Synthesis of Syndiotactic Polystyrene Macromonomers End Capped with Bromine Atoms: a Tool for a New Investigation of Styrene Polymerization Mechanism Catalyzed by Monocyclopentadienyl Titanium Compounds

Abstract

Syndiotactic polystyrene samples end capped with bromine atoms in benzylic position have been synthesized by terminating the syndiospecific styrene polymerization catalyzed by Cp*TiBn3/B(C6F5)3/Al-Oct3 (Cp* = η5-C5(CH3)5; Bn = CH2Ph, Oct = (CH2)7CH3) with N-bromosuccinimide (NBS) and 2-bromo-2-phenylethylisocyanate (BPEI). The chemical structure of the different polymer end groups was assessed by mono and bidimensional NMR techniques. The evaluation of the relative concentrations of the end groups permitted to give new insights into the bromination reaction and syndiospecific styrene polymerization mechanism. Aspecific bromination of the methine carbon of the growing polymer chain bound to the titanium catalyst was observed when NBS was used as terminating agent. The formation of Ti(III) hydrido complexes in the activation reaction was proved as well as the role of these species as active catalyst in syndiospecific styrene polymerization.

Well‐defined Syndiotactic Polystyrene‐b‐Atactic Polystyrene Stereoblock Polymers

“…[11] Accordingly, syndioselective styrene polymerization was performed using Cp à Ti(CH 2 Ph) 3 /B(C 6 F 5 ) 3 /Al(n-Oct) 3 (1:1:x) (x ¼ 1-20) as catalytic system and NBS as the in situ halogenating endcapping agent. A series of preliminary experiments was carried out to assess the overall degree of control over the polymerization and bromination efficiency of sPS chains.…”

“…Regarding the bromination end-capping efficiency, the reference experiment we selected (i.e., that reported under optimized conditions by Grassi and coworkers) [11] involved the utilization of 88 mmol of titanium precursor and afforded a syndiotactic (> 99% rrrrr, as assessed by 13 C NMR) polystyrene with 80% of chains end-capped with Br (''sPS-Br''; PS-11, Table 1). Varying the tris(n-octyl)aluminum-to-titanium ratio from 1 to 20 did not affect this bromination efficiency (PS-11, -12, -13, Table 1).…”

“…[11] Among them, N-bromosuccinimide (NBS) proved the most efficient, giving up to 80% endcapping efficiency. Although these in situ functionalized sPS macromers were not fully brominated, we reckoned they should represent suitable starting materials for the preparation of sPS-b-aPS stereoblock copolymers.…”

“…It is worth mentioning that true catalytic species involved in syndioselective styrene polymerizations promoted by Ti(IV)-based precursors are commonly accepted to be Ti(III) species deriving from the in situ reduction of the Ti(IV) precursor; yet, their exact identity and detailed reduction pathways still remain elusive despite extensive investigations. [12,14] [11] ). …”

Syndiotactic Polystyrene‐block‐Poly(methyl methacrylate) Copolymer via Click Chemistry

“…Recently, we reported the synthesis of sPS samples end‐capped with bromine atoms by controlled syndiospecific polymerization of styrene catalyzed by Cp*Ti(CH 2 Ph) 3 /B(C 6 F 5 ) 3 /Al( n ‐Oct) 3 (Cp* = pentamethylcyclopentadienyl) terminated with brominating agents as Br 2 , N ‐bromosuccinimide (NBS), and 2‐bromo‐2‐phenylethylisocyanate 11. Among these, NBS was proved to be the most effective leading to a end‐capping efficiency of 80%.…”