European Polymer Journal 47 (2011) 1636–1645

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European Polymer Journal

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Polymerization of styrene with tetradentate chelated a-diiminenickel(II) complexes/MAO catalyst systems: Catalytic behaviorand microstructure of polystyrene

Lidong Li, Clara S.B. Gomes, Patrícia S. Lopes, Pedro T. Gomes ⇑, Hermínio P. Diogo,José R. AscensoCentro de Química Estrutural, Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais,1049-001 Lisboa, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 January 2011Received in revised form 29 April 2011Accepted 21 May 2011Available online 30 May 2011

Keywords:a-diimine ligandLate transition metalMicrostructureNickel complexPolyolefinPolystyrene

0014-3057/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.eurpolymj.2011.05.015

⇑ Corresponding author. Tel./fax: +351 21841961E-mail address: pedro.t.gomes@ist.utl.pt (P.T. Go

The recently reported N,N,N,N-tetradentate chelated a-diimine Ni(II) complexes 1–3 weredemonstrated to be efficient precatalysts for the polymerization of styrene in the presenceof MAO, producing isotactic-enriched heterotactic polystyrenes (isotactic contents in therange 73–75.5%). The corresponding sequence distributions were determined by NMRspectroscopy, and fitted to six probabilistic models of chain propagation. A better confor-mity to a Bovey–first order Markov chain propagation model was obtained, indicating achain-end control mechanism. A comparison of the microstructural features with otherpolystyrenes obtained by several different types of initiation indicates that the polysty-renes are formed by a coordination insertion polymerization mechanism at the nickelcenters.

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1. Introduction

Polystyrene (PS) is one of the most widely used com-modity polymers (8% of the world polymer market), featur-ing good stiffness, transparency and excellentprocessability [1]. Styrene can be polymerized by all theknown polymerization mechanisms, i.e., free-radical, cat-ionic, anionic, and coordination addition mechanisms, eachleading to polystyrenes with different stereoregularities.The radical, cationic and anionic initiated polymerizationsgenerally afford atactic polystyrenes. Polystyrenes withcontrolled stereoregularities are mainly synthesised via acoordination addition mechanism, which has been a chal-lenging subject in homogeneous polymerization catalysis.

Isospecific polymerization of styrene is mainly pro-moted by heterogeneous catalyst systems, and just a fewcases were achieved by homogeneous systems [2]. In

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2.mes).

general, the homogeneous catalysts are based on early-transition metal titanium systems, affording syndiotacticpolystyrenes [1,3].

One of the first examples of nickel-based homogeneouscatalysts for oligo-/polymerization of styrene was the cat-ionic allyl Ni(II) complex [Ni(g3-CH2C(Me)CH2)(COD)][PF6](COD = 1,5-cyclooctadiene) [4]. In the absence of Lewisacids, this aluminum-free catalyst, acting alone or modi-fied by in situ addition of phosphines, gave isotactic-en-riched heterotactic polymers with low degree ofpolymerization [2b,4,5]. Addition of bulky phosphines,such as PCy3 (Cy = cyclohexyl) or P(o-tol)3 (o-tol = o-tolyl),afforded highly isotactic oligo/polymers (>90%) [2a,b].Other single-component cationic allyl Ni complexes, suchas [Ni(g3-CH2C(R1)CH2)(PR2iPr2)2][BPh4] (R1 = H, Me;R2 = Me, Ph) and [Ni(g3-CH2C(R)CH2)(EPh3)x][BPh4](R = H, Me; E = As (x = 2), Sb (x = 3)), were also reportedto produce atactic oligo-/polystyrenes [6]. Several indenylNi(II) complexes, with the general formula [Ni(R-Ind)(PPh3)Cl] (R-Ind = g-1-R-indenyl; R = alkyl, cycloalkyl,

Scheme 1. New precatalysts (1–3) and Brookhart-type precatalyst (4)used in this work.

L. Li et al. / European Polymer Journal 47 (2011) 1636–1645 1637

alkylamine) were reported, which, either in the presence orabsence of further amounts of phosphine, and after halideabstraction with salts such as AgBPh4, AgBF4 or NaBPh4, areable to catalytically convert styrene into atactic oligo-/polystyrenes [7,8]. Similar results were obtained with N-heterocyclic carbene (NHC) analogues [Ni(R-Ind)(NHC)Cl][9]. Conversely, the cationic indenyl Ni(II) complexes[Ni(g3:g1-Ind(CH2)xNR2)Cl] (x = 1, 2; NR2 = 2-pyridinyl, 2-pyrrolidinyl, NiPr2) and [Ni(g-2-MeInd)(PRiPr2)2][BPh4](R = Me, Ph) behaved as single-component catalysts inthe polymerization of styrene, leading to atactic polysty-rene [10]. High molecular weight atactic polystyrenes wereformed when catalyst systems of the type [NiCp(NHC)Cl]/MAO were used [11].

