Binary Copolymerization of p-Methylstyrene with Butadiene and

The synthesis of p-methylstyrene–butadiene and p-methylstyrene–isoprene binary copolymers promoted by the titanium complexes Ti(η5-C5H5)-(κ2-MBM...
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Binary Copolymerization of p‑Methylstyrene with Butadiene and Isoprene Catalyzed by Titanium Compounds Showing Different Stereoselectivity Antonio Buonerba, Maria Fienga, Stefano Milione, Cinzia Cuomo, Alfonso Grassi, Antonio Proto, and Carmine Capacchione* Dipartimento di Chimica e Biologia, Università degli Studi di Salerno, via Giovanni Paolo II, 132 I-84084 Fisciano (SA), Italy S Supporting Information *

ABSTRACT: The synthesis of p-methylstyrene−butadiene and p-methylstyrene−isoprene binary copolymers promoted by the titanium complexes Ti(η5-C5H5)-(κ2-MBMP)Cl (1) (MBMP = 2,2′-methylenebis(6-tert-butyl-4-methylphenoxo)) and chloro{1,4-dithiabutanediyl-2,2′-bis(4,6-di-tert-butylphenoxy)}titanium (2) activated by methylaluminoxane (MAO) is reported. Syndiotactic poly(p-methylstyrene)-co-cis-1,4-poly(butadiene) and syndiotactic poly(p-methylstyrene)-co-cis-1,4-polyisoprene were obtained using catalyst 1, whereas isotactic poly(pmethylstyrene)-co-trans-1,4-poly(butadiene) and isotactic poly(p-methylstyrene)-co-trans-1,4-polyisoprene were obtained using the catalyst 2. 13C NMR analysis of the copolymer microstructure allowed to assess the monomer block lengths and distribution in the polymer chain, revealing a blocky distribution of the two monomers along the polymer chain in the presence of the catalyst 1 and a random distribution with the catalyst 2 for both binary copolymers.

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INTRODUCTION The copolymerization can be considered one of the simplest ways to obtain new polymeric materials because of the possibility to tune the properties of the resulting polymer by simply controlling the content and the distribution of the different monomers along the polymer chain. In this area, the styrene− butadiene (SB) copolymers represent by far the most relevant industrial product with styrene−butadiene rubber (SBR) and the thermoplastic block copolymers (SBS) obtained by free radical emulsion processes or solution processes based on living anionic polymerization catalyzed by alkyllithium compounds.1 Such polymerization techniques, however, suffer from the lack of steric control, leading to stereoirregular styrene sequences as well as stereo- and regioirregular diene incorporation.2 In these cases the microstructure of the resulting polymers largely depends on the polymerization conditions such as the solvent, monomer concentration, temperature, and the presence of Lewis base additives. Indeed, the most efficient tool to obtain a stereoregular polymer is the use of transition metal complexes that promote the polymerization via coordination−insertion mechanism.3 In particular, among the polymers obtained by homogeneous catalysis promoted by transition metal complexes, the discovery by Ishihara et al.4 that the homogeneous system consisting of a combination of η5-Cp′TiX3 (Cp′ = cyclopentadienyl or alkylsubstituted cyclopentadienyl; X = alogen, alkoxy or alkyl) with methylaluminoxane (MAO) promotes the syndioselective © XXXX American Chemical Society

polymerization of styrene has brought forth a growing interest in such material for its unique proprieties.5 In addition, more recently were also developed efficient homogeneous catalysts for the synthesis of highly isotactic polystyrene (iPS).6 Some of these catalysts efficiently copolymerize butadiene and styrene giving copolymers with unique microstructural features.7 In particular, we have reported that the catalytic system Ti(η5-C5H5)-(κ2-MBMP) Cl (1) (MBMP = 2,2′-methylenebis(6-tert-butyl-4-methylphenoxo)) activated by MAO promotes the copolymerization of styrene (S) with 1,3-butadiene (B) or isoprene (I), giving in the first case multiblock copolymers containing segments of cis-1,4polybutadiene and crystalline syndiotactic polystyrene and in the latter case amorphous copolymers with short syndiotactic polystyrene homosequences.8 We also succeeded in synthesizing isotactic poly(styrene)-co-trans-1,4-poly(butadiene) using the chloro{1,4-dithiabutanediyl-2,2′-bis(4,6-di-tert-butylphenoxy)}titanium complex (2) activated by MAO. In this last case the resulting copolymers have a random distribution of the two monomers along the polymer backbone and, despite the presence of short isotactic polystyrene segments, are completely amorphous.9 We have shown, practically, that the catalytic systems based on titanium compounds 1 and 2 can give butadiene−styrene copolymers with opposite stereoselectivity. The synthesis of Received: August 1, 2013 Revised: September 30, 2013

