| Due to the continuous rise in the consumption of petroleum resources, there are urgent challenges of progressively exchanging petroleum based polymeric materials with biocompatible, biodegradable, renewable and sustainable alternatives. Intensive research interests have been concentrated on polymers based on natural resources, a-methylene-y-butyrolactone (MBL) or tulipalin A is one simplest member of a natural substance, which is found in tulips. Despite a lot of research has been conducted in the field of renewable materials, there were few works concentrating on the preparation of polymer particles based on such bio-renewable monomers, especially for monodisperse polymer particles.Conventionally, polymeric microspheres are prepared through heterogeneous polymerization techniques, including suspension, emulsion, and dispersion polymerization. These widely used methods have a common drawback:steric stabilizers or surfactants. Generally, the surfactants used in heterogeneous polymerization have brought out a lot of problems, such as the impurity in the resulting polymers, environmental pollution, and dispersion instability due to migration of stabilizer molecules.In our previous research, our research group had developed a stabilizer-free precipitation polymerization process. A stable colloidal dispersion could be formed without the presence of any surfactants, and the as-formed polymer particles were highly uniform with controllable particle size. The aim of the present research is to expand the scope of monomers that adapted to stabilizer-free precipitation copolymerization process, and develop a novel method for the preparation of monodisperse, impurity-free poly(?-methylene-?-butyrolactone-co-styrene) (poly(MBL-co-St)) particles based on MBL and St. Stabilizer-free precipitation copolymerization of MBL and styrene (St) was carried out in isoamyl acetate with BPO as initiator at 80?.The influence of monomer feed ratio, initiator concentration and reaction time on the size and morphology of the obtained polymer particle was investigated in details. These two monomers have totally different molecular structure and there was big difference in their polymerization reactivity. Therefore, the monomer feed ratio of MBL to St had big influence on the polymerization process and the morphology of as-prepared polymer particles. The monomer conversion decreased dramatically with decreasing monomer feed ratio of MBL to St. When the monomer feed ratio of MBL to St is 2:1, the monomer conversion could reach up to 98%, while the monomer conversions were only 66% and 55% with the MBL/St monomer feed ratio of 1:1 and 1:2, respectively. Poly(MBL-co-St) polymer particles with different size and morphology were produced at varying monomers feed ratios. It is worth pointing out that the size distribution of the polymer particles was very broad when with higher MBL feed ratio and meanwhile the size of the particles were very small. The number average particle "coagulation" diameter (Dn) decreased from 1880nm to 1040 nm when the molar ratio of MBL to St decreased from 2:1 to 1:1. With further increasing content of St in the feed monomers, coagulation of the copolymer particles was inhibited efficiently and separated polymer particles with narrow size distribution were successfully formed. Particularly, monodisperse spherical polymer particles were formed when the ratio of [MBL] to [St] was 1:2, and the diameter of the polymer particles were in the range of 785-2350 nm.The initiator concentration played an important rule during the polymerization process and higher initiator concentration would favor the increase of monomer conversion. The monomer conversion increased steadily with increasing BPO concentration at all MBL/St monomer feed ratio. With increasing initiator concentration, the particle size increased gradually but the surface of the polymer particles became a little rougher.The formation process of polymer particle was studied to get a deep insight into the polymerization process of this reaction system. The mechanism for the stability of the polymer particle formed by the stabilizer-free precipitation polymerization system was different from the traditional suspension, emulsion and dispersion polymerization system, in which the stabilization of the obtained polymer particles was mainly due to the surfactants or stabilizers used in the reaction system. It was found that the polymer particle formation mechanism, growth process and stabilization of the poly(MBL-co-St) particles was similar to that of conventional precipitation polymerization. After a short nucleation stage (10 min), the amount of polymer particles remained constant and the particle growth mainly came from the capturing of newly formed polymer chains. The particle size increased steadily with increasing monomer conversion, and polymer particles with narrow size distribution and spherical morphology were finally obtained. The number average diameter of the obtained polymer particles increased gradually from 785 nm (at 10 minute) to 2620 nm (at 180 minute), and meanwhile, the particle size coefficient variation (CV) decreased from about 5.5% to 3.0%.Subsequent investigations showed that polymer particles with different size and morphology, including spherical, potato shaped and irregularly flower-shaped beads, were produced with different monomer feed ratio. The relationship between the surface morphology of such polymers particles and the monomers monomer feed ratio, monomer concentration and solvent used were investigated in details. These experimental results enrich the polymer particle morphology control by reaction parameters.The FTIR and 1H NMR results demonstrated that the content of MBL in the resulted poly(MBL-co-St) was higher than the initial monomer feed ratio due to higher reactivity of MBL. When the molar feed ratio of MBL to St was 1:2, the molar ratio of MBL to St in the copolymer was 1.3:1. With increasing content of MBL in the monomers (from 1:1 to 2:1), the molar ratio of MBL to St in the copolymer increased dramatically from 1.9:1 to 3.1:1.This stabilizer-free precipitation polymerization method provides a facile and efficient way to get impurity-free polymer particles based on renewable monomer MBL. Furthermore, the Tg of the as-prepared poly(MBL-co-St) was in the range of 152-173?, which was much higher than that of PS homopolymer due to the presence of MBL in the copolymer chain, endowing the polymer particles with high heat resistance. Compared with conventional precipitation polymerization process, the monomer concentration was extremely high in the reaction system, and the monomer conversion was also very high. There were no any surfactants or steric stabilizers in the reaction system, and no cross linking agent was added in the reaction system.Preparation of monodisperse polymer microspheres in the absence of stabilizers has important theoretical and practical value. On the one hand, the preparation of polymer particles not only has important theoretical value, but also has important practical significance. The present work further expands the scope of monomers that adapted to the stabilizer-free precipitation polymerization system. On the other hand, the present work provides a facile and efficient way to get impurity-free polymer particles based on a bio-renewable monomer. |
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