Stereoselective copolymerization of styrene with terpenes catalyzed by an ansa-lanthanidocene catalyst: Access to new syndiotactic polystyrene-based materials

: The copolymerization of bio-renewable β -myrcene or β -farnesene with styrene was examined using an ansa -neodymocene catalyst, affording two series of copolymers with high styrene content and unprecedented syndioregularity of the polystyrene sequences. The incorporation of terpene in the copolymers ranged from 5.6 to 30.8 mol % ( β -myrcene) and from 2.5 to 9.8 mol % ( β -farnesene), respectively. NMR spectroscopy and DSC analyses suggested that the microstructure of the copolymers consists of 1,4- and 3,4-poly(terpene) units randomly distributed along syndiotactic polystyrene chains. The thermal properties of the copolymers are strongly dependent on the terpene content, which is easily controlled by the initial feed. The terpolymerization of styrene with β -myrcene in the presence of ethylene was also examined.


Introduction
The sustainability and possible eco-compatibility of bio-based polymers has raised much interest in finding inexpensive alternatives to petroleum-sourced materials.Among the wide variety of renewable monomers, terpenes are particularly interesting because of their abundance in nature, and their reactive conjugated 1,3-diene framework.For instance, β-myrcene, or to a lesser extent β-farnesene, can polymerize via anionic or radical polymerization to form elastomeric materials [1].In a few rare cases, the coordination (catalytic) polymerization of such monomers has been reported [2][3][4][5][6].On the other hand, syndiotactic polystyrene (sPS) is a very attractive material due to its high melting point (ca.270 • C), high crystallinity, low dielectric constant and good chemical and heat resistances [7,8].However, its processability is limited by its high melting point and its brittleness.Syndioselective copolymerization of styrene with small amounts of a second monomer is one of the most common ways to tune the properties of sPS, and thus to improve its processability [9][10][11].A few examples of styrene-myrcene polymers have already been reported, mostly via anionic or radical copolymerizations [12][13][14].To our knowledge, only one paper has addressed the coordination copolymerization of styrene with β-myrcene (My) in the presence of the ternary system Cp * La(BH 4 ) 2 (THF) 2 /nBuEtMg/AliBu 3 , affording poly(S-co-1,4-trans-My) containing high amounts of myrcene (66-96 mol %) [15].Yet, nothing has been reported about styrene-myrcene copolymers containing stereoregular PS sequences, nor about the copolymerization of styrene with β-farnesene.

Copolymerizations of Styrene with β-Myrcene and Farnesene
A series of styrene/β-myrcene and styrene/farnesene copolymerizations catalyzed by 1 were performed (Scheme 1, Table 1).Scheme 1. Styrene/β-myrcene and styrene/farnesene copolymerizations catalyzed by complex 1 (Table 1, entries 4-10 and entries [13][14][15][16][17]. As judged by the relatively narrow monomodal molecular weight distributions (ÐM = 1.3-2.7),true copolymers were produced.Depending on the quantity of incorporated β-myrcene, the resulting copolymers were either white powders or transparent hard elastomeric materials.On the other hand, all poly(S-co-Fa) samples were white powders.The catalyst productivity decreased with an increase of the [terpene]0/[St]0 feed ratio (from 127 to 6.4 kg•mol −1 •h −1 and from 198 to 28 As judged by the relatively narrow monomodal molecular weight distributions (Ð M = 1.3-2.7),true copolymers were produced.Depending on the quantity of incorporated β-myrcene, the resulting copolymers were either white powders or transparent hard elastomeric materials.On the other hand, all poly(S-co-Fa) samples were white powders.The catalyst productivity decreased with an increase of the [terpene] 0 /[St] 0 feed ratio (from 127 to 6.4 kg•mol −1 •h −1 and from 198 to 28 kg•mol −1 •h −1 in the cases of styrene/β-myrcene and styrene/farnesene copolymerization, respectively, entries 4-10 and entries [13][14][15][16][17]; this is consistent with the fact that 1 is poorly active towards β-myrcene and farnesene homopolymerization (entries 1-2 and 11-12).On the other hand, as the [terpene] 0 /[St] 0 feed ratio was raised, the terpene content in the resulting copolymers increased as expected; thus, a wide range of compositions was obtained (My content = 5.6-30.8mol % and Fa content = 1.5-9.8mol %).It should be noted that, under similar copolymerization conditions, the incorporation of farnesene in the copolymer is 2-3 times lower than the incorporation of β-myrcene (for instance, compare entries 4 and 13 or 10 and 17).This difference is due to the fact that commercially available