| Conductive polymer is a kind of attractive material since it was invented in1977. After over three decades, it is still an emerging research area due to its wide application in light-emitting device, supercapacitors, electrochromic devices and organic photovoltaic cells. So that the growing demand for organic conjugated material for their use in semiconductor decvices, for electronic and optoelectronic technologies, low band gap conjugated polymer are among the most promising material. In addition, it can be easily tailored by rationally designed and synthesized. As far as its synthesis methods are concerned, it mainly includes chemical oxidation polymerization, electropolymerization and photo-polymerization but recently some groups make arduous efforts to pay much attention on solid-state polymerization (SSP), which appeared early in the1960s. This method deserves much attention due to its special features such as solvent free and oxidant or additional potential free, so this method is necessarily required through traditional methods. Besides, solid-state polymerization is a commonly practiced technique to obtain high weight crystalline polymers such as commercial poly(ethylene-terephtalate) and poly(aryl carbonate). The technique of method involves heating the monomer crystal below their melting point without using any catalyst or solvent. On the other hand, the physical state of the monomer is very important in determining both the reaction kinetics and the morphological, crystallographic and molecular structures of the resulting polymer. Till now, only a few examples of thiophene derivatives which are suitable for SSP were successfully realized since pioneer work which was done by Wudl and co-worker, who first successfully made poly(3,4-ethylenedioxythiophene)(PEDOT) with spontaneous SSP in2003.Because solid-state polymerization is still a subject of considerable research interest and the lack of stability of these material and complicated synthetic routes necessary for their preparation limit their potential application in electronic devices. So that, this dissertation is continuously devoted to the design and synthesis of some polymers by the development of new synthetic methodologyFirstly, in chapter two several parrel, stradegy thiophene derivatives containing different heteroatom linkers were rationally designed, synthesized and studied as the potential candidates for SSP. Our study reveals that heteroatoms like C, Si and P are good linkers between 3,4-ethylenedioxythiophene (EDOT) units and most monomers can be spontaneous polymerized successfully through annealing at a moderate temperature. Specially, in this work we find that: particularly a short intermolecular Hal-Hal in crystal packing not only within but also a little bit longer their double van der Waals radius of halogen is suitable for SSP, because these monomers, which have a flexible short single heteroatom linkers, may rotate and elongate freely under heating so that the distance of this halogen with halogen atom of neighboring monomer in crystal unit is close enough to form its polymer chain, this problem plays an important role in SSP and help us to design, synthesize and study other candidates for the SSP. Detailed characterizations of these corresponding polymers were carried out and several key crystal structures were analyzed as following:Synthesis of monomers and their behavior under SSP:As mentioned above, various modified structures with Si, C or P linker containing monomers were synthesized. Although, we have to get into trouble in the final bromination step, but we devote to the suitable synthetic methodology of the monomer so that mono bromo substituted EDOT was synthesized as starting material in case of Si, C or P linker containing monomers.However, these obtained dihalogen-substituted monomers were examined under SSP and those successful samples released bromine or iodine vapor. According to SSP results, only three monomers can successfully form conducting polymers through SSP, indicating single hetero atoms are effective linkers in our design strategy. Surprisingly, two dibromo-substituted failed to form respective polymers under SSP while diiodo-substituted monomers worked well. It is noted that previous studies showed that in case of halogen substituted EDOT derivatives or analogs, bromo or iodo did not have big difference because of the formation of the same polymer matrix under SSP. However, it is not true in our case of the examined X2-Si-EDOT or X2-P-EDOT systems. According to these observations, it is clear that iodo-substituted monomers incline to form corresponding polymer under SSP whereas sometimes bromo-substituted monomers do not have ability to make it. These results disappoint us a litter bit because bromination is more economic and easier approach when compared with iodination.Crystallographic X-ray