Double-bond-containing polyallene-based triblock copolymers via phenoxyallene and (meth)acrylate

A series of ABA triblock copolymers, consisting of double-bond-containing poly(phenoxyallene) (PPOA), poly(methyl methacrylate) (PMMA), or poly(butyl acrylate) (PBA) segments, were synthesized by sequential free radical polymerization and atom transfer radical polymerization (ATRP). A new bifunctional initiator bearing azo and halogen-containing ATRP initiating groups was first prepared followed by initiating conventional free radical homopolymerization of phenoxyallene with cumulated double bond to give a PPOA-based macroinitiator with ATRP initiating groups at both ends. Next, PMMA-b-PPOA-b-PMMA and PBA-b-PPOA-b-PBA triblock copolymers were synthesized by ATRP of methyl methacrylate and n-butyl acrylate initiated by the PPOA-based macroinitiator through the site transformation strategy. These double-bond-containing triblock copolymers are stable under UV irradiation and free radical circumstances.

now, which may be attributed to the unusual cumulated double bonds. Therefore, the applications of polyallene as reactive polymers have been certainly limited. In order to enlarge the application range of polyallene, it is necessary to prepare tailor-made polymeric architectures containing polyallene segment. To realize this goal, block copolymer with a stable covalent-bonded linkage between two different segments is a good and convenient choice. The interest in block copolymers arises mainly from their unique solution and associative properties as a consequence of their molecular structure. To our best knowledge, none has reported the block copolymer via allene derivative and common vinyl monomer until now.
Generally, two strategies have been employed to synthesize the block copolymers: one is the sequential feeding of different monomers through living polymerization, including anionic [17][18][19] , cationic 20,21 , group transfer 22 , living radical polymerization [23][24][25][26] ; on the other hand, the site transformation strategy was used to synthesize the block copolymers via different polymerization mechanisms 27 . Thus, bi-and multi-functional initiators have been designed and prepared in order to satisfy different polymerization mechanism demands of the monomers with different chemical structures [28][29][30][31][32][33] . By this way, end-functionalized macroinitiator was first prepared and subsequently used to initiate the polymerization of the second monomer by another polymerization mechanism. In principle, any block copolymer difficult to be prepared by single polymerization mechanism could be designed and synthesized by this approach.
Recently, metal-catalyzed living radical polymerization including ATRP [34][35][36][37][38][39] and SET-LRP [40][41][42][43][44][45] has been widely used for the synthesis of polymers with various structures, especially for the synthesis of block polymers. However, ATRP still does not work for some monomers though it has many advantages compared to other polymerization methods when used for block copolymerization. An alternative method to overcome this limitation is to combine ATRP with conventional radical polymerization by using bifunctional initiator.
In light of those aforementioned considerations, a new bifunctional initiator containing azo and ATRP initiating groups was first synthesized in the present work, which could sequentially initiate the polymerizations of allene derivative and commonly used vinyl monomer under different conditions as shown in Fig. 1. The free radical method is an easiest way to realize the polymerization of allene derivative, while ATRP is an attractive approach for common vinyl monomer. In this work, we reported the first example of triblock copolymers containing phenoxyallene and (meth)acrylate repeated units prepared by a combination of conventional azo-initiated free radical polymerization and ATRP, which showed high stability under free radical and UV irradiation surroundings.

Results and Discussion
Synthesis and characterization of azo-ATRP bifunctional initiator. As mentioned above, it was difficult to prepare block copolymers of allene derivatives and vinyl monomers by single polymerization mechanism, i.e. sequential feeding, because nickel-catalyzed living coordination polymerization is only suitable for allene derivatives [3][4][5][6][7][8][9] , diene 12 , isocyanide 13 , and thiophene 14,15 , not suitable for common vinyl monomers. Therefore, the "site transformation" strategy is the only feasible approach to construct block copolymers of allene derivatives and vinyl monomers, in which end-functionalized polyallene can be employed as macromolecular initiator or mediation agent to initiate or mediate the polymerization of the second monomer with diverse polymerization techniques such as ATRP [34][35][36] , SET-LRP 40,41 , and RAFT polymerization 46,47 . ATRP appears to be the most popular process among different reversible-deactivation radical polymerization (RDRP) techniques because of its mild reactions conditions, tolerance of monomer functionalities, and variety of monomers, and it has been extensively used to prepare copolymers with different architectures, making it not difficult to synthesize polyallene-based block copolymers. Indeed, our group has reported the synthesis of well-defined polyallene-based graft copolymers via the combination of Ni-catalyzed living coordination polymerization and ATRP [48][49][50] .
