Steric Hindrance Drives the Boron‐Initiated Polymerization of Dienyltriphenylarsonium Ylides to Photoluminescent C5‐Polymers

Abstract A series of alkyl‐subsituted dienyltriphenylarsonium ylides were synthesized and used as monomers in borane‐initiated polymerization to obtain practically pure C5‐polymers (main‐chain grows by five carbon atoms at a time). The impact of triethylborane (Et3B), tributylborane (Bu3B), tri‐sec‐butylborane (s‐Bu3B), and triphenylborane (Ph3B) initiators on C5 polymerization was studied. Based on NMR and SEC results, we have shown that all synthesized polymers have C5 units with a unique unsaturated backbone where two conjugated double bonds are separated by one methylene. The synthesized C5‐polymers possess predictable molecular weights and narrow molecular weight distributions (M n,NMR=2.8 −11.9 kg mol−1, Ð=1.04–1.24). It has been found that increasing the steric hindrance of both the monomer and the initiator can facilitate the formation of more C5 repeating units, thus driving the polymerization to almost pure C5‐polymer (up to 95.8 %). The polymerization mechanism was studied by 11B NMR and confirmed by DFT calculations. The synthesized C5‐polymers are amorphous with tunable glass‐transition temperatures by adjusting the substituents of monomers, ranging from +30.1 °C to −38.4 °C. Furthermore, they possess blue photoluminescence that changes to yellow illuminating the polymers for 5 days with UV radiation of 365 nm (IIE, isomerization induced emission).


Introduction
Exploring new polymerization methods always attracts great attention because it can offer new polymeric structures with unprecedented properties.B oron-initiated polymerization of ylides is an emerging C1 polymerization discovered by Shea et al.,who coined the name polyhomologation (main chain built up by one carbon atom at atime) [1] (Scheme 1A). Polyhomologation has already been applied to the synthesis of various polymethylene-based homo-and block copolymers with different topologies and properties. [2] Next, ab oron-catalyzed polymerization of allylic arsonium ylides (C3 polymerization) was reported, [3] where the main chain grows by three carbon atoms at atime producing C3-polymers (e.g., polypropenylene) (Scheme 1B). Recently,wereported anew boron-catalyzed polymerization of ylides,w hich is the polymerization of dienyltriphenylarsonium ylides initiated by triethylborane (Et 3 B), called C5 polymerization (main-chain elongation by five carbon atoms at at ime) (Scheme 1C). [4] C5 polymerization has generated new polymeric materials,C5-polymers,which have aunique unsaturated backbone structure where the two conjugated double bonds are separated by one methylene.H owever,i no ur previous work, [4] the synthesized C5-polymers contained mainly C5 (up to 84.1 %), but also C1 and C3 segments.T he following possible mechanism (Scheme 1C)h as been proposed to explain the presence of C1 and C3 segments:t he ylide monomer reacts with Et 3 Btoform an ate complex, which by 1,2-migration gives borane 1 with simustaneous remonval of triphenylarsine.T he borane 1 may experience once or twice [1,3]-sigmatropic rearrangement that leads to isomeric borane 2 or 3.D uring the polymerization, each cycle comprises one (C3 units), two (C5 units) or none (C1 units) [1,3]-sigmatropic rearrangement, eventually resulting in at hree-armed star terpolymer,which by oxidation/hydrolysis affords ahydroxylterminated polymer.Therefore,the key to achieving ahigher C5 segment ratio or even pure C5-polymer is to ensure two [1,3]-sigmatropic rearrangements of the intermediates in each polymerization cycle.