Analysis:In the single crystals of the three successful monomers and a typical unsuccessful sample were obtained and studied by X-ray analysis for further understanding of the structural requirements for SSP. Before getting monomers’crystal structures, we expected that their intermolecule halogen distances are within their double van der Waals radius of halogen (3.7A for Br-Br and4.0A for I-I), like previous reports. However, what surprised us is that the distances between neighboring halogens are a little bit longer. Their nearest halogen distances are4.296,4.343, and4.092A for Br2-C-EDOT, I2-Si-EDOT and I2-P-EDOT respectively, which can have SSP successfully. We attribute such especial phenomena to its flexible linker. Such featured structure may rotate or elongate freely under heating, resulting in the close to halogen atom of neighboring monomer, which plays an important role in SSP, especially in our examined system. In this work, combing with their crystal structures analysis, it is clear that heteroatom types and substitution groups have great effect on their SSP behavior and this finding may help us to gain a deeper understanding of SSP. Furthermore, taking thiophene moiety into consideration, our design strategy along with EDOT parallel direction may broaden SSP application, suggesting a promising approach for designing other thiophene monomers suitable for SSP.Secondly, among all these monomers developed so far, symmetrical dihalogen-substituted monomers and3,4-ethylenedioxythiophene (EDOT) framework are necessary. Meanwhile, it is noted that all successful dihalogen-substituted monomers are modified through EDOT molecule’s vertical direction. In addition, the modification on EDOT whether based on EDOT framework or its derivative, is a kind of burdensome work involving multi-step synthesis. Due to such restriction based on present design monomer strategy, recently, we successfully designed a monomer along with EDOT parallel direction through introducing a flexible linker between EDOT units and obtained corresponding conductive polymers by SSP. Here, we further examined the flexible linker length effect among these thiophene derivatives and checked their behavior under SSP. This study not only revealed fundamental information in SSP, but also broadened the monomer design concept and obtained novel conductive polymers as well. Moreover, we want to emphasize here two points:1, taking the introduced non-conjugated flexible linker in most cases into consideration, our developed strategy still has big shortcoming of having poor conductivity when compared with famous PEDOT. Maybe conjugated linkers would work well under SSP but need to be verified soon;2. we are more interested in the developing new synthesis method and studying structure information other than their application at present.With this purpose, in chapter three several EDOT derivatives with different length by simply tuning linker atoms number were also rationally designed, synthesized and studied as potential candidates for SSP. We examined up to four-atom length and also investigated different halogen substituted effect. Detailed characterizations of their corresponding polymers were also carried out and crystals of several monomers were obtained for structures analysis:+With synthesis of monomers and solid state polymerization:Three designed monomers were synthesized from other materials. Because of the difficulty of the final bromination step, monobromo substituted EDOT was synthesized as starting material in most cases. Tow monomers were obtained through Mannich reaction to link the thiophene moiety of2-bromo or iodo substituted3,4-ethylenedioxythiophene. Another compound was obtained by the reaction of mono-iodo substituted EDOT with1,2-bis-dimethylsilanyl-ethane with the help of Li-reagent of LDA.After that, these resulting dihalogen-substituted monomers were polymerized through SSP. According to SSP results, except for one monomer, others monomers can be successfully formed conducting polymers through SSP, it is shown that indicating linker distance play a key role in their behavior under SSP. Previously, we introduced one-atom linker between EDOT units and got great success under SSP. Therefore, we can draw a conclusion that flexible linker of single bond with up to three atoms is appropriate length between EDOT units for solid SSP. Although we failed to obtain EDOT monomers with two-atom-length spacer, it can be expected that such structure should be a successful candidate for SSP. Therefore, though flexible linker along parallel direction proves to be an excellent design strategy, the maximum length would be no more than three atoms according to these results. Detailed discussion will be done in the following crystal analysis section. It is interesting that after replacement of methyl group by long alkyl chain of butyl, the suitable temperature for it requires lower about20℃to complete SSP while monomer with methyl group needs100℃. Such phenomena indicates that the substitution effect on the flexible spacer have definitely influenced on their SSP behavior as well, which encourage us to further explore much more acutely controllable and fine tunable monomers for SSP.