Controlled Radical Grafting: Nitroxyl-Mediated Maleation of Model

Mark E. Scott,† J. Scott Parent,*,† John Dupont,† and Ralph A. Whitney‡. Departments of Chemical Engineering and of Chemistry, Queen's Univers...
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Ind. Eng. Chem. Res. 2003, 42, 3662-3670

Controlled Radical Grafting: Nitroxyl-Mediated Maleation of Model Hydrocarbons Mark E. Scott,† J. Scott Parent,*,† John Dupont,† and Ralph A. Whitney‡ Departments of Chemical Engineering and of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6

An exploration of nitroxyl-mediated polymerization principles for the synthesis of maleated polyethylene (PE) and polybutadiene (PBD) is presented. Thermolysis and disproportionation rates for eight alkoxyamines of relevance to PE and PBD modification are reported, including heptan-3-yl- and hept-5-en-3-yl-based alkoxyamines of 2,2,6,6-tetramethylpiperidinyloxyl, 2,2,5trimethyl-4-phenyl-3-azahexane-3-oxyl, and 1,1,3,3-tetramethylisoindolin-2-yloxyl. The mediation of heptan-3-yl radical activity was complicated by extensive disproportionation of the alkoxyamines to olefin and the corresponding hydroxylamine. The hept-5-en-3-yl-based alkoxyamines underwent C-O bond homolysis without incurring significant disproportionation, and the addition of maleic anhydride (MAn) to the liberated allylic radical proceeded at a rate comparable to that of nitroxyl trapping. However, rapid disproportionation of the succinyl-based alkoxyamines arrested grafting after a single MAn addition. The consequences for controlled radical grafting of maleic anhydride to polymers are discussed. Introduction The introduction of single, pendant anhydride groups onto nonpolar polymers is a common means of preparing blend compatibilization agents and phase adhesion promoters.1,2 Radical-mediated grafting has been widely adopted for this purpose because it employs relatively inexpensive reagents and is amenable for use in conventional processing equipment.3 However, the primary difficulty in developing maleic anhydride (MAn) or vinyltrimethoxysilane (VTMS) grafted derivatives of high-density polyethylene (PE) and 1,4-polybutadiene (PBD) is the cross-linking incurred as a result of macroradical combination.4 In the case of PBD modifications, the high molecular weight of the elastomer, coupled with the propensity of allylic radicals to terminate by combination, leads to severe gel formation. We have explored the potential to adapt nitroxylmediated polymerization chemistry5,6 for the purpose of improving the selectivity of PE and PBD graft modification. Scheme 1 illustrates an ideal controlled radical grafting (CRG) process for the maleation of highdensity PE. Reversible termination of alkyl and succinyl radicals by a suitable nitroxyl should, in theory,7,8 limit the steady-state radical concentration to such a level that the rate of termination (a bimolecular radical process) is reduced disproportionately relative to the rate of grafting. A graft propagation cycle is comprised of both monomer addition and hydrogen atom abstraction reactions (intramolecular abstraction is omitted from Scheme 1 for clarity), with the overall order of the MAn addition rate being intermediate between zero and one.9 Therefore, nitroxyl mediation of radical concentrations may establish a pseudoliving grafting process * To whom correspondence should be addressed. Tel.: (613) 533-6266. Fax: (613) 533-6637. E-mail: parent@ chee.queensu.ca. † Department of Chemical Engineering. ‡ Department of Chemistry.

wherein comparatively high graft levels are achieved while incurring a relatively low extent of polymer crosslinking. The development of a nitroxyl-mediated grafting process requires the consideration of factors beyond those associated with monomer polymerization. Most notably, the nitroxyl must regulate the concentration of a variety of carbon-centered radicals, including those derived from the polymer and those resulting from monomer addition. Also, the kinetics of monomer addition and hydrogen atom abstraction reactions that constitute the graft propagation cycle must be sufficiently rapid to compete with the rate of radical trapping by the nitroxyl-mediating agent. Furthermore, CRG process development must be concerned with alkoxyamine disproportionation (Scheme 2) because not only does this process lead to the loss of a dormant site but also air oxidation of the hydroxylamine to nitroxyl will displace alkoxyamine dissociation equilibria toward the dormant states.10-12 Although important insights into the disproportionation of benzylic- and acrylic-based alkoxyamines have been gained,12,13 the alkyl and allylic systems that are relevant to CRG processes have received comparatively little attention. In high-density PE modifications, the mediation of secondary aliphatic carbon radicals is required, while the modification of PBD involves the mediation of allylic radicals. The challenges associated with the analysis of polymers containing a low concentration of reactive functional groups have led us to explore the CRG concept using appropriate model compounds. Given the detailed structural analysis that is needed to evaluate CRG principles, a model system that is amenable to chromatographic separation and full structural characterization is of considerable value. Our approach is based on the premise that a successful CRG of a model compound is a precondition for success in a polymeric system. We have selected heptan-3-ylalkoxyamine as a representative of the PE reactivity and hept-5-en-3-ylal-

10.1021/ie020888v CCC: $25.00 © 2003 American Chemical Society Published on Web 07/10/2003