In this paper, as eries of new alkyl-subsituted dienyltriphenylarsonium ylides were synthesized and applied in the C5 polymerization to improve the C5 segment content. The impact of af ew borane initiators,t riethylborane (Et 3 B), tributylborane (Bu 3 B), tri-sec-butylborane (s-Bu 3 B), and triphenylborane (Ph 3 B), at ah igher C5 segment content, was studied. It was found that the steric hindrance of both the monomer and initiator promotes the two [1,3]-sigmatropic rearrangements of the intermediates at each polymerization cycle to form more C5 segments,t hus driving the polymerization to almost pure C5-polymer.The proposed mechanism of the polymerization was studied and confirmed by nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations.The thermal properties of the synthesized C5-polymers with different substituents were also evaluated by differential scanning calorimetry (DSC) measurements. Moreover,u nusual photoluminescence properties of the synthesized C5-polymers were found by fluorescence spectroscopy.

Results and Discussion
As eries of dienyltriphenylarsonium ylide salts with different substituents on the conjugated double bond was successfully synthesized, as confirmed by 1 HNMR, 13 CNMR, 19 FNMR, and 1 H-1 HCOSY (Scheme S1, Figures S1-S4). The corresponding ylide monomers were then generated in situ by deprotonation of the salts with n-butyllithium (n-BuLi)i n tetrahydrofuran (THF) at À78 8 8Cunder an argon atmosphere.
In order to further improve the C5 segment ratio of the C5-polymer,t he polymerizations of as eries of new ylides (Ylide 2t oY lide 5, Scheme 2) initiated by Et 3 Bu nder the condition of [Ylide] 0 /[Et 3 B] 0 = 105/1 at 50 8 8C ( Table 1, Entries 3t o6)w ere studied. Thei ns itu generated ylide monomers were initially heated from À78 8 8Ct o0 8 8Ca nd stirred for 30 min, followed by the addition of Et 3 B. Then, the mixtures were placed at a508 8Coil bath immediately to start the polymerizations.W hen the deep red solutions turn into colorless solutions,the ylides were entirely consumed, and the polymerizations were completed. After oxidation/hydrolysis, the corresponding hydroxyl-terminated linear polymers were obtained. All synthesized polymers characterized by NMR and SEC (Figures S7-S10) were found to be C5-polymers with major C5 segments.A ll SEC traces are symmetrical, monomodal, and narrow ( = 1.08-1.18), as shown in Figures S7C-S10C.T he molecular weights of the C5-polymers calculated by NMR were very close to the theoretical ones (Table 1, . All these results indicate the living character of the Et 3 B-initiated C5 polymerization. Based on the 1 HNMR and 1 H-1 HC OSY spectra (Figures S5 and S7-S10), all synthesized polymers are C5-polymers with dominant C5 segments,r anging from 82.6 %t o9 2.6 % ( Table 1, Entries 2-6). It has been observed that when the substituents on the conjugated double bond of the ylides change from methyl, ethyl, propyl, and pentyl to heptyl, the C5 segment ratios gradually increased from 82.6 %(MeDEY-2), 86.3 %( EtDEY-1), 86.5 %( PrDEY-1), and 87.5 %( Pen-DEY-1) to 92.6 %(HepDEY-1) ( Table 1). This is because the long-chain aliphatic substituent on the ylide causes av ery severe steric hindrance on the side chain of the propagating polymer during the polymerization. Thes teric hindrance inhibits the formation of the C1 and C3 segments and promotes C5 segments.Asshown in Figure S11, we obtained the optimized conformations and relative Gibbs free energy (DG)o ft he intermediates forming C1, C3, and C5 segments by using the dispersion-corrected BP86 (BP86-D3BJ) density functional theory (DFT) method with the def2tzvpp basis set and CPCM (THF) solvent model. Thef ree energy of all C3 intermediates is higher than that of the C1 and C5 intermediates,w hich thermodynamically support the lowest C3 segment content in almost all synthesized polymers (Table 1). Thefree energy of the C1 intermediates is very close to that of the C5 intermediates.However,inall cases of Table 1, the C5 segment ratios are much higher than that of C1. This is because the formation of C5 segments reduces the crowding of the propagating three-armed polymeric borane.Conversely,t he C1 segments will produce long side chains,w hich will cause steric hindrance.
