Isospeciﬁc Polymerization of Halide- and Amino-Substituted Styrenes Using a Bis(phenolate) Titanium Catalyst

: Isospeciﬁc polymerization of polar styrenes is a challenge of polymer science. Particu-larly challenging are monomers bearing electron-withdrawing substituents or bulky substituents. Here, we report the coordination polymerization of halide- and amino-functionalized styrenes including para -ﬂuorostyrene ( p FS), para -chlorostyrene ( p ClS), para -bromostyrene ( p BrS), and para -(N,N-diethylamino)styrene (DMAS) using 2,2 (cid:48) -sulfur-bridged bis(phenolate) titanium precursor ( 1 ). The combination of 1 and [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 provides crystalline poly( p FS)s with perfect isotacticity ( mmmm > 95%) and high molecular weights ( ≤ 16.0 × 10 4 g mol − 1 ). Upon activation with a large excess of DMAO, 1 reaches polymerization activity of 5.58 × 10 5 g mol Ti − 1 h − 1 producing isotactic poly( p FS)s featuring higher molecular weights ( ≤ 39.6 × 10 4 g mol − 1 ). The distinguished performance of the 1 /DMAO system has been extended to the polymerization of p ClS and p BrS, both usually involve halogen abstraction during the polymerization, to produce isotactic and high molecular weight ( M n = 32.2 × 10 4 vs. 13.7 × 10 4 g mol − 1 ) polymers in good activities (2.18 × 10 5 vs. 1.31 × 10 5 g mol Ti − 1 h − 1 ). Surprisingly, 1 /DMAO is nearly inactive for DMAS polymerization, on contrary, the system 1/ [Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 displays isoselectivity ( mmmm > 95%) albeit in a moderate activity. S.L. D.C.; D.C.;

In contrast, the isotactic polymerization of polar styrene monomers develops relatively slowly [17,41], mainly due to the lack of highly efficient catalysts, which struggle to provide high activity while maintaining the sterically crowded coordination sphere required by the isotactic selectivity, although isotactic-enriched polystyrene has been synthesized long before with Ziegler-Natta catalysts or anionic catalysts [42,43]. However, the potential applications of functionalized isotactic polystyrenes as optically active, helical, etc., functional materials have led researchers to expend a lot of effort in this research field. In 2015, the first homogeneous single-site β-diketiminato rare-earth metal catalysts were developed by our group, showing high isoselectivity for ortho-methoxystyrene without masking reagents via the unique "self-assisted" mechanism; however, they are nearly inactive towards other polar styrene derivatives [44,45]. Recently, we developed a series of racemic isopropylidenebridged bis(benz[e]indenyl) rare-earth metal alkyl complexes, which served as effective catalysts for the isoselective polymerizations of styrene, para/meta-methoxystyrenes, paramethylthiostyrene, and para-vinylphenyldimethylsilanol, etc. [46]. Unfortunately, they were virtually inert to the halide-and amino-functionalized styrenes. Probably because the coordination sphere of these catalysts is more open to allow more monomer coordination, in particular in the inert M-σ-X (X = functional group) mode. The bulkier ansa-bridged bis(indenyl) allyl yttrium and neodymium complexes developed by Carpentier's group are also inactive [47,48], where the vacant coordination site is too crowded to facilitate the coordination of bulky amino-functionalized styrenes. In addition, the low Lewis acidity of neutral catalysts inhibits the coordination of the electron-deficient double bond of halostyrene to the metal ion. Thus, we turned to the 2,2 -sulfur-bridged bis(phenolato) titanium dichloro complex with a higher Lewis acidity (1) (Figure 1) [49][50][51][52], an isospecific catalyst for styrene polymerization reported by Okuda and co-workers, and examined its catalytic performance for the polymerization of polar styrenes, and in particular, of halostyrenes, since the resultant halogen styrene polymers feature improved corrosion resistance, heat resistance, flame retardancy, etc. [53][54][55][56][57]. Moreover, the halogen groups can be easily converted into other functionalities to impart different properties to the polymers [58]. In addition, this catalyst performs well in catalysing the (co)polymerization of conjugated dienes [59,60], which provides a possible route for the direct synthesis of halide-functionalized butyl rubber with faster vulcanization, high energy absorption, and low elastic modulus [61,62], and halide-functionalized butadiene-styrene rubber possessing better compatibility with polar fillers which could be used to fabricate tires with low-rolling resistance and good wet-skid resistance [63][64][65][66]. Herein, we report the polymerization behaviour of para-fluorostyrene (pFS), para-chlorostyrene (pClS), para-bromostyrene (pBrS), and para-(N,N-diethylamino)styrene (DMAS) by using complex 1 activated by organoborate, alkyl aluminium, MAO and dried MAO (DMAO).
