Primary reactions in the electron-induced polymerization of acrylates

__ -_ l!B

& *H

Nuclear Instruments and Methods in Physics Research B 105 (1995) 154-158

NOMB

Beam Interactions with Materials A Atoms

ELSEVIER

Primary reactions in the electron-induced polymerization of acrylates W. Knolle *, R. Mehnert Institutfir Oberfliichenmodifizierung

e. V. (IOM), Permoserstrape

15, D-04303 Leipzig, Germany

Abstract Using electron pulse radiolysis with optical detection the mechanism of the polymerization of tripropyleneglycol diacrylate (TPGDA) induced by electron-irradiation was studied at room temperature. The dose per pulse was 50 or 130 Gy corresponding to the electron pulse lengths of 5 and 15 ns, respectively. Short-lived transients, such as TPGDA radical cations and radical anions as well as different types of radicals were observed in neat TPGDA. The reactions of the different ions were studied separately in solutions of TPGDA in tetrahydrofuran and in n-butylchloride. TPGDA cations produced by charge transfer from n-butylchloride primary cations undergo both rapid deprotonation and ion-molecule reactions with TPGDA. Deprotonation leads to TPGDA vinyl-type radicals. In tetrahydrofuran TPGDA radical anions are produced by reaction of the solvated electron with the acrylate. Fast dimerization of the anions is observed. Proton abstraction from the solvent leads to alkyl-type radicals. Addition of the different radicals to TPGDA leads to the chain start of the radical was estimated for the starting reaction. In the presence of oxygen the polymerization. An upper bound of lo5 dm3mol-‘s-l formation of peroxyl radicals is observed and the polymerization is inhibited.

1. Introduction Multifunctional acrylates are widely used in radiation curable formulations and as crosslinking agents in industrial curing applications. Usually several reaction steps precede the formation of radicals which initiate the polymerization or form active sites for crosslinking. To gain an understanding of the primary mechanism is of industrial interest, e.g. for the optimization of curing formulations. Electron pulse radiolysis is an effective tool to directly observe transients which can be characterized by their absorption spectra and their lifetimes. From the time profiles of the transient spectra information can be deduced about the nature of the transients, their participation in a chemical reaction and their kinetic characteristics. A few pulse radiolysis studies have been published on acrylate polymerization kinetics. Acrylate radical anions [l] and a-carboxyalkyl radicals [2] were observed as intermediates. To avoid rapid polymerization of the irradiated sample Schmidt and Decker [l] carried out experiments using low temperature liquid or frozen methyltetrahydrofuran solutions containing acrylates. They were able to identify radical anions, their dimeric form and unspecified radical species. Wojmirovits et al. [2] performed studies at room temperature in cyclohexane solution containing 1,6 hexanediol diacrylate (HDDA) but also in neat HDDA. They assigned a transient light absorption decreasing with the wavelength in the 280-400 nm range as due to a-carboxyalkyl radicals but suffered from the limited time resolution (So-2500 ns pulses) of their pulse radiolysis apparatus. From the effect of electron (Ccl,) or cation scavengers (ethanol, triethylamine) on intermediates observed in electronirradiated neat acrylates it was concluded that anionic as well as cationic species precede the formation of radicals [3]. A detailed study of the cationic pathway of radical formation in the system TPGDA/n-butylchloride was presented recently [4]. Here we summarize the results of the cationic as well as the anionic pathway.

2. Experimental The pulse radiolysis experiments were carried out using 5 or 1.5 ns electron pulses of the 11 MeV linear accelerator of the Institut flir Oberfbachenmodifizierung. Corresponding to the pulse width the dose per pulse was 50 or 130 Gy. The optical detection system consisted of a pulsed xenon lamp, different suprasil reaction vessels, a high-intensity grating monochroma-

* Corresponding author. Tel. + 49 341 235 2588. fax + 49 341 235 2584/3400, 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00537-4

e-mail [email protected].

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tor, lP28 photomultipliers for emission or absorption measurements, a fast silicon photodiode detector and fast transient recorders. Linac operation and data acquisition were done in a computer controlled mode. All experiments were performed at room temperature. TPGDA was obtained from UCB, Speciality Chemicals Division; n-butylchloride and ethanol were Merck spectrograde products. Tetrahydrofuran (spectrograde) was obtained from Aldrich.