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Scheme 1

recovered by filtration, washed with an excess of ethanol, and dried in vacuo at room temperature. Copolymerization of 1,3-Butadiene and p-Methylstyrene. The copolymerization runs were carried out following a standard procedure. A 100 mL flask equipped with a magnetic bar was charged with 14.6 mL of MAO (10 wt % toluene solution, 22 mmol, Al/Ti = 1000), 5 mL of a butadiene solution 3.0 M (0.81 g, 15 mmol) in toluene, a variable amount of p-methylstyrene, and the proper amount of toluene to reach a total volume of 40 mL. The reaction was started by injection of a toluene solution (2 mL) of titanium complex. The run was terminated after 1 h by introducing ethanol (15 mL) and the antioxidant (WingstayK). The polymer was coagulated in ethanol (200 mL) acidified with aqueous HCl, recovered by filtration, washed with an excess of ethanol, and dried in vacuo at 30 °C. Copolymerization of Isoprene and p-Methylstyrene. The copolymerization runs were carried out following a standard procedure. A 100 mL flask equipped with a magnetic bar was charged with 29.0 mL of MAO (10 wt % toluene solution, 0.044 mol, Al/Ti = 1000), 4 mL of isoprene (2.72 g, 40 mmol), a variable amount of p-methylstyrene, and the proper amount of toluene to reach a total volume of 80 mL. After equilibration of the solution at 50 °C the reaction was started by injection of a toluene solution (4 mL) of titanium complex. The run was terminated after 1 h by introducing ethanol (30 mL) and the antioxidant (WingstayK). The polymer was coagulated in ethanol (200 mL) acidified with aqueous HCl, recovered by filtration, washed with an excess of ethanol, and dried in vacuo at 30 °C. Characterization of the Polymers. The 13C NMR spectra of the polymer samples were recorded with a Bruker AVANCE 300 spectrometer (300 MHz for 1H and 75 MHz for 13C). Using 5 mm o.d. NMR tubes, polymer samples (30 mg) were dissolved in CDCl3 (0.7 mL) and analyzed at room temperature. Chemical shifts were referenced to TMS and calculated by using the residual isotopic impurities of the deuterated solvents. The microstructure of the copolymer samples was assessed by comparing the observed resonances

p-methylstyrene−butadiene and p-methylstyrene−isoprene copolymers is of particular interest because of the expected change of the thermal properties and the possibility of using the methyl in para position to further functionalize the polymer.10 Here we report on the binary copolymerization of p-methylstyrene (pMS) with butadiene and with isoprene using the catalytic systems 1 and 2 and the complete characterization of the resulting copolymers (Scheme 1).

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EXPERIMENTAL PART

Materials. All manipulations of air- and/or water-sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk techniques or a MBraun drybox. Commercial grade toluene (Carlo Erba) was dried over calcium chloride, refluxed 48 h under a nitrogen atmosphere over sodium, and distilled before using. Polymerization grade 1,3-butadiene, purchased from Rivoira, was dried by passing through a column filled with activated molecular sieves (4 Å). Isoprene (Sigma-Aldrich) was purified by distillation over calcium hydride under a nitrogen atmosphere. p-Methylstyrene (SigmaAldrich) was purified by distillation over calcium hydride under reduced pressure. Methylaluminoxane, purchased from Sigma-Aldrich as a 10 wt % solution in toluene, was used as received. The titanium complexes 1 and 2 were prepared according to the literature procedure.6d,11 Polymerization of p-Methylstyrene. The polymerization runs were carried out following a standard procedure. A 100 mL flask equipped with a magnetic bar was charged with toluene (12 mL), p-methylstyrene (5 mL), and the proper amount of MAO (10 wt % solution in toluene). After equilibration of the solution at the polymerization temperature the reaction was started by injection of a toluene solution (5 mL) of 1 (11 mg, 22 μmol). The run was terminated after 10 min by introducing ethanol (15 mL). The polymer was coagulated in ethanol (200 mL) acidified with aqueous HCl, B

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with the data reported in the literature7e,8a,9a or by calculation of the chemical shifts following the rules reported in the literature.16,17 The thermal analysis of the polymers was carried out on TA Instruments DSC Q20 using a heating rate of 10 °C/min. The average molecular weights of the polymer samples were determined at 30 °C with a 150C Waters GPC equipped with JASCO 875-UV (254 nm) and WGE-DR BURES ETA1002 refractive index detectors and three PSS columns set consisting of 105, 104, and 100 Å (pore size)−5 μm (particle size) column. CHCl3 was used as carrier solvent with a flow rate of 1.0 mL/min. The calibration curve was established with polystyrene standards. Powder wide-angle X-ray diffraction (WAXD) patterns were obtained, in reflection, with an automatic Bruker D8 powder diffractometer using the nickel-filtered Cu Kα radiation.

