Lithium amides as initiators of anionic polymerisation of methyl methacrylate

PII:

Eur. Polym. J. Vol. 34, No. 12, pp. 1877±1887, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $ - see front matter S0014-3057(98)00026-3

LITHIUM AMIDES AS INITIATORS OF ANIONIC POLYMERISATION OF METHYL METHACRYLATE S. A. COUPER, R. E. MULVEY and D. C. SHERRINGTON* Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL, U.K. (Received 17 March 1997; accepted in ®nal form 9 October 1997) AbstractÐReaction of BunLi with isopropylcyclohexylamine and 2,6-dimethylpiperidine allows isolation of the corresponding lithium amides. Both of these are e€ective initiators of polymerisation of methyl methacrylate (MMA). A more convenient procedure however is to generate the lithium amides in situ and six secondary amines have been used in this way. Polymerisations in THF at ÿ788C, both with and without added TMEDA Lewis base, yield polymer quantitatively with a high degree of syndiotacticity. Mn values, however, are somewhat larger than expected for clean living systems and likewise molecular weight distributions are rather broad. Similar reactions at 258C give much lower yields of polymer, somewhat reduced syndiotacticity and similar molecular weight e€ects. Using diisopropylaminolithium as the initiator in toluene solution at ÿ788C yields highly isotactic polymer essentially quantitatively. Again the Mn and molecular weight distributions are higher than ideal. Adding trace levels of TMEDA to this polymerisation switches the microstructure of the polymer to high levels of syndiotacticity, but the initiator eciency is reduced at the same time resulting in a very high Mn and a broad molecular weight distribution. Addition of varying levels of LiOBut to the polymerisation in toluene retains the isotactic selectivity of the propagation and improves the initiator eciency signi®cantly. The results are discussed in the light of current understanding of anionic vinyl polymerisations. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Although considerable progress has been made in recent years [1, 2] in controlling the tacticity of polymers derived from methacrylate and acrylate monomers, the ability to do this in a versatile manner under ambient conditions remains an elusive target. Since, for example, the glass transition temperature of poly(methyl methacrylate) (PMMA) varies from 0458C to 01158C for the highly isotactic versus the highly syndiotactic forms respectively [3], considerable industrial as well as academic interest exists in this area. Although low temperature polymerisation of MMA via free radical intermediates generates a relatively high level of syndiotacticity (via thermodynamic control), use of various anionic polymerisation techniques has not only allowed some degree of ``living'' character to be imported to these polymerisations, but has also o€ered most ¯exibility in terms of tacticity control. Judicious choice of alkali metal organometallics has been crucial in this respect. For example, Hatada and co-workers have developed highly stereoregular polymerisations of MMA using Grignard reagents as initiators, producing both highly isotactic [4±6] and highly syndiotactic [7] polymer as required. The formation of highly syndiotactic polymer with considerable eciency in low cost non-polar media such as toluene has been an important development. Whereas simple alkali metal alkyls are highly solvent depen*To whom all correspondence should be addressed.

dent as initiators, more complex species, particularly those involving organoaluminiums [8, 9], can yield tacticity control apparently contrary to the ``normal'' solvent dependence. A combination of a trialkylaluminium species with an alkyllithium initiator is particularly e€ective in generating syndiotactic placement in MMA polymerisations in toluene [10, 11]. Recently Ballard and his coworkers [12] have described a mixed Al/Li system with intense steric crowding around the active centre, and have coined the term ``screened anionic polymerisation.'' This system facilitates good stereochemical control even at ambient temperature. The use of additives in combination with anionic initiators has also proved very important in controlling tacticity. Lochman and his co-workers have exploited alkali metal alkoxides in this context. [13± 16] Particularly in combination with the initiator, alithiobutyric ester enolate, which is a model for the active centre in a methacrylate polymerisation, good control of the polymer microstructure can be achieved under ambient conditions. Teyssie and his co-workers [17, 18] have exploited salts such as LiCl in a similar way, proposing the formation of a bridged structure involving LiCl complexed with the active centre. Other additives have also been reported [19, 20] including crown ethers [21]. The relationship between group transfer polymerisation (GTP) of methacrylates and anionic propagation of these monomers is one where considerable debate remains [22, 23], particularly regarding the degree of covalent bonding in the proposed active species. The emergence of zirco-

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nium [24] and samarium [25] metallocene-type initiators has added a new dimension to these polymerisations and potentially o€ers a mechanistic bridge to ethylene and propylene polymerisations. The early use of simple alkali metal amides (NaNH2 [26], KNH2 [27]) as initiators of anionic polymerisation was complicated by the presence of excess liquid NH3 as the solvent. Use of species such as lithium diethylamide and diphenylamide was also described some time ago for a number of monomers, including MMA, but these proved to be inecient initiators. [28±30] Since these early attempts considerable re®nement in the synthesis and use of lithium amides in organic synthesis has occurred, and yet further investigation of these as initiators of anionic polymerisation has been rather sparse [31, 32]. One example has been found in the more recent literature, and ecient polymerisation of MMA has been reported [33]. In addition, a second paper has appeared during the processing of this manuscript [53]. The past decade has witnessed an escalation in the number of structural studies devoted to lithium amides, as described in a number of recent reviews [34±36]. In general, they have a strong propensity towards aggregation through the self-association of highly polar Li±N bonds. Rarely are their component Li+ and R2Nÿ ions completely solvent separated, though they are often presented as such. The degree of aggregation is largely dictated by the steric bulk of the amide ``R'' substituents as well as by the nature and number of any additional donor solvent ligands present. Monomeric structures occur only in the presence of an exceptionally bulky anion and/or donor ligands that co-ordinate strongly to the metal. The principles governing the aggregation phenomena of lithium amides, which tends to involve small cyclic (LiN)2 or 3 building blocks, are well established and have been discussed in detail elsewhere [34±37]. With our own expertise in the synthesis of well de®ned lithium±nitrogen complexes [37] it was thought to be timely to examine a range of lithium amides as potential initiators of MMA polymerisation since residual aggregation around the Li centre may o€er unusual stereochemical control. This paper reports our ®rst ®ndings in this context.

