towards controlled cationic polymer growth from inorganic oxide defects: directing the mechanism of...

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Polymer xxx (2014) 1e7

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Polymer

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Towards controlled cationic polymer growth from inorganic oxidedefects: Directing the mechanism of polystyrene grafting fromg-irradiated silica

Francesca D'Acunzo a, *, Patrizia Gentili b, Giancarlo Masci b, Ornella Ursini a

a Institute of Chemical Methodologies, CNR, Via Salaria Km. 29.300 00015 Monterotondo Rome, Italyb Department of Chemistry, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy

a r t i c l e i n f o

Article history:Received 7 June 2014Received in revised form7 August 2014Accepted 13 August 2014Available online xxx

Keywords:Controlled polymerizationHybrid materialIrradiation

* Corresponding author. Tel.: þ39 (0)6 90672344.E-mail address: [email protected] (F. D'Acu

http://dx.doi.org/10.1016/j.polymer.2014.08.0410032-3861/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: D'Acunzomechanism of polystyrene grafting from g-i

a b s t r a c t

Most studies on surface-initiated controlled polymerizations for the synthesis of polymeric covalentorganic-inorganic hybrid materials focus on chemical methods requiring specific modifications of theinorganic substrate. Few mechanistically-aware approaches have been undertaken towards exploitingthe reactivity of defects induced by physical techniques such as ionizing radiations or UVeVis light.Within this framework, we take grafted polymerization of styrene from g-irradiated silica as a mecha-nistic testing ground where para- and diamagnetic silica defects are present, and polymerization pro-ceeds through both radical and cationic mechanisms, resulting in a bimodal molecular weightdistribution. We show that these mechanistic intricacies can be sorted out by resorting to the chemicalarsenal developed in the last decades for controlled polymerizations. Specifically, we obtained a silica-polystyrene grafted material by cationic grafting from at 30 �C, a unimodal molecular weight distribu-tion, and a relatively high molecular weight (Mn ¼ 7.4 kDa) with a PDI of 1.68.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Hybrid inorganic-organic polymeric materials have beenintensively investigated over the years as fillers of enhancedcompatibility in composite materials. A finer degree of polymerstructure and composition control has been achieved in hybridmaterials for chromatographic applications. The potential for evenmore demanding applications, such as in the construction of opto-electronic devices [1], drives current research towards morphologycontrol, and enhanced physical and chemical stability of theresulting material with efficient and stable charge exchange be-tween the organic and inorganic components. In these respects,better performance from covalently attached polymer layers oninorganic surfaces is foreseen than from physisorbed layers. Cova-lent silica hybrids can be obtained through solegel and graftingapproaches based on silylated organic compounds, click chemistryon azide- or alkyne-functionalized silica, radical reactions of thiol-modified silica, or metal-catalyzed modifications of highly reactivesilica precursors (see Ref. [2] and references therein). Covalent

nzo).

F, et al., Towards controlledrradiated silica, Polymer (201

immobilization of polymers requires the encounter of polymerchains with active groups on the inorganic surface, a process thatbecomes increasingly hindered as the polymer layer grows inthickness and concentration. On the other hand, in a grafting fromprocess the coverage of the inorganic substrate is achieved by smallmolecules, and further chain growth results from the encounter ofmonomers (small molecules) with reactive groups on the polymerchain end. In short, covalent hybrid inorganic-organic materialsobtained by polymer growth from the inorganic substrate shouldoffer, in principle, several advantages in terms of surface coverageand control, as well as physical stability and favorable electronicproperties. If polymerization initiation from the surface of thesubstrate is fast and chain breaking reactions are retarded orinhibited, controlled grafted polymerization is achieved. Therefore,controlled radical (NMP, ATRP, RAFT) [3e7], and catalyst-transfer(KCTP) [8] polymerization methods have been employed in thefield of hybrid materials. All of these methods require some kind ofspecific modification of the inorganic substrate, resulting in asurface-initiated polymerization reaction. An attractive alternativeis to activate the desired inorganic substrate towards surface-initiated polymerizations by resorting to physical rather thanchemical means, i.e. by making use of ionizing radiation and/orlight of appropriate wavelength, depending on the material. Inmany cases, the inorganic component in a hybrid system is an

cationic polymer growth from inorganic oxide defects: Directing the4), http://dx.doi.org/10.1016/j.polymer.2014.08.041