the presence of a sharp singlet for the 13C signal of the ipsocarbon at 142.8 ppm.12 The number average molecular weight (Mn) is independent of the MAO concentration and decreases as polymerization temperature (entries 2, 4, and 5 of Table 1) is increased, suggesting fast chain transfer to aluminum; this trend is opposite to that found for the productivity that increases with polymerization temperature, suggesting a faster increase of specific propagation rate versus chain transfer rate. The isospecific polymerization of substituted styrene monomers catalyzed by 2/MAO was previously reported,13 where higher polymerization activity of pMS vs styrene was actually observed with the latter catalyst. p-Methylstyrene−Butadiene Copolymerization. Syndiotactic poly(p-methylstyrene)-co-cis-1,4-poly(butadiene) polymers in a wide range of composition (xpMS = 0.19−0.70) were obtained at 50 °C by 1/MAO under the experimental conditions used for pMS polymerization (see Table 2). Gel permeation chromatography (GPC) analysis of these copolymers showed that the molecular weight distributions are monomodal with a PDI values close to 3, consistent with their copolymeric nature. The copolymerization experiments were carried out with increasing concentration of pMS, leaving butadiene constant. The p-methylstyrene−butadiene copolymers are solid at high concentration of pMS (entries 11−14) and waxy when the butadiene concentration is increased to 67.0 mol % (entries 8−10); all copolymers are completely soluble in hexane, THF, or chloroform, also when pMS concentration approaches as high values as xpMS = 0.70. As expected, the Tg values increase by increasing the pMS content. The DSC curve of the pMS-B copolymers by 1/MAO, in which the pMS concentration is lower than 50 mol %, shows two Tg at −89 and 66 °C as expected for a phase-separated polymeric material. The Tg value of the PB domains is very close to that of poly(butadiene), whereas that of the styrenic domains is significantly lower than that of syndiotactic poly(pMS) expected at 105 °C.4b The Tg at 66 °C does not change in the range of pMS mole fraction 0.19−0.33 and suggests a partial inclusion of the butadiene units which softens the styrenic domains. When the mole fractions of the two monomers are comparable, both the Tg traces broaden and decrease in temperature until to be practically undetectable in the case of the PB domains. As the pMS mole fraction is increased to 0.66 or higher, the Tg of the styrenic domains slightly increased to 69 °C, remaining still significantly lower than that of poly(p-methylstyrene), in agreement with the short block length of this monomer in the pMS−butadiene copolymers. It is worth noting that the

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RESULTS AND DISCUSSION Polymerization of p-Methylstyrene Catalyzed by 1/MAO. The performance of 1/MAO in p-methylstyrene polymerization was explored under different experimental conditions: the main results are summarized in Table 1 where data are compared with those of styrene under the same experimental conditions. Table 1. p-Methylstyrene Homopolymerization Catalyzed by 1/MAO entrya

monomer

T (°C)

Al/Ti

yields

conv (%)

Mn × 103

Mw/Mn

1 2 3 4 5 6

pMS pMS pMS pMS pMS S

50 50 50 30 70 50

500 1000 1500 1000 1000 1000

0.865 1.334 2.301 0.247 4.335 1.927

19 30 51 5 96 42

22 22 21 32 5

2.0 1.9 1.8 2.1 2.4

Experimental conditions: 11 mg of precatalyst (22 μmol, 5.5 × 10−4 M), MAO (10 wt % toluene solution), 5.0 mL of monomer, toluene (35 mL), 50 °C; polymerization time 10 min.

a

The monomer conversion increased with both polymerization temperature and MAO/1 molar ratio and resulted quantitative in 10 min at 70 °C when using 1000 equiv of MAO. The head-to-head comparison of the entries 2 and 6 shows that the productivity of the catalyst is comparable to that observed in styrene polymerization. The distribution curve of the average molecular weights of the poly(p-methylstyrene) samples is monomodal, and the polydispersity indexes (PDI = Mw/Mn) are very close to those found for single site catalysts. The 13C NMR analysis of the poly(p-methylstyrene)s showed that 1/MAO is highly syndiospecific as testified by

Table 2. p-Methylstyrene−Butadiene Copolymerizations by 1/MAO compositionb (mol %)

yield a

Tg (°C)

entry

[pMS] (M)

[pMS]/[B]

(g)

(%)

pMS

B1,4

B1,2

Mw (×10 Da)