farnesene is a mixture of α-and β-isomers (ca.75:25 mixture of α/β; see Figure S21); however, only the β-isomer was incorporated, as evidenced from the 1 H NMR data (see the Supporting Information, Figure S12), whereas the initial ratio [Fa] 0 /[St] 0 was calculated considering the entire amount of farnesene (i.e., both α-and β-isomers).We assume that the trisubstituted C 3 =C bond in the α-isomer, as compared to the disubstituted C 3 =C one in the β-isomer, most likely accounts for lower reactivity of the α-isomer (selective polymerization of β-farnesene was already reported in the literature; see [6]).In addition, an increase in polymerization temperature logically induced an increase in productivity (6 times higher when going from 60 • C to 120 • C in the case of styrene/β-myrcene copolymerization), but also an increase in the amount of terpene incorporated in the copolymer (from 10.9 to 19.7 mol % and from 2.0 to 3.4 mol % for β-myrcene and β-farnesene, respectively; entries 6 and 7 and entries 13-14, respectively).
The reactivity ratios of r ST = 2.06 and r MY = 0.37 were determined according to the Fineman−Ross equation (Supporting Information, Figure S8) [20].These values indicate a preference for the insertion of styrene, regardless of the last inserted monomer unit, consistent with the fact that 1 is 3 orders of magnitude more productive in catalyzing styrene homopolymerization than in catalyzing β-myrcene homopolymerization (entries 1 and 3) [21].
Microstructures of the copolymers were examined by 13 C{ 1 H} NMR spectroscopy (Figure 2).Single and relatively sharp signals in the ipso carbon (δ 145.2 ppm) and methylene (δ 43.8 ppm) regions of the spectra are indicative of the presence of poly(S-co-My) and poly(S-co-Fa) with sPS sequences.These signals broadened with an increase of terpene content, due to the increasing presence of styrene-terpene junctions (see Figure 3 for poly(S-co-My) copolymers and Supporting Information, Figure S14, for poly(S-co-Fa) copolymers).Note that the spectra were recorded using a regular NMR sequence that does not necessarily return quantitative signals; thus, the syndiotacticity content was not quantified.In addition, the presence of both 1,4-and 3,4-units for both copolymers confirms that complex 1 is not regioselective towards β-myrcene or β-farnesene.The non-regioselectivity of 1 in those St/My and St/Fa copolymerizations contrasts with styrene/isoprene copolymers containing regular trans-1,4-polyisoprene units obtained in the presence of {CpCMe 2 Flu}Nd(C 3 H 5 )(THF) [17].We assume that this arises from the more sterically hindered C 3 =C bond in the higher β-myrcene and β-farnesene monomers and/or from the presence of 2,7-tert-butyl substituents on the fluorenyl moiety of catalyst 1.The presence of a unique T g for all co-and terpolymers suggests a random distribution of comonomers within sPS segments (consistent with the aforementioned broadening of 13 C NMR signals due to increasing presence of styrene-terpene junctions).
As expected, properties of the copolymers were dependent on their composition, which is easily tunable by adjusting the monomer feed ratio.The T g values significantly decreased with the increase of β-myrcene and β-farnesene contents (Figure 4), and poly(S-co-My) materials incorporating between 10 and 20 mol % of β-myrcene units were completely amorphous.As compared with isoprene-based copolymers [17], a wider range of thermal properties is potentially available using these terpenes as comonomers.As shown in Figure 4, the T g values of the three copolymers were similar at low comonomer incorporation levels (T g = 70-77 • C for 3-9 mol % of comonomer inserted).For higher amounts of comonomer incorporated (15-30 mol %), the T g values of poly(S-co-My) copolymers monotonously decreased, while those of poly(S-co-1,4-trans-IP) reached a plateau at ca. 60 • C.     As expected, properties of the copolymers were dependent on their composition, which is easily tunable by adjusting the monomer feed ratio.The Tg values significantly decreased with the increase of β-myrcene and β-farnesene contents (Figure 4), and poly(S-co-My) materials incorporating between 10 and 20 mol % of β-myrcene units were completely amorphous.As compared with isoprene-based copolymers [17], a wider range of thermal properties is potentially available using these terpenes as comonomers.As shown in Figure 4, the Tg values of the three copolymers were similar at low comonomer incorporation levels (Tg = 70-77 °C for 3-9 mol % of comonomer inserted).For higher amounts of comonomer incorporated (15-30 mol %), the Tg values of poly(S-co-My) copolymers monotonously decreased, while those of poly(S-co-1,4-trans-IP) reached a plateau at ca. 60 °C.

