Synthesis of Soluble Star-Shaped Polymers via In and Out Approach by Ring-Opening Metathesis Polymerization (ROMP) of Norbornene: Factors Affecting the Synthesis

Abstract

The methods for one-pot synthesis of ‘soluble’ star-shaped polymers by sequential living ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and cross-linking (CL) reagent using Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 have been explored. The method (called the “in and out” or core-first approach) basically consists of (i) the living ROMP of NBE (formation of arm), (ii) reaction with CL (formation of core), (iii) additional living ROMP of NBE (propagating arms from the core, formation of star), (iv) end-modification via Wittig-type cleavage of metal–carbon double bonds containing polymer chain with aldehyde. Two different approaches in the core formation step (reaction with CL mixed with/without NBE) for synthesis of the high molecular weight star-shaped ROMP polymers with more branching, unimodal molecular weight distributions have been explored in detail. The method (reacting CL with NBE in the core formation step) under highly diluted conditions afforded the high molecular weight polymers with unimodal molecular weight distributions.

Phenoxide-Modified Half-Titanocenes Supported on Star-Shaped ROMP Polymers as Catalyst Precursors for Ethylene Copolymerization

“…As summarized in Table , the resultant star-shaped ROMP polymers possessed apparently high molecular weights (1.01–2.00 × 10 5 ) in comparison to the linear ROMP polymers (ca. 5100 as 50 equiv of poly­(NBE)-Ar) with unimodal molecular weight distributions (1.23–1.44), and the M n values in the resultant star ROMP polymers are close to those reported previously with different end groups. ,− As demonstrated previously, − these results suggest that the resultant polymers are star-shaped ROMP polymers containing poly­(NBE) arms. As reported previously, the M n values increased upon increasing the cross-linkers (CL, run 1 vs run 3) or adding NBE in the core formation (second) step (runs 1–4).…”

“…A toluene solution containing Mo cat was added to a toluene solution containing 25 equiv of norbornene (NBE) and subsequent addition of 10 or 15 equiv of a cross-linking agent (CL), 1,4,4a,5,8,8a-hexahydro-1,4,5,8- exo - endo -dimethanonaphthalene (CL, exo : endo = 0.15:1.00), followed by addition of 25 equiv (or 50 equiv) of NBE and termination with 2,6- i Pr 2 -4-Me 3 SiO-C 6 H 2 CHO (excess), affording the targeted star-shaped ROMP polymer consisting of ring-opened poly­(NBE) arms in high yields (Table ). , In some experimental runs, 5 equiv of NBE was also added in the core formation step (reaction with CL) to modify the core size; the core formation with CL and NBE would probably form a core with a rather large size. ,, As reported previously, , dilution in toluene is a prerequisite for obtaining a star-shaped ROMP polymer with uniform molecular weight distributions; the resultant polymers, expressed as star-poly­(NBE)-Ar , were isolated as white precipitates by pouring the reaction mixture into cold n -hexane. The resultant star-shaped polymers are highly soluble in ordinary organic solvents such as toluene, THF, dichloromethane, and chloroform.…”

“…Star-shaped polymers consisting of linear arms connected at a central branched core are one of the simplest nonlinear polymers and studies on synthesis by the living polymerization technique have attracted considerable attention. − Synthesis by adopting the living ROMP has been employed by sequential addition of monomers and cross-linkers (CLs) ,− and two major approaches (shown in Scheme ) such as (i) an arm (brush) first approach using a ruthenium carbene catalyst, − and (ii) an in and out (core first) approach using molybdenum alkylidene catalyst − are known. Related examples for the synthesis of cross-linked ROMP polymers (insoluble in common organic solvents) applied as monolithic materials have also been reported. ,, The one-pot synthesis (in and out approach) by the molybdenum catalyst Mo­(CHCMe 2 Ph)­(N-2,6- i Pr 2 C 6 H 3 )­(O t Bu) 2 ( Mo cat ) enables an introduction of a functionality to the polymer chain ends (star surface) via a Wittig-type reaction of a metal–carbon double bond containing polymer chain with certain aldehydes. − The resultant polymers possessed a spherical shape with controlled diameters corresponding to the NBE repeat units, as shown by TEM micrographs (exemplified in Scheme ). ,, …”

Star-Shaped ROMP Polymers Coated with Oligothiophenes That Exhibit Unique Emission

“…The resultant star-shaped polymers with different end-groups (runs 2, 6, 8, 10, 14) possessed similar M n values (1.36–1.55 × 10 5 ) with unimodal molecular weight distributions ( M w / M n = 1.29–1.54), which are also close to those of the reported star ROMP polymers terminated with 4-pyridinecarboxaldehyde. 32 − 34 …”

“… a According to the method employed previously, 29 , 35 the number of arms containing oligothiophene moieties in the resultant star-shaped ROMP polymers could be speculated on the basis of GPC data for poly(NBE) 25 (linear ROMP polymer prepared with 25 equiv of NBE, M n = 2350 by GPC in THF vs polystyrene standards) and poly(NBE) 50 ( M n = 4710). 34 It was thus speculated that the resultant star-shaped ROMP polymers [runs 2 and 4, M n = 15 100, 236 000 (for 3T-Ph); runs 10 and 12, M n = 15 000, 279 000 (for MP3T), respectively] possessed ca. 36 (3T-Ph), 55 (MP3T) poly(NBE) arms containing the oligothiophene moiety.…”