EXPERIMENTAL

Materials Tetrahydrofuran (THF) (Aldrich) toluene (Aldrich) and n-hexane (Aldrich) were all dried from sodium benzophenone. Methanol and methylene chloride (Aldrich) were used as supplied. Methyl methacrylate (Aldrich) was passed through a darkened column containing basic alumina and 4 AÊ molecular sieves. Phenylnaphthylamine (Aldrich) was sublimed before use. Diisopropylamine, 2,6dimethylpiperidine, 2,2,6,6-tetramethylpiperidine, hexamethyldi-silazane, isopropylcyclohexylamine and tetramethylethylenediamine (Aldrich) were all fractionally distilled onto 4 AÊ molecular sieves over which they were stored. BunLi (Aldrich) was standardised by titration with BusOH using 1,10-phenanthroline as indicator [38]. Ethyl tiglate (Lancaster) and ButOH (Aldrich) were used as supplied.

Synthesis of solid lithium amides Initially solid lithium amides were synthesised as a prospective strategy for using these as pre-formed initiators.

Isopropylcyclohexylaminolithium (THF solvate) Isopropylcyclohexylamine (3.34 ml, 20 mmol) was dripped into a stirred solution of BunLi (20 mmol, typically 12.5 ml, 1.6 M in hexane) which caused an exothermic reaction and the formation of a pale yellow solution. Upon the addition of THF (1.62 ml, 20 mmol) there was negligible change to either solution colour or ¯uidity. Progressive vacuum reduction of the solution caused the rapid precipitation of an o€-white solid. At this point the pressure was equilibrated and the suspension heated until complete dissolution of the solid occurred. The hot, pale yellow solution was left to cool, then left standing at 08C overnight resulting in the deposition of a batch of colourless needle crystals. These were isolated by ®ltration, washed with chilled hexane and dried in vacuo. M.Pt. = 73±758C. Elemental microanalysis (%) for C13H26ONLi: C, 71.2; H, 11.9; N, 6.4; Li 3.2. Found: C, 69.2; H, 10.8; N, 5.3; Li, 3.0. FTIR (cmÿ1, nujol): 2960, 2920, 2850, 1470, 1460, 1440, 1160, 1050. (On exposure of the mull to air, a band at 3690 cmÿ1 was seen, due to LiOH). 1 H NMR (400 MHz, d8-THF, 258C) (d, ppm): 0.96 (q, 2H, CH (3,5)); 0.97 (d, 6H, CH(CH3)2; 1.06 (q, 1H, CHax (4)); 1.26 (q, 2H, CHax (2,6)); 1.57 (d, 1H, CHeq (4)); 1.68 (d, 2H, CHeq (3.5)); 1.77 (m, 4H, OCH2CH2 (THF)); 1.83 (d, 2H, CHeq (2.6)), 2.44 (t, 1H, CHax (1)); 3.08 (septet, 1H, CH(CH3)2); 3.65 (m, 4H, OCH2 (THF)). 13 C NMR (100 MHz, d8-THF, 258C, 1 H decoupled): (d, ppm) 25.63 (OCH2CH2 (THF)); 27.82 (CH(CH3)2); 28.16 (CH2 ring); 28.51, (CH2 ring); 39.77 (CH2 ring); 51.97 (CH(CH3)2); 62.03 (3H ring); 68.42 (OCH2 (THF)).

2,6-dimethylpiperidinolithium vate)

(TMEDA

hemi-sol-

2,6-Dimethylpiperidine (2.7 ml, 20 mmol) was added dropwise to a stirred solution of BunLi (20 mmol, typically 12.5 ml, 1.6 M in hexane) resulting in an exothermic reaction with gas evolution to leave a viscous yellow solution. This thinned considerably on the addition of TMEDA (3.5 ml, 20 mmol) with the solution developing a deeper orange colour. Upon progressive vacuum reduction of the solution a point was reached where precipitation of a yellow solid commenced. At this stage the volume reduction was stopped, the pressure equalised and the suspension heated until complete dissolution of the solid had occurred. The hot, red solution was left standing to cool to ambient temperature resulting in the deposition of a large crop of cubic, yellow crystals. These were isolated by ®ltration, washed with chilled hexane and toluene and dried in vacuo. M.Pt. = 90±928C. Elemental microanalysis (%) for C20H44N4Li2: C, 67.8; H, 12.4; N, 15.8; Li, 4.0. Found: C, 67.2; H, 12.8; N, 15.0; Li, 3.6. FTIR (cmÿ1, nujol): 2960, 2910, 2860, 1460, 1370, 1350, 1305, 1210, 1170, 1110, 1090, 1020, 950, 880. (On exposure of the mull to air a sharp band at 3690 cmÿ1 was seen, due to LiOH, and a second band at 3590 cmÿ1 due to reformed -NH). 1 H NMR (400 MHz, d8-THF, 258C) (d, ppm): 0.60 (m, 2H, CHax (3,5)); 1.03 (d, 6H, CH(CH3)2); 1.40 (m, 3H, CHax (3.5)); 1.03 (d, 6H, CH(CH3)2; 1.40 (m, 3H, CHax (4), CHeq (3,5)); 1.73 (m, 1H, CHeq (4)); 2.14 (2, 6H, N(CH3)2); 2.30 (s, 2H, NCH2); 2.68 (pentet, 2H, CHax (2,6)). 13 C NMR (100 MHz, d8-THF, 258C 1 H decoupled) (d, ppm): 28.15 (CH(CH3)2; 28.44 (CH2 ring); 39.85 (CH2 ring); 46.37 (N(CH3)2); 59.0 (NCH2; 61.2 (CH(CH3)2). 7 Li NMR (d8-THF, ÿ758C): ÿ1.35 (s, weak); 0.28 (s, strong).