A series of Ni salts and Ni organometallic compoundswere screened, which were active towards the polymeriza-tion of styrene, in the presence of MAO as cocatalyst,affording atactic or partially isotactic polystyrenes [12].These authors found that [Ni(acac)2] (acac = acetylaceto-nate) and NiCl2 were the most active, whereas phosphineadducts [NiCl2(PR3)2] exhibited the lowest activities.Impregnation of [Ni(acac)2] on MAO-supported silica gavea highly isospecific catalytic system, the cocatalyst havinga marked influence on the isotacticity of the final polymer[13]. Recently, a comparison of the performances of Brook-hart-type a-diimine Ni(II) and bis(imino)pyridine Fe(II)complexes on the polymerization of styrene, when treatedwith MAO and triisobutylaluminum, revealed that theFe(II) complexes produced syndiotactic polystyrenes withlow activities, whereas the use of the Ni(II) complexesled to more active systems and atactic polymers [14]. Atype of bis(b-ketoamide) nickel(II) complexes based onpyrazolone derivatives, demonstrated to be active in thepolymerization of styrene, when activated with MAO, lead-ing to moderate molecular weight atactic polystyrenes[15]. Additionally, the latter authors reported two typesof bidentate N,N-chelating nickel(II) complexes, anilide-imine nickel(II) bromides and b-diketiminate nickel(II)bromides, both being able to polymerize styrene to giveheterotactic (isotactic enriched) polymers upon activationwith MAO [16]. A catalyst system based on nickel(II) tetra-phenylporphyrin and MAO was recently reported to pro-duce polystyrenes with isotactic contents (m) in therange 59–64%, and relatively narrow molecular weight dis-tributions Mw/Mn � 1.50–1.80 [17].

In a recent work, we have synthesised a series of tetra-dentate chelated a-diimine nickel(II) complexes [NiBr2

(ArN@C(An)–C(An)@NAr)] (where Ar = 2-(1-benzyl-1H-1,2,3-triazol-4-yl)phenyl (1), 2-(1-(1-phenylethyl)-1H-1,2,3-triazol-4-yl)phenyl (2), 2-(1-phenyl-1H-1,2,3-tria-zol-4-yl)phenyl (3); An = acenaphthenic backbone), asrepresented in Scheme 1, and preliminary tests showedthat complexes 1–3 are precatalysts for the polymerizationof norbornene and styrene when activated with MAO [18].Further studies on the polymerization of norbornene cata-lyzed by 1–3/MAO systems concluded that it follows acoordination addition mechanism [19]. In the presentwork, we study in more detail the performances of catalystsystems 1–3/MAO in the polymerization of styrene, in tol-uene, and compare them with those of a typical Brookhart-type a-diimine nickel(II) precatalyst 4 (Scheme 1). The

microstructures of the obtained polystyrenes were deter-mined by 13C{1H} NMR spectroscopy, and their sequencedistributions analyzed. To the best of our knowledge thisis the first study on the microstructure of polystyrenes ob-tained by Brookhart type catalysts.

2. Experimental

2.1. Materials

All manipulations dealing with air- or moisture-sensi-tive materials were carried out under inert atmosphereusing a dual vacuum/nitrogen line and standard Schlenktechniques. Unless otherwise stated, all reagents were pur-chased from commercial suppliers (e.g. Acros, Aldrich, Flu-ka) and used as received. All solvents were used underinert atmosphere and purified prior to use. Toluene waspurified by refluxing over sodium/benzophenone ketyl,and distilled prior to use. Styrene was dried over calciumhydride for 2 days, distilled trap-to-trap under vacuum,at room temperature, and kept by storage at �20 �C. Meth-ylaluminoxane (MAO) of the type PMAO-IP (7 wt.% Al) waspurchased from AkzoNobel. The nickel precatalysts [NiBr2

(ArN@C(An)–C(An)@NAr)] (where Ar = 2-(1-benzyl-1H-1,2,3-triazol-4-yl)phenyl (1), 2-(1-(1-phenylethyl)-1H-1,2,3-triazol-4-yl)phenyl (2), and 2-(1-phenyl-1H-1,2,3-triazol-4-yl)phenyl (3); An = acenaphthenic backbone)were prepared as reported in a previous publication [18].The deuterated solvent 1,1,2,2-tetrachloroethane-d2 wasdried by storage over 4 Å molecular sieves and degassedby the freeze–pump-thaw method.

2.2. Polymerization of styrene

The polymerizations of styrene were conducted in100 ml Schlenk tubes, which were flamed in vacuum andback-filled with nitrogen. The desired amounts of solvent(toluene), styrene and catalyst suspension in the solvent(1.0 lmol/ml) were injected, by this order, via syringe.The polymerization was initiated by addition of MAO intoluene via syringe. At the desired reaction time, the

1638 L. Li et al. / European Polymer Journal 47 (2011) 1636–1645

polymerization was terminated by the addition of HCl-acidified methanol (5% (v/v)). The polymer was isolatedby filtration, washed with methanol, and dried in vacuumat 50 �C.