The chemical structure of bifunctional initiator 1 was examined by FT-IR, 1 H NMR, 13 C NMR, and elemental analysis in detail. As shown in Fig. 2A, the IR absorptions at 2237 and 523 cm −1 denoted the presence of -C≡ N group and C-Br bond, respectively, while the stretching vibration of carbonyl appeared at 1756 cm −1 . Figure 3A shows 1 H NMR spectrum of bifunctional initiator 1, which exhibited all typical proton resonance signals related with azo and ATRP initiating groups. The peaks at 4.59 ('b' , 1 proton) and 1.95 ('d' , 3 protons) ppm were attributed to 4 protons of O 2 CCH(CH 3 )Br initiating group. The resonance signals located at 2.61 ('c' , 4 protons) and 1.75 ('e' , 3 protons) ppm corresponded to 7 protons of CH 3 C(CN)CH 2 CH 2 CO 2 adjacent to azo group. The distinct carbon resonance signals appearing at 21.4 ('k') and 39.4 ('g') ppm in 13 C NMR spectrum ( Fig. 3B) further proved the existence of Br-containing ATRP initiating group. Moreover, the elemental analysis result was well consistent with the theoretical value. Thus, all aforementioned results evidenced the successful synthesis of bifunctional initiator 1, which can combine free radical polymerization with ATRP to prepare the block copolymers unavailable via single polymerization mechanism.
Synthesis and characterization of Br-PPOA-Br macroinitiator. Since azo group may induce the chain transfer of propagating radicals during ATRP, conventional free radical polymerization of phenoxyallene (POA) initiated by bifunctional initiator 1 should be firstly conducted. Therefore, solution free radical polymerization of POA was performed in toluene at 75 °C under Ar using the as-prepared 1 as initiator in order to obtain Br-PPOA-Br 2 macroinitiator.
The resulting homopolymer was characterized by GPC, FT-IR, 1 H NMR, 13 C NMR, and elemental analysis. The homopolymer showed a unimodal elution peak in GPC retention curve with a broad molecular weight distribution of 1.95, which was the characteristic of free radical polymerization. The peaks at 1643 and 1675 cm −1 in FT-IR spectrum after homopolymerization of POA (Fig. 1B) corresponded to the double bonds originating from the cumulated double bonds of allene derivatives. The stretching vibration peak of carbonyl appeared at 1761 cm −1 , this indicating the presence of ATRP initiation group in PPOA homopolymer. Figure 4A shows 1 H NMR spectrum of the homopolymer, which displayed that PPOA homopolymer comprised both 1,2-and 2,3-polymerized units (labeled as y and x in Fig. 1). PPOA homopolymer contained 28% of 1,2-polymerized units and 72% of 2,3-polymerized units, i.e. less 1,2-polymerized units due to the steric hindrance, via the integration area ratio of 1,2-polymerized units (peak 'd' at 5.00 ppm) to 2,3-polymerized units (peak 'f ' at 2.55 ppm). The resonance signals at 1.61 ppm ("h") belonged to 3 protons of CH 3 CCN previously adjacent to azo group in bifunctional initiator 1, which clearly demonstrated the polymerization of POA was indeed initiated by the azo group in compound 1. The signals originating from 4 protons of O 2 CCH(CH 3 )Br initiating group were located at 4.57 ('e' , 1 proton) and 1.93 ('g' , 3 protons), respectively, which illustrated that the halogen-containing ATRP initiating group was inert during the free radical polymerization of POA. Figure 4B shows 13    CO 2 ) ppm, respectively. Thus, we can confirm the formation of PPOA homopolymer containing terminal ATRP initiating group.
Though polystyrene-calibrated GPC provided a relative M n of 9,400 g/mol, the 'absolute' molecular weight (M n,GPC/MALS ) was determined by GPC/MALS with a value of 9,200 g/mol, which was very close with that obtained from conventional GPC because the structure of PPOA was similar with that of polystyrene. Generally, the termination of free radical polymerization includes coupling and disproportionation and the ratio between both mechanisms can be determined by measuring the exact number of characteristic end group. Herein, if there was only coupling termination, the number of O 2 CCH(CH 3 )Br end group should be 2.0; if there was only disproportionation termination, the number of O 2 CCH(CH 3 )Br end group should be 1.0; if there was both coupling and disproportionation termination, the number of O 2 CCH(CH 3 )Br end group should be between 1.0 and 2.0. Given that PPOA homopolymer possessed two O 2 CCH(CH 3 )Br initiating groups at both ends, the bromine content (Br%) should be 1.74% ( = 160/9,200). In the current case, elemental analysis afforded a Br% of 1.72% for PPOA homopolymer, which was very close to the theoretical value (1.74%). This fact strongly verified that PPOA homopolymer possessed two O 2 CCH(CH 3 )Br initiating groups at both ends, i.e. Br-PPOA-Br 2 macroinitiator. Besides, the macroinitiator can dissolve in common organic solvents including THF, CHCl 3 , toluene, and DMF, which is convenient for the subsequent ATRP; but it can not dissolve in n-hexane, methanol, and water.
