Study On Kinetics Of RAFT Photopolymerization Of N-Isopropylacrylamide (NIPAM) And Applied To Photosynthesis The Hydrogels

Responsive hydrogels are sensitive to the external stimulus such as temperature, pH, composition of solution, light, ionic strength, magnetic field, electric field and some biochemical molecules. N-Isopropylacrylamide (NIPAM) is an extremely important nonionic acrylamide monomer. Poly(NIPAM) (PNIPAM) possesses a readily accessible lower critical solution temperature (LCST) of 32℃in water, which is close to human body temperature (37℃) and as such has, for example, been evaluated in drug delivery applications. Reversible addition fragmentation chain transfer (RAFT) polymerization has proven itself to be an extremely versatile controlled/"living"free radical polymerization technique (CLRP). It has been shown to be applicable to the controlled polymerization of a wide-range of monomers, under a wide range of conditions to yield well-defined polymers or copolymers with both low polydispersity and functionalized end group as well as polymers with complex architectures. In this paper, kinetics of RAFT photopolymerization of NIPAM was studied. Using this technique of photo-induced RAFT polymerization, several hydrogels which include PNIPAM hydrogels, PNIPAM-g-PNIPAM and PNIPAM-g-PDMAA comb hydrogels and PNIPAM-g-PDMAA comb/porous hydrogels have been prepared. Their temperature behaviors, swelling ratio and dewelling rate were studied. Obtained conclusions were worth in academic and industrial to synthesize and characterize intelligent hydrogels. The followings are main conclusions of our work:1. In this paper, photo-induced RAFT polymerization of NIPAM were studied in the presence of S-dodecyl-S'-(α,α'-dimethyl-α''-acetic acid) trithiocarbonate (DDMAT) as a RAFT agent and (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TPO) as a photoinitiator, at room temperature. The variations factors such as the ratio of chain transfer agent and initiator ([CTA]/[I]) were discussed in the paper, which effects on the monomer conversions and molecular weights of the polymers. Additions of TPO photoinitiator remarkably suppress retardation effect in RAFT process, thus shorten initialization period and accelerate overall RAFT process. The experimental results showed that when [CTA]/[I] was 15:1, [I]/[monomer] was 0.5 wt%, The kinetic characteristic of the living polymerization was evidenced by narrow molecular weight distribution (PDI = 1.30~1.45) , linear increase of molecular weight with increasing conversion, well-controlled molecular weight, and first-order polymerization kinetics. The NIPAM homopolymers prepared by RAFT photopolymerization (PNIPAM-DDMAT) had ability of chains extension.2. The conventional Poly(N-isopylacrylamide) hydrogels take more than several hours to days for completion of volume shrinking, which is the main drawback for their practical usage, such as on–off valves and artificial muscles, and was an important topic to be solved. For resolving this problem, herein, several gels were prepared by reversible addition-fragmentation chain transfer (RAFT) photopolymerization.a. Functional poly(N-isopylacrylamide) hydrogels have been prepared by the technique of photo-induced RAFT polymerization of NIPAM in the presence of DDMAT as a chain transfer agent and N,N'-methylenebisacylamide (BIS) as a cross-linker at different temperatures (50℃and 25℃) respectively. The hydrogels (RAFT gels) synthesized at the same temperatures showed accelerating to shrinking kinetics compared with the hydrogels synthesized by conventional photopolymerization. It could be attributed to the presence of dangling chains mainly caused by DDMAT. The RAFT gels synthesized at 50℃showed more excellent water releasing performance than one synthesized at 25℃. According to the mechanism of RAFT photopolymerization, re-grafting of the hydrogels were studied. It indicated more excellent water releasing performance than before.b. The NIPAM homopolymers (PNIPAM-DDMAT) were prepared by RAFT photopolymerization first. Then PNIPAM-g-PNIPAM comb hydrogels were prepared by photopolymerization in the presence of PNIPAM-DDMAT as a macro-CTA and BIS as a cross-linker. The results showed that the grafting chain amount and length had a significant influence on the deswelling kinetics and swelling ratio of gels. After chain extention of PNIPAM-g-PNIPAM comb hydrogels, gels indicated more excellent water releasing performance than before.c.Poly(N,N`-dimethylacrylamide) (PDMAA) is a kind of polymer with excellent biocompatibility. Firstly, the macro-CTA of PDMAA (PDMAA-DDMAT) were prepared by photo-induced RAFT polymerization. Then PNIPAM-g-PDMAA comb/porous hydrogels were prepared by RAFT photopolymerization in the presence of PDMAA-DDMAT, BIS as a cross-linker and poly(ethylene glycol)(PEG) as a pore-forming agent at 20℃. The grafting chain amount had a significant influence on the gel rapid LCST (lower critical solution temperature), deswelling kinetics and swelling ratio. The formation of a porous structure has been shown to effectively enhance the deswelling rate of PNIPAM hydrogels. But it has no effect on the swelling ratio and deswelling rate of the comb hydrogels.