2.3. Characterization

2.3.1. Microstructure determinationNuclear magnetic resonance (NMR) spectra were re-

corded on a Bruker Avance III 500 (1H, 500.130 MHz; 13C,125.758 MHz) (UltraShield magnet) spectrometer. Spectrawere referenced internally using the residual protio sol-vent resonance relative to tetramethylsilane (d = 0). The1H and 13C{1H} NMR spectra of the polystyrenes in1,1,2,2-tetrachloroethane-d2 solutions were run, at 80 or115 �C. All chemical shifts are quoted in d (ppm).

The measurement of observed intensities, for tetrad andpentad sequences, were performed by electronic integra-tion within the range of the methylene and ipso carbons13C{1H} NMR resonances, respectively, the correspondingvalues being normalized for each of the resonances. Sixprobabilistic models of chain propagation (Price–Bernoulli,Price–first order Markov, Price–second order Markov, Bo-vey–Bernoulli, Bovey–first order Markov, Bovey–secondorder Markov) [20] were tested in the simulation of the ob-served 13C NMR resonance intensities. The calculatedintensities were determined by non-linear least-square fit-tings to the observed intensities with the Microsoft Excel�

Solver tool, considering simultaneously the correspondingtetrad and pentads equations, and by optimization of thereaction probabilities (P) for each model.

2.3.2. Molecular weight measurementsThe polystyrenes number-average molecular weight

(Mn), weight-average molecular weight (Mw), and polydis-persity indexes (Mw/Mn), were determined by gel perme-ation chromatography/size-exclusion chromatography(GPC/SEC), using a Waters 150 CV gel permeation chro-matograph, equipped with a series of a PL Polypore guardcolumn (50 � 7.5 mm) and two PL Polypore columns(300 � 7.5 mm), using THF as eluent and a flow-rate of1.0 mL/min (nominal), at 35 �C (nominal); the detectorwas a differential refractometer. The GPC/SEC system wascalibrated with polystyrene standards (TSK Tosoh Co.).

2.3.3. Thermal analysisDifferential scanning calorimetry (DSC) experiments on

the polymer samples were carried out in a MTDSC2920 TAapparatus operating as a conventional DSC. The baselinewas calibrated scanning the temperature domain of theexperiments with an empty crucible. The temperaturecalibration was performed taking the onset of the endo-thermic melting peak, at the heating rate of the measure-ments, using five calibration standards, and the enthalpyscale was calibrated using indium as a standard referencematerial. Polystyrene samples with masses in the rangeof 4–6 mg were weighed in aluminum crucibles. All runswere performed under helium atmosphere, to increasethermal contact between the crucible and the apparatussensor, at a heating rate of 10 �C/min. The temperatureprogram consisted of two sequential temperature ramps,

the first one from room temperature up to 140 �C (or to270 �C in the case of a few selected samples), followed bycooling down to 40 �C, and a subsequent second ramp upto the same upper limit. The second heating cycle was cho-sen to extract the thermal parameters.

3. Results and discussion

3.1. Polymerization of styrene

All the three complexes 1–3 are active catalysts for thepolymerization of styrene upon activation with MAO. Theresults of the catalytic reactions and corresponding poly-mer properties are summarized in Table 1. In the absenceof MAO, complexes 1–3 are not active for styrene polymer-ization (entries 4, 17 and 29, Table 1), whereas MAO aloneis able to polymerize styrene above 30 �C, but in very lowyields, the resulting polymers being multimodal and withbroad molecular weight distributions (entry 3, Table 1).The latter polystyrenes are constituted by two separateddistributions, the larger fraction (89%) being bimodal, witha Mn = 1.43 � 104 g mol�1 and Mw/Mn = 3.77, and the smal-ler fraction (11%) being nearly monodisperse, with aMn = 8.36 � 105 g mol�1 (Mw/Mn = 1.09). The catalyticactivities of MAO are about 100 times lower than thoseof catalyst systems 1–3/MAO, at 50 and 70 �C. Therefore,we can neglect the amount of PS originated by the cocata-lyst in comparison with that produced by the 1–3/MAOcatalyst systems.

