Organoselenium Compounds Applied In "Living"/Controlled Radical Polymerization

“Living”/Controlled Free Radical Polymerization (LRP) integrates the advantages ofliving polymerization and radical polymerization, and is the research hotspot in the field ofpolymer chemistry. So far, there are a variety of mature LRP methods. Reversible AdditionFragmentation-Chain Transfer (RAFT) polymerization was among them and showedmany advantages, such as highly controlled polymerization process, powerful designability of molecular structure, wide monomer range, and mild polymerization condition.RAFT polymerization was also carried out in heterogeneous system, which showed strongpotential industrial application prospect. However, RAFT agent has been limited to thedithiocarbonyl compounds (dithiocarbamate, xanthan, trithiocarbonate and dithioester,etc.). Same RAFT agent was only effective for specific type monomers, which restrictedthe application of the RAFT polymerization to some extent. Therefore, searching of RAFTreagents which can be used in wide range monomer type has practical significance for thefurther application of RAFT polymerization.As known as chalcogen elements, selenium, oxygen, sulfur and tellurium belong tothe same group in the periodic table, and show similar chemical properties. Thecompounds derived from these elements are widely used in free radical reaction, especiallyin LRP. There are different types of chalcogen compounds, which are used as the effectivemediators. Dithioesters can be used as RAFT reagents, thioethers and selenides can beused as iniferters, tellurides can be used to regulate Organotellurium-mediated LivingRadical Polymerization (TERP), and nitrogen oxygen stable free radicals can be used inStable Free Radical Polymerization (SFRP). Selenium has similar outer sphere electrons assulfur. Furthermore, the hollow4d orbit in selenium offers more flexible reactionpossibility through free radical way. Thus, it would be valuable to exploring the radicalpolymerization behavior under the mediator of organoselenium compounds.Furthermore, the medical and health care function of organoselenium compounds hasattracted the attentions of scientists and commodity markets. Organoselenium compoundsand their polymers have wide and unique application prospect in fluorescent probe, polymer self-assembly, drug release, photoelectric materials, catalyst and organic synthesis,especially synthesis of natural products. However, the characteristics of organoseleniumcompounds, such as volatile, unpleasant smell, toxic, difficult for storage and easy to causeenvironmental pollution, affect and restrict their further application. Thus, synthesis oforganoselenium polymer was interested by polymer scientists. The traditionalpost-modification methods were unable to control the molecular weight and seleniumcontent of obtained organoselenium polymer, which limited the research and application oforganoselenium polymer.This thesis is focus on the research of polymerization behavior of typical vinylmonomers using organoselenium compounds as mediators. The research included thefollowing aspects.(1) P,P-Diphenyl phosphinodiselenoic acid benzyl ester (DPPSB) was synthesized.Polymerization of styrene using DPPSB as a mediator under UV-vis irradiation at30oC inbulk or in all kinds of solvents was investigated. Polymerization results presented theiniferter polymerization features: the GPC determined molecular weights with moderatemolecular weight distribution were closed to the theoretical ones, which revealed that themolecular weights of PS can be controlled both by the molar ratio of monomer to DPPSBand by the conversion; molecular weight distribution widened with the increasingconversion. The chain extension investigations using these obtained polymers as themacro-mediators were also performed. The GPC traces of these chain extended polymersshowed one peak, which implied the high degree of the functional groups in originalmacro-mediators. The influence of DPPSB concentration on polymerization controllabilitywas investigated. The polymerization of MA and MMA were also controlled in the presentof DPPSB.31P NMR spectra showed that P-Se bond of DPPSB had decomposed in thepolymerization system.1H NMR spectra showed that the polymer chain had theC-SeCH2Ph structure. UV-vis spectrum showd that polymer contained selenide structure.After oxidation reaction with30%H2O2,1H NMR spectra showed that carbon-carbondouble bond was at the polymer chain end,31P NMR spectra showed that phosphorus atomwas still in the polymer chain, GPC test showed that the Mn,GPCwas