Anionic polymerisation using lithium amides

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Polymerisation of MMA initiated by lithium amides formed in situ

Reaction of 2,6-dimethylpiperidinolithium TMEDA with ethyl tiglate

All manipulations were carried out under argon in the glove box. Lithium amides were generated in situ initially by addition of BunLi to the appropriate amine dissolved in THF or toluene. MMA was then added and the polymerisation allowed to proceed for a given time (usually four hours) before the reaction was terminated and the polymer formed isolated. The procedure is exempli®ed as follows. Diisopropylamine (2.8  10ÿ4 mol) was dissolved in freshly distilled THF (20 ml)) in a Schlenk tube with te¯on taps and chilled to ÿ788C in a methanol/CO2 slush bath. BunLi (2.8  10ÿ4 mol typically 0.13 ml, 1.6 M in hexane) was added by syringe to this solution and system left magnetically stirred for ®ve minutes. MMA (3 ml, 2.8  10ÿ2 mol) was slowly dripped into the initiator solution over a period of two minutes to avoid any elevation in temperature. The resulting solution was then stirred for 4 h. The reaction was quenched and the polymer formed was precipitated by pouring the reaction mixture into a stirred solution of n-hexane (200 ml) and methanol (2 ml) at room temperature. The solid polymer was collected on a glass sinter and allowed to dry at ambient temperature overnight. Finally, the product was dried to constant weight under reduced pressure at 1108C (re¯uxing toluene) in a drying pistol containing P2O5. In those reactions where the viscosity was very high or partial gelation occurred, dichloromethane (10 ml) was added to increase the ¯uidity before working up the reaction and isolating the polymer as before. Polymerisations in toluene as the solvent were run identically; and likewise polymerisations performed at 258C (room temperature). In the latter case however the solution of in situ formed lithium amide was allowed to equilibrate to room temperature still in the glove box before the MMA was added. When polymerisations were carried out in the presence of TMEDA as a Lewis base, 0.03 ml (2.8  10ÿ4 mol) of the latter were added to the in situ formed lithium amide immediately prior to the addition of the MMA. When ButOLi was used as an additive, a known weight of the latter was introduced into the reaction vessel under dry argon and then dissolved in freshly distilled toluene (20 ml). The solution was chilled to ÿ788C and then diisopropylamine (0.03 ml, 2.8  10ÿ4 mol) was added, followed by BunLi (2.8  10ÿ4 mol, typically 0.13 ml, 1.6M in hexane). The procedure was then the same as described above.

A stirred solution of BunLi (10 mmol, typically 6.7 ml, 1.6 M in hexane) was diluted with additional hexane (5 ml). To this 2,6-dimethylpiperidine (1.35 ml, 10 mmol) was introduced dropwise to produce a ¯uid yellow solution with the evolution of gas and heat. The solution deepened in colour upon the addition of TMEDA (1.51 ml, 10 mmol) and was left stirring for two minutes. As the initial few drops of ethyl tiglate (1.20 ml, 10 mmol) were added, there was a very obvious darkening of the solution to a deep red with the evolution of heat, but as the remainder of the reactant was syringed into the solution the colour faded to a deep orange hue. Upon standing overnight at 48C a large crop of clear, cubic crystals was deposited. These were isolated by ®ltration, washed with chilled hexane and dried in vacuo. M.Pt. = 98±1008C; Elemental microanalysis (%) for C13H27O2N2Li: C, 62.4; H, 10.8; N, 11.2; Li, 2.8. Found: C, 61.3; H, 9.4; N, 6.9; Li, 2.7. FTIR (cmÿ1, nujol): 2920, 2860, 1610, 1590, 1560, 1460, 1120, 800. (On exposing the mull to air, a sharp peak at 3690 cmÿ1 was seen due to LiOH). 1 H NMR (400 MHz, d8-THF, 258C) (d, ppm): 1.30 (t, 3H, OCH2CH3); 1.87 (s, 4H), NCH2 (TMEDA)); 2.00 (s, 1.5H C.C(CH3) (E-isomer)); 2.05 (s, 12H, N(CH3)2 (TMEDA)): 2.22 (s, 1.5H, C.C(CH3) (Z-isomer)); 3.95 (q, 2H, OCH2); 4.5±4.9 (m's, 2H, CH.CH2 (E + Z-isomers)); 7.25 (m, 1.5H, CH.CH2 (Z-isomer)); 7.77 (m, 1.5H, CH.CH2 (E-isomer)). 13 C NMR (100 MHz, d8-THF, 258C, 1 H decoupled) (d, ppm): 11.69 (OCH2CH3); 16.23 (C.C(CH3)); 46.29 (N(CH3)2 (TMEDA)); 57.62 (NCH2 (TMEDA)); 58.79 (OCH2); 94.24 (CH.CH2); 139.30 (CH.CH2). (Note: C.C(CH3) not observed).