Because the substituents on the triazolyl rings of com-plexes 1–3 possess similar electronic and steric properties,their influences on the polymerization activity and molecu-lar weight of the final polymers are comparable. In principle,under the same reaction conditions, the activities decreasein the order 2 > 1 > 3, e.g. 1.69 (entry 20) > 1.00 (entry7) > 0.99 � 105 g PS�(mol�Ni)�1 h�1 (entry 32), the molecu-lar weights being similar. As shown in Table 1 (Supplemen-tary material Figs. S1 and S2), the polymerization activitieschange very noticeably with the [Al]/[Ni] molar ratio, as anincrease from 200 to 1000 corresponds to an activity ca.2.5 times higher, whereas the polymer molecular weightsand polydispersities show only slight variations. The effectof an increase of the reaction temperature is also significant,since there is a net enhancement of the catalytic activity anda clear decrease of Mn with the temperature (Table 1; Sup-plementary material Fig. S3), although polydispersities re-main approximately constant. The effect on Mn is expecteddue to more favorable chain transfer reactions (b-hydrogenelimination) at higher temperatures. In general, the num-ber-average molecular weights of the polystyrenes are rela-tively low, varying from 5200 to 17,600 g mol�1, the greatmajority of the corresponding polydispersities being in therange of 1.68–1.90.

A comparison of the catalyst systems 1–3/MAO featuresin the polymerization of styrene with those of related typ-ical a-diimine nickel(II) catalysts reported in the literature,such as [NiBr2(ArN@C(An)-C(An)@NAr)]/MAO (whereAr = 2,6-Me2C6H3 or 2-CF3C6H4, An = acenaphthenic back-bone), under polymerization conditions: bulk, 50 �C, sty-rene:MAO:AliBu3:Ni = 100,000:300:100:1 [14], shows the

Table 1Polymerization of styrene promoted by 1–4/MAO catalyst systems. Polymerization conditions: nickel complex, 5 lmol; styrene, 5.0 g; solvent, toluene; totalvolume, 25 ml; reaction time, 3 h.

Entry Cat. T (�C) nAl/nNi Yield (g) Conv. (%) Activitya � 10�5 Mnb � 10�3 Mw/Mn

b Tgc (�C)

1d 30 Traces2d 50 0.01 0.2 0.013d 70 0.05 1.0 0.03 7.7 (89%)e 3.77e 95.3

836.2 (11%) 1.254 1 30 0 0 0 05 1 30 50 1.10 22.0 0.73 16.1 1.746 1 30 100 1.44 28.8 0.96 14.3 1.977 1 30 200 1.50 30.0 1.00 15.5 1.71 93.48 1 30 500 2.58 51.6 1.72 17.5 1.769 1 30 1000 3.72 74.4 2.48 17.6 1.69

10 1 30 2000 4.18 83.6 2.79 16.3 1.7111 1 50 50 1.36 27.2 0.91 12.0 1.7012 1 50 100 1.86 37.2 1.24 11.5 1.7313 1 50 200 2.09 41.8 1.39 11.5 1.68 91.514 1 70 50 1.83 36.6 1.22 6.1 2.28 88.415 1 70 100 2.34 46.8 1.56 6.1 2.17 78.216 1 70 200 2.66 53.2 1.77 5.2 2.29 82.117 2 30 0 0 0 018 2 30 50 2.15 43.0 1.43 15.7 1.6919 2 30 100 2.17 43.4 1.45 15.1 1.7720 2 30 200 2.53 50.6 1.69 14.8 1.69 95.2

21f 2 30 200 0.96 19.2 0.64 15.7 1.7022g 2 30 200 0.78 15.6 0.52 14.5 1.7523 2 50 50 1.42 28.4 0.95 12.7 1.7124 2 50 100 1.73 34.6 1.15 13.0 1.6825 2 50 200 2.53 50.6 1.69 12.2 1.70 95.826 2 70 50 0.88 17.6 0.59 8.5 1.6827 2 70 100 2.90 58.0 1.93 8.5 1.6928 2 70 200 2.95 59.0 1.97 8.0 1.70 85.529 3 30 0 0 0 030 3 30 50 0.49 9.8 0.33 16.0 1.8931 3 30 100 0.90 18.0 0.60 16.0 1.7332 3 30 200 1.48 29.6 0.99 14.8 1.75 95.533 3 50 50 1.64 32.8 1.09 12.3 1.6934 3 50 100 1.76 35.2 1.17 11.9 1.6835 3 50 200 1.85 37.0 1.23 11.5 1.70 95.736 3 70 50 1.96 39.2 1.31 9.2 1.6837 3 70 100 2.01 40.2 1.34 8.7 1.7238 3 70 200 3.14 62.8 2.09 7.4 1.84 91.139 4 30 200 0.03 0.6 0.02 19.7 1.7440 4 50 200 0.21 4.2 0.14 16.2 1.73 103.341 4 70 200 0.22 4.4 0.15 12.7 1.77 94.6

a g PS�(mol Ni)�1 h�1.b g mol�1, determined by GPC/SEC.c Determined by DSC.d nNi = 0 mol, nAl = 1.0 mmol.e Bimodal peak.f Five equivalents of butylated hydroxytoluene (BHT) were added.g Five equivalents of galvinoxyl were added.