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F. D'Acunzo et al. / Polymer xxx (2014) 1e72

oxide, such as TiO2, ZnO or silica. Such materials are typically non-stoichiometric and oftentimes oxygen-deficient [9e12], dependingon their processing or defect engineering history. Relaxation ofoxygen vacancies results in different materials features, from twotrapped electrons substituting for O2� in oxygen-defective MgO, tointermediate covalent-ionic bond character in TiO2, to covalentSieSi bonds in silica [9]. Radiation damage imparted to silica by g-rays includes oxygen vacancies formed through knock-on pro-cesses, trapping of a hole at oxygen-deficient centers [13e16], andtransformation of pre-existing SieH, SieOH, SieX centers. As aresult, radiolized silica exhibits, among others, paramagnetic de-fects of the ≡Si$ and ≡SiO$ type, as well as E0g centers (Fig. 1). Thesehave been described as asymmetric, positively-charged two-siliconcenters with an oxygen vacancy. One paramagnetic silicon atom inthe E0g center is three-fold coordinated, and features an unpairedelectron in a dangling bond; the second cationic diamagnetic Si iscoordinated to a backside oxygen [17]. E0g centers can be long-livedenough to be involved in the reactivity of radiolized silica [14]. EPRstudies of radiation-induced defects in silica trace back to the1950's [17], while the mechanisms involved in their reactivity, theirannealing by diffusing mobile species, and their stability have beenthe object of recent investigations [13,14].

It is well-known that exposure to g-rays results in enhancedadsorption of polymeric materials on silica, both quantitatively, andin terms of stability of the material [18,19]. However, the potentialfor paramagnetic and diamagnetic centers to act as radical and/orcationic sites (Fig. 1) for covalent binding and concomitant het-erogeneous polymerization initiation has not been fully explored.Previous reports describing polymerization and grafting of vinylmonomers with pre-radiolized silica [18] are difficult to interpret ina mechanistic sense, since many variables such as temperature, thepresence of oxygen, which heavily determines the fate of activesites in g-irradiated silica, and reaction time, are not easilydecoupled. Based on a retrospective reading of Fukano's body ofdata [20e22], we have undertaken a mechanistically-awareapproach to grafting of polyvinyls from g-irradiated silica. In ourprevious work on this subject [23], we have drawn on Fukano'sinvestigation, and we have determined the yields, structures, andmolecular weight distributions of free and immobilized polymer, aswell as the structure and relative abundance of reaction by-products as a function of experimental conditions. We have thusshown that by-product amounts, grafting efficiency (i.e. the ratio ofnon-extractable polymer to total polymer obtained), and polymermolecular weight distribution are strongly dependent on silica pre-irradiation atmosphere, as well as on polymerization conditions.These initial results encouraged us to proceed towards under-standing and, possibly, directing the mechanism of styrene poly-merization initiated at the surface of radiolized silica, so as toultimately provide an alternative approach towards controlledpolymerizations with high grafting efficiency. The present workconstitutes a step forward towards this end, in that it tackles theinterplay of radical vs. cationic polymerization initiation and inhi-bition, silica active sites quenching, and chain transfer in thepolymerization of styrene from g-irradiated silica surface.

SiO

OO Si+

OO

OO

Fig. 1. Pictorial of positively-charged E0g centers in silica. Bond lengths and angles arenot drawn to scale.

Please cite this article in press as: D'Acunzo F, et al., Towards controlledmechanism of polystyrene grafting from g-irradiated silica, Polymer (201

2. Note on nomenclature

We define some abbreviations to designate the different mate-rials and polymer fractions in this study. Polystyrene is abbreviatedas PS; free polymers are indicated as F, and grafted (unextractable)ones as G. Furthermore, since two ranges of molecular weights areencountered in this investigation, we label L (Low) and H (High) thePS of molecular weights around 10 kDa and 1000 kDa, respectively.For example, PSF-H is free polystyrene of higher molecular weight,while PSG-L is unextractable polystyrene of lower molecularweight.

3. Experimental

g-irradiation and polymerization experiments were carried outin duplicate. ATR FT-IR measurements were in triplicate on eachsample, and mean results are presented. Single GPC runs wereacquired for each sample. For 13C- and 29Si- solid state NMR of thehybrid materials and for GCeMS analysis of by-products, we referto our previous publication on the subject [23].