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3663 Scheme 1

Scheme 2

Scheme 3

koxyamine as a model for PBD. Similar to their linear counterparts, cyclooctane and cyclooctene generate a limited number of isomeric activation and disproportionation products, and they have also been evaluated. The recent surge of activity in the controlled radical polymerization field has led to the development of a number of nitroxyls that may be CRG candidates (Scheme 3). 2,2,6,6-Tetramethylpiperidinyloxyl (TEMPO) was selected based on its widespread use in the literature, while 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO) was chosen because of its superior stability at high temperature. 2,2,5-Trimethyl-4-phenyl-3-azahexane-3oxyl (TIPNO) was selected for its reported ability to mediate the radical copolymerization of styrene with MAn.14 Our aim was to assess the ability of the aforementioned nitroxyls to support an effective CRG process using measurements of the rates of alkoxyamine thermolysis and disproportionation. In this work, the rates of nitroxyl exchange and alkoxyamine disproportionation are reported for eight model systems. Those systems demonstrating the greatest promise have been examined for grafting activity with respect to MAn, vinyltriethoxysilane, and styrene addition. Experimental Section Materials. The following reagents were used as received from Sigma-Aldrich (Oakville, Ontario, Canada): dicumyl peroxide (98%), benzoyl peroxide (97%), TEMPO (98%), copper powder (99%), copper(II) bromide (99%), N,N,N′,N′,N′′-pentamethyldiethylenetriamine (99%), cyclooctane (99+%), cis-cyclooctene (95%), cis-2-heptene (97%), trans-2-heptene (99%), cis-3-heptene (96%), trans-

3-heptene (99%), trans-2-pentenal (95%), phosphorus tribromide (99%), ethylmagnesium chloride (2.0 M in ether), platinum on activated carbon (10 wt %), 2,2′dipyridyl (99+%), cis,cis-1,3-cyclooctadiene (98%), and VTMS (97%). N-Bromosuccinimide (Aldrich, 99%) was recrystallized from water and dried under vacuum. MAn (Aldrich, 99%) was recrystallized from chloroform and dried under vacuum. Styrene was distilled under reduced pressure and tetrahydrofuran was distilled over CaH2 prior to use. TIPNO was prepared according to Benoit et al.15 TMIO was prepared according to Griffiths et al. and was characterized by melting point analysis (128 °C).16 trans-5-Bromo-3-heptene was prepared according to Fu and Cook.17 Activated acidic alumina (Anachemia, 80-200 mesh) and neutral silica (Merck, 70-230 mesh) were used as received. Instrumentation and Analysis. All NMR spectra were recorded with a Bruker AM-300 spectrometer in CDCl3 or C6D6 with chemical shifts reported relative to tetramethylsilane. Mass spectrometry was carried out using a Fisons VG Quattro triple-quadrupole mass spectrometer. Samples were ionized by electrospray ionization, fast atom bombardment (FAB), electron ionization, or chemical ionization using isobutane or ammonia, as indicated. High-resolution mass spectrometry was performed by the Facility for Mass Spectrometry at the University of Ottawa. Elemental analysis was conducted by Guelph Chemical Laboratories Ltd. (Guelph, Ontario, Canada). Normal-phase high-pressure liquid chromatography (HPLC) was performed using a Waters model 400 instrument equipped with UV-vis and refractive index detectors (Supelcosil PLC-Si column). Gas chromatography (GC) measurements were conducted using a Hewlett-Packard split-splitless 5890 series II instrument equipped with a flame ionization detector. 3-Bromocyclooctene. The brominated alkene was synthesized according to a general method outlined by Greenwood and Kellert.18 A solution of carbon tetrachloride (80.0 mL), cis-cyclooctene (14.33 g, 0.1403 mol), N-bromosuccinimide (15.24 g, 0.0856 mol), and benzoyl peroxide (0.0636 g, 0.26 mmol) was degassed by three freeze/pump/thaw cycles and heated to reflux (bp ) 78 °C) under nitrogen for 2 h. The solution was filtered, and the solvent and light initiator fragments were removed by Kugelrohr distillation (52 °C, 11 mmHg). The resulting yellow residue was distilled further by