Synthesis of lithium tert-butoxide BunLi (10 mmol; typically 6.7 ml 1.6 M in hexane) was introduced to a Schlenk vessel under an inert argon atmosphere and stirred. From a hot syringe, molten ButOH (1 ml, 10 mmol) was added dropwise causing a vigorous exothermic reaction, ultimately leaving a clear, colourless solution. This was slowly concentrated by removal of solvent in vacuo until a suspension was produced. After pressure equilibration the system was heated until complete dissolution had occurred. On leaving to cool to room temperature a large crop of colourless ButOLi needle-like crystals were deposited. These were isolated by ®ltration and washed with chilled hexane before drying in vacuo. The yield was essentially quantitative; M.Pt.>3008C. Elemental microanalysis (%) for C4H9OLi: C, 60.0; H, 11.2; Li, 8.8. Found: C, 58.4; H, 10.7; Li, 9.2. FTIR (cmÿ1, nujol): 1950, 2930, 2850, 1470, 1350, 1210, 970, 580. 1 H NMR (400 MHz, d8-toluene, 258C) (d, ppm): 1.27 singlet.

Inert atmosphere technique Reactions and manipulations outside the glove box (see below) were carried out under dry argon using standard Schlenk techniques [39]. Air was evacuated using an Edwards oil pump and replaced by argon previously passed through two columns, one containing P2O5 and the other 4 AÊ molecular sieves. Storage vessels were then sealed and transferred to the glove box. Here they were stored under argon until required.

The glove box The glove box was a Faircrest Mark IV model, made of mild steel and ®tted with neoprene gloves. Samples were transferred into and out of the box via two evacuable ports: one large (50 cm  20 cm approx.) and one small (10 cm  20 cm approx.). A working atmosphere was maintained by constantly recirculating the argon through external columns. The two columns, one containing molecular sieves and one containing a copper catalyst, remove moisture and oxygen respectively by operating at ambient temperature and 1008C respectively. A positive pressure of two inches of water within the box was maintained via a ¯ute ®lled with paran oil. The moisture level within the box was measured in parts per million by a Shaw Moisture Meter, Model SHA, which operates via a variable meter, attached to a sensor which has a metal core coated in a hydroscopic dielectric. Moisture levels within the box were maintained by the columns at 1±2 ppm. The glove box columns were regenerated regularly (every 3±4 weeks) using forming gas (20% H2, 80% N2) which was passed through the copper column at 2508C to reduce any oxide formed during recirculation. The molecular sieve column was regenerated by removal of moisture under vacuum at 1808C.

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Scheme 1. Structural formulae of secondary amines used in forming lithium amides.

Instrumentation Infra-red (FTIR) spectra were recorded on a PerkinElmer 457 grating spectrometer. Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AMX400 MHz spectrometer operating at 400.136 MHz for 1 H and 100.625 MHz for 13 C. Spectra were referenced to the internal solvent peaks which were in turn referenced to tetramethyl silane at 0 ppm. Elemental microanalyses were performed on a Perkin-Elmer 240 elemental analyzer,

and Li analyses were obtained from atomic absorption ¯ame spectrophotometry using a Phillips PU9100X instrument.

Molecular weight determination Polymer molecular weights were obtained using gel permeation chromatography (GPC). Samples were dissolved in THF and analyzed at 258C using 2  PL gel mixed bed B columns (10 mm, 30 cm). The ¯ow rate was 1 ml minÿ1

Anionic polymerisation using lithium amides and monitoring was via a refractive-index detector. The columns were calibrated using polystyrene standards and the universal calibration procedure adopted to convert elution volumes to poly(methylmethacrylate) molecular weights. Mn and Mw values were calculated in the usual way.

Tacticity determination [40] The tacticity of poly(MMA) samples was quanti®ed from 1 H NMR spectra recorded at 400.136 MHz in d1chloroform at 258C. The resonance from the a-CH3 protons was taken as diagnostic of the con®guration of backbone pseudo-stereogenic centres. The resonance at 0.75± 0.85 ppm was assigned to syndiotactic CH3- groups, at 0.95±1.00 ppm to heterotactic CH3-groups and at 1.1± 1.2 ppm to isotactic CH3-groups. In each case these were singlets though their broadness varied a little. Integration of these peaks allowed quantitative assignment of the % syndio, hetero and isotactic placements respectively. In the case of samples displaying relatively high syndiotacticity the methylene resonance appears as a singlet at 1.75± 1.8 ppm, but with samples showing rather high isotacticity this resonance is revealed as an AB quartet (two doublets at 1.45±1.5 and 2.1 ppm respectively) centred around 01.8 ppm. Interestingly all polymer samples were either >60% syndiotactic or >65% isotactic. In all cases the methoxy ester hydrogens appear as a singlet at 3.5± 3.6 ppm.