3.1. Materials

Silica gel (SigmaeAldrich) 70e230 mesh, 100 Aː pore size,300 m2 g�1 was dried in an oven at 180 �C for two days prior toirradiation. Stabilizer-free styrene (Fluka, >99.5%) was obtained bywashing repeatedly with alkali, followed by deionized water. Sty-rene was then dried overnight over magnesium sulfate, anddistilled under reduced pressure.

3.2. g-irradiation and polymerization experiments

The radiolysis and polymerization experiments were conductedas previously described [23]. g-irradiation (100 KGy) was carriedout in a sealed, evacuated glass vial in a 60Co source from AtomicEnergy of Canada at a dose rate of 1 kGy h�1. Irradiated silica wasused within 5 min of irradiation, and transferred rapidly into around-bottomed cylindrical glass ampoule (1 cm diameter, 15 cmheight) containing 2 mL of stabilizer-free styrene. The slurry wassubjected to three frozen-thaw cycles, the ampulewas sealed undervacuum, and then kept in a thermostat at 30 �C for 20 h. Shaking orstirring was unnecessary, since the amount of styrene is such thatsilica is impregnated, resulting in a uniform slurry with no settlingof the solid phase.

The reaction crude was diluted with toluene and centrifuged inorder to recover the supernatant. This procedure was repeated fivetimes with fresh toluene aliquots, totaling 10 mL of extracts. Silicawas then extracted with toluene for at least 10 h in a soxhletextractor, and the resulting solution was concentrated to about5 mL. All extracts were combined. Free polystyrene was precipi-tated in a 10- to 15-fold excess methanol.

Thorough removal of strongly adsorbed silica from the toluene-extracted silica was achieved by soxhlet extraction withchloroform.

Grafted polystyrene was removed from thoroughly desorbedsilica-polystyrene samples (0.5e1.0 g) by refluxing for 2 h in 10%aqueous KOH until silica was completely dissolved. The polymerwas recovered by filtration and washed with deionized water untilneutral, then rinsed with methanol and allowed to dry under astream of nitrogen.

3.3. Gas chromatographyemass spectrometry (GCeMS)

GCeMS analyses were performedwith a Finnigan Trace GC Ultraequipped with a Thermo Scientific TRACE TR5ms column (5%


Fig. 2. g-irradiated silica-initiated polymerization of styrene through radical and cationic mechanisms: formation of PSF and PSG. SET ¼ Single Electron Transfer. Alternative routesfor PSF formation that do not involve chain transfer to monomer from grafted growing chains are the radical thermal autopolymerization of styrene (1.5% yield, see Table 1, firstcolumn), and bulk initiation by radiation-induced species such as trapped hydrogen atoms or silyl-peroxy radicals. Hþ loss by 2 results in inactive grafted products, and may transferthe chain to monomer in the bulk.

F. D'Acunzo et al. / Polymer xxx (2014) 1e7 3

phenylmethylpolysiloxane, 15 m � 0.25 mm ID, 0.25 mm filmthickness), using a temperature gradient of 35 �Ce300 �C, and aTrace DSQ single quadrupole MS operating at 70 eV ionizationpotential with helium as the carrier gas.

3.4. Gel permeation chromatography (GPC)

GPC measurements were performed with two Polymer Labo-ratories PLgel Mixed-B columns (10 mm, 300 � 7.5 mm) thermo-stated at 25 ± 0.2 �C and a Knauer K-2501 UV detector operating at260 nm or a Shimadzu RID-10A refractive index detector (Shi-madzu, Kyoto, Japan) thermostated at 30 �C. Near-monodispersepolystyrene standards (Shodex, from 3.25 to 1570 kDa) were usedfor calibration. The eluent was 0.25% (w/v) tetrabutylammoniumbromide THF at a flow rate of 0.8mLmin�1. The samples for analysiswere prepared in the eluent at concentration 1 mg mL�1. Injectionvolume was 100 mL. Chromatograms are reported normalizing toone the intensity of the highest peak (normalized intensity).

3.5. Attenuated total reflectance (ATR) FT-IR

ATR FT-IR spectra were recorded with a Thermo ScientificNicolet iS10 spectrometer equipped with a triglycine sulfate de-tector (DTGS), and acquired with Omnic vers. 8.1.10 software. Thespectra were the result of 64 scans with a spectral resolution of4 cm�1. A Smart iTR ATR accessory equipped with a diamond ATRcrystal was used. An atmospheric suppression algorithm wasapplied to all spectra.