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Kugelrohr distillation (60 °C, 0.30 mmHg) to give 3-bromocyclooctene (4.81 g, 30% yield). 1H NMR (CDCl ): δ 1.33, 1.39 (m, 2H, -CH CH CH 3 2 2 2 CHBr-), 1.54, 1.60 (m, 2H, -CH2CH2CHBr-), 1.74, 1.67 (m, 2H, dCHCH2CH2-), 2.05, 2.29 (m, 2H, -CH2CH2CH2CHBr-), 2.14, 2.23 (m, 2H, dCHCH2CH2-), 4.96 (m, 1H, -CHBr-), 5.61 (m, 1H, -CHdCHCHBr-), 5.80 (m, 1H, -CHdCHCHBr-). 2-(Cyclooct-2-enyloxy)-1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindole (TMIO-cyclooctene). The alkoxyamine was synthesized according to a general method outlined by Matyjaszewski et al.19 3-Bromocyclooct-1-ene (0.1776 g, 0.934 mmol), TMIO (0.2099 g, 1.103 mmol), copper powder (0.0638 g, 1.004 mmol), copper dibromide (0.0046 g, 0.021 mmol), and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (0.0100 g, 0.058 mmol) were added to benzene (1.25 mL). The mixture was freeze/thaw degassed and heated to 75 °C for 4 h under nitrogen. The solution was concentrated in vacuo, and excess nitroxyl was removed by Kugelrohr distillation (75 °C, 0.30 mmHg). Hexane (2 mL) was added to the residue, which was then applied to an alumina column. The column was eluted with hexane and concentrated in vacuo to yield a colorless liquid (0.0927 g, 33% yield). 1H NMR (CDCl ): δ 1.36 (m, 12H, -C(CH ) -), 1.303 3 2 1.75 (m, 6H, -CH2CH2CH2CH2CHO-), 2.10 (m, 2H, dCHCH2CH2-), 2.20 (m, 2H, -CH2CHO-), 4.75 (m, 1H, -CHO-), 5.68 (m, 2H, -CHdCH-), 7.12, 7.22 (m, 2 × 2H, Ar-H). Calcd for C20H29NO (CI, M+1): m/e 300.2327. Found: m/e 300.2339. 2-(Cyclooct-2-enyloxy)-1,1,6,6-tetramethylpiperidine (TEMPO-cyclooctene). The alkoxyamine was synthesized according to a general method outlined by Matyjaszewski et al.19 3-Bromocyclooct-1-ene (0.4399 g, 2.314 mmol), TEMPO (0.4279 g, 2.739 mmol), copper powder (0.1508 g, 2.373 mmol), copper dibromide (0.0036 g, 0.016 mmol), and 2,2′-dipyridyl (0.0147 g, 0.094 mmol) were added to benzene (3.1 mL). The mixture was freeze/thaw degassed and heated to 75 °C for 5.5 h under nitrogen. The solution was concentrated in vacuo, and excess nitroxyl was removed by Kugelrohr distillation (85 °C, 0.40 mmHg). Hexane (2 mL) was added to the residue, which was then applied to an alumina column. The column was eluted with a 5% ethyl acetate, 95% hexane solution and concentrated in vacuo to yield a colorless liquid (0.1553 g, 25% yield). 1H NMR (CDCl ): δ 1.18 (s, 12H, N-C(CH ) -), 3 3 2 1.20-1.60 (m, 6H, -CH2CH2CH2CH2CHO-), 1.45 (m, 6H, -C(CH3)2CH2CH2CH2C(CH3)2-), 2.08 (m, 2H, dCHCH2CH2-), 2.17 (m, 2H, -CH2CHO-), 4.65 (m, 1H, -CHO-), 5.60 (m, 2H, -CHdCH-). Calcd for C17H31NO (CI, M+1): m/e 266.2484. Found: m/e 266.2478. 1-(1-Ethylpent-2-enyloxy)-2,2,6,6-tetramethylpiperidine (TEMPO-heptene). The alkoxyamine was synthesized according to a general method outlined by Matyjaszewski et al.19 trans-5-Bromo-3-heptene (1.2247 g, 6.92 mmol), TEMPO (1.2884 g, 8.25 mmol), copper powder (0.6680 g, 10.51 mmol), copper dibromide (0.0317 g, 0.14 mmol), and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (0.1008 g, 0.58 mmol) were added to benzene (12.0 mL). The mixture was freeze/thaw degassed and heated to 75 °C for 14 h under nitrogen. The solution was concentrated in vacuo, and the residue was diluted in hexane (2 mL) and applied to a silica column. The column was eluted using hexanes, and the eluent

was concentrated in vacuo to yield a colorless liquid (0.2588 g, 15% yield). 1H NMR (CDCl ): δ 0.85 (t, 3H, CH CH CH(ONR )), 3 3 2 2 0.99 (t, 3H, CH3CH2-), 1.09 (m, 4H, NC(CH3)2CH2-), 1.12 (s, 12H, NC(CH3)2-), 1.17 (m, 2H, NC(CH3)2CH2CH2-), 2.06 (m, 2H, dCHCH2CH3), 1.43, 1.76 (m, 2H, -CH2CHO-), 3.90 (Z isomer, d of t, 0.72H, -CHO-), 4.36 (E isomer, d of t, 0.28H, -CHO-), 5.33 (m, 1H, -CHdCHCH(ONR2)), 5.49 (m, 1H, -CHdCHCH(ONR2)). Calcd for C16H31NO: C, 75.83; H, 12.33; N, 5.53; O, 6.31. Found: C, 75.29; H, 12.42; N, 5.40; O, 6.89. 1-(1-Ethylpent-2-enyloxy)-1,1,3,3-tetramethyl-2,3dihydro-1H-isoindole (TMIO-heptene). The alkoxyamine was synthesized according to a general method outlined by Matyjaszewski et al.19 trans-5-Bromo-3heptene (1.2307 g, 6.95 mmol), TMIO (1.3206 g, 6.94 mmol), copper powder (0.6785 g, 10.68 mmol), copper dibromide (0.0323 g, 0.14 mmol), and N,N,N′,N′,N′′pentamethyldiethylenetriamine (0.1015 g, 0.59 mmol) were added to benzene (13.0 mL). The mixture was freeze/thaw degassed and heated to 75 °C for 7.5 h under nitrogen. The solution was concentrated in vacuo, and the residue was diluted in hexane (2 mL) and applied to an alumina column. The column was eluted using hexanes, and the eluent was concentrated in vacuo to yield a colorless liquid (0.4135 g, 21% yield). 1H NMR (CDCl ): δ 0.96 (t, 3H, CH CH CH(ONR )), 3 3 2 2 1.02 (t, 3H, CH3CH2-), 1.33, 1.40, 1.44, 1.54 (s, 12H, NC(CH3)2-), 2.09 (m, 2H, dCHCH2CH3), 1.49, 1.84 (m, 2H, -CH2CHO-), 4.05 (Z isomer, d of t, 0.83H, -CHO-), 4.50 (E isomer, d of t, 0.17H, -CHO-), 5.40 (m, 1H, -CHdCHCH(ONR2)), 5.61 (m, 1H, -CHdCHCH(ONR2)), 7.10, 7.23 (m, 2 × 2H, ArH). Calcd for C19H29NO (CI, M+1): m/e 288.2327. Found: m/e 288.2335. N-tert-Butyl-O-(1-ethylpent-2-enyl)-N-(2-methyl1-phenylpropyl)hydroxylamine (TIPNO-heptene). The alkoxyamine was synthesized according to a general method outlined by Matyjaszewski et al.19 trans-5Bromo-3-heptene (1.4985 g, 8.46 mmol), TIPNO (2.8103 g, 12.75 mmol), copper powder (0.8204 g, 12.91 mmol), copper dibromide (0.0396 g, 0.18 mmol), and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (0.1260 g, 0.73 mmol) were added to benzene (16.0 mL). The mixture was freeze/thaw degassed and heated to 70 °C for 4 h under nitrogen. The reaction solution was concentrated in vacuo, and the residue was fractionated by Kugelrohr distillation (80 °C, 0.40 mmHg) to give a slightly yellow residue, which was diluted in hexane (2 mL), applied to a silica column, and eluted using hexanes. The eluent was concentrated in vacuo to yield a colorless liquid (0.3175 g, 10% yield). 1H NMR (CDCl ): δ 0.42, 0.47, 1.06, 1.21 (d, 6H, 3 (CH3)2CH-), 0.85, 0.90 (t, 3H, CH3CH2CH(ONR2)), 0.94, 1.00 (s, 9H, (CH3)3-), 1.03, 1.06 (t, 3H, CH3CH2CHd), 1.45, 1.56, 1.85 (m, 2H, CH3CH2CH(ONR2)), 2.11 (m, 2H, CH3CH2CHd), 2.17 (m, 1H, (CH3)2CH-), 3.31, 3.42 (d, 1H, NCH(Ph)-), 3.97, 4.19 (Z isomer, d of t, 0.94H, -CHO-), 4.47, 4.59 (E isomer, d of t, 0.10H, -CHO-), 5.43, 5.54 (m, 1H, CH3CH2CHdCH-), 5.55, 5.63 (m, 1H, CH3CH2CHdCH-), 7.13-7.42 (m, 5H, ArH). Calcd for C21H35NO (FAB, M+1): m/e 318.2797. Found: m/e 318.2798. 1-Cyclooctyloxy-2,2,6,6-tetramethylpiperidine (TEMPO-Cyclooctane). TEMPO (0.5821 g, 3.73 mmol) and dicumyl peroxide (0.4997 g, 1.85 mmol) were dissolved in cyclooctane (10.0183 g). The solution was degassed and heated to 123 °C for 6t1/2 (t1/2 at 123 °C

