Indium(III) halides as exceptionally active, water-tolerant catalysts for cationic polymerization of styrenics

Abstract

The (co)polymerization of styrenes, α-methylstyrene, and indene is efficiently promoted by InBr₃ or InCl₃, in combination with an alkyl halide. This convenient route employs unpurified (wet) monomers, proceeds at slightly higher than room temperatures, and uses amounts as low as 20 ppm of indium pre-catalyst.

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Low molecular weight resins (M_n = ca. 10³ g mol⁻¹) obtained from cationic oligomerization of styrenic derivatives (or a mixture of chemically related monomers, e.g. a steam cracking C₉ fraction) are produced on a large scale for commodity applications such as paints, adhesives, …¹ Common catalysts/co-initiators used in industries revolve around the strong Lewis acids BF₃ and AlCl₃, loaded at ca. 200–1000 ppm and combined with controlled amounts of water that acts as the initiator.² Those systems suffer, however, from two main limitations that contribute to a significant cost in the process: (i) the presence of excessive amounts of water in the medium eventually leads to catalyst deactivation, which enforces to operate under rigorously controlled degree of humidity in the monomer feeds and the reactor; and (ii) the strong reaction exothermicity must be evacuated by cooling below room temperature, to limit transfer reactions responsible for the formation of undesired lower molecular weight materials. In order to optimize the energy consumed in the oligomerization process and improve its overall flexibility and efficiency, it is desirable to implement catalyst systems that allow: (i) good conversion and control over the molecular mass of the oligomers with a broad range of styrenic monomers, (ii) small loadings of a water-tolerant catalyst, and (iii) an operating temperature slightly higher than ambient conditions.

Recent years have witnessed a boom in the search for water-tolerant Lewis acidic catalysts, mainly for applications in fine chemicals synthesis.³ Also, a few classes of Lewis acids have been shown to polymerize styrenics in the presence of large amounts of water to prepare latexes, the most notable of which are rare-earth triflates and tris(pentafluorophenyl)borane.^4,5 Yet, none of these systems are at this stage potentially applicable because of the excessive catalyst loadings (3 to 40 mol%) employed. Herein we report the highly efficient oligomerization of styrenics using catalyst systems based on indium(III) halides. This convenient route employs unpurified (wet) monomers, proceeds at slightly higher than room temperatures, and uses very low amounts (as low as 20 ppm) of indium pre-catalyst.

Our initial interest in indium(III) halides was inspired by recent works describing the use of such simple salts or other organoindium compounds as catalysts in fine chemicals synthesis and the strong tolerance featured by some of these catalysts towards functionalities and water.^6–8 On the other hand, the use of indium catalysts in polymerization is thus far restricted to the ring-opening polymerization of lactones and lactides for the production of microstructurally controlled biodegradable polyesters.⁹ No mention is made of their use for oligo-/polymerizing olefins or styrenic monomers. In our preliminary investigations, InBr₃ was combined with various amounts of p-tolylethanol or 1-indanol¹⁰ to initiate the cationic oligomerization of p-methylstyrene (pMeSt) and indene (Ind), respectively, in p-xylene solutions, under air. These two monomers were selected as model substrates since they have a quite different reactivity, i.e. stability of propagating carbocation and relative polymerizability. It is known that the latter polymerizability depends on the intrinsic nature of the monomer (steric hindrance, electronic effects; pMeSt is thus generally more easily polymerized than Ind) but it is also significantly affected by the nature of the catalytic system and reaction conditions.^2,11InBr₃ (500 ppm vs. monomer), when associated with 5 mol% of p-tolylethanol toward pMeSt, did not fully dissolve in the reaction medium but gave complete monomer conversion after 17 h (non-optimized reaction time) and a polymer with a relatively narrow molecular weight distribution. Yet, reducing the InBr₃ loading to 200 ppm reached the limits of this system, as the reaction was not completed after 17 h (entry 3) (Table 1). Similar observations were made for the oligomerization of Ind performed with the InBr₃/1-indanol system but with incomplete conversions under the same conditions (60–90%) (entries 8–10), evidencing the expectedly lower reactivity of this monomer (as compared to pMeSt).

Table 1 Oligomerization of p-methylstyrene (pMeSt) and indene (Ind) promoted by InBr₃/co-initiator systems^a