Quantitative analysis of polystyrene to silica weight ratio wasperformed with TQ Analyst vers. 8.0.2.97 (Thermo Scientific) soft-ware through calibration with silicaepolystyrene blends in the0e30% interval. The polystyrene to silica ratio was determined asthe ratio of the 1057 cm�1 to 699 cm�1 peak heights with a fixedlinear two-point baseline. Correlation coefficients of 0.980 forlinear calibration, and of 0.962 for cross-validation (one pointremoved per cycle) were obtained.

4. Results and discussion

When styrene is incubated with silica g-irradiated in vacuum,two polymer fractions are observed, i.e. a free (PSF) and a non-extractable one (PSG) [23,24]. Both fractions exhibit a bimodalmolecular weight distribution, which Fukano ascribed to theconcomitant action of two polymerizationmechanisms, radical andcationic [20,21]. Bimodal molecular weight distributions of poly-styrene have been observed in other instances [25e33] even incontrolled polymerization conditions, and they have been ratio-nalized in different ways, depending on the polymerization system.In the forthcoming discussion and in the Supporting Information,number-average (Mn) andweight-average (Mw)molecularweightsare provided for PS-H, while for some PS-L samples peak molecularweights (Mp) only could be determined due to overlap with nonpolymeric low molecular weight peaks (see Fig. 3 for details).

As shown in Fig. 1, two silicon atoms of different character arepresent in E0g centers [17] of radiolized silica, one radical and theother cationic. E0g centers may, therefore, be viewed as potential


Fig. 3. GPC traces of PS obtained at 30 �C (dashed line) and 125 �C (solid line) with g-irradiated silica. Chromatograms are reported normalizing to one the intensity of thehighest peak (normalized intensity). (a) Free polymer; (b) Polymer degrafted from thesilica-polystyrene hybrids. Solid line traces were obtained with a refractive index de-tector, and the analysis is truncated at the low molecular weight end due to instru-mental artifacts. Dashed line traces were obtained with a UVeVis detector, and tailingof small molecules may overlap with PS-L.

Table 1Effect of silica g-irradiation, DMAC (a Lewis base), BQ (a radical trap), and DNB (aradical and electron trap) on free and grafted polymerization of styrene at 30 �C:polymer yields, grafting efficiency and molecular weights.

Additive e e DMAC BQ DNB

Temperature 30 �Ca

(125 �C)a30 �C(125 �C)

30 �C 30 �C 30 �C

PSF yieldb (w/w%) 1.5%(83%)

12%(34%)

6.0% 3.5% 1.5%

Total PS yieldc (w/w%) 1.5% 15%(36%)

6.0% 5.0% 2.5%

PSG/silicad (w/w%) e

(2.0%)6 ± 1%(4.5%)

e 2.0 ± 0.2% 1.0 ± 0.1%

Grafting efficiencye (w/w%) e

(1%)21%(7%)

e 23% 26%

Silica 1 g irradiated 100 kGy; Styrene 2 mL (1.812 g; 17.4 mmol); DMAC 200 mL(2.2 mmol); BQ 2.2 mg (0.02 mmol); DNB 3.4 mg (0.02 mmol).

a No silica irradiation.b Free PS yield: free PS/styrene initial amount (w/w%); ±0.5% range on of two

replicates.c Total PS yield: free and grafted (unextractable) PS/styrene initial amount

(w/w%).d ± standard deviation of eight FTIR measurements (four per each of two replicate

samples).e Grafting efficiency: grafted/total PS (w/w%).


sites for either radical, or cationic styrene polymerization initiation.Furthermore, as already outlined in the introductory paragraph ofthis report, other paramagnetic centers may be present that couldinitiate radical processes. Fig. 2 shows a possible scenario for theformation of PSF and PSG through radical and cationic mechanisms(PS-H and PS-L, respectively). In addition to radiolized silica-initiated polymerization, one should also keep in mind that sty-rene autopolymerizes thermally via a well-known radical initiationroute, and that free radical species may be trapped by g-irradiatedsilica, independently of silica-induced initiation. Furthermore,other radiation-induced species [13] migrating from the bulk ofsilica to its surface, may initiate free polymerization independentlyof grafting.