RESULTS AND DISCUSSION

Synthesis and characterisation of lithium amide initiators Solid samples of isopropylcyclohexylaminolithium and 2,6-dimethylpiperidinolithium were synthesised under stringently-dry and anaerobic conditions as described in Section 2. The former synthesis was carried out in the presence of THF, so the amide crystallised as a THF solvate (Li±THF molar ratio, 1:1). This was established by a 1 H NMR spectrum of the re-dissolved solid which showed the characteristic CH2 multiplets of THF at 1.77 and 3.65 ppm. Giving rise to a set of discrete, well-separated resonances, the isopropylcyclohexylamino group could also be clearly identi®ed and assigned from this spectrum. Based on comparisons with related (structurally authenticated) lithium amides, most notably the dimer (diisopropylaminolithium THF)2 [41], one can be reasonably con®dent that this new amide solvate adopts a similar dimeric (NLi)2 cyclic ring structure with terminal Li±O (THF) bonds. TMEDA was used in place of THF in the latter synthesis. Thus, the isolated crystals of 2,6dimethylpiperidinolithium were found to contain 0.5 molar equivalents of co-ordinated TMEDA (i.e. Li±TMEDA ratio, 2:1). Again the presence and amount of co-ordinated solvent was clearly discernible from the 1 H NMR spectrum which revealed characteristic TMEDA resonances at 2.14 ppm (Me) and 2.30 ppm (CH2). Interestingly, two distinct resonances were observed in 7 Li NMR spectra. Their relative integrals were found to be concentration dependent, indicating the presence of two distinct lithium solution species in dynamic equilibrium as opposed to a single solution species with two distinct lithium environments. A similar

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dynamic process, previously noted in THF/pentane mixtures of closely related 2,2,6,6-tetramethylpiperidinolithium, was attributed to a dimer _ monomer equilibrium [42]. Initiation of MMA polymerisation using lithium amides Our original strategy was to synthesise and isolate, if necessary at low temperature, structurally well-de®ned solid lithium amides, and then to use predetermined quantities of these as initiators. However, it soon became apparent that synthesising and isolating representative examples of these initiators was itself going to be a signi®cant undertaking. The two solid amides described above were however used in preliminary experiments to assess their e€ectiveness as initiators, and it was quite apparent in semi-quantitative experiments that they were indeed ecacious. However, the handling of these species, and particularly the quantitative manipulation of the very small quantities required for initiation reactions proved to be highly problematical. Rapid initiation of polymerization using these structurally well-de®ned pre-formed Li amides is a signi®cant result, and adds con®dence to the view that similar species generated in situ in solution are also the active initiating moieties. Since, therefore, progress in the study of polymerisations using preformed amides was likely to be slow, it was decided to modify our strategy and adopt a procedure in which each lithium amide was generated in situ, via a 1:1 molar reaction of appropriate levels of each secondary amine (Scheme 1) with BunLi. The reaction of the latter with secondary amines is fast even at low temperature and it was anticipated that this would not create problems in polymerisations. An equilibration time of ®ve minutes was found to be more than adequate and was adopted as routine. Rapid colour formation was observed for two of the six amines used. Dimethylpiperidinolithium displayed a pale yellow colour, while the pale purple solution of phenylnaphthylamine (in THF) was rapidly replaced by a deep yellow on addition of the BunLi. Even at ÿ788C the former pale yellow colour disappeared as soon as the ®rst drop of MMA was added to this initiator, while the deep yellow of the phenylnaphthylaminolithium became more golden on addition of MMA. Polymerisation of MMA in THF at ÿ788C 1,1-Diphenylhexyllithium produced by the in-situ reaction of BunLi with 1,1-diphenylethylene is a well studied [33] alkyllithium initiator which combines a bulky structure and additive-free aspects of initiation which we are also seeking in the amidolithium species in the present work. First studied by Wiles and Bywater [43] this initiator therefore o€ers a valuable benchmark with which the results found for the various aminolithium species can be compared. Long, Allen and McGrath [33] used diphenylhexyllithium to initiate MMA polymerisation in THF at ÿ788C and showed it to be very ecient, yielding a polymer with Mw/Mn=1.2 and a stereochemical make-up of iso/hetero/syndio of 1/18/ 81%.

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S. A. Couper et al. Table 1. Polymerisation of MMA at ÿ788C in THFa

Precursor amineb

Polymer yield (%)

Molecular weight Mn

Diisopropylamine Dimethylpiperidine Tetramethylpiperidine Phenylnaphthylamine Hexamethyldisilazane Hexamethyldisilazanec Isopropylcyclohexylamine

0100 0100 0100 0100 18 0100 0100

43,000 13,900 154,000 169,000 86,800 461,000 53,600

Mw 52,600 19,200 340,000 884,000 201,000 1310,000 65,400

Tacticity (%) Mw/Mn 1.2 1.4 2.2 5.2 2.3 2.8 1.2

iso

hetero

syndio

00 00 00 3 00 4 00

19 23 20 17 18 21 19

81 77 80 80 82 75 81

a

Reaction time, 4 h. Reacted with BunLi (1:1). Reaction time 24 h.

b c

The results in Table 1 show that all the lithium amides examined, except that derived from hexamethyldisilazane, are very ecient initiators at the concentration used (01.2  10ÿ2 M) in THF at ÿ788C. Polymer yields are all essentially quantitative. The concentration of MMA adopted (01.2 M) would predict a number average degree of polymerisation (DPn) of 100 if the initiation step is fast and quantitative, and the propagation essentially ``living'' in nature. This corresponds to a Mn of 010,000. In practice all the lithium amides studied yield polymers with molecular weights far in excess of the predicted value. The most likely reason for this is loss of active initiator molecules via quenching side-reactions (see polymerisations at 258C). These appear to continue during propagation since Mw/Mn data are generally well above unity as well. In the case of diisopropylaminolithium and isopropyl-cyclohexylamino lithium, Mw/Mn are relatively small, 1.2, though Mn are 4±5 times larger than predicted. These data suggest that initiator molecules are destroyed early on, but that propagations are relatively clean with regard to side reactions. Dimethylpiperidinolithium is the only species which shows a high level of initiator eciency yielding a polymer with Mn reasonably close to the theoretical value. The polydispersity, Mw/Mn is also reasonable. The lithium amide derived from hexamethyldisilazane displays very unusual behaviour. The polymer yield at 4 h is only 18%, but a parallel reaction left for 24 h gave 0100% yield of polymer. This suggests that the initiation process is slow in this case, since arguably the propagating centre will be similar in all cases. This lithium amide is known to adopt a trimeric ring structure in the solid state [44, 45], and a dimeric ring when associated with two molecules of THF in the solid state [46] and breakdown of these structures in excess THF may be slow. However, diisopropylaminolithium also adopts a similar structure with THF [41] and since this initiator gives a high yield of polymer, the stable aggregate argument is not a telling one. Although Asao and co-workers [47] have reported the amide from hexamethyldisilazane to be a poor nucleophile relative to other lithium amides possibly as a result of pp±dp interactions between the N and Si atoms [48], in fact there is little real evidence for such interaction. A more likely reason is simply that the Si±N±Si backbone is rather bulky and in¯exible, and is stabilised by Coulombic interactions

