatic synthesis reaction were represented, especially the synthesis method, function of the products and potential application of the chemoenzymatic reaction involved ATRP. Subsequently, the crystallization behavior of polymers were introduced briefly, including the crystallization process, configuration model of crystallization, impact factors, morphology of the crystal under the polarized optical microscope (POM)and effects of their crystallization behavior and assembling structure to their properties. At last, we introduced the self-assembly behavior of the amphiphilic block copolymers, including the research importance, objects, method of preparing micelle and the advantages of using micelle as the drug deliverer.In Chapter 2, we developed a novel bifunctional initator which could be used as a brige between the enzymatic condensation polymerization and ATRP reaction. This novel initiator 2, 2, 2-trichloroethanol (TCE) contains a single primary alcohol group to initate enzymatic condensation polymerization and an activated trichloromethyl group, an effective initiating group for ATRP. The trichloromethyl group terminated polyester poly(2, 2, 2-trichloroethanol 10-hydroxydecanoic acid) (P(TCE-10-HD)) was resulted from TCE-initiated enzymatic polymerization of 10-hydroxydecanoic acid (10-HD) catalyzed by Novozyme 435 and used as the macroinitiator of the subsequent ATRP reaction. The compositions of the resulting polymer were confirmed by NMR and FTIR analysis. All of these analysis results proved that the product yield and structure could be ensured. Also the kinetics of the enzymatic polymerization was studied using GPC analysis. And we found out the best reaction time of this enzymatic polymerization. It was found that many properties of the block copolymers with polyester blocks, such as drug permeability, biodegradation and mechamical properties, are significantly affected by their crystallization behavior and assembling structure. So we discussed the crystallization behavior of the polyester P(TCE-10-HD). The crystallization behavior of this polyester was studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The morphology and crystal growth of the polymers were observed by polarized optical microscope (POM). The isothermal melt-crystallization of P(TCE-10-HD) was observed by POM, and the spherulites could be observed. The results of DSC, XRD and POM proved that P(TCE-10-HD) has good crystallization property. As far as we know there is no report about the crystallization of P(TCE-10-HD) before. Furthermore, in ordere to study the crystallization properties, non-isothermal crystallization kinetics of P(TCE-10-HD) has been carried out of with DSC techniques by Avrami, Ozawa and Mo Zhishen. The activation energy value of P(TCE-10-HD)was determined by the Kissinger method.In Chapter 3, we use the P(TCE-10-HD) as the macroiniator of the ATRP reaction of glycidyl methacrylate (GMA). The well defined diblock copolymer P(TCE-10-HD)-b-PGMA could be obtained successfully. The compositions of the resulting polymers were confirmed by NMR, FTIR and GPC analysis. Also the kinetics of ATRP reaction was studied. All of these analysis results proved that this new method we used to prepare the diblock copolymer was simple and convenient and the molecular weight and structure of the product was the as what we expected. We also discussed the crystallization behavior of the diblock copolymer P(TCE-10-HD)-b-PGMA with different Mn (different Mn of PGMA block). The crystallization behavior of these polymers was studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The morphology and crystal growth of the polymers were observed by polarized optical microscope (POM). We observed the crystallization of P(TCE-10-HD)-b-PGMA by the solvent volatilizing and did not find any crystal with well-defined crystallinenature. Because the PGMA block is amorphous, it could affect the crystallization behavior of diblock copolymer P(TCE-10-HD)-b-PGMA, the crystallization property was getting worse with increasing the weight fraction of PGMA block. We use the same method to investigate the non-isothermal crystallization behavior of the diblock copolymer P(TCE-10-HD)-b-PGMA via DSC. Avarmi equation, Ozawa approach and a new method developed by Mo Zhishen were adopted to study the non-isothermal crystallization of P(TCE-10-HD)-b-PGMA, and activation energy value of this diblock copolymer was determined according to the Kissinger method. However, the crystallization behavior of P(TCE-10-HD) block in the diblock copolymer is more complicated than the crystallization behavior of polyester P(TCE-10-HD) because of the existence of PGMA block which is amorphous. So the further researches of the crystallization behavior of diblock copolymer P(TCE-10-HD)-b-PGMA will be needed.In Chapter 4 we studied the self-assembly behavior of the amphiphilic diblock copolymer P(TCE-10-HD)-b-PGMA obtained by the method discribed in Chapter 3. At first, the critical micelle concentration (CMC) of the diblock copolymer P(TCE-10-HD)-b-PGMA was determined by a fluorescence probe technique using pyrene and polymer solutions with different concentration. We determined the CMC of two copolymers with different Mn and compared the CMC value got from them. The result is that the weight fraction of the hydrophilic block can affect the CMC of copolymers, and the copolymers with lower weight fraction of hydrophilic block could form micelles easier and have a lower CMC value. The fluorescence measurement also proved that the amphiphilic block copolymer could self-assemble in mixed solvents and form nanoparticles. Also the Dynamic light scattering (DLS) measurement confirmed the presence of vesicles in solution and provided an average diameter of the nanoparticles. We also used atomic force microscopy (AFM), transmission electron microscopy (TEM) and scan electron microscopy (SEM) to observe the morphologies of the nanoparticles. The results proved that the nanoparticls formed by the amphiphilic diblock copolymer P(TCE-10-HD)-b-PGMA in mixed solvents are vesicles. Furthermore, because this amphiphilic diblock copolymer is biocompatible and biodegradable, so we could presume that it has the potential application in drug delivery and controlled release. So, we do some primary experiments about the drug-loaded and releasing amount. Our group will do more and further researches about the controlled release behavior of this diblock copolymer in the future.