In Fig. 2a and b, styrene binds directly to paramagnetic ordiamagnetic sites on silica, resulting in a carbon radical 1 or cation 2that initiates polymer growth in a grafting from process. Radical orcationic chain transfer to free monomer may also occur, whichwould terminate the growth of the grafted polymer and initiatepolymerization in the bulk (“chain transfer to monomer” processesin Fig 2). With reference to Fig. 2b0 and 2c, styrene may also beoxidatively activated towards polymerization through single elec-tron transfer (SET) to cationic or E0g centers, resulting in a free styrylradical cation 3 that could bind to g-irradiated silica, yielding 1 or 2for grafted polymer growth. It is worth noting that the fate of 3 is acomplex one, since it can react with a second styrene molecule,yielding a distonic radical cation that evolves in a complex mixtureof cyclic and oligomeric products, besides initiating a free radicalpolymerization in the bulk through the intermediate 4-phenyltetralin benzyl radical [23,34e36].


In an attempt to disentangle the possible mechanisms outlinedin Fig. 2, we have carried out the polymerization reaction at 30 �C inthe presence of additives that should interact with reactive cationicand/or radical intermediates in different ways. We chose 30 �C asthe reaction temperature so as to set the radical autopolymeriza-tion to a convenient rate [34,37]. Dimethylacetamide (DMAC) hasbeen known to interact with electrophiles (carbonium ion chainends, in the present case) as an electron pair donor [38,39] resultingin “controlled” cationic polymerizations through the inhibition ofchain-terminating side reactions such as chain transfer and indanylend-groups formation. Benzoquinone (BQ) is an established styrylradical trap [40], and 1,4-dinitrobenzene (DNB) is a well-knownradical polymerization retarder or inhibitor [41] and an electrontrap in electron transfer reactions [42]. The outcome of the poly-merization reactions with and without these additives is summa-rized in Table 1. Previously published data from our group [23]obtained at 125 �C are also reported in Table 1 for comparison.Polymerization conditions have been chosen in view of the mech-anistic study presented in this manuscript and are not optimizedfor synthetic purposes, so that polymer yields are quite low. ATR-FTIR spectra of representative silica samples after styrene poly-merization and thorough removal of PSF by soxhlet extraction arepresented in the Supporting Information.

4.1. Temperature effect on styrene polymerization with pristine andradiolized silica

Styrene polymerizationwas carried out with dry, non-radiolizedsilica, so as to obtain a “baseline” for the thermal autopolymeri-zation of styrene (Table 1) No PSG is detected by FTIR at 30 �C incontrast with 2% w/w PSG on silica when the reaction is carried outat 125 �C [23]. The PSF yield at 30 �C is also far smaller (1.5%) than at125 �C (83%). This result is ascribed to the temperature dependenceof radical self-initiation [34] and propagation rates [37]. Pre-irradiation of silica strongly enhances the yield of PSF (12%) at30 �C with respect to pristine silica (1.5%), in contrast to what weobserved at 125 �C, at which temperature the yield of PSF decreasesfrom 83% to 34% upon silica pre-irradiation, in favor of other side-products [23]. As a consequence of the opposite effect of silica pre-irradiation at 30 and 125 �C, an increase in PSF yield of only aboutthree-fold (12%e34%) is observed with radiolized silica upon


Fig. 4. GPC traces of free (dashed line) and grafted (solid line) PS with (a) DNB and (b)BQ. Chromatograms are reported normalizing to one the intensity of the highest peak(normalized intensity).


increasing the temperature from 30 �C to 125 �C. In fact, based on aGPC peak area estimate, at the lowest temperature, PSF-H(Mn ¼ 449 kDa) is less abundant with respect to PSF-L(Mp ¼ 6.60 kDa), which accounts for the overall lower free poly-mer yield (Fig. 3a). As for the weight percentage of PSG on pre-irradiated silica, no significant difference is observed at 30 �C (6%w/w%) vs. 125 �C (4.5% w/w%). Moreover, in contrast to PSF, PSG-L(Mp ¼ 8.89 kDa) is about as abundant as PSG-H (Mn ¼ 727 kDa) at30 �C as well as at 125 �C (Fig. 3b). Therefore, it appears that in thegrafted radical vs. cationic polymerization initiation from silicadefects, temperature effects are minor. In the bulk, instead, theeffect of temperature on styrene autopolymerization is predomi-nant over any other irradiated silica-induced effect on radicalpolymerization. In terms of molecular weight, PSF-L and PSG-L aretemperature-independent (Fig. 3), since chain-terminating re-actions are fast and require much lower temperatures to be sloweddown. PSF-H (Fig 3a) and PSG-H (Fig 3b), on the other hand, resultfrom temperature-dependent radical processes, and a slight in-crease in molecular weight is observed upon decreasing the tem-perature from 125 �C to 30 �C.