Run	Monomer	InBr3 (ppm)	Initiator	Initiator (mol%)	Solvent additive	Conv. (%)	M n,theo ^b/g mol^−1	M n,exp ^c/g mol^−1	M w/Mn^c
a Reactions conducted in p-xylene solution (50 wt%) at 60 °C over 17 h (reaction time period not optimized) under air with technical grade reagents (100–150 ppm H2O) using typically 75 mL of pMeSt or Ind. b Number average molecular weight calculated from the initiator content. c Number average molecular weight and molecular weight distribution, as determined by GPC in THF at 23 °C vs.polystyrene standards. d 200 μL of iPr2O were added.
1	pMeSt	500	pMePhCH(Me)OH	5.0	—	100	2500	2500	1.7
2		500	pMePhCH(Me)OH	2.0	—	100	6025	5100	1.7
3		200	pMePhCH(Me)OH	5.0	—	95	2375	3100	1.7
4		20	tBuCl	5.0	—	100	2450	9100	1.9
5		20	PhCHBrMe	5.0	iPr2O ^d	100	2550	2000	1.4
6		20	PhCHBrMe	1.0	iPr2O ^d	100	12 000	7000	1.2
7		20	PhCHBrMe	0.1	iPr2O ^d	100	118 200	11 400	1.9
8	Ind	500	1-Indanol	5.0	—	90	2200	1800	1.9
9		500	1-Indanol	2.0	—	60	3600	2400	2.2
10		200	1-Indanol	5.0	—	85	2100	1500	1.8
11		100	PhCHBrMe	5.0	—	100	2500	1500	1.3
12		100	PhCHBrMe	1.0	—	100	11 800	2100	1.7
13		100	PhCHBrMe	0.1	—	100	116 200	3000	2.5
14		20	PhCHBrMe	5.0	iPr2O ^d	100	2500	1000	1.8
15		20	PhCHBrMe	1.0	iPr2O ^d	100	11 800	2200	1.8
16		20	PhCHBrMe	0.1	iPr2O ^d	100	116 200	3600	2.1

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A considerable enhancement of activities was observed when, instead of an alcohol derivative, an alkyl halide was used as initiator in combination with InBr₃. For instance, in the oligomerization of pMeSt with tert-butyl chloride, the InBr₃ loading needed to be reduced down to 20 ppm and, even under these very low catalyst loading conditions, strong exothermicity and eventual loss of temperature control could not be systematically avoided (entry 4). The higher efficiency of tBuCl or PhCHBrMe initiators, compared to pMePhCH(Me)OH, can be mostly ascribed to the high affinity of InX₃ in abstracting halogen atoms rather than a hydroxyl group.¹² Experimental M_n values for the polymerization of pMeSt with 5.0 and 2.0 mol% of pMePhCH(Me)OH correlate well with theoretical ones (entries 1 and 2). This indicates that controlled initiation takes place by pMePhCH(Me)OH and competitive initiation by water is negligible at high initiator concentrations. With tBuCl as initiator (entry 3), the molecular weight is higher than with pMePhCH(Me)OH at the same initiator concentration, which suggests incomplete initiation in this case.

To get a better control over the reactions promoted with alkyl halide initiators, the use of a donor co-solvent was explored. Such a base would interact with the Lewis acidic indium catalyst and, in turn, fully solubilize the latter, as well as allow to decrease the instantaneous concentration of Lewis acid providing a better control over molecular weight and narrowing molecular weight distributions.^12,13 In fact, addition of 200 μL of either THF, acetone, tert-butyl methyl ether or diethyl ether to 6 mg (500 ppm) of InBr₃ was found to inhibit strongly the catalytic activity (pMeSt conversions < 20% at 60 °C, 17 h). However, with the more sterically hindered di(isopropyl)ether as co-solvent/additive, the catalyst system was fully soluble and performed quite efficiently. Under those conditions, combinations of 20 ppm of InBr₃ with 1-bromo-phenethyl (0.1–5 mol%) as the initiator quantitatively converted pMeSt at 60 °C, giving oligomers with relatively narrow molecular weight distributions (entries 5–7). Due to the lower reactivity of Ind (as compared to pMeSt), the oligomerizations of this monomer were better controlled in terms of exothermicity; complete conversions to oligo(1-indene) with M_n values in the targeted range were achieved using 100 ppm of InBr₃ combined with 1-bromo-phenethyl (0.1–5 mol%), in the absence of any donor co-solvent (entries 11–13). Yet, the catalyst loading could be reduced down to 20 ppm of InBr₃, in the presence of a small amount of iPr₂O to ensure solubility of the catalyst precursor (therefore useable as a dilute stock solution), and complete conversions of Ind were achieved with as low as 0.1 mol% of PhCHBrMe (entries 14–16).

