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

: 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(CHCMe 2 Ph)(N-2,6- i Pr 2 C 6 H 3 )(O t Bu) 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.

For example, as shown in Scheme 2, pyridine modified 'soluble' star-shaped polymers, starpoly(NBE)n-Py (n = 25, 50), were used as the effective ligand for ruthenium catalyzed chemoselective hydrogen transfer reduction of various ketones, such as 5-hexen-2-one, 2-allyl-cyclohexanone, 5isopropenyl-2-methylcyclohexanone (dihydrocarvone), and the activity by RuCl2(PPh3)4 increased with addition of the star polymer ligand [20]. The catalyst could be recovered quantitatively by pouring the reaction mixture into methanol and could be reused without further purification; both the activity and the selectivity did not decrease in the several recycle runs [20]. Moreover, the observed catalytic activities were close to those using the supported catalysts with linear poly(NBE) containing pyridine at the chain end, suggesting that unique characteristics observed in the linear ROMP polymers could be preserved by placing the pyridine moiety on the star surface (and the star polymer supported catalysts showed better efficiency in the catalyst recycle) [20].

Scheme 2.
Chemoselective hydrogen transfer reactions of ketones by efficient recyclable ruthenium catalyst supported by star-polymer ligand [20]. Scheme 1. Two approaches employed for synthesis of star-shaped polymers by living ring-opening metathesis polymerization (ROMP) of cyclic olefins by molybdenum, ruthenium catalysts. NBE = Norbornene, CL= Cross-linking.
For example, as shown in Scheme 2, pyridine modified 'soluble' star-shaped polymers, star-poly(NBE) n -Py (n = 25, 50), were used as the effective ligand for ruthenium catalyzed chemoselective hydrogen transfer reduction of various ketones, such as 5-hexen-2-one, 2-allyl-cyclohexanone, 5-isopropenyl-2-methylcyclohexanone (dihydrocarvone), and the activity by RuCl 2 (PPh 3 ) 4 increased with addition of the star polymer ligand [20]. The catalyst could be recovered quantitatively by pouring the reaction mixture into methanol and could be reused without further purification; both the activity and the selectivity did not decrease in the several recycle runs [20]. Moreover, the observed catalytic activities were close to those using the supported catalysts with linear poly(NBE) containing pyridine at the chain end, suggesting that unique characteristics observed in the linear ROMP polymers could be preserved by placing the pyridine moiety on the star surface (and the star polymer supported catalysts showed better efficiency in the catalyst recycle) [20]. viscous oil) [29]. By contrast, the one-pot approach using a molybdenum catalyst (in and out approach) [17][18][19][20][21], Mo(CHCMe2Ph)(N-2,6i Pr2C6H3)(O t Bu)2 (Mo cat.), does not require tedious purification steps, and enables an introduction of a functionality to the polymer chain ends (star surface) via a Wittig-type reaction of metal-carbon double bond containing a polymer chain with certain aldehyde [12,16,[17][18][19][20][21][36][37][38]. Scheme 1. Two approaches employed for synthesis of star-shaped polymers by living ring-opening metathesis polymerization (ROMP) of cyclic olefins by molybdenum, ruthenium catalysts. NBE = Norbornene, CL= Cross-linking.
For example, as shown in Scheme 2, pyridine modified 'soluble' star-shaped polymers, starpoly(NBE)n-Py (n = 25, 50), were used as the effective ligand for ruthenium catalyzed chemoselective hydrogen transfer reduction of various ketones, such as 5-hexen-2-one, 2-allyl-cyclohexanone, 5isopropenyl-2-methylcyclohexanone (dihydrocarvone), and the activity by RuCl2(PPh3)4 increased with addition of the star polymer ligand [20]. The catalyst could be recovered quantitatively by pouring the reaction mixture into methanol and could be reused without further purification; both the activity and the selectivity did not decrease in the several recycle runs [20]. Moreover, the observed catalytic activities were close to those using the supported catalysts with linear poly(NBE) containing pyridine at the chain end, suggesting that unique characteristics observed in the linear ROMP polymers could be preserved by placing the pyridine moiety on the star surface (and the star polymer supported catalysts showed better efficiency in the catalyst recycle) [20].