4.2. Effect of radical and electron scavengers on styrenepolymerization with g-irradiated silica

Total polymer yields (Table 1) decrease in the presence ofradical/electron traps BQ and DNB, the larger effect being that ofDNB, which affords only 2.5% total (i.e. free and grafted) polymer,while 5% is the total yield with BQ. Fig. 4a and b shows that both BQand DNB completely suppress the formation of both PSF-H andPSG-H, while allowing formation of PSF-L and PSG-L. Since theformation of PS-H is associated with a radical polymerizationmechanism [20e22], both BQ and DNB are efficient enough to blockradical reaction pathways in Fig. 2 by trapping propagating radicals1. However, DNB also partially inhibits the formation of PSLresulting in a lower overall PS yield than that obtained with BQ. Infact, SET initiation routes in Fig. 2b0 and c should be blocked by DNBacting as an electron acceptor [42], thus making DNB a more effi-cient inhibitor than BQ. The latter should leave SET routes b0 and c,and cationic intermediates 2, unaffected. Specifically, DNB shouldprevent conversion of Si cations to Si radicals by SET, so that Siradicals are not available for fast recombinationwith 3. At the sametime, radical chain growth stemming from coupling of 3 withdiamagnetic sites on silica is blocked by DNB acting as a radical trap.No difference is observed between molecular weights and molec-ular weight distributions with either BQ or DNB, and without ad-ditives (Figs. 3 and 4). In fact, Mp of PSF-L all range between 6.54and 7.39 kDa, and PSG-L between 8.13 and 8.89 kDa.

Since grafting efficiency values in the absence (21%) or in thepresence of either additive (23% with BQ; 26% with DNB) seemindependent of extent of polymerization, we propose that PSF-Land PSG-L both stem from a common intermediate that suffersno interference from BQ and DNB. Intermediate 1 is, therefore,ruled out for these polymer fractions. Furthermore, pathway b inFig. 2 is unaffected by radical and electron scavengers. From allthese considerations, we infer that PSF-L and PSG-L form throughcationic intermediates arising from either direct quenching bystyrene of a cationic silicon center, be it a E0g center or other(Fig. 2b), or from a one-electron oxidation of monomer mole-cules by reactive sites on radiolized silica (Fig. 2b and c). Thegrowth of the bound polymer chains can undergo termination bychain transfer (e.g. proton transfer) to free monomers withconsequent initiation of cationic bulk polymerization. In addi-tion, other termination reactions are likely to occur, such asrearrangements and cyclization, that do not transfer polymeri-zation to the bulk.


4.3. Effect of DMAC on styrene polymerization with radiolized silica

DMAC (Table 1: 2.2 mmol/17.4 mmol styrene) suppresses theformation of PSF-L, while leaving PSF-H unaffected (Fig. 5a), withMn¼ 404 kDa vs 445 kDa of the additive-free PSF. In fact, DMAC hasbeen shown to interact with carbocationic intermediates as a Lewisbase [43], while it does not interfere with radical polymerization.As for the non-extractable polymer, the effect is quite different. Infact, of the 6% grafting in the additive-free reaction, about half canbe attributed to PSG-H (Fig. 3), so we expected to obtain around 3%PSG-H through the radical mechanism. Grafting is below detectionby FTIR (see Supporting Information), instead, in the presence ofDMAC (Table 1), most probably due to inactivation of those sites ong-irradiated silica that initiate grafting, e.g. E0g centers. When DMACis used in a much smaller amount (Table 2A: 0.002 mmol/17.4mmol styrene) PSF yield (10%) and PS grafting (6.8%) are similarto those obtained in the absence of additives (Table 1: 12% and 6%respectively). PSF-L (Mp ¼ 5.6 kDa) and PSG-L (Mp ¼ 7.3 kDa) arealso detected (Fig. 5b), even though free and grafted PS-H arepredominant. In other words, if DMAC concentration is low enough,quenching of initiating sites on irradiated silica is partial, andsurface-initiated grafted polymerization is restored. However, for-mation of PS-L, both free and grafted, is still substantially inhibitedrelative to PS-H.