These results evidence a spectacular improvement—of ca. 2 orders of magnitude—of the catalytic activity as compared to the current state-of-the art in this chemistry. In fact, cationic oligomerization of pMeSt and Ind with other efficient water-tolerant systems such as Bi(OTf)₃ or B(C₆F₅)₃ with alcohol initiators required at least 2000 ppm of catalyst and also larger amounts of initiator (typically 5 mol%) to proceed under such conditions.¹⁴ The efficiency of this new indium catalysis was also probed in the copolymerization of styrene (St) with α-methylstyrene (α-MeSt), a common formulation used to prepare a variety of tackifying resins. Representative results are summarized in Table 2. The reactions were performed on technical-grade monomers and solvent that contained 100–200 ppm water. The residual moisture allowed the reaction to proceed without adding another initiator but, in this case, relatively large amounts of InBr₃ (500 ppm) were required to reach full conversions (entries 17 and 18). On the other hand, the use of an alkyl halide such as tBuCl or PhCHBrMe allowed reduction of the InBr₃ catalyst loading down to 20 ppm (entries 19–23). Changing the α-MeSt/St ratio in the monomer feed allowed manipulation of the molecular weight of the final materials, with higher M_n values obtained expectedly with increasing amounts of styrene, still keeping the polydispersities relatively narrow (compare entries 22 and 23). It is well-known in the fine chemicals catalysis literature that InX₃ (X = Br, Cl, Otf, …) salts can display quite different catalytic abilities.^6–8 We found that InCl₃, the cheapest of the indium salts, performed also well in the co-oligomerization of α-MeSt and St. The activity of the InCl₃/tBuCl combination is somewhat lower than that of InBr₃/RX systems and the reactions are best carried out at 60 °C (entries 24–27); a kinetic monitoring revealed that, under these conditions, the reactions are completed within 100 min (see ESI†). Preliminary mechanistic investigations are consistent with a carbocationic process. ¹H and ¹³C NMR analyses of oligostyrenes (purposely prepared from the InCl₃/tBuCl system, as their structure is inherently simpler to analyze) showed the co-existence of macromolecules with tBuCH₂ and CH₃ end-groups at one terminus (see ESI†, note that the nature and origin of the other chain terminus were not fully established at this stage), respectively indicative of initiation by a tBu⁺ group and transfer to monomer as well. The deep red color of the media observed during the course of indene oligomerizations is also indicative of the formation of such carbocationic species. The generation of the active species from the InCl₃/tBuCl combination was studied by NMR spectroscopy. Variable temperature ¹H NMR analyses proved uninformative regarding the formation of carbocationic species, most likely due to the transient nature of the latter. In contrast, a ¹¹⁵In NMR monitoring (Fig. 1)¹⁵ revealed the transformation of the precursor InCl₃ (which is silent in NMR under the analytical conditions used, most probably due to extreme broadening of the signal) to InCl₄⁻ (δ = 447 ppm; independently prepared from HCl and InCl₃, see ESI†) upon addition of 1 equiv. of tBuCl in iPr₂O; apparition of the latter signal proceeded within 2 min at room temperature, indicating a rapid reaction. The same transformation of InBr₃ to InBr₄⁻ (δ = 187 ppm) was observed upon treatment of the neutral precursor with 1 equiv. of PhCHBrMe (see ESI†). Formation of these tetrahalogenoindate species is consistent with abstraction of the halide from the initiator to produce the corresponding putative carbocation.

Table 2 Co-oligomerization of α-methylstyrene (α-MeSt) and styrene (St) promoted by InX₃/co-initiator systems^a

Run	St/α-MeSt	InX3	InX3 (ppm)	Solvent	Initiator	Initiator (mol%)	Temp./°C	Conv. (%)	M n ^b/g mol^−1	M w/Mn^b	T g ^c/°C
a Reactions conducted in p-xylene solution (50 wt%) at 60 °C over 17 h (reaction time period not optimized). b Number average molecular weight and molecular weight distribution, as determined by GPC in THF at 23 °C vs.polystyrene standards. c Determined by DSC. d 200 μL of iPr2O were added.
17	30 : 70	InBr3	500	—	—	—	40	95	750	1.8	nd
18			500	—	—	—	50	100	640	1.7	36
19			20	iPr2O ^d	tBuCl	1.0	40	100	550	1.3	16
20			20	iPr2O ^d	PhCHBrMe	1.0	40	95	750	1.4	29
21			20	iPr2O ^d	PhCHBrMe	0.5	40	95	750	1.4	39
22			20	iPr2O ^d	PhCHBrMe	0.1	40	80	750	1.4	48
23	70 : 30		20	iPr2O ^d	PhCHBrMe	0.1	40	60	1200	1.6	nd
24	30 : 70	InCl3	100	—	tBuCl	1.0	60	100	450	1.1	29
25			20	iPr2O ^d	tBuCl	0.5	60	90	620	1.3	nd
26	70 : 30		100	—	tBuCl	0.5	60	100	900	1.5	44
27			20	iPr2O ^d	tBuCl	0.5	60	75	1200	1.6	nd
28			100	—	tBuCl	1.0	50	100	1100	1.3	42

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Fig. 1 ¹¹⁵In NMR spectra (87.7 MHz, 23 °C) of a solution of InCl₃ in iPr₂O ([In] = 40 mmol L⁻¹) (bottom), and of the same solution 2 h (middle) and 18 h (top) after addition of 1 equiv. of tBuCl at 23 °C.

This work was financially supported by the Total & Cray Valley companies. We gratefully thank Mr Jean-Paul Guegan (UMR CNRS Sciences Chimiques de Rennes) for performing ¹¹⁵In NMR studies and Dr Corinne Roqueta (Cray Valley) for fruitful discussions.

Notes and references

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10. 1-p-Tolylethanol and 1-indanol, the corresponding secondary alcohols derived from p-methylstyrene and indene, were selected as initiators for structure homogeneity.
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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization of polymers, NMR monitorings and supplementary figures. See DOI: 10.1039/c1py00145k

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