Nickel-mediated Polymerization of Methyl Methacrylate

Abstract:

The Ni(II) complexes [Ni(5-C₅H₃ R₂)(X)(NHC)] 1a–f combined with MAO was tested in methylmethacrylate (MMA) polymerization. The complex 1f, bearing the bulky 2,6-diisopropenylphenyl substituents in the NHC ligand was found to be the most effective in the polymerization of MMA with TOF up to 200 h^-1 resulting in a syndiotactic, high molecular weight PMMAs which can be explained by anionic, MAO-centered polymerization mechanism.

Introduction:

A great deal of attention is currently being paid to polymers containing polar monomers, which may give rise to new high-performance materials with high adhesion and toughness and good dyeing and moisture adsorption properties.¹ Metal-based catalysts tolerant of polar functionalities, which perform homopolymerization, and if possible copolymerization with nonpolar olefins, are being sought. Late transition metal complexes look promising because of their lower oxophilicity,² and probable tolerance against polar monomers, and against impurities in polar olefins polymerization.

Acrylates are polymerized and copolymerized for many different uses including coatings, textiles, adhesives, and paper.³ Commercial poly(methyl methacrylate) has been produced since 1927.⁴ Like many other polar monomers, acrylates are commonly polymerized by 18radical⁵ or anionic mechanisms. In addition, polymerization of acrylates with late transition metal complexes has been studied.⁶ Metallocene group IV complexes are known to be excellent for this type of polymerization.

Half-sandwich nickel(II) complexes with N heterocyclic carbenes (NHC) of the general formula [Ni(5-C5H4R)(X)(NHC)] (R = H or alkyl, X= Cl, Br, I) was synthesized by reacting nickelocene or its derivatives and suitable imidazolium salts . The diamagnetic property of these compounds helps in showing some C-C bond forming reactions. But, complexes 1 are very active in aryl dehalogenation and aryl amination, hydrothiolation of alkynes and oxidation of secondary alcohols as a precatalyst.

Experimental:

Materials and synthesis:

Methyl methacrylate (MMA)

Methyl acrylate (MA),

[Ni(acac)₂],

Toluene,

Purified THF, and hexane

1,3-bis(1,1-dimethylbut-3- enyl)cyclopentadiene

complexes 1a–d and 1f

[Ni(5-C5H5)(CH3CN)(IMes)]+(PF6)− [5]

[Ni(5-C5H5)(Cl) (PPh3)]

MAO (10% wt. solution in toluene)

Synthesis of 1e:

A hexane solution of n-BuLi (2.5 mL, 5.1 mmol) and a THF (5 mL) solution of 1,3-bis(1,1-dimethylbut-3-enyl) cyclopentadiene (4.83 mmol) was added and the mixture was stirred for 2 h at ambient temperature. This solution was added to the solution of [Ni(acac)₂] (1.199 g, 4.67 mmol) in THF (10 mL) at −78 ^oC. A color change immediately from green to red is observed and a suspension of 1,3-dimesitylimidazolinium chloride [12] (1.693 g, 4.96 mmol) in THF (10 mL) was quickly added at this temperature. The reaction mixture was allowed to warm up to ambient temperature and stirred for a further 2 h. The volatiles were removed under reduced pressure. The solid residue was extracted with hexane (20 mL) and filtered through Celite. Complex 1e was isolated by crystallization as a red, microcrystalline solid.

Polymerization:

14mg of Complex 1f(0.0255 mmol) dissolved in 15ml of toluene in a schlenk tube with a magnetic stirrer in it. To this solution, MAO ((5.10 mL, 10% wt. in toluene, 7.65 mmol) which is red in color was added by a gas tight pipette which results in a brown solution. The obtained brown solution was stirred at ambient temperature for half an hour. Now MMA(2.72 mL, 0.0255 mol) was added and the apparatus is placed in a oil bath maintaining 50^oC with vigorous stirring. The reaction mixture was now quenched with excess of CH₃OH (200 mL) and then filtered. PMMA was collected by filteration and washed with CH₃OH and kept for over night drying. The obtained polymer is purified with small volume of CHCl₃ and stirred overnight with 10% aq. HCl. The organic and the aqueous phases are separated and the organic phase is poured into excess of CH₃OH. A white solid PMMA was isolated by filteration.

2.4. Characterization

• NMR spectr at ambient temperature on a Mercury-400BB spectrometer operating at 400 MHz for 1H NMR was recorded and at 101 MHz for 13C NMR was recorded.
• EI (70 eV) mass spectra on an AMD-604 spectrometer was recorded.
• MALDI-TOF mass spectra w with a Bruker Daltonics ultrafleXtremeTM mass spectrometer using HABA matrix was recorded.
• The average molecular weights were measured on a LabAlliance liquid chromatograph equipped with a Jordi Gel DVB Mixed Bed column (250 mm × 10 m) using CH2Cl2 as the mobile phase at 30 â-¦C and calibrated with standard PMMAs.