Furthermore,ithas been observed that when the aliphatic substituents on the intermediates become longer from methyl to heptyl, the relative free energies of the C5 intermediates decrease from 1.80 kcal mol À1 to À0.83 kcal mol À1 (Figure S11a-e), which means that the C5 intermediates are more and more stable than the C1 and C3 intermediates,t hus thermodynamically supporting the formation of more C5 segments.T he results are consistent with the increase in the  [4] C5 segment ratios of the corresponding C5-polymers from 82.6 %( MeDEY-2) to 92.6 %( HepDEY-1) ( Table 1, Entries 2-6). However,the free energy difference from 1.80 kcal mol À1 to À0.83 kcal mol À1 is very small, so the energy cannot be considered as the only factor for the formation of C5 segments.Inaddition, the free energy of the C5 intermediate with al onger pentyl substituent is À0.52 kcal mol À1 ,w hich is higher than that of C5 intermediates with apropyl substituent of À0.89 kcal mol À1 .H owever,t he C5 segment ratio of the PenDEY-1 (Table 1, Entry 5) is 87.5 %, which is still higher than 86.5 %o ft he PrDEY-1 (Table 1, Entry 4). From all the above results,i tc an be concluded that the steric hindrance mainly facilitates the formation of the C5 segments,t hereby driving the polymerization of dienyltriphenylarsonium ylide close to the pure C5-polymer. Thei nfluence of the borane initiator on the C5 segment ratio was also studied. In addition to Et 3 B, tributylborane (Bu 3 B), tri-sec-butylborane (s-Bu 3 B), and triphenylborane (Ph 3 B) were selected as initiators for the polymerization of Ylide 1( [Ylide 1] 0 /[borane] 0 = 105/1, Table 1, Entries 7-9). All obtained polymers characterized by 1 HNMR, 1 H-1 H COSY,a nd SEC (Figures 1a nd S12-S13) were found to be C5-polymers,p ossessing narrow molecular weight distributions ( = 1.18-1.24). As shown in Table 1, the highest C5 segment ratio is 85.9 %o btained by the s-Bu 3 B-initiated polymerization of Ylide 1( MeDEY-5, Table 1, Entry 9), the second is 84.3 %obtained by Ph 3 B-initiated polymerization of Ylide 1(MeDEY-3, Table 1, Entry 7), and the third is 83.2 % provided by Bu 3 B-initiated polymerization of Ylide 1( Me-DEY-4, Table 1, Entry 8). All data are higher than 82.6 %o f the C5 segment ratio of the C5-polymer synthesized by Et 3 Binitiated polymerization of Ylide 1(MeDEY-2). It is because s-Bu 3 B, Ph 3 B, and Bu 3 Bh ave greater steric hindrances than Et 3 B. Thel arger steric hindrance will push the intermediates to undergo twice the [1,3]-sigmatropic rearrangements forming the C5 segments for releasing the crowding at the beginning of the polymerization. Ph 3 Bh as the largest steric hindrance,but the strongest Lewis acidity of Ph 3 Binhibits the [1,3]-sigmatropic rearrangement of borane,t hus relatively suppressing the formation of C5 segments.Incontrast, s-Bu 3 B has the second-largest steric hindrance but lower Lewis acidity,s oM eDEY-5 has the highest C5 segment ratio of 85.9 %. Ther elative free energy of the corresponding C5 intermediates (Figures S11a and S11f-h) calculated by DFT for the formation of the C5 segments thermodynamically supports the observed order of the C5 segment ratios: . Thef ree energy of the C5 intermediate (s-Bu 3 B) is the lowest (À1.27 kcal mol À1 ,F igure S11h), which is consistent with the highest C5 segments ratio (MeDEY-5, 85.9 %). TheC 5i ntermediate (Ph 3 B) has the second lowest free energy of À1.02 kcal mol À1 ( (Figure S11a), which agrees with the lowest C5 segment ratio (MeDEY-2, 82.6 %). Furthermore,the free energy of the C5 intermediates (Ph 3 Ba nd s-Bu 3 B) is even lower than that of their corresponding C1 intermediates (Figures S11fa nd S11h), which means that the polymerizations tend to form C5 segments rather than C1 and C3 segments in both cases.Above all, s-Bu 3 Bperformed the best initiation in the polymerization of Ylide 1f or achieving ahigher C5 segment ratio. Figure 1A shows ar epresentative 1 HNMR spectrum of the C5-polymer (MeDEY-5, Table 1 Table 1, Entries 10 and 11) were carried out. Theo btained polymers are confirmed to be C5-polymers,p ossessing predictable molecular weights and narrow molecular weight distributions (M n,NMR = 6.3 and 11.9 kg mol À1 , = 1.09 and 1.12). As shown in Figure 1C,when the feeding molar ratio of Ylide 1tos-Bu 3 Bincreases,the SEC traces shift from the low molecular weight to high molecular weight elusion volumes, and they are all symmetrical, monomodal, and narrow.T he small shoulder peaks on the SEC traces can be attributable to as mall amount of double bonds on the polymer chains, especially the pendant double bonds of the C1 and C3 repeating units,w hich are participating in side reactions during the oxidation of the 3-armed borane star with NaOH/ H 2 O 2 . [5] These results indicate the living nature of the polymerization initiated by s-Bu 3 B. As summarized in Table 1, when the molecular weight of the synthesized C5polymer increases from 3.4 kg mol À1 to 6.3 kg mol À1 ,t he C5 segment ratio increases from 85.9 %to86.9 %. This is because the higher molecular weight leads to more severe crowding of the propagating polymeric borane,w hich will push the intermediate to undergo twice the [1,3]-sigmatropic rearrangement to release the crowding,thus generating more C5 segments.H owever,w hen the molecular weight was further increased to 11.9 kg mol À1 ,the C5 segment ratio decreased to 86.7 %, nearly the same as the one of 6.3 kg mol À1 .Inaddition, it was observed that the end of all obtained C5-polymers is aC 5m onomer unit connected to the hydroxyl group.A ll above results indicate that the formation of the C1 and C3 segments occurs mainly at the beginning of the polymerization, when the steric hindrance of the propagating polymeric borane is not severe at this time,t hereby allowing the crowding caused by the C1 and C3 segments.When the steric hindrance caused by the molecular weight reaches asufficient level, at the end of the polymerization, only C5 segments are produced.
In our proposed mechanism of the polymerization of dienyltriphenylarsonium ylides,t he key process is the rearrangements between the three intermediates for forming the C1, C3, and C5 segments.I no rder to demonstrate the formation of the three intermediates, 11 BNMR measurements of Et 3 Ba nd am ixture of Et 3 Ba nd Ylide 1( [Ylide 1] 0 /[Et 3 B] 0 = 1/1) at 25 8 8Ci nC DCl 3 were carried out. As shown in Figure S18A, the characteristic boron signal of Et 3 B is as inglet at 86.94 ppm. When Et 3 Bw as mixed with equimolar Ylide 1, three overlapped singlets corresponding to the characteristic signals of the three produced borane intermediates are located at À1.46 ppm, À1.50 ppm, and À1.53 ppm ( Figure S18B). Furthermore,t he DFT calculations (Gaussian 16, BP86 + D3 (BJ) DFT hybrid functional, def2tzvpp basis set, and CPCM (THF) solvent model) were performed to obtain the relative Gibbs free energy (DG), enthalpy (DH), and electronic energy (DE)for the rearrangement process of the Et 3 B-initiated polymerization of Ylide 1and optimized structures of key transition states,asshown in Figure 2. Based on the calculations,the C1 intermediate (IN-1) undergoes a[ 1,3]-sigmatropic rearrangement via at ransition state (TS-1) with 11.9 kcal mol À1 free energy barrier to form the C3 intermediate (IN-2), which further generates the C5 intermediate (IN-3) through atransition state (TS-2) with 12.2 kcal mol À1 free energy barrier. Another reaction path in which IN-1 may directly experience a[ 1,5]-sigmatropic rearrangement to generate IN-3 is also possible.B ased on the calculation results,the IN-1 should experience atransition state (TS-3) with af ree energy barrier of 53.8 kcal mol À1 , which is much higher than that of the previous reaction path of 11.9 kcal mol À1 and 12.2 kcal mol À1 .T herefore,t he DFT calculations support that the C1 intermediate should undergo a[1,3]-sigmatropic rearrangement leading to C3 intermediate, which will undergo a[ 1,3]-sigmatropic rearrangement again to form the C5 intermediate.During the polymerization, each cycle comprises one or two [1,3]-sigmatropic rearrangements, or none of them, which eventually leads to at hree-armed terpolymer with as mall amount of C1 and C3 segments, which by oxidation/hydrolysis affords ah ydroxyl-terminated terpolymer containing the C1, C3, and C5 segments.