[49]. This e€ect, together with structural aggregation, is a more reasonable rationale for the behaviour of this amide. Examination of the tacticity data in Table 1 shows that all the polymers produced at ÿ788C in THF possess stereochemical compositions very close to that of the benchmark polymer produced using diphenylhexyllithium as initiator [33]. The observed high degree of syndiotacticity is consistent with the fact that the solvent, THF, is a co-ordinating Lewis base. This is able to achieve complete solvation of the Li counterion during propagation, the active centre most likely being essentially a lithium enolate solvent separated ion pair. Absence of any co-ordination between the Li ion and the functionalities on the polymer chain means that the orientation of the incoming monomer and its placement in the chain are controlled solely by the stereochemistry of the last group on the chain. Earlier groups on the polymers and the Li counterion are not involved. The overall stereochemistry of the chain should therefore exhibit Bernoullian statistical character. Placing all the tacticity data (%) in Table 1 on Bernoullian distribution curves [50] (triad probability versus meso placement probability, Pm, for racemic±racemic, meso±meso, and racemic±meso/ meso±racemic diad pairs) shows excellent adherence to these statistics with all the data falling around Pm=0.2. The high syndiotacticity is con®rmed in each case by the appearance of a strong singlet at 01.8 ppm in the 1 H NMR spectra of these polymers arising from the magnetic equivalence of the two backbone CH2 protons in the largely syndiotactic chain. Polymerisation of MMA in THF at 258C Table 2 summarises the data for polymerisations in THF where the in situ pre-formed initiator was allowed to warm to 258C before the monomer was introduced. The most important change is that polymer yields (at 4 h) all drop considerably, except in the case of the amide derived from hexamethylsilazane where the yield actually doubles. The drop in yield is also accompanied by a broadening in the molecular weight distribution, at least in the case of those amides previously showing a rather narrow dispersity at ÿ788C. These features suggest a growing contribution from side-reactions when the temperature is allowed to rise to 258C, and these no doubt continue during the 4 h polymerisation

Anionic polymerisation using lithium amides

1883

Table 2. Polymerisation of MMA at 258C in THFa Precursor amineb

Diisopropylamine Dimethylpiperidine Tetramethylpiperidine Phenylnaphthylamine Hexamethyldisilazane Isopropylcyclohexylamine

Polymer yield (%)

18 9 9 8 40 5

Molecular weight

Tacticity (%)

Mn

Mw

Mw/Mn

iso

hetero

syndio

33,500 30,800 17,700 19,400 63,000 31,800

101,000 82,100 37,800 37,200 108,000 75,500

3.0 2.7 2.1 1.9 1.7 2.4

6 8 8 4 7 11

30 30 30 33 33 30

64 62 62 63 60 59

a

Reaction time, 4 h. Reacted with BunLi (1:1).

b

period. Instability of solvent separated lithium enolate ion pairs at 258C allowing attack on the carbonyl group etc is not unexpected. The increase in reaction temperature also in¯uences the chain microstructure with a drop in the overall syndiotactic content of the polymers. The data still display an adherence to Bernoullian statistics [50], but the greater mobility of polymer chains at 258C leads to a loss in order in the overall stereochemistry. The improved performance of the amide derived from hexamethyldisilazane tends to con®rm the argument that this species has lower nucleophilicity than the other lithium amides, and arguably therefore higher stability. On raising the temperature the nucleophilic reactivity would be expected to rise and hence also the rate of initiation by this species. The limited polymer yield (40%), the somewhat elevated molecular weight and still rather poor polydispersity show that this initiator system still remain far from ideal even at 258C. The results do, however, give some indication regarding how more useful ambient temperature initiators might be evolved. Polymerisation of MMA in THF at ÿ788C and 258C in the presence of TMEDA The remarkable parallel between the data in Tables 1 and 3 (ÿ788C) and those in Tables 2 and 4 (258C) show that addition of equimolar levels of TMEDA (relative to initiator) has no practical in¯uence on these lithium amide initiated polymerisations in THF. TMEDA is potentially a didentate ligand and Lewis base, and might potentially displace THF from metal ion co-ordination spheres. In fact, recent work has suggested that the opposite can in fact occur, even at low temperature [51]. If this were the case with the lithium enolates involved in the present work then addition of TMEDA