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for dicumyl peroxide is 295 min) under nitrogen. Excess cyclooctane and light initiator fragments were removed by Kugelrohr distillation (70 °C, 0.30 mmHg) to give a yellow residue, which was taken up in hexane (2 mL) and applied to an alumina column. The column was eluted with a 1% ethyl acetate, 99% hexane solution, and the eluent was concentrated in vacuo to yield a colorless liquid (0.3821 g, 39% yield). 1H NMR (CDCl ): δ 1.15 (s, 12H, NC(CH ) -), 1.303 3 2 1.80 (m, 10H, -CH2CH2CH2CH2CHO-), 1.45 (m, 6H, -C(CH3)2CH2CH2CH2C(CH3)2-), 1.65-1.80, 1.96-2.15 (m, 4H, -CH2CHO-), 3.85 (m, 1H, -CHO-). Calcd for C17H33NO (CI, M+1): m/e 268.2640. Found: m/e 268.2636. 2-(Cyclooctyloxy)-1,1,3,3-tetramethyl-2,3-dihydro1H-isoindole (TMIO-Cyclooctane). TMIO (0.1414 g, 0.743 mmol) and dicumyl peroxide (0.1000 g, 0.370 mmol) were dissolved in cyclooctane (2.2272 g). The solution was degassed and heated to 123 °C for 6t1/2 (t1/2 at 123 °C for dicumyl peroxide is 295 min) under nitrogen. Excess cyclooctane and light initiator fragments were removed by Kugelrohr distillation (88 °C, 0.30 mmHg) to give a yellow residue, which was diluted in hexane (2 mL) and applied to a silica column. The column was eluted with hexanes and concentrated in vacuo to yield a colorless liquid (0.0312 g, 14% yield). 1H NMR (CDCl ): δ 1.35 (s, 12H, NC(CH ) -), 1.523 3 2 1.65, 1.70-1.85 (m, 10H, -CH2CH2CH2CH2CHO-), 1.70-1.85, 1.96-2.09 (m, 4H, -CH2CHO-), 3.92 (m, 1H, -CHO-), 7.12, 7.22 (d, 2 × 2H, ArH). Calcd for C20H31NO (CI, M+1): m/e 302.2484. Found: m/e 302.2479. 1-(1-Ethylpentyloxy)-2,2,6,6-tetramethylpiperidine (TEMPO-heptane). TEMPO-heptene (0.1200 g, 0.47 mmol) in hexane (10.0 mL) was hydrogenated over Pt on activated carbon (0.0150 g, 10 wt %) for 19.5 h at 43 psi. The solution was filtered through Celite and concentrated in vacuo. The residue was diluted in hexane (2 mL), applied to a silica column, and eluted using hexanes. The eluent was concentrated in vacuo to yield a colorless liquid (0.0847 g, 71% yield). 1H NMR (CDCl ): δ 0.87 (t, 3H, CH CH CH(ONR )), 3 3 2 2 0.92 (t, 3H, CH3CH2-), 1.12 (s, 12H, NC(CH3)2-), 1.32 (m, 4H, NC(CH3)2CH2-), 1.33 (m, 2H, CH3CH2CH2-), 1.45 (m, 2H, NC(CH3)2CH2CH2-), 1.48 (m, 2H, CH3CH2CH2-), 1.39-1.51, 1.62-1.84 (m, 4H, -CH(ONR2)CH2CH3, -CH2CH(ONR2)CH2CH3), 3.68 (t of t, 1H, -CH(ONR2)-). Calcd for C16H33NO: C, 75.23; H, 13.02; N, 5.48; O, 6.26. Found: C, 74.77; H, 13.43; N, 5.07; O, 6.73. 2-(1-Ethylpentyloxy)-1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindole (TMIO-heptane). TMIO-heptene (0.2046 g, 0.81 mmol) in hexane (12.0 mL) was hydrogenated over Pt on activated carbon (0.0274 g, 10 wt %) for 18 h at 40 psi. The solution was filtered through Celite and concentrated in vacuo. The residue was diluted in hexane (2 mL), applied to an alumina column, and eluted using hexanes. The eluent was concentrated in vacuo to yield a colorless liquid (0.2029 g, 98% yield). 1H NMR (CDCl ): δ 0.96, 0.97 (t, 2 × 3H, CH CH -, 3 3 2 CH3CH2CH(ONR2)), 1.38 (s, 12H, NC(CH3)2-), 1.40 (m, 2H, CH3CH2CH2-), 1.43 (m, 2H, CH3CH2CH2-), 1.621.72 (m, 4H, -CH(ONR2)CH2CH3, -CH2CH(ONR2)CH2CH3), 7.11, 7.25 (m, 2 × 2H, ArH). Calcd for C19H31NO (CI, M+1): m/e 290.2484. Found: m/e 290.2483. N-tert-Butyl-O-(1-ethylpentyl)-N-2-methyl-1-phenylpropyl)hydroxylamine (TIPNO-heptane). TIPNOheptene (0.0790 g, 0.249 mmol) in hexane (20.0 mL) was