Terpolymerizations of Styrene with β-Myrcene and Ethylene
A series of styrene/β-myrcene/ethylene terpolymerizations was also performed under the same conditions (Scheme 2, Table 2).The remarkably narrow and monomodal molecular weight distributions suggested the formation of true terpolymers (Ð M = 1.3-1.6).Complex 1 was moderately productive for styrene/β-myrcene/ethylene terpolymerizations (64-250 kg•mol −1 •h −1 ), and the productivity decreased with an increase in the [My] 0 /[St] 0 feed ratio.However, it is noteworthy that these productivities were significantly higher than those obtained for styrene/β-myrcene copolymerizations under the same conditions (6-127 kg•mol −1 •h −1 , compare Tables 1 and 2).This difference in productivity between co-and terpolymerizations is even more pronounced with the increase of [My] 0 /[St] 0 , as productivity values were 2-10 fold larger in terpolymerization than in the corresponding copolymerization.This can be accounted for by the fact that ethylene is a smaller, less sterically demanding monomer than styrene or β-myrcene and, thus, is more easily inserted.Hence, ethylene acts as a "spacer" that facilitates subsequent insertions of styrene or β-myrcene monomer, resulting in higher overall productivities than in copolymerizations without ethylene [22][23][24].Under the given conditions, the level of β-myrcene incorporated in terpolymers was equivalent to that observed for styrene/β-myrcene copolymers.In addition, the amount of ethylene incorporated in terpolymers increased (from 18.8 up to 29.2 mol %) with an increase in β-myrcene content.[17] copolymers as a function of diene (terpene) comonomer content (Table 1, entries 3,4,6,8,10,13,15,16).

Terpolymerizations of Styrene with β-Myrcene and Ethylene
A series of styrene/β-myrcene/ethylene terpolymerizations was also performed under the same conditions (Scheme 2, Table 2).The remarkably narrow and monomodal molecular weight distributions suggested the formation of true terpolymers (ÐM = 1.3-1.6).Complex 1 was moderately productive for styrene/β-myrcene/ethylene terpolymerizations (64-250 kg•mol −1 •h −1 ), and the productivity decreased with an increase in the [My]0/[St]0 feed ratio.However, it is noteworthy that these productivities were significantly higher than those obtained for styrene/β-myrcene copolymerizations under the same conditions (6-127 kg•mol −1 •h −1 , compare Tables 1 and 2).This difference in productivity between co-and terpolymerizations is even more pronounced with the increase of [My]0/[St]0, as productivity values were 2-10 fold larger in terpolymerization than in the corresponding copolymerization.This can be accounted for by the fact that ethylene is a smaller, less sterically demanding monomer than styrene or β-myrcene and, thus, is more easily inserted.Hence, ethylene acts as a "spacer" that facilitates subsequent insertions of styrene or β-myrcene monomer, resulting in higher overall productivities than in copolymerizations without ethylene [22][23][24].Under the given conditions, the level of β-myrcene incorporated in terpolymers was equivalent to that observed for styrene/β-myrcene copolymers.In addition, the amount of ethylene incorporated in terpolymers increased (from 18.8 up to 29.2 mol %) with an increase in β-myrcene content.Scheme 2. Styrene/β-myrcene/ethylene terpolymerizations catalyzed by 1. Scheme 2. Styrene/β-myrcene/ethylene terpolymerizations catalyzed by 1. Table 2. Styrene/β-myrcene/ethylene terpolymerizations catalyzed by 1 a .The poly(S-co-E-co-My) terpolymers were also characterized by 13 C{ 1 H} NMR spectroscopy (Supporting Information, Figure S18).As previously observed for styrene/β-myrcene copolymers, PS sequences in the terpolymers were predominantly syndiotactic (see the ipso carbon signal at δ 145.2 ppm) and the myrcene units were present as both 1,4-and 3,4-units (for instance, see the resonances at δ 150.9 and 138.6-136.7 ppm assigned to the C2 atom of the backbone of 3,4-and 1,4-insertions, respectively).Again, the T g values were significantly lower in poly(S-co-E-co-My) terpolymers than in poly(S-co-E-co-1,4-trans-IP) terpolymers (at a given ethylene incorporation level, ca.20 mol %) [17].The T g values were also lower in poly(S-co-E-co-My) terpolymers than in poly(S-co-My) copolymers containing similar β-myrcene incorporation contents.