L. Li et al. / European Polymer Journal 47 (2011) 1636–1645 1639

catalytic activities obtained with precatalysts 1–3 are com-parable or higher than those of Brookhart’s system. Thecorresponding molecular weights are approximately thesame, and the molecular weight distributions are signifi-cantly narrower in the case of the present work (Mw/Mn

ca. 1.7 vs. 2.4–2.9, at 50 �C). Since our results with precat-alysts 1–3 were obtained using different conditions in rela-tion to the latter catalyst systems, we decided to make anexperimental comparison with the Brookhart-type catalyst4/MAO, under the same polymerization parameters (Table1). It is evident that the catalyst systems 1–3/MAO are sub-stantially more active than 4/MAO, e.g., 1.23 (entry 35) vs.0.14 � 105 g PS�(mol�Ni)�1 h�1 (entry 40) and 2.09 (entry38) vs. 0.15 � 105 g PS�(mol�Ni)�1 h�1 (entry 41). The

molecular weights of the polystyrenes obtained by thelatter system are slightly higher than those of 1–3/MAO,and the corresponding molecular weight distributionsapproximately the same.

The addition of free radical traps such as 2,6-di-tert-bu-tyl-4-methylphenol (BHT) or galvinoxyl to the reactionmedium (5 equivalents in relation to the Ni precatalyst)led to some decrease in the catalyst activity, but not to acomplete deactivation, e.g. 0.64 (entry 21, Table 1) or0.52 � 105 g PS�(mol Ni)�1 h�1 (entry 22, Table 1) vs.1.69 � 105 g PS�(mol Ni)�1 h�1 (entry 20, Table 1), whilethe molecular weights and molecular weight distributionsof the resulting polymers remained constant. However, theuse of radical traps in metal-based systems employing

1640 L. Li et al. / European Polymer Journal 47 (2011) 1636–1645

MAO can lead to wrong conclusions, since radical traps canreact with metal hydrides and halt or slow down metal-centered non-radical reactions [21] or fail to intercept rad-ical reactions that proceed in the presence of MAO [22].Additionally, it is known that, in nickel and palladiumorganometallic chemistry, phenols such as 2,4,6-trimethyl-phenol or BHT can also react with metal-alkyl groups togive stable phenolate ligands [23] or form stable coordina-tion adducts [24]. These types of reactions can, in fact, con-tribute to the observed decrease in the catalyst activity,and thus in the polymerization yield, in the experimentswith the radical traps used in this work (BHT and galvin-oxyl), turning them inconclusive as probes for a radicalmechanism.

Further insights into the propagation mechanism can beobtained by the analysis of the polystyrene microstruc-tures and their comparison with those of polystyrenes syn-thesised by other different types of initiation.

3.2. Microstructure analysis

All the polystyrenes obtained with 1–3/MAO catalystsare readily soluble in acetone, chloroform and tetrahydro-furan, which is indicative of a relatively low stereoregular-ity. The DSC analyses performed on these polymers areconsistent with the latter conclusion, since they onlyshowed a thermal event ascribed to a glass transitiontemperature (Tg) in the range 78–96 �C (Table 1), and noendothermic events attributed to melting were detectedup to 270 �C, which is typical of an amorphous behavior.The Tg variation with Mn roughly follows the trendexpected for amorphous low molecular weight polysty-renes [25].

Unlike the controlled stereoregular polymerization ofa-olefins with metallocenes of variable symmetries [26],the relationship between the stereoregularities of polysty-renes with the molecular structures of nickel precatalystsstill remains unclear. To gain insight into this aspect, thedetermination and analysis of the microstructures of thepolystyrenes is of major importance. It has been demon-strated that 13C NMR spectroscopy is a powerful tool tocharacterize the microstructure of polystyrene [20a,27].Based on the analyses of sequence distributions of differentdiastereomers, up to tetramers and pentamers, as modelcompounds, and epimerized isotactic polystyrenes, it wasfound that 13C resonances of methylene carbons and aro-matic ipso carbons of polystyrene are sensitive to the se-quence distribution. The methylene carbon 13Cresonances, appearing at ca. d 44, show a good resolutionover a 5 ppm range, which enable the assignment of tetr-ads and hexads, while the aromatic ipso carbon 13C reso-nances, at ca. d 146, are spread over a range of 2 ppmand are sensitive to the assignment of pentads. More re-cently, Feil and Harder assigned the heptad sequences forpredominantly syndiotactic polystyrenes using the ipsocarbon 13C resonance at a high magnetic field NMR spec-trometer (600 MHz) [28].