Scheme 2.
Chemoselective hydrogen transfer reactions of ketones by efficient recyclable ruthenium catalyst supported by star-polymer ligand [20].
Since it was demonstrated that, as also described above, these ring-opened polymers, poly(NBE), possessed a rather linear nature compared to ordinary polymers prepared by vinyl additions exemplified by poly(acrylamide) due to the cyclic structure in the main chain [39], it is thus highly expected that these "surface-modified" star polymers can be used as advanced materials that cannot be achieved by ordinary end-modified linear and bottle brush polymers. However, previous reports [17][18][19][20], by adopting so-called Method 1 (shown in Scheme 1), only demonstrated synthesis of the 'soluble' polymers with a limited number of arms under limited conditions (5 or 10 equivalent of CL [17], shown below in Table 1) due to a difficulty of the molecular weight control, as also described below. Therefore, the development of a controlled synthesis of 'soluble' star-shaped ROMP polymers having more arms has been an important subject in terms of application of these star-shaped polymers as functional materials. Recently, we reported an improved protocol for controlled synthesis of 'soluble' star-shaped polymers with more arms (branching) by the living ROMP technique using the Mo-alkylidene initiator by sequential additions of norbornene (NBE) and the cross-linker (CL), expressed as Method 1 in Schemes 1 and 3 (shown below), upon addition of CL (15 and 20 equivalent to Mo) [21]. It turned out that the polydispersity index (PDI, M w /M n ) values became low with decreasing the monomer concentration (under diluted conditions) in the core formation step (reaction with CL), shown in Scheme 1 (introduced as Method 1) [21]. It also turned out that the M n values in the resultant polymers increased upon increasing the amount of NBE in the third step (3rd reaction in Method 1) consisting of unimodal molecular weight distributions (M w /M n = 1.17, 1.28, shown below in Table 1).
In this paper, we have explored two modified approaches (Methods 2 and 3, shown in Scheme 3) in the core formation step (varying cross-linking density, core size) for preparation of the polymers with different core sizes upon increasing the amount of CL and/or by mixing CL and NBE. We thus herein introduce factors affecting the precise synthesis of star-shaped ROMP polymers with (relatively) unimodal molecular weight distributions, by adopting the living ROMP technique. Since most methods (controlled radical, ROMP, etc.) face a difficulty of preparation of the polymers with "unimodal" molecular weight distributions and require separation for the obtainment of the desired star-shaped polymers with relatively controlled molecular weights [12,29], the report should be helpful for precise synthesis under optimized conditions. It has been known in the synthesis of star-shaped polymers using vinyl monomer and divinyl monomer (CL) by controlled radical polymerization that the degree of cross-linking or density of cross-linking (network structure, core size and density) can be tuned in the core formation step by combination of propagation and cross-linking (timing of adding CL during propagation) [5]. In this sense, the core size in the star-shaped ROMP polymers would be controlled (modified) by adopting combined ROMP of CL and NBE in the core formation step (2nd reaction, Methods 2 and 3, Scheme 3). Table 1

Results and Discussion
Synthesis of Pyridine Modified Star-Shaped Ring-Opened Poly(norbornene) As described in the introductory section, the method for synthesis of the titled star-shaped polymers using a molybdenum-alkylidene catalyst (Mo cat.), the so-called in and out (core first) approach, consists of (i) the living ROMP of norbornene (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), and (iv) end-modification via Wittig-type cleavage of metal-carbon double bonds containing polymer chain with aldehyde. In this method, an exclusive introduction of functionality into the ROMP polymer chain end can be easily achieved through a cleavage of the ROMP polymer-metal double bonds (of Schrock type Mo-alkylidene) with aldehyde, yielding a carbon-carbon double bond via a Wittig-like reaction [8][9][10]12,16]. Three modified approaches shown in Scheme 3 have thus been considered, because the approaches would prepare the polymers with different core sizes upon increasing the amount of CL (Method 1, Scheme 1) [21] and/or by mixing CL and NBE in the core formation step (varying cross-linking density, core size), as widely employed by the ATRP technique [5].
It has been known in the synthesis of star-shaped polymers using vinyl monomer and divinyl monomer (CL) by controlled radical polymerization that the degree of cross-linking or density of cross-linking (network structure, core size and density) can be tuned in the core formation step by combination of propagation and cross-linking (timing of adding CL during propagation) [5]. In this sense, the core size in the star-shaped ROMP polymers would be controlled (modified) by adopting combined ROMP of CL and NBE in the core formation step (2nd reaction, Methods 2 and 3, Scheme 3). Table 1 22)) were apparently larger than that with the increase of the linear poly(NBE) (2354, from 25 mer (M n = 2354) to 50 mer (M n = 4708)), clearly indicating that the resultant polymers are star-shaped ROMP polymers consisting of core and poly(NBE) arms. It also turned out that the M n values in the resultant polymers were higher than those prepared by Method 1 in most cases. Moreover, the observed increases in the M n value by Method 2 (e.g., 46,000 from 145,000 (runs 16, 25 equivalent NBE in the third step) to 191,000 (runs 18, 50 equivalent of NBE in the third step); 49,000 from 153,000 (runs 20, 25 equivalent of NBE in the third step) to 202,000 (runs 22, 50 equivalent of NBE in the third step)) are apparently larger than those by Method 1 (e.g., 19,000 from 137,000 (runs 15, 25 equivalent of NBE in the third step) to 156,000 (runs 17, 50 equivalent of NBE in the third step); 15,000 from 149,000 (runs 19, 25 equivalent of NBE in the third step) to 164,000 (runs 21, 50 equivalent of NBE in the third step)). It is thus strongly suggested that the resultant star-shaped ROMP polymers possess more branching (arms) than those prepared by Method 1. The resultant polymers prepared by Method 2 are soluble in toluene, tetrahydrofuran (THF), chloroform, and dichloromethane at room temperature.  16,18). It seems that the extent of a higher molecular GPC trace was decreased upon increasing the amount of toluene (or decreasing the monomer concentration); we speculate that this would probably be in order to avoid so-called star-star coupling (further intermolecular metathesis of the core molecules). Therefore, as observed in Method 1, control of monomer concentration in the 2nd step (core formation step) plays an important role for the obtainment of star-shaped ROMP polymers with unimodal molecular weight distributions. Table 2 Table 1. Table 2 Table 1. In order to obtain the higher molecular weight polymers (with more branching) with unimodal molecular weight distributions, two step reactions were attempted in the core formation step (Method 3, Scheme 3). In this method, 10 equivalent of CL and 5.0 equivalent of NBE were added into a solution of the initial ROMP with NBE (1st step), and the solution was stirred for 10 min (expecting formation of the initial core), and 10 equivalent of CL was then further added into the solution and stirring of the mixture was continued for a certain period in the 2nd step. The results are summarized in Table 3. Table 3. Synthesis of star-shaped polymers by living ring-opening metathesis polymerization (ROMP) using Mo(CHCMe 2 Ph)(N-2,6-i Pr 2 C 6 H 3 )(O t Bu) 2 (Method 3) a .