Mw/Mn

nSc

8 9 10 11 12 13 14

0.19 0.38 0.75 1.12 1.50 1.88 3.75

0.5 1 2 3 4 5 10

0.91 0.92 1.25 1.82 2.65 1.59 2.68

54 35 28 29 33 16 14

19.3 19.7 33.2 48.5 57.6 66.8 69.5

70.3 67.7 57.3 45.0 36.5 29.0 26.6

10.4 12.6 9.5 6.9 5.9 4.2 3.9

61 63 43 51 57 37 59

3.8 3.6 2.8 2.9 3.0 3.3 3.0

3.7 n.d.d 6.5 10.7 13.9 25.6 n.d.d

3

PB

sP(pMS)

−89 −90 −78 −90

65 66 66 62 61 68 69

Polymerization conditions: 11 mg of complex (22 μmol, 5.5 × 10−4 M), 0.81 g of butadiene (0.015 mol, 0.38 M), 21 mL of MAO (Al/Ti = 1500), 40 mL of toluene, 50 °C, polymerization time: 1 h. bCopolymer composition determined by 1H NMR analysis. cAverage styrene block lengths, see Supporting Information for details. dNot determined.

a

C

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range of compositions (xpMS = 0.29−0.93; Table 3) Notably, the catalyst 2 is more active than 1 at room temperature. The molecular weight distributions are monomodal with PDI values close to 2, and all the polymers are solid soluble in the most common organic solvents. The Tg values increased as the pMS concentration was increased in the copolymers and reached a value of 98 °C, close to that of the isotactic poly(p-methylstyrene),4b when xpMS is 0.94 (see Figure S9 in the SI). Notably, the presence of pMS along the polymer chain causes an increase of the Tg compared to the corresponding styrene−butadiene copolymers obtained in the presence of the same catalytic system.9a As expected from the stereospecificity properties of the catalyst 1 and 2, the microstructure of the p-methylstyrene− butadiene copolymers resulting from the two catalysts are completely different. Actually, catalyst 1 is syndiospecific in styrene polymerization and exhibits cis-1,4-selectivity in butadiene polymerization, whereas the catalyst 2 is isospecific in styrene polymerization and trans-1,4-selective in butadiene polymerization. For both the catalysts such stereoselectivity is retained when the two monomers, namely p-methylstyrene and butadiene, are copolymerized as envisaged by inspection of the 13 C NMR spectra of the copolymers. Considering that the methyl group in para-position does not significantly affect the 13C NMR chemical shifts of the carbon atoms in the main chain,10d−10f the attributions of the 13C signals were performed using the previously reported data for the styrene−butadiene copolymers.8−9a As a matter of fact, the 13C NMR spectrum of the pMS-B copolymers by catalyst 1 (Figure 2, Figures S1 and S2 in the SI) exhibits signals attributed to the CCC (C = cis-1,4-butadiene) and SSS (S = p-methylstyrene; see Scheme 2 and Table 4) triads as the most intense revealing that butadiene homosequences are primarily cis-1,4 and the styrene homosequences are in syndiotactic arrangement. Conversely, the pMS−B copolymers by catalyst 2 produce signals due to isotactic styrene triad SSS and TT (T = trans-1,4-butadiene) corresponding to trans-1,4butadiene dyad, respectively (see Scheme 2, Table 5, Figure 3, Figures S5 and S6 in the SI). In detail, the 13C NMR spectrum of the pMS−butadiene copolymers by catalyst 1 showed pMS homosequences in syndiotactic arrangement with diagnostic signals for SS1S and SS2S at 40.4 and 44.1 ppm, respectively, and for CC1C/CC4C at 27.6 ppm (Table 4). Isolated 1,2vinylbutadiene (denoted with V, see Scheme 2) units in the range 5−9 mol % were also identified (Figure 2, Table 4). Isolated trans-1,4-butadiene units are incorporated exclusively in the CTC heterosequence and detected in amount

related styrene−butadiene copolymers prepared in the presence of the same catalytic system show a melting point due to the sPS blocks while in the case of pMS-butadiene only the glass transition temperatures for both blocks are observed. The WAXD analysis14 of the samples 11−14 (Table 2) showed the presence of the crystalline form I of syndiotactic poly-pMS block; however, the absence of endo peaks attributed to the melting of this form (Figure 1d−g) is probably due to the

Figure 1. DSC traces of multiblock copolymers pMS-B synthesized with the catalytic system 1 reported in Table 2 of entry: (a) 8, (b) 9, (c) 10, (d) 11, (e) 12, (f) 13, and (g) 14.

formation of crystalline clathrate forms obtained in presence of toluene as polymerization solvent.15 The copolymerization of pMS with B was also performed using the 2/MAO catalyst obtaining copolymers in a wide