could not have any in¯uence at all. As we shall see this is not the case when toluene is the bulk solvent. Polymerisation of MMA at ÿ788C in toluene initiated by diisopropylaminolithium Since diisopropylaminolithium yielded the cleanest of the polymerisations studied in THF it was decided to select this initiator for a careful study of analogous reactions in toluene. In general terms it is well known that the structures of active centres are more complex in solvents of low dielectric constant with the counterion often intimately involved in the structure. In addition, the presence of small quantities of potentially co-ordinating additives, or of salts, can have a remarkable in¯uence on the polymerisation itself and on the yield, molecular weight and tacticity of the polymer product. The ®rst entry in Table 5 shows the e€ect of switching from THF (Table 1) to toluene as the solvent, all other factors remaining the same. The yield of polymer in 4 h remains essentially quantitative, the Mn value is closer to the theoretical value of 010,000, but the most important di€erence is the shift to a highly isotactic backbone structure shown unambiguously by the appropriate resonances in the a-methyl region of the 1 H NMR spectrum of the polymer. This is consistent with the general view that when the Li ion is not co-ordinated by solvent (as in the case of toluene) its coordinative demands are satis®ed somewhat by complexation not only with the enolate oxygen centre of the ultimate group, but also with the oxygen of carbonyl group in the penultimate unit of the polymer backbone. An ``isotactic'' approach of incoming monomer i.e. to form a meso diad with the ester side groups on the same side of the chain, may well be favoured by additional co-ordination of the Li counterion to the oxygen atom of the car-

Table 3. Polymerisation of MMA at ÿ788C in THFa in the presence of TMEDAb Precursor aminec

Diisopropylamine Dimethylpiperidine Tetramethylpiperidine Phenylnaphthylamine Hexamethyldisilazane Isopropylcyclohexylamine a

Reaction time, 4 h. Amine/TMEDA = 1:1. Reacted with BunLi (1:1).

b c

Polymer yield (%)

0100 0100 0100 0100 20 0100

Molecular weight

Tacticity (%)

Mn

Mw

Mw/Mn

iso

hetero

syndio

82,200 30,000 90,100 265,000 78,000 52,900

115,000 38,900 182,000 696,000 296,000 66,300

1.4 1.3 2.0 2.6 3.8 1.3

00 00 00 00 3 00

18 21 18 15 18 20

82 79 82 85 79 80

1884

S. A. Couper et al. Table 4. Polymerisation of MMA at 258C in THFa in the presence of TMEDAb Polymer yield (%)

Precursor aminec

Diisopropylamine Dimethylpiperidine Tetramethylpiperidine Phenylnaphthylamine Hexamethyldisilazane Isopropylcyclohexylamine

13 14 10 31 40 9

Molecular weight

Tacticity %

Mn

Mw

Mw/Mn

iso

hetero

syndio

16,100 20,200 14,600 17,000 51,600 12,800

61,700 67,800 45,200 25,800 95,200 43,100

3.8 3.4 3.1 1.5 1.9 3.4

5 6 5 2 4 2

32 31 32 31 31 34

63 63 63 67 65 64

a

Reaction time, 4 h. Amine/TMEDA = 1:1. Reacted with BunLi (1:1).

b c

bonyl group on the monomer, resulting in interatomic distances being minimised before backbone bond formation. In the event of a ``syndiotactic'' approach by the monomer i.e. to form a racemic diad, it is possible that after backbone bond formation and subsequent production of the new active centre, interatomic distances for co-ordination to Li are once more minimised by bond rotation around the a±b (terminal) backbone bond, again producing a meso diad. Although the system favours high isotacticity such behaviour occurs whether the endgroup is meso in orientation to the penultimate group or not, and as such should obey Bernoullian statistics. The tacticity data in entry one Table 4 does indeed fall on the Bernoullian distribution curves [50] this time at high Pm (10.85) indicative of the high level of isotacticity. The latter is also re¯ected in the magnetic inequivalence of the two protons in the CH2 backbone groups. This gives rise to a clear AB quartet centred around 1.8 ppm in the 1 H NMR spectrum of the polymer. The Mn value of this polymer (21800) is closer to the theoretical value (10,000) than that of the polymer formed in THF (43000) under similar conditions. This suggests an increase in initiator eciency in toluene and most probably re¯ects the formation of a less reactive, more stable, and most likely more aggregated solution state structure for the initiator in toluene relative to the structure in THF. Diisopropylaminolithium in its uncomplexed form adopts an in®nite helix structure in the solid state [52] while the corresponding 1:1 THF complex forms a more usual dimer structure [41]. These di€erences are likely to be re¯ected in solution in toluene and THF respectively and so support the

argument for a more aggregated more stable structure in toluene. The data in entry two Table 5 show that addition of TMEDA to the toluene system in a 1:1 molar ratio with initiator causes considerable changes. Firstly, and very interestingly, the tacticity of the product polymer is seen to shift predominantly to be syndiotactic, with the data being very similar to that for polymers produced in THF solvent at 258C (Table 2). The data is again in accordance with Bernoullian statistics [50] with Pm 0 0.2. This switch in microstructure with toluene as the bulk solvent has useful technical potential. It is induced solely by the addition of a low level of TMEDA, and must arise from some intimate interaction of the latter with the Li counterion at the active centre. Whatever the structure formed, the co-ordinative control by Li originally favouring meso placement of incoming monomer in toluene is lifted, and the steric control is more like that seen in THF solvent. It is unlikely that TMEDA co-ordination to Li results in the generation of a fully solvent separated ion pair (Li enolate) as in THF solvent or in mixtures of non-donor and donor solvents, since the latter systems involve a large excess of donor molecules. With TMEDA limited to a stoichiometric level relative to the initiator (and active centre) it is more likely that chelation to Li removes the need for the counterion to increase its co-ordination sphere by interaction with incoming monomer. Some interaction with the penultimate carbonyl group of the chain may be retained i.e. the active centre remains somewhat more structured than is the case for fully developed solvent separated ion pairs in bulk THF. This would also explain why the