3. Results and discussion

3.1. System n-butylchloride/

TPGDA -

cationic precursor reactions

After electron pulse irradiation of n-butylchloride two types of radical cations are observed: n-butylchloride parent radical cations BuCl+ and butene radical cations [5]. Electrons generate non-reactive chloride ions and butyl radicals within a few picoseconds by dissociative electron capture (reaction 1) e- ------+

B&l

-

C,H,&,

C,Hj,

&H9,

(1)

Cl-.

Because of the lower ionization potential of TPGDA positive charge transfer is expected to proceed from butyl chloride radical cations to the acrylate solute. With increasing solute concentration a faster decay of the BuCl cation absorption was found. The rate constant for this diffusion-controlled reaction (k = 9 X 10’ dm’mol-Is-‘, reaction (2)) was determined from a Stern-Volmer-plot. C,H,Cl+ C,H;+

+ TPGDA TPGDA -

C,H,Cl+ C,H,

Pa)

TPGDA;,

+ TPGDA’.

(2b)

Charge transfer to the vinyl group of the acrylate is expected

and an alkene cation-like

structure is formed:

0

TPGDA’

: H,d-cH--C&

(3)

‘O-R

The spectrum of the short-lived component observed below 500 nm (Fig. 1, spectrum at 0 p.s) is attributed to the TPGDA cation (reaction (3)). This assignment is supported by the following arguments: the diffusion-controlled reaction of the acrylate with the BuCl cations, the decrease of the absorption if cation scavengers (e.g. ethanol) are added, the insensitivity to oxygen, the fast unimolecular decay and the similarity with spectra of alkene radical cations [6].

L

3ocl

400

I

500

340nm

600

400nm

1

700

800

900

wavelength I nm Fig. 1. Transient optical absorption spectra observed after electron pulse irradiation (times as indicated) 2 X IO-’ mol dm-3 TPGDA. Insets: time profiles of the optical density at different wavelength.

of de-aerated

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containing

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At higher solute concentrations ( > 5 X lo-’ mol dm- 3 TPGDA) two transients with different decay rates are observed in the spectrum, which are affected by ethanol addition, but which are insensitive to oxygen. These transients are assigned to dimer cations of different structures (reaction (4)).

H+=CH-COOR H#?+ --dH-COOR H,&-dH

+

(4a)

resonance stabilized (DCI)

H,C=CH

c=o

c=o

I

I

OR

OR

H,&CH-CH,-eH

covalently

I c=o

A=0

OR

OR

I

I

(distonic

(4b)

bonded radical cation (DCZ)

In analogy to alkene and styrene radical cations [7] the fast-decaying near infrared absorption band (Fig. 1, inset at 900 nm) is assigned to the resonance charge transfer stabilized dimer cation (DCl), whereas the visible absorption band around 440 nm is assigned to the distonic radical cation (DC2). Visible absorption bands are characteristic for covalently bonded alkene-type dimers [8]. However, for TPGDA concentrations up to 10-l mol dmp3 the decay rate of the solute cation of about 4 X lo7 SC' remains unchanged. This leads to the conclusion that the TPGDA radical cation transforms mainly by deprotonation to a vinyl-type radical (reaction (5)). Hz&-H-COOR

3

H,C=d-COOR.

(Rl)

In comparison to acrylate radical cations, the corresponding acrylate radicals show longer lifetimes. At low TPGDA concentrations (< 5 X lop3 mot dme3), where reaction (5) is the main source of radicals, a long-lived absorption is found below 360 nm (not shown, see Ref. [4] for details). This absorption is completely removed by ethanol addition and it is strongly affected by oxygen. The assignment of this absorption to the vinyl-type radical structure (Rl) is supported by the observation of a long-lived 400 nm absorption band in oxygen saturated TPGDA/n-BuCl solutions. Visible absorption bands have been found to be characteristic for vinyl peroxyl radicals [9]. Electron irradiation of deaerated n-BuCl solutions containing at least 0.2 mol dmv3 TPGDA generates an insoluble polymeric acrylate. At these solute concentrations in the pulse radiolysis experiments a long-lived absorption part with a shoulder around 310 nm is observed (Fig. 1, spectrum at 7 ps), showing a grow-in followed by a decay. The time constant for the growth depends on the TPGDA concentration. This supports the assumption that the product of reaction (6) and of subsequent additions of TPGDA molecules (that means the propagating radical) is observed in this region. H,C=C

*

+ H,C=CH

c=o

I c=o I

OR

OR

-

H,C=C-CH,--dH

I c=o I

I=0

OR

OR

(R2).