Table 3. p-Methylstyrene−Butadiene Copolymerization by 2/MAO compositionb (mol %)

yield entry 15 16 17 18 19 20 21

a

[pMS] (M)

[pMS]/[B]

(g)

(%)

pMS

tB1,4

cB1,4

Mw × (103 Da)

Mw/Mn

nSc

Tg (°C)

0.04 0.07 0.19 0.38 0.75 1.12 3.75

0.1 0.2 0.5 1 2 3 10

0.34 0.75 0.87 2.03 3.13 4.43 18.2

34.7 65.0 51.2 78.5 71.9 72.2 98.3

28.6 35.4 48.7 62.7 77.3 83.8 93.6

67.5 60.8 47.9 36.2 17.8 16.2 6.4

3.9 3.8 3.4 1.1 4.9

21 32 33 45 123 128 270

1.8 2.1 1.7 1.7 2.2 1.7 2.6

1.9 2.0 2.9 3.2 3.7 6.5 7.2

−23 −11 −0.3 23 34 51 98

a Polymerization conditions: 14 mg of complex (22 μmol, 5.6 × 10−4 M), 0.81 g of butadiene (0.015 mol, 0.38 M), 14.6 mL of MAO (Al/Ti = 1000), 40 mL of toluene, 25 °C polymerization time: 1 h. bCopolymer composition determined by 1H NMR analysis. cAverage styrene block lengths; see Supporting Information for details.

D

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Table 4. 13C NMR Chemical Shift Determined for the Monomer Triads in the pMS−B Copolymer Produced in the Presence of 1/MAO chemical shift (ppm)

Figure 2. Aliphatic region of the 13C NMR solution spectrum (CDCl3) of pMS-co-butadiene polymers obtained in the presence of 1/MAO (entry 10, Table 2). The signals are labeled according to the numbering and symbols used in Table 4; the signal marked with Me at 21.2 ppm is the methyl in para-position of the aromatic ring.

Scheme 2

a

line

sequence

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

CS2C SS1S CV2C CS2S SS2V SV2C CV2S SS2S CST1 CVT1 CS1S SCV1 CS1C CV1S SV1C CV1C VC1V SSC1 CC1C/CC4C C4SC CC4S CC4VC

S−Bb

pMS−B

45.7 43.9 43.7 42.8 40.1−41.6c 40.1−41.6c 40.1−41.6c 40.6 40.1 38.2 37.4 35.8 35.7 35.9−36.1 33.1−33.9 34.3 33.4 32.5 27.4 25.1 25.0 24.9

45.5 44.1 44.0 42.7 41.9 41.6 41.1 40.4 40.0 38.4 37.6 36.2 36.0 35.9 35.0 34.5 33.7 32.9 27.6 25.5 25.3 25.2

a

The numbering and symbols are those indicated in Scheme 2a; the styrene unit is inserted in the triad with the methine atom on the right as shown for the TST triad in Scheme 2b. bChemical shift values of the butadiene−styrene copolymer obtained with same catalytic system.8b cCalculated by other authors.16

comparable to that observed in the homopolymer (4−13 mol %) (see Table 4 for the complete assignments). The average poly-pMS block lengths (ns) were thus calculated using the 13C NMR assignments and the method previously reported (see the Supporting Information).7e Styrene block lengths of about 4 up to 25 were found in the copolymers with styrene molar fraction in the range xpMS = 0.19−070 (entries 8−14 in Table 2). 13 C NMR analysis of the copolymers by 2-MAO shows that the p-methylstyrene-co-butadiene polymers have a different microstructure. As a matter of fact, the chemo- and stereoselectivity of this catalyst lead to isotactic p-methylstyrene homosequences SSS with diagnostic signals SS1S = 43.1 ppm and SS2S = 40.4 ppm; the butadiene units are mainly enchained as trans-1,4-butenyl units (T4T/TT1 = 32.9) with a lower amount of cis-units (1−5%). The aliphatic region of the spectrum of the copolymer with xpMS = 0.49 is shown in Figure 3, and the relative assignments are reported in Table 5. Notably, in this case the average poly-pMS block lengths calculated using the 13C NMR assignments show styrene block lengths of 2 up to 7 in the copolymers with styrene molar fraction in the range xpMS = 0.29−0.93, resulting in random rather than a blocky copolymer as in the case of the catalyst 1. p-Methylstyrene−Isoprene Copolymerization. The unique behavior of such catalytic systems in the copolymerization of dienes and styrene and the very few examples, in the scientific literature, of catalytic systems able to promote the binary copolymerization of styrenic monomers with isoprene17 prompted us to investigate the behavior of pMS and isoprene copolymerization catalyzed by 1 and 2. In Tables 6 and 7, the