Table 5. Polymerisation of MMA at ÿ788C in toluene initiated by diisopropylaminolithiuma Polymer yieldb (%)

Additive

Type ÿ TMEDA ButOLi ButOLi ButOLi ButOLi a

Initiator/ additive (mol/ mol) ÿ 1:1 1:1 1:2 1:5 1:10

0100 0100 0100 0100 0100 0100

Molecular weight

Mn

Mw

Mw/Mn

iso

hetero

syndio

21,800 233,000 9,800 11,800 33,200 10,000

53,000 95,200 23,400 29,400 113,000 25,000

2.4 4.1 2.4 2.5 3.4 2.5

74 7 69 71 78 65

22 31 28 24 16 18

4 62 3 5 6 17

Initiator formed from 1:1 diisopropylamine/BunLi. Reaction time, 4 h.

b

Tacticity (%)

Anionic polymerisation using lithium amides

1885

Scheme 2. Reaction of 2,6-dimethylpiperidinolithium TMEDA with ethyl tiglate.

level of syndiotacticity achieved (062%) is less than that observed when THF is used as a bulk solvent (typically 075±80%) at ÿ788C. Another important di€erence emerging when TMEDA is used as an additive in toluene versus THF as the solvent is the very low initiator eciency. The molecular weight data in entry two Table 5 indicates that the Mn is around twenty times higher than the predicted value of 10,000, and the dispersity also much higher at 04. This implies a higher level of side-reactions both before initiation and during propagation, or the involvement of some complex TMEDA initiator structure in solution which allows only a partial net use of nucleophilic N centres. There does exist precedent in the literature for such structures [51]. Overall, however, the e€ect is a very negative one as far as control of polymerisation is concerned, because while the facile switch in tacticity from iso- to syndiotactic polymer on addition of TMEDA is attractive, this is only so if it goes hand-in-hand with good control of molecular weight and molecular weight distribution.

A broadness in the molecular weight distribution is not uncommon in hydrocarbon solvents with organolithium initiators and has been remedied in the past by addition of suitable salts [16]. In the present work LiOBut was examined as a potential s-complexant. The results are shown as entries 3±6 in Table 5 where an increasing ratio of initiator/ LiOBut of 1:1 to 1:10 has been used. The in¯uence on the tacticity of the polymer is negligible with similar microstructure to that produced in pure toluene being generated (070±75% isotactic). In contrast LiOBut does improve the initiator eciency with, in three instances a close approach to the target Mn of 10,000 being achieved, albeit with dispersity of 02.5. This suggests that stabilisation of the initiator species is achieved by formation of discrete adducts in solution. In the case of alkyllithium with lithium alkoxides, adducts have been successfully isolated and characterised [14], and Lochmann and MuÈller have quanti®ed the reduction in reactivity of the resultant species to be a factor of 010 [15]. The overall e€ect is to generate a much cleaner polymerisation system, and this seems to be the case in the present work as well. Stabilisation of the

1886

S. A. Couper et al.

active centre by complexation of LiOBut during propagation would also be expected to reduce backbiting and cyclic termination of the growing chain. This does not seem to be a signi®cant factor in this work however, since the molecular weight distribution seems little a€ected even when a very large excess of salt is added. Mechanism of initiation reaction By analogy with initiation using lithium alkyls it is believed that lithium amides function by initial attack of the amide anionic centre on the methylene carbon of MMA, to generate the corresponding lithium enolate (either solvent separated or further complexed in some way). In all, 36 attempts were made with in situ generated lithium amides to isolate compounds derived from the above addition reaction. As well as MMA itself, methyl and ethyl crotonate, and methyl and ethyl tiglate were employed as the electrophilic centre. Only in the reaction of dimethylpiperidinolithium with ethyl tiglate in the presence of TMEDA was a stable species isolated and characterised (see experimental section). Initially it was believed that this was the anticipated adduct (I) (Scheme 2). However, a full assessment of all the characterisation data showed it to be a mixture of the E and Z geometric isomers (II) arising from proton abstraction of the b-methyl group by the bulky dimethylpiperidino anion. Presumably abstraction from this methyl group is favoured by electron delocalisation onto the ester group via a conjugated enolate structure. Failure to isolate any of the anticipated adducts was disappointing, but in the case of MMA there is no experimental evidence which points to any initiation mechanism other than proposed addition, analogous to the addition of lithium alkyls.

SUMMARY

In-situ generated lithium amides are e€ective initiators of polymerisation of methyl methacrylate. In THF at ÿ788C highly syndiotactic polymer is formed in essentially quantitative yield using a variety of amides. The reactions are however not clean living systems and this is re¯ected in the molecular weight data. Corresponding reactions in toluene at ÿ788C yield isotactic polymer again in quantitative yield, but again with low initiator eciency. Addition of trace levels of TMEDA switches the tacticity to syndiotactic in toluene but worsens the initiator eciency. In contrast, addition of varying levels of LiOBut in toluene maintains the isotactic microstructure of the polymer and improves the initiator eciency signi®cantly. The system still, however, falls short of being clean and living.

AcknowledgementsÐWe acknowledge the receipt of a CASE award from the EPSRC for S. A. C. and appreciate the support of Zeneca Resins and, in particular, Dr D. M. Haddleton and Dr N. Richards then of that Company. GPC molecular weight data were provided by RAPRA.

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