catalysts inhibits the coordination o the metal ion. Thus, we turned to th complex with a higher Lewis acid styrene polymerization reported by performance for the polymerization since the resultant halogen styrene p resistance, flame retardancy, etc. [5 converted into other functionalities addition, this catalyst performs we dienes [59,60], which provides a functionalized butyl rubber with fa elastic modulus [61,62], and halide better compatibility with polar fille rolling resistance and good wet polymerization behaviour of parabromostyrene (pBrS), and para-(N,N activated by organoborate, alkyl alu

Results and Discussion
At first, a combination of 1 with [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 was chosen as the catalyst for polymerization of para-fluorostyrene (pFS) because organoborate-based activators are reported to be more efficient reagents than MAO. The polymerization was performed at 25 • C in a toluene solution to reach 54% conversion in 30 min. On increasing the reaction temperature from 25 to 80 • C, the highest catalytic activity of 1.12 × 10 5 g mol Ti −1 h −1 was observed at 40 • C ( Table 1, entries 1-4), probably because the Ti(IV) active species is readily reduced to the inert Ti(III) by aluminium alkyls at high reaction temperatures. Subsequently, a kinetics investigation was carried out at 40 • C under a pFS-to-1 ratio of 1000:1. Increasing the reaction time from 15 to 120 min resulted in an obvious increase in monomer conversion from 30% to 79% (Table 1, entries 5-7). The molecular weight distributions of the resultant polymers consequently broadened from 1.53 to 1.89, but the molecular weights (M n = 15.1-16.0 × 10 4 g mol −1 ) remained nearly constant (Figure 2), indicating the chain transfer reaction accompanying the polymerization process.  (Figure 2), indicating the chain transfer reaction accompanying the polymerization process.    (Table 1, entries 5-7).
The catalytic system 1/MAO has been reported to exhibit high catalytic activity for isospecific styrene polymerization [49]. Therefore, complex 1 activated by 2000 equivalents of MAO was utilized to catalyse pFS polymerization at 40 °C; however, only 25% of the monomer was consumed in 2 h (Table 2, entry 1). In contrast, the combination of 1 and 2000 equivalents of DMAO showed a much higher catalytic activity (5.51 × 10 5 g molTi −1 h −1 vs. 1.51 × 10 5 g molTi −1 h −1 ) with 90% monomer conversion under identical conditions ( Table 2, entry 2). This was attributed mainly to the absence of free AlMe3, which is able to interact with the active species leading to an inactive dimethyl-bridged species [67][68][69]. Notably, a large excess of DMAO against complex 1 is necessary in order 1000:1. Increasing the reaction time from 15 to 120 min resulted monomer conversion from 30% to 79% (Table 1, entries 5distributions of the resultant polymers consequently broadene molecular weights (Mn = 15.1-16.0 × 10 4 g mol −1 ) remained n indicating the chain transfer reaction accompanying the polym  The catalytic system 1/MAO has been reported to exhibit isospecific styrene polymerization [49]. Therefore, compl  The catalytic system 1/MAO has been reported to exhibit high catalytic activity for isospecific styrene polymerization [49]. Therefore, complex 1 activated by 2000 equivalents of MAO was utilized to catalyse pFS polymerization at 40 • C; however, only 25% of the monomer was consumed in 2 h ( Table 2, entry 1). In contrast, the combination of 1 and 2000 equivalents of DMAO showed a much higher catalytic activity (5.51 × 10 5 g mol Ti −1 h −1 vs. 1.51 × 10 5 g mol Ti −1 h −1 ) with 90% monomer conversion under identical conditions (Table 2, entry 2). This was attributed mainly to the absence of free AlMe 3 , which is able to interact with the active species leading to an inactive dimethylbridged species [67][68][69]. Notably, a large excess of DMAO against complex 1 is necessary in order to obtain a highly active species for pFS polymerization. Whenever all complex 1 molecules were converted into the cationic active species, further increasing DMAO loading amount did not improve the catalytic activity (Table 2, entries 2, 6, and 7). Increasing reaction temperature accelerated the polymerization process to a certain degree, but too-high a temperature led to declined polymerization activity from 5.51 × 10 5 g mol Ti −1 h −1 at 40 • C to 4.54 × 10 5 g mol Ti −1 h −1 at 60 • C, along with a dramatic decrease in molecular weight from 37.4 × 10 5 g mol Ti −1 to 13.4 × 10 5 g mol Ti −1 h −1 ( Table 2, entries 2, 8 