Table 5. 13C NMR Chemical Shift Determined for the Monomer Triads in the pMS−B Copolymer Produced in the Presence of 2/MAO chemical shift (ppm) a

line

sequence

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

TS2T CS2T SS1T SS1S SS2T TS2S SS2S TST1 CST1 STS1 TS1S T4T/TT1 T4ST ST4S T4SS C4T/CT1 C4S

S−Bb

pMS−B

45.9 45.6 43.7 43.2 43.4 42.5 40.9 40.3 39.6 36.0 35.4 33.0 30.6 30.5 30.2 27.6 25.4

45.4 45.1 44.0 43.6 43.1 42.0 40.3 39.6 39.1 35.9 35.4 32.9 30.7 30.5 30.3 27.6 25.4

a

The numbering and symbols are those indicated in Scheme 2a; the styrene unit is inserted in the triad with the methine atom on the right as shown for the TST triad in Scheme 2b. bChemical shift values of the butadiene−styrene copolymer obtained with same catalytic system.9a

results of pMS−isoprene copolymerization in the presence of the catalysts 1 and 2 are reported, respectively. E

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(entry 22, Table 6; entry 34, Table 7) copolymers with a very different composition were obtained, namely xpMS = 0.19 for and xpMS = 0.92 for the catalyst 1 and 2, respectively. In analogy to pMS-B copolymers obtained with the same catalytic system, the Tg increases with pMS content (see Figure S10), raising 57 °C in the case of the copolymer containing 92.1% of pMS (entry 34, Table 7). A deeper insight into the microstructural features of the pMS-co-I polymers came also in this case by a detailed 13C NMR analysis of the copolymers and by the comparison with the corresponding styrene-co-isoprene polymers. In Figure 4, the spectrum of the aliphatic region of the copolymer with xpMS = 0.40 obtained in the presence of the catalyst 1/MAO (entry 24, Table 6) is shown.

Figure 3. Aliphatic region of the 13C NMR solution spectrum (CDCl3) of pMS-co-butadiene polymers obtained in the presence of 2/MAO (entry 17, Table 3). The signals are labeled according to the numbering and symbols used in Table 5; the signal marked with Me at 21.2 ppm is the methyl in the para-position of the aromatic ring.

As in the case of pMS-butadiene copolymerization, a wide range of composition has been obtained (xpMS = 0.19−0.69), and for all polymers the molecular weights distributions are monomodal with a PDI values close to 2, consistent with the material being copolymeric in nature. All copolymers are completely soluble in hexane, THF, or chloroform, also when pMS concentration approaches high values as xpMS = 0.69. As expected, the Tg values increase by increasing the pMS content (see Table 6 and Figure S8), reaching the value of 25 °C for the copolymer containing the highest pMS content (68.7 mol %S; entry 27, Table 6). However, in this case only one value of Tg was observed probably due to the presence of shorter syndiotactic poly-pMS segments that cannot give rise to a separated polymeric phase. Furthermore, the ability of the catalyst 2 in the copolymerization of pMS with isoprene was also explored. The results are reported in Table 7. As in the case of catalyst 1, a wide range of composition could be synthesized (xpMS = 0.20−0.92). Differently from the catalyst 1, the copolymers obtained in the presence of the catalyst 2 show a drastic increase of the molecular weight by increasing the [pMS]/[I] ratio in the feed, and therefore the incorporation of p-methylstyrene into the polymer chain is higher. As for catalyst 1, the molecular weight distribution is rather narrow (1.2−1.7), confirming the copolymeric nature of the material. It is worth noting that the catalyst 2 vis-á-vis the catalyst 1 is, probably because of the different stereoselectivity, less reactive toward isoprene and requires very low [pMS]/[I] ratio for higher isoprene incorporation. Under the same reaction conditions and with the same [pMS]/[I] = 1 in the feed

Figure 4. Aliphatic region of the 13C NMR solution spectrum (CDCl3) of pMS-co-isoprene polymers obtained in the presence of 1/MAO (entry 24, Table 6). The signals are labeled according to the numbering and symbols used in Table 8. The signals marked with asterisk are probably due to chain end groups.