In the present work, five typical polystyrenes obtainedby our catalyst systems 1–3/MAO in toluene (entries 7,13, 16, 28 and 38, Table 1), and a polystyrene sample ob-tained by a Brookhart-type catalyst 4/MAO in toluene (en-

try 41, Table 1) were selected to be characterized by 1H and13C{1H} NMR spectroscopy in a 500 MHz NMR spectrome-ter, at 80 or 115 �C. Apparently, no significant influence ofthe temperature on the resolution of the 13C resonanceswas observed. The spectra of the three samples obtainedusing each one of the three different nickel precatalysts1–3, under the same other experimental conditions (en-tries 16, 28 and 38, Table 1), are very similar, being thatcorresponding to the PS obtained with 1/MAO (entry 16)represented in Fig. 1.

The assignments of the tetrads, in the methylene carbonregion, and the pentads resonances, in the aromatic ipsocarbon region, are represented in Fig. 1b and c, and Table2 lists their corresponding observed 13C NMR resonancenormalized intensities (%). It can be seen that the threesamples have similar sequence distributions (entries 16,28 and 38, Table 2), with isotactic contents (m) of 73–74%, which presumably stems from the close electronicand steric nature of the three precatalysts 1–3 or, alterna-tively, it is due to a weak relationship between the stereo-regularity of the polymers with the molecular structure ofthe catalysts (e.g. end group control mechanism). An in-crease of the reaction temperature has also a minor influ-ence on the sequence distribution (entries 7, 13 and 16,Table 2, performed with 1/MAO at 30, 50 and 70 �C, respec-tively) as a very slight decrease of the isotactic content isobserved. It is worth noting that the spectra of the polysty-rene sample obtained with the Brookhart-type precatalyst4 exhibits approximately the same features as those of pre-catalysts 1–3 (Supplementary material Figs. S4 and S5), thetetrads and pentads distribution being comparable (entry41, Table 2; Supplementary material Figs. S6 and S7).

We have tested six probabilistic models of chain propa-gation: Price–Bernoulli (pure catalytic-site control), Price–first order Markov, Price–second order Markov (bothmixed catalytic-site and chain-end control), Bovey–Ber-noulli, Bovey–first order Markov, Bovey–second order Mar-kov (all the three pure chain-end control) [20a,b]. All thefive polystyrene samples obtained by catalyst systems1–3/MAO in toluene show very similar microstructures(entries 7, 13, 16, 28 and 38, Table 2). The calculated inten-sities can be found in Tables S1–S5 of the Supplementarymaterial, and the corresponding derived reaction probabil-ities and mean deviations between observed and calcu-lated intensities (R) [20b] are summarized in Table 3. Ascan be seen from the values of the fitting parameters R, acatalyst-site control mechanism for the propagation canbe excluded (RT = 5.2–5.6). For all the samples, the betterfitting is obtained for a Bovey–first order Markov model(Pm/r = 0.27–0.30, Pr/m = 0.70–0.74, RT = 1.3–2.0), which issupported by a sum of Pm/r and Pr/m very close to unity,pointing out to a pure chain-end control mechanism condi-tioned by the relative configuration of the last two chainunits (bonded to nickel). Nevertheless, a Price–second or-der Markov or a Bovey–second order Markov models can-not be completely disregarded since they show meandeviations that are not that different from the one of theBovey–first order Markov model. In the case of the sampleobtained by Brookhart-type precatalyst 4, remarkably sim-ilar fittings to those of precatalysts 1–3 are observed (entry41, Table 3; Supplementary material Table S6). The better

Fig. 1. 13C{1H} NMR spectrum of a polystyrene obtained with catalyst 1/MAO (entry 13, Table 1). (a) Assignment of the resonances; (b) assignment of themost probable tetrads in the methylene carbon region; (c) assignment of the most probable pentads in the aromatic ipso carbon region.

L. Li et al. / European Polymer Journal 47 (2011) 1636–1645 1641

fitting is also obtained for a Bovey–first order Markov mod-el (Pm/r = 0.30, Pr/m = 0.73, RT = 1.6), indicating that the pure

chain-end control mechanism also dominates the poly-merization reaction, as in the case of systems 1–3/MAO.

Table 2Observed 13C NMR resonance normalized intensities (%) of pentads and tetrads, and corresponding derived values of triads and diads, for the polystyrenesobtained with 1–3/MAO catalyst systems (entries 7, 13, 16, 28 and 38) and Brookhart-type catalyst 4/MAO (entry 41), in toluene.