4.4. Directing the mechanism of styrene polymerization andgrafting

We ran the polymerization and grafting reaction in the presenceof a fixed amount of DNB (so as to inhibit any SET reactions) anddecreasing amounts of DMAC (so as to avoid complete suppressionof surface initiation). In these conditions, any grafted or free


Fig. 5. GPC traces of PSF (dashed lines) and PSG (solid lines) with (a) DMAC 2.2 mmol/17.4 mmol styrene; (b) DMAC 0.002 mmol/17.4 mmol styrene; (c) DMAC 0.002 mmoland DNB 0.02 mmol/17.4 mmol styrene. Chromatograms are reported normalizing toone the intensity of the highest peak (normalized intensity).

Table 2Effect of DMAC, BQ and DNB in different amounts and combinations on free andgrafted polymerization of styrene at 30 �C in the presence of g-irradiated silica:polymer yields and grafting efficiency.

A B C D E

DMACa DMACb:BQ 10:1

DMACb:DNB 10:1

DMACc:DNB 1:1

DMACa:DNB 0.1:1

PSF yieldd (w/w%) 10% <1% 1.0% 2.0% 6.0%Total PS yielde (w/w%) 14% <1% 1.0% 3.0% 8.0%PSG/silicaf (w/w%) 6.8 ± 0.1% e <1% 2.0 ± 0.4% 4.0 ± 0.5%Grafting efficiencyg

(w/w%)26% e e 34% 32%

Silica 1 g irradiared 100 kGy; Styrene 2 mL (1.812 g; 17.4 mmol); Polymerizationtemperature: 30 �C; BQ 2.2 mg (0.02 mmol); DNB 3.4 mg (0.02 mmol).

a DMAC 0.2 mL (0.002 mmol).b DMAC 20 mL (0.2 mmol).c DMAC 2 mL (0.02 mmol).d Free PS yield: free PS/styrene initial amount (w/w%); ±0.5% range on two

replicates.e Total PS yield: free and grafted (unextractable) PS/styrene initial amount (w/w

%).f ± standard deviation of eight FTIR measurements (four per each of two replicate

samples).g Grafting efficiency: grafted/total PS (w/w%).


polymer should be formed through the mechanism in Fig 2b. Re-sults in Table 2 clearly show that both free and grafted polymeryields benefit from decreasing the DMAC to DNB ratio. In fact, theyield of PSF increases from 1% to 6%, and the w/w% of PSG reaches4% upon a decrease of the DMAC:DNB ratio by 100-fold. The lattergrafting percent value is comparable to that obtained with no in-hibitors present, i.e. in uncontrolled conditions yielding a bimodalmolecular weight distribution. Furthermore, the yield of PSF andthe PS grafting % are higher with DMAC/DNB 0.1:1 (Table 2E: 6% and4% respectively) than with DNB alone (Table 1: 1.5% and 1%), whichimplies that DMAC does benefit the cationic polymerizationthrough stabilization of the growing chains both in the bulk, and inthe grafting from process, as long as electron transfer reactions thatinactivate irradiated silica active sites are inhibited. It is worthnoting that using BQ in combination with DMAC has no beneficialeffect (Table 2B) on either free or grafted polymerization. In otherwords, the action of an electron trap, rather than that of a radicaltrap, is necessary to avoid quenching of irradiated silica reactivesites.


In sum, we have shown that it is possible to control the cationicgrowth of polystyrene from defects on irradiated silica by stabilizingcarbocationic intermediates with a lewis base (DMAC) at optimizedconcentrations. This result has been achieved while avoiding silicadefects inactivation and inhibiting free radical polymerization by theaddition of a radical and electron trap, so that a unimodal molecularweight distribution is obtained. The molecular weights of the poly-mers, free (Mn ¼ 6.35 kDa; PDI¼ 1.73) and grafted (Mn¼ 7.42 kDa;PDI¼ 1.68), are in linewith those obtained by homogeneous cationicpolymerizations by other Authors at room temperature, typically intheMn range 2e10 kDa [44e46]with PDI values of 1.4e1.7. From thepoint of view of monomer conversion, GCeMS analysis reveals that,contrary towhat is observed at 125 �C [23], non-polymerized styreneremains unreacted at 30�, i.e. no significant amounts of by-products(namely, those deriving from styrene thermal auto-initiation) areobserved. This means that in a synthetically-rather thanmechanistically-oriented study, conditions may be found (i.e. tem-perature, solvent, reversible electron pair-donating species) to slowdown chain-terminating reactions vs. chain propagation, thusimparting a “living” character to this grafted cationic polymerization.