It is worth noting that also in the case of the pMS−isoprene copolymerization the diene monomer is polymerized with both good 1,4-chemoselectivity (see Table 6) and stereoselectivity (90% cis-units, entry 24, Table 6) as qualitatively evident from the intensity of the methyl signals of the different isoprene units (peaks 58, 59, and 60 in Figure 4). A complete assignment was made by comparison with the styrene-co-isoprene copolymers made with the same catalytic system, and the results are reported in Table 8. As for the styrene−isoprene copolymers the isoprene with trans stereochemistry is present only as isolated units along the chain backbone, and the presence of the narrow signal at 44.1 ppm and at 40.8 ppm of respectively the methylene SS1S and methine SS2S of the triads relative to poly(p-methylstyrene)

Table 6. p-Methylstyrene−Isoprene Copolymerization by 1/MAO compositionb (mol %)

yield entrya

[pMS] (M)

[pMS]/[I]

(g)

(%)

pMS

I1,4

I3,4

Mw (×103 Da)

Mw/Mn

nsc

Tg (°C)

22 23 24 25 26 27

0.50 0.75 1.00 1.25 1.50 1.75

1.0 1.5 2.0 2.5 3.0 3.5

0.69 0.84 0.90 1.09 1.26 1.28

18 17 15 15 15 13

19.4 36.0 40.0 56.3 59.6 68.7

75.3 60.0 56.4 40.3 37.8 28.5

5.3 4.0 3.6 3.4 2.8 2.6

61 55 53 56 42 63

1.8 2.0 2.2 2.5 2.2 2.3

3.5 n.d.d 4.4 6.5 6.1 7.4

−30 −10 n.r.e 19 21 25

a Polymerization conditions: 11 mg of complex (22 μmol, 5.5 × 10−4 M), 1.36 g of isoprene (0.020 mol, 0.50 M), 21 mL of MAO (Al/Ti = 1500), 40 mL of toluene, polymerization time: 1 h. bCopolymer composition determined by 1H NMR analysis. cAverage styrene block lengths; see Supporting Information for details. dNot determined. eNot resolved.

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Table 7. p-Methylstyrene−Isoprene Copolymerization by 2/MAO compositionb (mol %)

conversion entry 28 29 30 31 32 33d 34d

a

[pMS] (M)

[pMS]/[I]

(g)

(%)

pMS

I1.4

I3.4

Mw (×103 Da)

Mw/Mn

ns c

Tg (°C)

0.01 0.02 0.03 0.05 0.10 0.25 0.50

0.02 0.03 0.05 0.1 0.2 0.5 1.0

0.06 0.20 0.38 0.55 0.88 2.59 1.54

2.0 6.9 12.8 17.2 24.1 50.9 41.4

20.1 29.3 48.2 55.3 67.7 81.5 92.1

67.5 69.2 48.9 44.4 30.9 18.5 7.9

12.4 1.5 2.9 0.3 1.3

14 12 18 23 35 64 81

1.7 1.5 1.3 1.3 1.3 1.4 1.2

1.2 1.5 2.2 2.5 3.6 12 45

−26 0.7 7.8 46 53 57

Polymerization conditions: 27 mg of complex (44 μmol, 5.5 × 10−4 M), 2.72 g of isoprene (0.040 mol, 0.50 M), 29.1 mL of MAO (Al/Ti = 1000), 80 mL of toluene, polymerization time: 1 h, temperature: 50 °C. bCopolymer composition determined by 1H NMR analysis. cAverage styrene block lengths; see Supporting Information for details. dPolymerization conditions: 14 mg of complex (22 μmol, 5.5 × 10−4 M), 1.36 g of isoprene (0.020 mol, 0.50 M), 14.6 mL of MAO (Al/Ti = 1000), 40 mL of toluene, polymerization time: 1 h, temperature: 25 °C.

a

Table 8. 13C NMR Chemical Shift Determined for the Monomer Triads in the pMS−I Copolymer Produced in the Presence of 1/MAO chemical shift (ppm) line

sequencea

S−Ib

pMS−I

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

VS1V CV3C TV3C CS2C VS1C′ SS1S CS1S, C′S1S CS2S SS2S CT1 C1′S SS1C C4SC/T4SC CS1C CC1C C′C1 SC1C/SC1S/VC1 C4C′ CC4C SC4′ C5 V5 T5

48.4 48.2 46.8 46.6 44.8 44.1 43.9 43.4 40.4 40.2 39.4 36.0 35.6−35.8 34.4 32.4 30.8 30.1 28.6 26.6 25.9 23.7 18.8 16.2

48.5 48.1 46.2 45.3 44.9 44.1 43.9 43.7 40.8 40.2 39.3 36.1 35.6−35.8 34.4 32.4 30.8 30.2 28.6 26.6 26.0 23.7 18.8 16.2

Figure 5. Aliphatic region of the 13C NMR solution spectrum (CDCl3) of pMS-co-isoprene polymer obtained in the presence of 2/MAO (entry 31, Table 7). The signals are labeled according to the numbering and symbols used in Table 9; the signal marked with Me at 21.2 ppm is the methyl in para-position of the aromatic ring. The signal marked with an asterisk is relative to residual solvent (toluene).