Based on our previously proposed isomerization-induced light emission, we further conducted the irradiation of MeDEY-6 solution (3.0 mg mL À1 in THF) under 365 nm UV light for 5d ays.A ss hown in Figure 4E,t he MeDEY-6 solution exhibits anew strong light emission at 494 nm after 5 days of UV irradiation. Ther ed-shift of the light emission indicates the formation of longer conjugated double bond chromophores,which is attributed to the UV-induced isomerization. As shown in Figure S20, the MeDEY-6 solution shows an ew UV-vis absorption band centered at around 368 nm, which is associated with p-p*e lectron transition from the longer conjugated double bond system induced by isomerization. Thel onger conjugated double bond system may be responsible for the extensive "electron delocalization" and for alower energy band gap,thus leading to fluorescence with alonger wavelength. Figure 4G shows alight yellow fluorescence emitted by the MeDEY-6 solution irradiated with 365 nm UV light for 5days,which is consistent with the redshift that appears in Figure 4E.I na ddition, the emission intensity of the MeDEY-6 solution after irradiation for 5days under 365 nm UV light is much weaker than before.This may be due to the fact that when the polymer molecule returns from the excited state to the ground state through such al ower energy band gap,m ore non-radiative transitions occur.

Conclusion
In summary,aseries of new alkyl-subsituted dienyltriphenylarsonium ylides were synthesized and used as the monomers in the borane-initiated C5 polymerization (mainchain grows by five carbon atoms at atime). Theimpact of the borane initiators,i ncluding triethylborane (Et 3 B), tributylborane (Bu 3 B), tri-sec-butylborane (s-Bu 3 B), and triphenylborane (Ph 3 B) on the achievement of pure C5 polymerization, was studied. It has been found that increasing the steric hindrance of both the monomer and the initiator can facilitate the formation of more C5 repeating units,t hereby driving the polymerization to almost pure C5-polymer (the C5 repeating units up to 95.8 %). All synthesized polymers have mainly C5 repeating units (tiny C1 and C3 repeating units), possessing predictable molecular weights and narrow molecular weight distributions (M n,NMR = 2.8 À11.9 kg mol À1 , = 1.04-1.24). Thek ey polymerization process that each cycle comprises one or two [1,3]-sigmatropic rearrangements, or none of them, which eventually leads to the C5-polymer having asmall amount of C1 and C3 segments,was proved by NMR and DFT calculations.All synthesized C5-polymers are amorphous with tunable glass-transition temperatures ranging from + 30.1 8 8CtoÀ38.4 8 8Cbyadjusting the substituents of monomers.Aphotoluminescence property of C5-polymers was discovered, which was attributed to the dual effect of their original conjugated double bond chromophores and the isomerization-induced longer conjugated double bond chromophores.T his work provides an ew path to pure C5 polymerization and opens new horizons towards novel polymeric materials and properties.