Sequences Entry 7 Entry 13 Entry 16 Entry 28 Entry 38 Entry 41

Pentads(mmmm) + (mmmr) 48.9 47.8 45.1 44.1 46.0 44.3(rmmr) 6.5 6.4 7.1 7.6 7.1 6.1(mmrm) + (mmrr) 22.7 23.4 24.1 23.6 23.7 24.3(rmrm) + (rrmr) 17.3 17.5 18.6 18.4 17.8 20.7(..rr..) 4.6 4.9 5.1 6.3 5.4 4.6

Tetrads(rmr) 4.6 5.5 4.7 5.3 4.8 7.7(rrr) 0.0 1.2 0.4 1.0 0.5 3.2(mmr) 30.2 31.3 33.3 31.8 32.1 31.1(rrm) + (mmm) 51.8 49.0 49.4 49.1 49.7 46.3(mrm) 13.4 13.0 12.2 12.8 12.9 11.7

Triadsa

(mm) 55.4 54.2 52.2 51.7 53.1 50.4(mr) 40.0 40.9 42.7 42.0 41.5 45.0(rr) 4.6 4.9 5.1 6.3 5.4 4.6

Diadsb

(m) 75.4 74.7 73.6 72.7 73.9 72.9(r) 24.6 25.3 26.4 27.3 26.1 27.1

a Calculated from: (mm) = (mmmm) + (mmmr) + (rmmr), (mr) = (mmrm) + (rmrm) + (mmrr) + (rmrr).b Calculated from: (m) = (mm) + 0.5 (mr).

Table 3Calculated reaction probabilities of the polystyrenes obtained with 1–3/MAO (entries 7, 13, 16, 28, 38) and 4/MAO (entry 41) catalyst systems, in toluene.

Model/Parameters Entry 7 Entry 13 Entry 16 Entry 28 Entry 38 Entry 41

Price/BernoulliPd 0.22 0.23 0.24 0.24 0.23 0.25RT

a 5.4 5.4 5.6 5.2 5.5 5.3

Price/Markov IPd/l 0.45 0.44 0.45 0.47 0.45 0.45Pl/d 0.21 0.23 0.24 0.24 0.23 0.26RT

a 2.9 2.8 3.2 2.8 2.9 3.0

Price/Markov IIPdd/d 0.36 0.40 0.43 0.39 0.41 0.34Pdl/d 0.00 0.02 0.00 0.00 0.00 0.06Pld/d 0.55 0.57 0.51 0.49 0.52 0.58Pll/d 0.22 0.23 0.25 0.25 0.24 0.24RT

a 1.8 2.0 2.3 2.1 2.1 2.4

Bovey/BernoulliPm 0.73 0.72 0.71 0.70 0.71 0.69RT

a 2.6 2.3 3.1 2.7 2.7 2.7

Bovey/Markov IPm/r 0.27 0.28 0.29 0.30 0.29 0.30Pr/m 0.74 0.73 0.72 0.70 0.72 0.73RT

a 1.7 1.3 2.0 1.7 1.7 1.6

Bovey/Markov IIPmm/m 0.75 0.73 0.71 0.71 0.72 0.71Pmr/m 0.70 0.71 0.66 0.65 0.68 0.71Prm/m 0.64 0.64 0.65 0.63 0.64 0.60Prr/m 1.00 0.99 1.00 1.00 1.00 0.91RT

a 1.9 2.1 2.4 2.1 2.1 2.4

a Mean deviation between observed and calculated intensities (Supplementary Tables S1-S6).

1642 L. Li et al. / European Polymer Journal 47 (2011) 1636–1645

The sequence distributions of polystyrenes samplesprepared by free radical polymerization are reported inthe literature for reactions initiated by 2,2’-azoisobutyro-nitrile (AIBN) [29a,b] or benzoyl peroxide (BPO) [29c,30],showing remarkable differences in relation to those ob-tained in the present work (Supplementary material

Table S7). These radical polystyrenes have isotactic con-tents of ca. 50%, typical of fully atactic polymers, and theirmicrostructures conform to a Bovey–Bernoulli propagationmodel. Atactic microstructures and a Bovey–Bernoullipropagation model are also obtained for the polystyrenessynthesised with classical anionic or cationic initiators,

Scheme 2. Coordination mechanism proposed for the polymerization of styrene catalyzed by 1–3/MAO systems (R = H (highly major), Me (minor)).

L. Li et al. / European Polymer Journal 47 (2011) 1636–1645 1643

such as LiBu (isotactic content of ca. 45%) and BF3�OEt2

(isotactic content of ca. 54%), whereas initiation by MAO,acting alone, gives rise to a slightly more isotactic polysty-rene (isotactic content of ca. 60%) [29a] (Supplementarymaterial Table S7).