hydrogenated over Pt on carbon (0.0150 g, 10 wt %) for 3.25 h at 42 psi. The solution was filtered through Celite and concentrated in vacuo. The residue was then diluted in hexane (2 mL), applied to a silica column, and eluted using hexanes. The eluent was concentrated in vacuo to yield a colorless liquid (0.0636 g, 80% yield). 1H NMR (CDCl ): δ 0.47, 1.19 (d, 6H, (CH ) CH-), 3 3 2 0.96, 0.97 (t, 2 × 3H, CH3CH2-, CH3CH2CH(ONR2)), 0.97 (s, 9H, (CH3)3-), 1.27-1.56 (m, 4H, CH3CH2CH2-, CH3CH2CH2-), 1.62-1.76, 1.89-2.09 (m, 4H, -CH(ONR2)CH2CH3, -CH2CH(ONR2)CH2CH3), 2.17 (m, 1H, (CH3)2CH-), 3.42 (d, 1H, NCH(Ph)-), 3.81 (m, 1H, -CHO-), 7.16-7.44 (m, 5H, ArH). Calcd for C21H37NO (CI, M+1): m/e 320.2953. Found: m/e 320.2952. Synthesis and Isolation of MAn Grafted Products. To a solution of TEMPO-heptene (0.0093 g, 0.037 mmol) in C6D6 (0.74 mL) was added MAn (0.0107 g, 0.109 mmol). The solution was freeze/thaw degassed, sealed in an NMR tube, and heated to 111 °C for 7 h. The solution was cooled and separated by HPLC using a 40% ethyl acetate, 60% hexane eluent to give the starting alkoxyamine, MAn, and heptene-g-MAn. To a solution of TIPNO-heptene (0.0101 g, 0.032 mmol) in C6D6 (0.64 mL) was added MAn (0.0104 g, 0.106 mmol). The solution was freeze/thaw degassed, sealed in an NMR tube, and heated to 111 °C for 19 min. The solution was cooled, concentrated in vacuo, and separated by HPLC using a 40% ethyl acetate, 60% hexane eluent to give the starting alkoxyamine, MAn, and heptene-g-MAn. 1H NMR (CDCl ): δ 0.96 (t, 3H, CH CH CH(MAn)), 3 3 2 1.01 (t, 3H, CH3CH2CHd), 1.64, 1.78 (m, 2H, CH3CH2CH(MAn)), 2.08 (m, 2H, CH3CH2CHd), 3.21 (m, 1H, -CHMAn), 5.33 (m, 1H, -CHdCHCH(MAn)), 5.69 (m, 1H, -CHdCHCH(MAn)), 6.59 (s, 1H, OC(dO)CHdC). Calcd for C11H14O3 (EI, M+): m/e 194.0943. Found: m/e 194.0947. Nitroxyl Exchange Reactions. A 20-fold excess of TMIO was added to a 0.01 M solution of TEMPO- or TIPNO-based alkoxyamine in benzene. A 20-fold excess of TEMPO was employed for nitroxyl exchanges of TMIO-based alkoxyamines. Each Pyrex ampule contained a 1.5-mL aliquot of solution, which was degassed by three freeze/pump/thaw cycles, then sealed in vacuo, and stored at -30 °C. Ampules were heated to the desired temperature in an isothermal hot air bath ((1 °C) for various time intervals, removed, and subsequently quenched in a liquid-nitrogen bath. All ampules were then stored at -30 °C until analyzed by GC. GC analysis for alkoxyamines required a 19095Z-221 HP-SIMDIST megabore column, with injector and detector temperatures of 120 and 300 °C, respectively, unless otherwise noted. Oven temperature profile for heptene-alkoxyamines: 35 °C for 4 min, ramp to 130 °C at 5 °C/min, hold for 10 min, and ramp to 250 °C at 20 °C/min. Oven temperature profile for heptanealkoxyamines: 40 °C for 4 min, ramp to 130 °C at 5 °C/min, hold for 2 min, and ramp to 250 °C at 20 °C/ min. For TEMPO-cyclooctane and TEMPO-cyclooctene, injection and detector temperatures were 225 and 300 °C, respectively, and the oven temperature profile was 35 °C for 4 min, ramp to 130 °C at 5 °C/min, hold for 10 min, ramp to 280 °C at 10 °C/min, and hold for 10 min. GC analysis for olefins required a Supelco SPB-1 microbore column, with injector and detector temperatures of 120 and 300 °C, respectively. Oven temperature