+Molecule weight:In SSP method, the most of polymers could not common be dissolved in organic solvent so it is very difficult to get the polymer’s molecule weight, but in this work it is the first time for us to get the polymer’s molecule weight and its dependence with temperature concerning of SSP. These results are very important for fully understanding SSP process, the extending monomer structures, further rational designing a system of new monomers and the tuning of the conductive polymers properties as well.Because of the insolubility of the P(3-alkyl-EDOT), so that the soluble P(3-alkyl-EDOT) was chosen as a typical model compound to further characterize especially concerning of molecular weight and its GPC profile was presented in supporting information. Previous MALDI-TOF mass spectrometry reveals that poly(2,3-Dihydro-thieno[3,4-b][1,4]dithiine) obtained through SSP containing up to5-14monomer units and elemental analysis of dedoped one indicates an average of six repeat units per chain, we get polymer molecular weights information of N2H4treated P(3-alkyl-EDOT) through GPC. Mn of3723g mol-1with PDI of2.68indicate that the P(3-alkyl-EDOT) has a repeat units of10.Meanwhile, in the case of four-atom linker length, though it would be get theoretical SSP success according to its crystal structure analysis, it fails under SSP because of its quite low melt point, indicating that at present we do not find length limit for the flexible chain. Combing with their crystal structure data, these observation and evidence not only may help to gain a deeper understanding of SSP but also enlarge thiophene-based SSP database. Furthermore, the substitution on the flexible linker moiety also plays an important role in the fine-tuning of their monomers’behavior under SSP and corresponding functional properties. And such kinds of research are underway in our lab. Anyway, both of the linker length, linker structure and sometime the melt point of monomer are key factors to determine their behavior under SSP.It is generally accepted that due to their electronic state difference, conjugated polymer share most properties with their conductive polymers but differ in other characteristics such as UV-visible, fluoresce spectra properties and of course including conductivity and so on. It seems that conjugated polymers play a brilliant role in organic electronics ranging from OLED, OPV, to OFET while conductive polymer like PEDOT-PSS as a typical sample mainly serves as electrode materials or as buffer layers in organic electronics devices. Triggered by SSP progress, we think about how to use such simple heat treatment method to synthesize neutral conjugated polymer directly. However, such facile method is lacking before2011.Recently, Officer and our group independently fortunately found PEDOT can be formed just by simple heating mono-bromo substituted3,4-ethylenedioxythiophene (Br-EDOT). Meanwhile, Swager T. M. group successfully synthesized corresponding polymers based on mono choloro-substituted EDOT derivatives in the existence of Lewis acid. We already verified such heat promoted polymerization is acid depending and named such type reaction as self acid-assisted polycondensation (SAAP). This interesting result encourages us to further design more complicated monomers and try to probe the success rate based on SAAP through changing thiophene derivatives structures, halogen effects as well as halogen positions in the designed monomers. In addition, soluble polymer was designed by introducing long alkyloxy chain in monomer because of no report concerning polymer molecule weight and lack of detailed characterization till now.Furthermore, it is generally believed that compared with symmetrical molecules, unsymmetrical molecules are really hard nuts to crack but play a big role in molecule design to make multi-functional polymers with unique optical, electronic and physical properties. Therefore, it is necessary to explore unsymmetrical monomer suitable for facile SAAP, which offers amazing advantages of solvent, trace metal free and oxidant or ex-ternal applied potential free. Here, we present our latest result and prove that SAAP is a versatile approach to synthesize different conjugated polymers with great potential in wide application.Finally, in chapter four we design and synthesise four mono-halogen substituted thiophene derivative monomers. Most synthesized monomers can be spontaneous polymerized successfully through simple annealing process at moderate temperature, indicating that