2.5. Crystal structure determination

The selected single crystals mounted in inert oil were transferred to the cold gas stream of the diffractometer. Diffraction data was collected at 100(2) K on the Oxford Diffraction Gemini A Ultra diffractometer with graphite-monochromated Mo-K radiation. Cell refinement, data collection, data reduction and analysis were performed with the CrysAlisPRO [13]. Empirical absorption correction using spherical harmonics was applied. The structure was solved in monoclinic space group P21/c by direct methods

using the SHELXS program . It is worth noting here that the skew angle β is very close to 90â-¦. Full-matrix least-squares refinement against F2 values was carried (SHELXL-97 and OLEX2.

Table 1

Crystal data, data collection and refinement parameters for complex 1e.

Complex 1e

Empirical formula C38H51ClN2Ni

Crystal size (mm) 0.07 × 0.07 × 0.40

M·(g mol−1) 629.96

Crystal system Monoclinic

Space group P21/c (no. 14)

Z 4

F(0 0 0) 1352

Temperature (K) 100(2)

Dcalc. (g cm−3) 1.251

Absorption coefficient (mm−1) 0.688

Radiation Mo-K ( = 0.71073A)Ëš

range (â-¦) 3.3–30.0

Index range −20 ≤ h ≤ 20; −13 ≤ k ≤ 13; −13 ≤ l ≤ 13

Reflections collected 37,962

Unique data 9684, Rint = 0.0355

Observed refl. [I > 2σ(I)] 8195

Data/restraints/parameters 9684/17/415

Goodness-of-fit on F2 a 1.043

Results and discussion:

Synthesis:

The series of Ni(II) complexes 1a–d and 1f (Scheme 1) was prepared from nickelocene or 1,1’ bis(allyl)nickelocene and the suitable imidazolium salt.

Complex 1e bearing the 1,3-disubstituted cyclopentadienyl ligand could not be obtained by this route. Therefore, it was synthesized form the pentamethylcyclopentadienyl

congener [4e] from [Ni(acac)₂] by the one-pot two-step procedure intermediate {(5-1,3-

R2C5H3)Ni(acac)} (Scheme 2).

Scheme 1. Ni(II) complexes used in this study, where R = allyl (1d) or 1,1-dimethyl-but-3-en-1-yl (1e); Mes = 2,4,6-trimethylphenyl, Dipp = 2,6-diisopropylphenyl.

Scheme 2. The synthesis of complex 1e, where R = 1,1-dimethyl-but-3-en-1-yl,

Mes = 2,4,6-trimethylphenyl.

• From the symmetry of the molecule, it is found that the geometry of the molecule was trigonal planar.
• The bond angles and the lengths between nickel and its substituents are approximately same compared to the related compounds.
• Due to week contact between H(29A) hydrogen of mesityl methyl group C(29) and the chlorine ion [H(29A)·Cl(1) 2.57 and C(29A)·Cl(1) 3.5346(15)A] it resulted in the formation of a week intra molecular C H·Cl hydrogen bond.

3.2. Polymerization:

Polymerization was performed under the similar environment of the styrene polymerization with an excess of commercial MAO. A toluene solution of complex 1 was treated with an excess of MAO (Al:Ni = 100:1) for 30 min at ambient temperature.

Then MMA (MMA:Ni = 1000:1) was added and the polymerization was run in a sealed Schlenk tube for 3 h at 50 â-¦C. The reaction mixture was separated as a homogenous mixture.

Molecular structure of complex 1e.

Polymerization of methyl methacrylate with complexes 1–3 and MAOa.

• The bromide analog 1b displayed slightly higher activity compared to 1a, while complex 1c bearing the alkyl-aryl NHC ligand was somewhat more productive than 1b in the productivity of the [Ni(Cp)(X)(NHC))]/MAO catalytic system.
• Substiuted cyclopentadienyl ligands was examined and complex 1d with allylcyclopentadienyl ligand gave the same result as 1a. It was reasoned that the allyl group might be too small to induce any effect. Therefore complex 1e with two bulky substituents was synthesized and tested to give the same conversion as 1d.
• By introducing the more bulky 2,6-diisopropylphenyl substituents in the NHC ligand (complex 1f) the yield of PMMA was 34% and when the excess of MAO was increased (Al:Ni = 300:1), the isolated yield of PMMA was increased to 60%.
• Changing the solvent resulted in a disappointing yield which was predicted to be due to the solubility problem.
• 1H and 13C NMR spectroscopy were used to determine the microstructure of PMMA.
• Syndiotactic-rich polymers were resulted toluene where atactic PMMA was obtained with hexane and this was because of the formation of MMA polymers via different mechanism in hexane and toluene.
• Isolated Methanol soluble oligomeric MMA were studied by MALDI-TOF MS which suggests more than one mechanism was operating the reaction.

Scheme 3. Rationale for the formation of poly(methyl methacrylate) with

[Ni(Cp)(X)(NHC)]/MAO.

• The structure of Ni complex had considerable effect on the overall yield of MMA with no influence on the molecular weight distribution or tacticity of the resulting polymer and the Al : Ni ratio do not effect the tacticity of the polymer.
• It was supposed that MMA polymerized by co ordinative anionic mechanism described in scheme 3.

Conclusion:

It can summarized that the complexes 1a-f and 2 can initiate polymerization of MMA in the presence of MAO with TOF up to 200h^-1. The results of PMMA with GPC, NMR and MS imply a anionic, MAO-centered mechanism of polymerization catalyzed by Ni(II) species.

References:

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