little different fromthat before oxidation reaction. All these results proved that P-Se bond of DPPSBdecomposed in the polymerization system to phosphorus free radicals and selenium freeradicals, and then polymerization was carried out by a iniferter mechanism. (2) A series of new selenocarbonats with different substituted degree of selenium, e.g.dibenzyl dithioselenocarbonate (DTSC), dibenzyl thiodiselenocarbonate (DTSC), dibenzyltriselenocarbonate (TSC), were synthesized. The polymerization of styrene in the presenceof these carbonates as mediaors and2,2-azobisisobutyronitrile (AIBN) as the free radicalinitiator at60oC in bulk were studied and compared with that in the presence of dibenzyltrithiocarbonate (TTC) as a RAFT agent. Polymerization results presented RAFTpolymerization characteristics: the GPC determined molecular weights with moderatemolecular weight distribution were closed to the theoretical ones, which revealed that themolecular weights of PS can be controlled both by the molar ratio of monomer to mediatorand by the conversion; molecular weight distributions were narrow. With the increase ofselenium atoms in mediator, polymerization rate slowed down, and long period ofinduction time appeared. The polymer chain extension experiments were carried outsuccessfully, which indicated that the polymer chains were still living.1H NMR spectraand UV-vis spectra showed that mediator structures remained in the polymer chains. TheGPC test showed that the molecular weights of polymer after oxidation with30%H2O2were about half of the original ones, which suggested that mediator structures located inthe middle of the polymer chains rather than the end. All these results proved thatselenocarbonates can be used as a new kind of RAFT agents. This new kind of RAFTagents can also be used to regulate the polymerization of MA and NIPAM. In MApolymerization system, with the increase of selenium atom number in mediator, molecularweights of polymer were still controlled and were closed to theoretical ones; molecularweight distributions were narrow except for TSC (Mw/Mn>1.70). In NIPAMpolymerization system, with the increase of selenium atom number in mediator, molecularweights of polymer were still controlled and were closed to theoretical ones, molecularweight distributions were also narrow, even when TSC as a RAFT agent (Mw/Mn=1.28).With selenocarbonates as RAFT agents, ABA type amphiphilic block polymersPS-b-PNIPAM-b-PS were synthesized in two steps.(3) A series of diselenocarbonate with different substitutions were synthesized andused as mediators for St, BA and VAc polymerization with AIBN as free radical initiator at60oC in bulk. The relationship between the structure of mediators and the controllabilityon polymerization of St, BA and VAc were summarized.(4) A series of diselenocarbamates were synthesized and used as RAFT agents for polymerization of St, BA and VAc with AIBN as free radical initiator at60oC in bulk. Therelationship between the structure of mediators and the controllability on polymerization ofSt, BA and VAc were summarized respectively, which have been explained in the view ofRAFT polymerization mechanism and electronic structures of these mediators.(5) In the presence of N, N-diethyl-Se-cyanomethyl diselenocarbamate (N1),O-ethyl-Se-cyanomethyl carbonate (O1), S-ethyl-Se-cyanomethyl thiodiselenocarbonate(S1), polymerization of St, BA and VAc with AIBN as radical initiator at60oC in bulkwas investigated and compared. These results indicated that O1can control thepolymerization of “highly activated monomer” St and “low activated monomer” VAc.Finally, O-phenyl-Se-(2-phenyl methyl acetate)-diselenocarbonate (O10) and N, N-(4-pyridyl)-Se-(2-phenyl methyl acetate)-diselenocarbamate (N10) were designed andsynthesized. The controllability on St, BA, VAc and MMA polymerization in the presenceof these two mediators were investigsted. Polymerization results showed that O10was auniversal mediator, not only for the LAM but also for the HAM.1H NMR spectra indicatedthat functionality degree of end group in the polymer chains obtained from O10mediatedpolymerization were very high, the Mn,NMRs were closed to their theoretical values. Blockpolymer were synthesized successfully by polymerization of a second monomer using thepolymer obtained from O10mediated polymerization as macro-mediator, and theirmolecular weights were closed to theoretical ones with relatively narrow molecular weightdistributions. The block polymer PVAc-b-PS with controllable molecular weight andmoderate molecular weight distribution was obtained finally.