Run
Toluene It turned out that, as observed in Table 2 as well as in Method 1 [21], the M n values in the resultant polymers increased over the reaction time in the 2nd step (40-110 min, runs 42-45). However, the PDI values (in the polymers prepared by Method 3) were larger (M w /M n = 1.88-4.42) than those prepared by Method 2 (M w /M n = 2.04-2.56); the PDI values did not improve upon increasing the amount of NBE in the 3rd step (runs 46-48). We thus speculate that this would be due to enhanced star-star coupling (intermolecular metathesis with the initial cores), and the approach in Method 3 is not suitable for this purpose.

Concluding Remarks
In this paper, two methods, which are different in the core formation step (2nd reaction), shown in Scheme 3, have been explored for one-pot synthesis of star-shaped ROMP polymers with more branching (arms). Although these reactions required conditions with high dilution for the obtainment of polymers with unimodal molecular weight distributions, as reported previously [21], the number of the poly(NBE) arms can be increased upon increasing the CL (higher CL/Mo ratio, 15, 20, Method 1, Scheme 3). Moreover, the core size can also be controlled upon copresence of NBE (use of CL and NBE in the 2nd step, Method 2, Scheme 3). It is thus possible to prepare high molecular weight star-shaped ROMP polymers with unimodal molecular weight distributions by adopting these approaches, especially by Methods 1 and 2. An introduction of a functional group into the ROMP polymer chain ends (star polymer surface) can also be attained by adopting these methods using molybdenum catalyst [12,[16][17][18][19][20][21], as described in the introduction. We thus believe that the method should be applied for controlled synthesis of functional materials as surface modified star-shaped polymers.
Molecular weights and the molecular weight distributions of the resultant polymers were measured by gel-permeation chromatography (GPC). HPLC-grade THF (Wako Pure Chemical Ind., Inc., Tokyo, Japan) was used for GPC and was degassed prior to use. GPC were performed at 40 • C on a Shimadzu SCL-10A using a RID-10A detector (Shimadzu Co. Ltd., Kyoto, Japan) in THF (containing 0.03 wt % 2,6-di-tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC GPC-806, 804 and 802, 30 cm × 8.0 mmϕ) were calibrated versus polystyrene standard samples. 1 H and 13 C NMR spectra were recorded on a Bruker AV500 spectrometer ( 1 H, 500.13 MHz; 13 C, 125.77 MHz), and all chemical shifts are quoted in ppm and are referenced to SiMe 4 . Obvious multiplicities and routine coupling constants are usually not listed, and all spectra were obtained in the solvent indicated at 25 • C.

General Polymerization Procedure
A typical polymerization procedure (run 35, Method 2, Table 2) is as follows. A toluene solution (1.0 g) containing Mo(CHCMe 2 Ph)(N-2,6-i Pr 2 C 6 H 3 )(O t Bu) 2 (2.00 × 10 −5 mol) was added in one portion to a rapidly stirred toluene solution (10.0 g) containing the norbornene (25 equiv. to Mo) at room temperature (25 • C), and the solution was stirred for 4 min. A toluene solution (4.0 g) containing 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphtalene (CL, 20 equivalent to Mo) and norbornene (NBE, 5 equivalent to Mo) was then added into the solution, and the mixture was stirred for the prescribed time (50 min). Then, a toluene solution (5.0 g) containing NBE (25 equivalent to Mo) was added in one portion to the mixture, and the reaction solution was further stirred for 15 min. The polymerization was quenched by adding 4-pyridine carboxaldehyde (ca. >10 mg, excess), and the solution was stirred for 1 h for completion. The mixture was then removed in vacuo until it was dissolved in the minimum amount of toluene. The solution was poured dropwise into methanol to afford pale white precipitates. The polymer was then collected by filtration and dried in vacuo. In Method 1, the basic procedure was the same except that a toluene solution containing only CL (15.0/20.0 equivalent to Mo) was added in the second step.
Funding: This project is partly supported by Grant-in-Aid for Challenging Exploratory Research (24656491, 15K14225), the Japan Society for the Promotion of Science (JSPS).