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Scheme 4

profile for cyclooctene and cyclooctadiene: 35 °C for 10 min, ramp to 250 °C at 20 °C/min, and hold for 16 min. Oven temperature profile for heptene: 40 °C for 4 min, ramp to 130 °C at 5 °C/min, hold for 2 min, and ramp to 250 °C at 20 °C/min. In every experiment the material balance was satisfied to within 5% of the initial alkoxyamine loading, thereby generating confidence not only that the experimental technique quantified the reaction products accurately but also that no unforeseen side reactions were overlooked. Additional precautions were taken to ensure that the acquired data reflected the rates of activation and disproportionation of the intended alkoxyamine. In the first place, a 20-fold excess of scavenging nitroxyl ensured that the regeneration of the initial alkoxyamine was insignificant. Kothe et al. have concluded that the rate of TEMPO-cumyl activation is accurately determined under conditions where the ratio of scavenging nitroxyl to alkoxyamine is greater than 25:1.8 We therefore limited our exchange conversions to a maximum of 60% in order to minimize the reformation of the original alkoxyamine from the exchanged product. Alkoxyamine Disproportionation Reactions. The kinetics of alkoxyamine disproportionation in the absence of added nitroxyl were monitored by 1H NMR. Solutions of alkoxyamine in C6D6 (0.01 M) were degassed by three freeze/pump/thaw cycles within a sealable NMR tube. The solutions were maintained at the desired temperature of (1 °C in a controlled air bath. Integrations of NMR spectra acquired periodically provided estimates of reagent and product concentrations. Kinetics of CRG of MAn. Alkoxyamines were purified by column chromatography immediately prior to use. Solutions of alkoxyamine (0.05 M) and the desired amount of MAn in C6D6 were charged to a sealable NMR tube and degassed by three freeze/pump/thaw cycles prior to heating. For the CRG of MAn onto TEMPOheptene, conversion was monitored by 1H NMR at various time intervals using integration of resonances at δ 2.68 (1H, -CHMAn) for heptene-g-MAn relative to those at δ 4.12 (Z isomer, 1H, -CHO-) and δ 4.55 (E isomer, 1H, -CHO-) for TEMPO-heptene. Analysis of the TIPNO-heptene system was based upon resonances at δ 2.68-2.82 (diasteromers, 1H, -CHMAn) for heptene-g-MAn relative to those at δ 4.15, 4.42, 4.68, and 4.80 (E/Z isomers/diastereomers, 1H, -CHO-) for TIPNO-heptene. Results and Discussion The C-O bond thermolysis rate for each alkoxyamine was determined by monitoring the extent of nitroxyl displacement by a second nitroxyl that was present in a 20-fold excess (Scheme 4). This nitroxyl exchange or radical crossover technique has been used to characterize the thermolysis rates for several styrenic and acrylic systems of relevance to controlled radical polymerization.20-23 Disproportionation during the nitroxyl exchange experiments was monitored by tracking the

Figure 1. Exchange data for TEMPO-heptane in the presence of excess TMIO in benzene at 160 °C (X ) alkoxyamine conversion): [, TEMPO-heptane; 9, TMIO-heptane; 2, total heptenes; s, regressions based on kact.

evolution of the corresponding olefinic products. Representative data acquired for a radical crossover experiment are presented in Figure 1 for the TEMPOheptane system. Consistent with other reports of alkoxyamine activation and disproportionation, both reactions followed first-order kinetics.8,24 That is, the rates of homolytic bond cleavage and disproportionation were proportional to the concentration of alkoxyamine. First-order log plots were prepared to derive rate constants for the activation (kact) and disproportionation (kex d ) processes (Scheme 4). The rate of alkoxyamine disproportionation in the absence of excess nitroxyl was also measured by independent NMR experiments, yielding a second series of first-order rate constants (kd). PE Model Compound Activation. The model compounds that relate to high-density PE modification are the heptan-3-ylalkoxyamines of TEMPO, TMIO, and TIPNO. Table 1 lists the first-order rate constants for homolytic C-O bond cleavage for this series of compounds along with their rates of disproportionation in isolation (kd) as well as in the presence of a 20-fold excess of the exchange nitroxyl (kex d ). Given the relatively high C-O bond dissociation energy of aliphatic alkoxyamines, their activation required more severe temperatures than those needed for styrenic and acrylic radical polymerizations.25 This is not considered to be a significant barrier to CRG process development, given that graft modifications must be conducted in the range of the resin’s melting point (approximately 130 °C). However, the heptane-3-ylalkoxyamines of TEMPO and TMIO yielded primarily disproportionation products at 160 °C, with nitroxyl exchange occurring relatively infrequently. The ratio of rate constants for disproportionation (kd) relative to activation (kact) for TEMPO-heptane and TMIO-heptane were kd/kact ) 1.0 and kd/kact ) 2.0, respectively, indicating that neither of these nitroxyls would be capable of supporting a viable CRG process. Rather than facilitating repeated monomer additions, alkoxyamine disproportionation would terminate the

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3667 Table 1. Rate Constants for the Thermolysis of PE Model Alkoxyamines alkoxyamine

kact (min-1)

TEMPO-cyclooctane TEMPO-heptane TEMPO-cyclohexaneb TIPNO-heptane TMIO-heptane

k433K ) 9.0 × 10-5 k433K ) 6.1 × 10-5 k418K ) 1.2 × 10-4 k433K ) 4.0 × 10-5

a Disproportionation in the presence of a 20-fold excess of nitroxyl. decomposition pathway exists with k418K ) 1.6 × 10-4 min-1.