Table 9. 13C NMR Chemical Shifts for the Monomer Triads in the pMS-I Copolymer by 2/MAO chemical shift (ppm)

a

The numbering and symbols are those indicated in Scheme 3a; the styrene unit is inserted in the triad with the methine atom on the left as shown for the TST triad in Scheme 3b. bChemical shift values of the isoprene−styrene copolymer obtained with same catalytic system.8a

homosequences confirms the syndiotactic arrangement of the pMS along the polymer chain. In Figure 5, the aliphatic portion of the 13C spectrum of the pMS-I copolymer obtained in the presence of the catalytic system 2/MAO is shown. Remarkably, in this case it is evident from a first inspection of the spectrum that isoprene is incorporated along the polymeric chain mainly with a trans-1,4-stereochemistry while the 3,4 and cis-1,4 units are in lower amount. In particular, the only signals detectable for isoprene cis-1,4 units and 3,4-units are those relative to the methyl of the isoprene unit respectively at 23.7 and 18.8 ppm, indicating

line

sequencea

S−Ic

pMS-I

61/62 63/64 65 66 67 68 69 70 71 72 73/74 75 76 77 78

TS2Tb TS2Sb SS1S SS2T SS2S TT1 TST1 SST1 TS1T SS1T ST4S/T4SS T4T C5 V5 T5

45.8 43.5 43.2 42.5 40.9 40.0 38.2 38.2 35.4 35.0 34.1 26.9 23.7d 18.8d 16.2d

45.7−45.4 44.1−43.6 43.1 42.3 40.4 39.9 37.8 37.6 35.7 34.5 34.2−34.0 27.0 23.7 18.7 16.2

a

The numbering and symbols are those indicated in Scheme 3a; the styrene unit is inserted in the triad with the methine atom on the left as shown for the TST triad in Scheme 3b. bThese signals are more complex in the 13C NMR spectrum probably because the resolution is at pentad level. cChemical shift values of the calculated isoprene− styrene copolymer.18 dChemical shift observed in the polyisoprene obtained with same catalytic system. G

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Scheme 3

Article

ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR, DSC, and WAXD of the copolymers and the details of the method for evaluation of the average pMS block lengths (nS). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax +39089-969603; e-mail [email protected] (C.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS For the financial support of this research the Universita′ degli Studi di Salerno (FARB 2012) and Ministero dell’Universita′ e della Ricerca Scientifica (MIUR, Roma − Italy; PRIN-2010: “Materiali Polimerici Nanostrutturati con strutture molecolari e cristalline mirate, per tecnologie avanzate e per l’ambiente.”) are gratefully acknowledged.

that these units are isolated along the polymer backbone. Furthermore, the methine and methylene carbon atoms of the pMS homosequences, SS1S and SS2S, are observed at 43.1 and 40.4 ppm, indicating an isotactic arrangement. The complete attribution of the signals in the 13C NMR spectrum is reported in Table 9. It is worth noting that, in analogy to pMS−butadiene copolymers, also in this case both systems give the corresponding copolymers with a wide range of compositions and furthermore also in this case the microstructural features of the resulting copolymers are strongly influenced by the catalytic system used, showing again an opposite stereoselectivity for both the monomers. Notably, in this case the average p-methylstyrene block lengths for copolymers obtained in the presence of catalysts 1 and 2 but with similar p-methylstyrene content (56.3%, entry 25, Table 6; and 55.3%, entry 31, Table 7) are respectively of 6.5 and 2.5, revealing still a stronger tendency of catalyst 1 to form longer pMS blocks compared to catalyst 2 but considerably shorter compared to pMS−B copolymerization with the same catalytic system.

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REFERENCES

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CONCLUSIONS In this work we have reported the binary copolymerization of p-methylstyrene with butadiene and isoprene promoted by two catalytic systems based on titanium complexes activated by methylaluminoxane. Notably both catalysts are able to promote the copolymerization of p-methylstyrene with butadiene and isoprene, giving the resulting copolymers with a wide range of compositions and in good yield. Furthermore, the complete characterization by 13C NMR of the resulting new copolymer showed that is possible to obtain copolymers with completely microstructural features by the rational choice of the catalytic system. As a matter of fact, on one hand the copolymers obtained by using the catalyst 1 show a high stereoselectivity for p-methylstyrene, butadiene, and isoprene, giving a polymer chain in which the styrenic monomer homosequences are syndiotactic and the diene monomers are inserted preferentially with 1,4-cis selectivity. On the other hand, the catalyst 2 gives copolymers with isotactic p-methylstyrene homosequences and the diene monomers preferentially with 1,4-trans selectivity. These results show not only the versatility of these catalysts in the copolymerization reaction with styrenic and diene monomers but also, notably, how it is possible to obtain materials with complete different microstructural features just by changing the ancillary ligand around the metal center. H

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I

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