The polymers produced in the present work with cata-lysts 1–4/MAO have considerably higher isotactic contents(73–75.5%) and chains conforming better to a Bovey–firstorder Markov propagation model. Although slightly moreisotactic, these microstructural features are similar tothose reported for polystyrenes obtained with other nickelcatalyst systems, such as the anilide-imine nickel(II) bro-mides/MAO [16a] and b-diketiminate nickel(II) bromides/MAO [16b] (m = 66–69% and 66%, respectively), the nicke-l(II) tetraphenylporphyrin/MAO (m = 60–64%) or thealuminum-free organometallic cationic allyl and benzylnickel complexes (m = 66–69%) [2b,4,5] (Supplementarymaterial Table S7), which polymerize styrene by a coordi-nation insertion mechanism, in which the last styrenicchain unit exhibits a g3-benzylic coordination to the Nicenter. Therefore, the mechanism of formation of the poly-styrenes in the presence of catalysts 1–4/MAO seems to oc-cur by a coordination insertion mechanism, alsodetermined for the polymerization of norbornene by cata-lysts 1–3/MAO. Additionally, as shown in Fig. 1a, the 13Cresonances of the methyl (C6) and methine (C5) of thesaturated terminal group are present in the range of d21.7–21.2 and 37.3–37.1, respectively, while those of the2-phenylvinyl terminal group (C1 and C2) lie at d 130.5and 133.9, respectively, consistent with the values ob-tained previously for oligo-/polymers of this type[2b,4,5]. The same conclusion is obtained in the 1H NMRspectra showing resonances at d 1.0–1.1 for the methylend group (–C6H3 protons), and d 6.0–6.2, correspondingto the 2-phenylvinyl end group (Ph–C1H@C2H– vinyl pro-tons) (Supplementary material Fig. S4). These terminalgroups suggest that the polymer chains are mainly initi-ated via a secondary migratory insertion of styrene intoNi–H bonds, and the chain transfer occurs via b-hydrogenelimination to Ni.

In the case of the typical Brookhart a-diimine nickel(II)or palladium(II)-based homogeneous catalysts, such as 4/MAO (Scheme 1), the polymerization initiators are meantto be 14-electron cationic methyl complexes [L2MMe]+ orthe hydride intermediate [L2MH]+ (M = Ni, Pd, L2 = biden-tate chelating a-diimine ligands), originated in the b-hydrogen elimination step (chain transfer) [31]. In thepresent tetradentate chelated a-diimine nickel precata-lysts 1–3, the corresponding species should be the satu-rated 18-electron [L4NiMe]+ or [L4MH]+ (L4 = tetradentatechelating ligands) species. When one or both of the chelat-ing triazolyl groups decoordinate from nickel (to remainuncoordinated or, alternatively, coordinate to the acidicaluminum centers of MAO), styrene can bind the latterspecies and undergo multiple insertion reactions (chaingrowth), to form a species such as [L^L3M(P)(olefin)]+ or[L2^L2M(P)(olefin)]+ (P = polymer alkyl chain), whose lastinserted styrene unit can interact in a g3-benzylic fashionwith Ni. Chain transfer occurs by b-hydrogen eliminationto the metal, giving rise to the abovementioned terminalgroups. This proposed mechanism is depicted in Scheme 2.

4. Conclusions

Complexes 1–3 when activated by MAO behave as effi-cient precatalysts for the polymerization of styrene in tol-uene, being more active than the conventional a-diiminenickel(II) Brookhart-type catalysts, namely 4/MAO, butshowing similar molecular weight and microstructuralfeatures.

The variation of the triazolyl substituents on the precat-alyst structure, in 1–3/MAO systems, have slight influenceson the catalytic activities and molecular weights, whereasthe reaction temperature has a marked effect on bothparameters, an increase of temperature leading to a risein activity and to a drop in Mn. Conversely, the catalyticactivities increase considerably with the [Al]/[Ni] molar ra-tio, but the polymer molecular weights and polydispersi-ties show small variations.

1644 L. Li et al. / European Polymer Journal 47 (2011) 1636–1645

The observed sequence distributions of the polymersobtained with 1–3/MAO catalyst systems are characteristicof isotactic-enriched heterotactic polystyrenes (isotacticcontents in the range 73–75.5%) that better conform to afirst-order Markov distribution, with Pm/r = 0.27–0.30 andPr/m = 0.70–0.74, indicating a chain-end control propaga-tion mechanism. Similar microstructures (Pm/r = 0.30 andPr/m = 0.73) are obtained for the polystyrenes obtainedwith the Brookhart-type catalyst 4/MAO. These micro-structural features are very different from those reportedin the literature for polystyrenes synthesised by classicalfree radical, ATRP, anionic or cationic initiation, but aresimilar to those exhibited by polystyrenes obtained by acoordination insertion mechanism, although slightly moreisotactic enriched than the latter ones. The 13C NMR end-group analysis supports the latter mechanism, in whichthe polymer chains are initiated via a secondary insertioninto the Ni–H bonds, and chain transfer via a b-hydrogenelimination.

Acknowledgements

We thank the Fundação para Ciência e Tecnologia forfinancial support (Projects POCI/QUI/59025/2004, PTDC/QUI/65474 and PTDC/EQU-EQU/110313/2009, co-financedby FEDER) and fellowships to L.L. and C.S.B.G. (SFRH/BPD/30733/2006 and SFRH/BPD/64423/2009, respectively).We also thank the Portuguese NMR Network (IST-UTLCenter).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.eurpolymj.2011.05.015.

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