and 9). to obtain a highly active species for pFS polymerization. Whenever all complex 1 molecules were converted into the cationic active species, further increasing DMAO loading amount did not improve the catalytic activity (Table 2, entries 2, 6, and 7). Increasing reaction temperature accelerated the polymerization process to a certain degree, but too-high a temperature led to declined polymerization activity from 5.51 × 10 5 g molTi −1 h −1 at 40 °C to 4.54 × 10 5 g molTi −1 h −1 at 60 °C, along with a dramatic decrease in molecular weight from 37.4 × 10 5 g molTi −1 to 13.4 × 10 5 g molTi −1 h −1 ( Table 2, entries 2, 8 and 9). All of the resulting poly(pFS)s are nearly insoluble in toluene, tetrahedrofuran (THF), chloroform, etc., at room temperature, but readily soluble in chlorobenzene, acetylene tetrachloride, etc., at high temperature. NMR spectroscopy analysis unambiguously indicated that the poly(pFS)s produced by both the catalytic systems 1/Al i Bu3/[Ph3C][B(C6F5)4] and 1/DMAO have an isotactic microstructure, evidenced by the quintet centred at δ 2.14 ppm and the multiplet centred at δ 1.52 ppm assigned to the methine and asymmetric methylene protons, respectively (Figure 3a). The perfect isotacticity is further confirmed by the sharp singlets at δ 43.78 ppm for methylene carbon and δ 41.03 ppm for methine carbon. All the peaks of the fluorine-substituted phenyl carbons split into doublets as a result of coupling with 19 F nuclei (Figure 3b). The ipsocarbon C3 shows a doublet at δ 141.62 ppm with a coupling constant of 4 J C-F = 3 Hz. The coupling constants of ortho-carbon C4 (δ 128.94 ppm, 3 JC-F = 8 Hz), meta-carbon C5 (δ 115.12 ppm, 2 JC-F = 21 Hz) and para-carbon C6 (δ 161.55 ppm, 1 JC-F = 244 Hz) are significantly enlarged due to gradually closing to the fluorine atom. The obtained isotactic polymer shows a high glass-transition temperature of around 104 °C and a melting temperature in the range of 242.9-247.8 °C ( Figure S20-S28), which are much lower than those observed in syndiotactic poly(pFS) [14].  All of the resulting poly(pFS)s are nearly insoluble in toluene, tetrahedrofuran (THF), chloroform, etc., at room temperature, but readily soluble in chlorobenzene, acetylene tetrachloride, etc., at high temperature. NMR spectroscopy analysis unambiguously indicated that the poly(pFS)s produced by both the catalytic systems 1/Al i Bu 3 /[Ph 3 C][B(C 6 F 5 ) 4 ] and 1/DMAO have an isotactic microstructure, evidenced by the quintet centred at δ 2.14 ppm and the multiplet centred at δ 1.52 ppm assigned to the methine and asymmetric methylene protons, respectively (Figure 3a). The perfect isotacticity is further confirmed by the sharp singlets at δ 43.78 ppm for methylene carbon and δ 41.03 ppm for methine carbon. All the peaks of the fluorine-substituted phenyl carbons split into doublets as a result of coupling with 19 F nuclei (Figure 3b). The ipso-carbon C3 shows a doublet at δ 141.62 ppm with a coupling constant of 4 J C-F = 3 Hz. The coupling constants of ortho-carbon C4 (δ 128.94 ppm, 3 J C-F = 8 Hz), meta-carbon C5 (δ 115.12 ppm, 2 J C-F = 21 Hz) and para-carbon C6 (δ 161.55 ppm, 1 J C-F = 244 Hz) are significantly enlarged due to gradually closing to the fluorine atom. The obtained isotactic polymer shows a high glass-transition temperature of around 104 • C and a melting temperature in the range of 242.9-247.8 • C ( Figure S20-S28), which are much lower than those observed in syndiotactic poly(pFS) [14]. Stimulated by the above results, the polymerizations of other chloro-styrene derivatives were studied using the system 1/DMAO. In a previous report, chloro-and bromo-substituted styrenes needed to be polymerized at low temperatures (≤0 °C) because the scandium cationic active species readily cleaved the C-X (X = Cl or Br) bond of halostyrene to generate the inert metal-halide species at high temperatures [15]. Surprisingly to us, para-chlorostyrene (pClS) was also converted into a perfect isotactic polymer at 40 °C with 63% conversion in 2 h (Table 3, entry 1). This may be due to the Lewis acidity of titanium cationic active species being lower than that of scandium, which would not cause C-X bond cleavage at higher temperatures. Even para-bromostyrene (pBrS) polymerization under similar conditions reached 57% monomer conversion in a prolonged reaction time (4 h) ( Table 3, entry 2). Their polymerization activities relate mainly to the electronics of the monomers following the trend pFS > pClS > pBrS, which is consistent with the natural bond orbital (NBO) charge