Instruments and measurements. 1H and 13 C{ 1 H} NMR spectra of co-and terpolymers were recorded on a Bruker AM-500 spectrometer (1,1,2,2-tetrachloroethane-d 2 , 60 • C, Bruker, Wissembourg, France).GPC analyses of β-myrcene-based polymers were performed in THF at 30 • C using PS standards for calibration.GPC analyses of farnesene-based polymers were performed in 1,2,4-trichlorobenzene at 135 • C using PS standards for calibration.Differential scanning calorimetry analyses were performed on a Setaram DSC 131 apparatus (SETARAM Instrumentation, Cranbury, NJ, USA), under continuous flow of helium and using aluminum capsules.Crystallization temperatures were measured during the first cooling (10 • C.min −1 ), and glass transition and melting temperatures were measured during the second heating (10 • C.min −1 ).
Typical procedure for styrene-terpene copolymerization.In a typical experiment (Table 1, entry 4), complex 1 (ca.8 mg), cyclohexane (7.6 mL) and (nBu) 2 Mg (0.1 mL of a 1.0 M solution in hexanes) were introduced in the glovebox in a Schlenk flask.The tube was capped with a septum.Out of the glovebox, the tube was heated with an oil bath at 60 • C.Under vigorous stirring, styrene (9.4 mL) and β-myrcene (1.8 mL) were added with syringes.When the desired polymerization time was reached, methanol (10 mL) was added to quench the reaction.The precipitated polymer was washed with methanol (ca.50 mL), filtered and dried under vacuum at 60 • C until constant weight was reached.
Typical procedure for styrene/β-myrcene/ethylene terpolymerization.In a typical experiment (Table 2, entry 1), a 300 mL-glass high-pressure reactor (TOP-Industrie, Vaux le Pénil, France) was charged with cyclohexane (41 mL) under argon and heated at the appropriate temperature by circulating water in a double mantle.Under an ethylene flow were introduced styrene (50 mL), β-myrcene (9.4 mL), and (nBu) 2 Mg (0.5 mL of a 1.0 M solution in hexanes) and the solution of complex 1 in toluene (ca.43 mg in 2 mL).The ethylene pressure in the reactor was kept constant at 2 bar with a back regulator and the reaction media was mechanically stirred.At the end of the polymerization, the reactor was vented to air, and the copolymer was precipitated in methanol (ca.500 mL), washed with methanol and dried under vacuum at 60 • C until constant weight was reached.
Calculation of β-myrcene and styrene fractions in poly(S-co-My) copolymers.The fraction of β-myrcene and styrene F My and F St in the copolymers was calculated with the following equations: F My = (A aliph − 3(A arom /5))/13.5 where A aliph is the area of aliphatic hydrogens (δ 0.5-2.5 ppm) and A arom is the area of aromatic hydrogens of styrene (δ 6.5-7.5 ppm).The fraction of β-myrcene can also be determined by considering the area of olefinic hydrogens A olefin (δ 4.0-5.5 ppm) but the uncertainty in the integration of the latter signals is more important than using the aliphatic hydrogens signals.
Calculation of β-farnesene and styrene fractions in poly(S-co-Fa) copolymers.The fraction of β-farnesene and styrene F Fa and F St in copolymers was calculated with the following equations: where A aliph is the area of aliphatic hydrogens (δ 0.5-2.5 ppm) and A arom is the area of aromatic hydrogens of styrene (δ 6.5-7.5 ppm).