4.5. Semi-quantitative considerations on PSG yields and silicasurface coverage

The mechanisms outlined in Fig. 2 involve heterogeneous re-actions between surface silica defects and adsorbed molecules. It isworthwhile estimating whether the number of silica defectsgenerated by pre-irradiation is compatible with the number ofpolymer molecules obtained. We consider the contribution fromthermal autopolymerization as negligible in this rough estimate,since (Table 1) it only accounts for 1.5% total PS yield. The actualidentity and surface concentration of silica defects involved ingrafting from and grafting to processes cannot be directly estimatedfrom the data available to us regarding the specific type of silica weused. In fact, g-irradiation damage consists of a number of bulkprocesses that are dependent on pre-existing defects and impu-rities, of which no detailed information is available. Furthermore,surface defects only, either directly caused by radiolysis, or gener-ated by migration from the bulk, are relevant to polymer grafting.However, let us consider that 8% yield of PS-L (Table 2E; Mn ca.7 k Da) corresponds to 1019 molecules g�1 silica, and 14% yield ofPS-H (Table 2A; Mn ca. 500 kDa) corresponds to order 1018



molecules g�1 silica. This means that 1018 to 1019 reactive surfacedefects g�1 silica should be present to account for the initiationmechanisms in Fig. 2. Messina et al. [14] obtained a 1016e1017 cm�3

E0g centers concentration value with 79 kGy g-irradiation of amor-phous silica, a concentration of defects that appears lower than thatrequired for surface-initiated polymerization. However, other de-fects than E0g centers can be involved in the mechanisms in Fig. 2;besides, differences in experimental conditions and, particularly, inthe history of the silica material could be accounted for thisdiscrepancy, not to mention the various approximations in ourcalculations. In short, the PS yields obtained are compatible withthe estimated surface concentration of defects on pre-irradiatedsilica. As for silica surface coverage, if we assume a hydrodynamicradius of 100 nm for solvated PS of 106 Da [47], the footprint of 5%grafting is estimated at order 1020 nm2 g�1, which corresponds to acomplete coverage of the silica we used in our experiments(300 m2 g�1 declared by the manufacturer, i.e 3 � 1020 nm2 g�1).

5. Conclusions

In this report we have taken a step further towards initiating thepolymerization of a vinyl monomer (styrene) directly from defects ofan inorganicoxide (g-radiation-inducedcationicandradical defects insilica) while directing the mechanism of the ensuing chain propaga-tion. In our previous investigation on this subject [23], we showedhow g-irradiation and polymerization conditions have a strong in-fluence on polymer yields, molecular weight distributions, and by-products formation. We also provided evidence of some deviationfrom the ideal grafted polystyrene structure, although no definitemechanism has been yet found to account for it. We have presentlyfurther confirmed that both a cationic, and a radical polymerizationmechanismoperate inparallel, althoughwithindifferent time frames,leading to two polymer fractions of distinct molecular weight distri-butions. This holds true both in the bulk, and in the grafted polymer.We have sketched a mechanistic framework that enables us to ratio-nally direct the polymerization reaction towards either the radical orthe cationic mechanism, thus resulting in a simple rather thanbimodal molecular weight distribution. This was achieved by resort-ing to a Lewis base and to electron or radical scavengers, also used incombination, to prevent undesired quenching of silica defects.Particularly, we have achieved the growth of polystyrene from g-irradiated silica through a cationic mechanism affording good poly-mer loading of the inorganic support with a satisfactory molecularweight andmolecularweight distribution in short reaction times andmild reaction conditions. The value of these results is in their provingthat stable, covalently-bound hybrid polymeric-inorganic materialscan be obtained by taking advantage of the reactivity of defects, andthat the chemical arsenal that was developed in the last decades forcontrolled polymerizations can be applied to these systems [48,49].This approach may provide alternatives, especially through defectsengineering, to the chemical modification of the inorganic substratethat is required for controlled polymerization processes.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2014.08.041.


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