grafting propagation sequence irreversibly. Moreover, oxidation of the hydroxylamine to the nitroxyl would displace the activation equilibrium toward the dormant state, thereby inhibiting further alkoxyamine homolysis. It is interesting to note that, even at comparatively low temperatures (110-130 °C), the TEMPO-cyclooctane alkoxyamine yielded only disproportionation product (Table 1). This inherent instability with respect to β-hydrogen transfer from the alkyl substituent to the nitroxyl is of interest because it relates to the mechanism of alkoxyamine disproportionation. In particular, whether the process is an in-cage radical disproportionation, a bulk radical process, or a concerted, nonradical reaction is of some consequence to CRG technology development. The data listed in Table 1 show that heptenyl-based alkoxyamine disproportionation is accelerated by the presence of excess nitroxyl because the rate constants acquired in the radical crossover experiments (kex d ) exceeded those measured for pure alkoxyamine solutions (kd) by a factor ranging from 3 to 16. This indicates that radical disproportionation in the bulk solution contributes significantly in these systems. Excess nitroxyl has a similar effect on the TEMPOcyclooctane system. However, the lack of significant exchange reactivity would suggest that TEMPO-cyclooctane does not disproportionate in the bulk solution but either through an in-cage radical process or by a concerted nonradical reaction. The rates of nitroxyl trapping are not at the diffusion limit,26,27 leading some to conclude that cage effects are not influential in nitroxyl-mediated polymerizations.13 On the basis of our data, we cannot make definitive statements regarding cage effects in the disproportionation of aliphatic alkoxyamines. However, we note that the instability of TEMPO-cyclooctane may be derived from an N-oxide Cope-elimination mechanism28 that has been reported for other systems.13 Styrenic and acrylic alkoxyamines of TIPNO are known to possess relatively low C-O bond dissociation energies.23 In the present case, the rate of homolytic cleavage for TIPNO-heptane was an order of magnitude greater than those of the corresponding TEMPOor TMIO-based alkoxyamines, and the kd/kact ratio of 0.002 at 145 °C confirmed its preference for nitroxyl exchange versus disproportionation. However, we observed a second alkoxyamine degradation reaction above 130 °C whose first-order kinetics were on the order of kact. Because of a lack of promising CRG results for this compound (see below), we have not characterized this degradation pathway. CRG of PE Model Compounds. The potential of TIPNO-heptane to support a CRG process was assessed by heating a degassed C6D6 solution of the purified alkoxyamine and MAn or VTMS to 145 °C and monitoring the system periodically by 1H NMR. Although this

-1 a kex d (min )

kd (min-1)

b

10-4

) 5.5 × 10-4 k403K ) 3.3 × 10-3 k433K ) 7.1 × 10-4

) 1.1 × k403K ) 8.3 × 10-4 k433K ) 8.8 × 10-5

k383K

k418K ) 2.8 × 10-6 k433K ) 8.2 × 10-5

k418K ) 4.6 × 10-5 c k433K ) 2.5 × 10-4

k383K

Extrapolated to 150 °C in tert-butylbenzene from ref 23. c A second Table 2. Rate Constants for the Thermolysis of PBD Model Alkoxyamines alkoxyamine TEMPO-cyclooctene TEMPO-heptene TEMPO-allyl b TEMPO-butene-3-yl b TIPNO-heptene TMIO-heptene

kact (min-1)

-1 a kex d (min )

k363K ) 8.4 × 10-4 k373K ) 1.5 × 10-3 k381K ) 4.8 × 10-3 k363K ) 2.6 × 10-3 k373K ) 1.2 × 10-2 k383K ) 2.3 × 10-2 k383K ) 1.5 × 10-3 k383K ) 8.3 × 10-2 k363K ) 5.5 × 10-3 k373K ) 2.0 × 10-2 k383K ) 4.2 × 10-2 k363K ) 1.2 × 10-3 k373K ) 2.1 × 10-3 k383K ) 8.1 × 10-3

k363K ) 1.5 × 10-5 k373K ) 3.2 × 10-5 k381K ) 2.2 × 10-4

a Disproportionation in the presence of a 20-fold excess of nitroxyl. b Interpolated values in tert-butylbenzene from ref 23.

alkoxyamine is activated by thermolysis at this temperature, no evidence of MAn or VTMS insertion into the C-O bond was observed, nor was there any evidence of monomer addition to the heptanyl substituent. This result suggests that secondary alkyl radicals are trapped much more rapidly by TIPNO than by monomer, resulting in the system being overwhelmed by heptanylnitroxyl radical combinations. Because only alkoxyamine disproportionation and degradation products were observed, we have concluded that the prospects for CRG to function effectively on PE using the nitroxyls tested are quite limited. PBD Model Compound Activation. The lower bond dissociation energy afforded by the allylic system results in much higher rates of homolytic bond cleavage for the heptenylalkoxyamines than those observed for their aliphatic analogues,29 as demonstrated by the data within Table 2. Comparisons between the TEMPO-, TMIO-, and TIPNO-heptene systems show that the reactivity decreased in the order of TIPNO > TEMPO > TMIO. This trend is consistent with a similar evaluation of cumyl-based alkoxyamines, for which C-O homolysis activation energies of TMIO-cumyl (118.9 kJ/ mol), TEMPO-cumyl (115.7 kJ/mol), and TIPNOcumyl (112.6 kJ/mol) have been reported.23 Further differences between the allyl- and alkylalkoxyamines studied in this work were noted in our assessment of disproportionation tendencies. Very little disproportionation was observed in any of the heptenylalkoxyamine experiments because the starting compound and the nitroxyl exchange product matched the initial alkoxyamine loading to within 5%. This was true whether excess nitroxyl was present in the system or whether the alkoxyamine was heated in isolation. Such a favorable balance between activation and disproportionation suggests that allylic radical concentrations may be mediated effectively by all three of the nitroxyls examined in this work.

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Figure 2.