Synthesis of PMMA-b-PPOA-b-PMMA triblock copolymer. PMMA-b-PPOA-b-PMMA 3 triblock copolymer was constructed via bulk ATRP of methyl methacrylate (MMA) initiated by Br-PPOA-Br 2 using CuBr/dHbpy as catalytic system at 50 °C (Table 1). All resulting products possessed higher molecular weights compared to that of Br-PPOA-Br 2 macroinitiator, which proved the occurrence of the polymerization of MMA monomer. The molecular weights of products ascended with longer polymerization time, a feature of ATRP 51 Fig. 5A, which shows that the conversion of MMA increased linearly with the extending of polymerization time. This first order polymerization kinetics clearly showed a constant number of propagating species during the polymerization, which is a typical character of ATRP 51 . Furthermore, the molecular weight of the obtained polymers linearly increased with the rising of the conversion of MMA as shown in Fig. 5B, which was also identical with the feature of ATRP 51 .
The obtained block copolymer was examined by 1 H NMR and all typical proton resonance signals of PPOA and PMMA segments could be found in Fig. 6. The characteristic signals of PMMA segments appeared at 0.84, 1.02, 1.23 ('h'), and 3.60 ('e') ppm, corresponding to the protons of CH 2 CCH 3 and CO 2 CH 3 of MMA repeated unit,  was conducted at 80 °C using Br-PPOA-Br 2 as macroinitiator and CuBr/PMDETA as catalytic system. It can be seen from Table 2 that all three resultants possessed higher molecular weights in comparison with that of Br-PPOA-Br 2 macroinitiator, this indicating the performance of ATRP of BA. Moreover, it was also found that the extension of polymerization time led to the raising of the molecular weights of the resulting products, this according with the characteristic of ATRP 51 . The conversions of BA monomer and the molecular weights of the resultants were determined by GC and GPC, respectively, so as to study the polymerization kinetics of bulk ATRP of BA. Figure Fig. 7B and it is obvious that the molecular weight is in first order with the conversion of BA monomer, which is consistent with the characteristic of ATRP 51 .
The chemical structure of PBA-b-PPOA-b-PBA 4 triblock copolymer was examined by 1 H NMR.   For Br-PPOA-Br 2 macroinitiator, it was exposed to UV irradiation plus benzophenone (photo-sensitizer) or AIBN (initiator for free radical polymerization) at ambient temperature (Table S1), respectively. As shown in Fig. 9, the peak value and shape of elution peak in GPC retention curves after UV irradiation, i.e. molecular weight and polydispersity, were almost same with those in original GPC curve, which meant Br-PPOA-Br 2 macroinitiator did not degrade or cross-link during the test.
More importantly, PMMA-b-PPOA-b-PMMA 3d triblock copolymer was exposed to UV irradiation same as the macroinitiator; furthermore, it was heated with AIBN or BPO at 80 °C (above the decomposition temperature of AIBN and BPO) as listed in Table S1. Excitingly, it can be seen from Fig. 10 that the peak value and shape of elution peak in GPC retention curves have no obvious change after the stability experiment, which clearly confirmed the double-bond-containing polyallene-based triblock polymers are stable under radical circumstance, either with UV irradiation or with heat initiation. This point implied that double bonds in polyallene-based copolymer are stable under common radical surrounding, which may faciliate the potential further application.

Conclusion
We have displayed the detailed synthesis of PMMA-b-PPOA-b-PMMA and PBA-b-PPOA-b-PBA triblock copolymers by the combination of free radical polymerization, ATRP and the site transformation strategy, employing a bifunctional initiator possessing azo and Br-containing ATRP initiating groups as starting material. Polyallene-based macroinitiator was prepared by free radical polymerization of phenoxyallene initiated by the bifunctional initiator, which contains two ATRP initiating groups at both ends evidenced by elemental analysis  and GPC/MALS. The target triblock copolymers were obtained by ATRP of MMA and BA initiated by the macroinitiator, respectively; and the polymerization process showed first order kinetics. This class of triblock copolymer is the first report of block copolymer via allene derivative and (meth)acrylate. The application of these double-bond-containing triblock copolymers is being explored in our group since they showed the exciting stability under radical circumstance, either with UV irradiation or with heat.
It is clear that the development of the as-prepared bifunctional initiator will make a significant contribution to the synthesis of polyallene-based block copolymers because PMMA and PBA segments in the current case can be easily extended to various hydrophilic and hydrophobic polymers for yielding diverse hydrophobic and amphiphilic block copolymers bearing double-bond-containing polyallene block.