they are suitable for SAAP procedure so that poly(3,4-ethylenedioxythiophene)(PEDOT) and thiophene based homopolymer were obtained by SAAP based on mono-halogen substituted thiophene derivative monomers. Detailed design monomer and characterizations of the obtained polymers were described following:+Synthesis of monomer and their behavior under SAAP:Five monomers were designed and synthesized and two monomers were obtained by bromination and iodination of bis-EDOT but three other monomers were synthesized through Stille coupling through dibromo substituted thiophene or derivatives and corresponding stannanes after that with the selective lithium reagent of LDA, borolane was introduced at5-position of3-Hexyloxy-thiophene then followed by Suzuki coupling, NBS brominationAll SAAP experiments were carried out in a plastic vial under60-100℃for24h. Compared with the typical5,7-dibromo-2,3-dihydrothieno[3,4-b][1,4]dioxine (DBEDOT)’s SSP procedure as mentioned above, instead of the generation of elementary halogen, hydrobromic acid is produced in our SAPP. Therefore, the obtained polymer PEDOT should be in neutral state because of nonexistence of strong oxidant in our experiment. In addition, such polymerization is self-accelerated process because of generation of hydrobromic acid and the proton is involved in their chain propagation. Our SAAP test shows that except for one monomer, others form corresponding polymers smoothly, indicating that SAAP can successfully form homopolymers through these designed unsymmetrical monomers and bromo-substitution position is a key factor for their success. In detail, halogen substituted in7-position of EDOT dimer (bis-EDOT) is effective, indicating SAAP can be employed successfully in oligmer especially in the case of dimer. Compared with monomer5-(5-bromothiophen-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine and5-bromo-7-(thiophen-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxine behaviors under SAAP, it is clear that bromo-substituted position on EDOT unit directly resulting in a successful SAAP, indicating bromo atom should be close to oxygen atom on the molecule level. Such fancy phenomenon is well explained by acid-assisted polymerization mechanism as proposed previously. Finally, the success of monomer5-bromo-4-(hexyloxy)-2,2’-bithiophene verifys that alkyloxy chain is an effective way for the synthesis of soluble polymer and more easily characterized homopolythiophen can be obtained through such facile method. In our study, we did not check chloro-substituted thiophene derivative under SAAP because C-Cl bond energy (328kJ/mol) is too strong to be broken by heat-treatment, resulting the failure of initiation of polymerization+Molecule weight of P(S-S-OHexy) vs temperature dependence:Because of the insolubility of the other three polymers, the soluble P(S-S-OHexy) was chosen as a typical model compound to further characterize especially concerning of molecular weight and its typical NMR was presented in supporting information. As can be seen that along with the temperature increase, Mn decrease drastically along with the DP with lower one order. In addition, the lower temperature usually results in lower PDI even to1.18, which is good news for us to rational control molecule distribution for different usage.In this chapter, PEDOTs and different homopolythiophen were prepared by SAAP based on mono-halogen substituted thiophen derivative monomers and the monomers’structure effects especially the bromo-substituted positions were investigated in detail. Our results proves that substituted halogen (bromo or iodo) position plays an important role SAAP and we intend to probe the relationship between monomers’structure and their success rate under SAAP. Our study reveals that halogen atoms, halogen positions and molecule structures have great effect on the monomers’behavior under SAAP. We can draw a conclusion that as far as monomer design is concerned, bromo or iodo atom has almost no difference in SAPP while bromo atom should close to alkyloxy or ethylenedioxy, which definitely guarantees its SAAP success. Our investigation not only helps to broaden SAAP database, but also supplies fundamental information to understand such promising polymerization approach. In addition, through simple temperature controlling, we can obtain low to high molecule weight scale polymers because of temperature dependence of molecule weight and distribution for SAAP.Furthermore, our success of preparation of unsymmetrical homopolythiophene may offer an alternative way to synthesize other conjugated polythiophenes, which may have plenty usage in lots of optical-electronic devices. Further expanding studies to other thiophene derivatives and their conjugated polymers on organic-electronic application field are under way in our lab. |
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