1H

NMR of the downfield region for the CRG of MAn onto TEMPO-heptene after 25 min at 111 °C. Solvent C6D6.

Cyclooctene-alkoxyamines were also examined (Table 2). An extrapolation of rate data acquired for TEMPOcyclooctene provided an activation rate constant of kact ) 8.4 × 10-3 min-1 at 110 °C, which was within the range of those recorded for acyclic alkoxyamines.23 Although the cyclooctene system may be representative of a broad range of allylic radical reactivity, the heptene compounds were selected for further study given that their linear structure was considered to be more representative of PBD. Within this heptene-alkoxyamine series, TIPNO-heptene and TEMPO-heptene demonstrated the highest activation rates and were tested as CRG compounds for MAn, VTMS, and styrene grafting. CRG of PBD Model Compounds. Activation of the allylalkoxyamines in the presence of MAn facilitated monomer addition. Figure 2 illustrates a 1H NMR spectrum of TEMPO-heptene after heating to 111 °C with 3 equiv of MAn. Apart from residual alkoxyamine and MAn, clear evidence exists of a graft-modified hydrocarbon (heptene-g-MAn) whose structure was determined through the characterization of the isolated compound. The pendant graft was not a conventional succinyl group but an unsaturated anhydride. Indeed, no evidence of a succinyl graft or succinyl-based alkoxyamine could be found. Kinetic studies in which TEMPO-heptene was heated to 111 °C with MAn under N2 were conducted in C6D6 (Figure 3). As the number of MAn equivalents relative to alkoxyamine was increased from 3 to 8, the rate of alkoxyamine conversion approached the rate of TEMPOheptene homolysis. This is the expected result, given that an increase in the monomer concentration should accelerate monomer addition to hept-5-ene-3-yl radicals relative to TEMPO/hept-5-en-3-yl radical combination. These observations are consistent with the mechanism illustrated in Scheme 5, which involves homolysis of the alkoxyamine, addition of MAn to generate a succinyl radical that may be trapped by TEMPO to give TEMPOSAn (succinic anhydride), or rapid disproportionation to yield the hydroxylamine and heptene-g-MAn. A nonradical disproportionation process may also contribute to irreversible termination. This CRG reaction generates a single, pendant anhydride graft per alkoxy-

Figure 3. Conversion versus time for the CRG of MAn onto TEMPO-heptene at 111 °C with (]) 8:1 MAn/TEMPO-heptene and ([) 3:1 MAn/TEMPO-heptene: s, C-O homolysis (kact ) 0.077 min-1). Solvent C6D6.

Scheme 5

amine, whereas a conventional grafting process can yield many grafts for each alkyl radical produced using a peroxide.30 This is an important result because the rate of TEMPO-SAn disproportionation is shown to be greater than that of chain transfer (Scheme 1), thereby

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3669

constitute the graft propagation cycle. The mediation of secondary alkyl radicals is problematic because of extensive alkoxyamine disproportionation. Although the activation of allylalkoxyamines facilitates MAn grafting to the hydrocarbon, disproportionation of the resulting succinyl radical limits the kinetic chain length to a single monomer addition. Acknowledgment Financial support from the National Sciences and Engineering Research Council of Canada (NSERC) and DuPont Canada is acknowledged. The authors are grateful to Dr. A. Rutter of Queen’s Analytical Services Group for GC analysis support. Figure 4. Conversion versus time for the CRG of MAn onto TIPNO-heptene at 111 °C with (]) 8:1 MAn/TIPNO-heptene and ([) 3:1 MAn/TIPNO-heptene: s, C-O homolysis (kact ) 0.126 min-1). Solvent C6D6.

arresting the graft propagation cycle after a single monomer addition. Although TEMPO has reportedly been used to mediate the copolymerization of styrene with various maleimides,31,32 similar TEMPO-acrylate alkoxyamines are known to undergo extensive disproportionation.13,33 Attempts made by Benoit et al. to mediate the copolymerization of styrene and MAn with TEMPO resulted in a nonliving system, but experiments employing TIPNO were more successful.14 However, succinyl-based alkoxyamines of TIPNO are susceptible to disproportionation, as demonstrated by Harth et al. through the addition of MAn and N-phenylmaleimide to a TIPNObased alkoxyamine-terminated polystyrene.34 Upon heating of their TIPNO-trapped succinyl radical to 125 °C, disproportionation introduced an unsaturated, functional end group to the polystyrene chains. The kinetics of MAn addition in the TIPNO-heptene system are illustrated in Figure 4. As observed previously, increasing the concentration of MAn in the system accelerated the conversion of TIPNO-heptene toward the rate of alkoxyamine homolysis. Only residual alkoxyamine, MAn, and heptene-g-MAn could be identified, with no clear evidence of a conventional succinyl graft or a succinyl alkoxyamine (TIPNO-SAn) observable at any time. Our final exploration of the CRG concept involved other monomers of industrial interest, namely, VTMS and styrene. Although TEMPO-heptene demonstrated reactivity with respect to MAn insertion, it did not behave similarly with respect to these monomers. That is, no insertion of VTMS or styrene into the C-O bond of TEMPO-heptene was observed when purified alkoxyamine was heated with these monomers under conditions identical with those used in the MAn studies. This suggests that the rates of vinylsilane and styrene addition to allylic radicals were insufficient to compete with nitroxyl trapping, ultimately resulting in a lack of grafting activity.35-37 Conclusions Adapting nitroxyl-mediated polymerization chemistry for the purposes of CRG involves several challenges, including the provision of appropriate activation and disproportionation rates for polymer- and monomerderived alkoxyamines, as well as accommodationg of the rates of monomer addition and chain transfer that

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Received for review November 4, 2002 Revised manuscript received May 30, 2003 Accepted June 2, 2003 IE020888V