of β-CH2 of these monomers; pBrS has the lowest electron density (−0.339) as compared to pClS (−0.346) and pFS (−0.353) ( Figure S38). On the other hand, Cl and Br atoms with larger atomic radii may also adversely affect the polymerization. Unexpectedly, 1/DMAO was virtually inactive for the polymerization of para-(N,N-diethylamino)styrene (DMAS). Switching to the catalytic system of 1/Al i Bu3/[Ph3C][B(C6F5)4], surprisingly, effective polymerization was achieved by converting 82% DMAS in 30 min in an isospecific manner, although the bulky dimethylamino group significantly decreased the activity (Table 3, entry 4). Thermal analyses revealed all the resultant isotactic poly(pClS), poly(pBrS), and poly(DMAS) Stimulated by the above results, the polymerizations of other chloro-styrene derivatives were studied using the system 1/DMAO. In a previous report, chloro-and bromosubstituted styrenes needed to be polymerized at low temperatures (≤0 • C) because the scandium cationic active species readily cleaved the C-X (X = Cl or Br) bond of halostyrene to generate the inert metal-halide species at high temperatures [15]. Surprisingly to us, para-chlorostyrene (pClS) was also converted into a perfect isotactic polymer at 40 • C with 63% conversion in 2 h (Table 3, entry 1). This may be due to the Lewis acidity of titanium cationic active species being lower than that of scandium, which would not cause C-X bond cleavage at higher temperatures. Even para-bromostyrene (pBrS) polymerization under similar conditions reached 57% monomer conversion in a prolonged reaction time (4 h) ( Table 3, entry 2). Their polymerization activities relate mainly to the electronics of the monomers following the trend pFS > pClS > pBrS, which is consistent with the natural bond orbital (NBO) charge of β-CH 2 of these monomers; pBrS has the lowest electron density (−0.339) as compared to pClS (−0.346) and pFS (−0.353) ( Figure S38). On the other hand, Cl and Br atoms with larger atomic radii may also adversely affect the polymerization. Unexpectedly, 1/DMAO was virtually inactive for the polymerization of para-(N,N-diethylamino)styrene (DMAS). Switching to the catalytic system of 1/Al i Bu 3 /[Ph 3 C][B(C 6 F 5 ) 4 ], surprisingly, effective polymerization was achieved by converting 82% DMAS in 30 min in an isospecific manner, although the bulky dimethylamino group significantly decreased the activity (Table 3, entry 4). Thermal analyses revealed all the resultant isotactic poly(pClS), poly(pBrS), and poly(DMAS) possess higher glass transition temperatures of 124.7, 135.1, and 130.7 • C, respectively, but the melting points were not observed in the DSC curves despite their perfect isotacticity (mmmm > 95%) ( Figure S39-S43).

Conclusions
We have demonstrated that the isospecific polymerizations of halostyrene and aminofunctionalized styrenes with high activity have been achieved using titanium bisphenolate catalyst 1. Compared with scandium, yttrium, and lutetium rare-earth metal-based catalysts, titanium catalysts have suitable Lewis acidity, which not only compensates for the low coordination ability of the double bond on halostyrenes, but also does not cause C-X bond cleavage at high temperatures. The polymerization activity is strongly influenced by the cocatalysts and the reaction temperature. Upon activation with Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 4 ], complex 1 furnishes perfect isospecific poly(pFS) with high molecular weight and narrow molecular weight distribution for the first time; however, the polymerization activity is relatively low. When DMAO is used as the cocatalyst, the pFS polymerization process is greatly accelerated under a suitable reaction temperature, resulting in an isotactic product with a higher molecular weight and a narrow molecular weight distribution. A reaction temperature over 40 • C is not detrimental to isoselectivity but decreases the polymerization activity and the molecular weight, probably due to the reduction of Ti (IV) to Ti (III) by alkyl aluminium and the chain transfer reaction. In addition, the combination of 1/DMAO also shows high catalytic activity and perfect isoselectivity for the polymerization of pClS and pBrS, but the isospecific polymerization of DMAS is only accomplished by the system 1/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 . This work paves a new avenue to access isotactic polyhalostyrenes, which can be easily transferred to other functionalized isotactic polystyrenes.

Conflicts of Interest:
The authors declare no conflict of interest.