(6)

This agrees well with pulse radiolysis experiments done in cyclohexane/l,6-hexanediol diacrylate solutions [2], where a long-lived transient decreasingly absorbing from 270 to 400 nm and possessing a shoulder at 310 nm was assigned to the o-carboxyalkyl radical of the growing chain. Spectral overlap in our experiments, e.g., with the absorption of the monomer radicals prevents a clearer separation of the radicals in the growing chain from other radical species. Thus a rate constant for reaction (6) can only be estimated, and an order of magnitude of lo4 to 10’ dm3mol-‘s-r is obtained.

3.2. System tetrahydrofiran

(THF) / TPGDA -

In THF fast electrons generate non-reactive cations (reaction (7)). e- ____--* THF -

solvated

anionic precursor reaction electrons

eitvt THF ‘, THF(H)+.

with a lifetime

of several

hundred

nanoseconds,

radicals

and

(7)

W. Knolle, R. Mehnert/Nucl.

2x10-*, 0

300

400

500

700

600

wavelength

after thepulse

800

900

I nm

Fig. 3. Transient optical absorption spectra observed concentration

and time after the pulse as indicated).

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Instr. and Meth. in Phys. Res. B 105 (1995) 154-158

after electron pulse irradiation of deaerated THF containing TPGDA Insets: time profiles of the optical density taken at 320 nm and 900 nm.

(TPGDA

If TPGDA is added the decay of the electron absorption becomes faster. For the reaction of the solvated electron with the acrylate (reaction (8)) a diffusion-controlled rate constant of k = 2 X 10” dm”mol- ‘sm. ’ is deduced. e ,;,,,, + TPGDA -

TPGDAI.

Radical anions of TPGDA are formed by electron attachment

to the carbonyl group.

0TPGDA-

0-

: H,C=CH--d<

H,d-CH=C<

(9)

O-R

O-R

The spectrum of the anion is given in Fig. 2 (spectra at 0 ps for two concentrations). The assignment of the short-lived transient observed immediately after the electron pulse to the acrylate anion is based on the following arguments: the diffusion-controlled reaction of the solvated electron with the acrylate, the stabilization of radical anions in THF, the decrease of the absorption if electron scavengers (e.g. CCL,, N,O) are added, and the existence of the radical anion of methyl methacrylate in gamma-irradiated glassy methyl tetrahydrofuran [lo]. It can be seen from Fig. 2, insert at 900 nm, that an infrared absorption band appears concomitant to the decay of the radical anion. A fast ion molecule reaction forming dianions (reaction (10)) is assumed. The infrared absorption band points to stabilization of the dimer by charge resonance. A partial overlap of the ion and the molecule in a sandwich-like structure is well known to result in an additional red or infrared absorption band [8]. Dimer radical anions have also been found in pulse radiolysis experiments in frozen methyl tetrahydrofuran [l]. TPGDA - -I- TPGDA -

(TPGDA),

.

(10)

A long-lived absorption below 320 nm with a different kinetic behaviour is observed after the decay of the anion, also (Fig. 2, spectrum at 0.5 ps, compare insets). The strong sensitivity towards oxygen of this long-lived transient points to a radical species. The radical anion can easily abstract a proton from the solvent leading to a radical (reaction (11)).

? H,C=CH

-

+H+

rearrangement _______ _--___,

H,C=CH I

* c-oI

- C-OH

OR

OR

H,C-eH

(1’) I

C=o

I (R3)

OR (R4)

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Nucl. Instr. and Meth. in Phys. Rex B 105 (1995) 154-158

Structure R3 is kinetically preferred, but a molecular rearrangement during or after the protonation may result into the thermodynamically more stable structure R4. With the means of optical spectroscopy alone it is not possible to distinguish between the two structures. Protonation is also assumed to be the reason for the decay of the dimer anion (reaction (12)). H,C=CH

H,C=CH

I

7

.c-0;.

I

s

I c=o

R3 (R4) +

(12)

‘O=C

OR

OR

I Hti=CH,

At higher solute concentrations a new transient light absorption appears in the spectrum (Fig. 2, spectrum at 4 ps, TPGDA concentration of 2 X lo-’ mol dm-3). It is assumed that the product of the addition of the radicals R3 or R4 to the acrylate double bond (reaction (13)) is observed in the wavelength region around 320 nm. Comparing these results with the results found in the BuCl experiments it seems clear that again the carboxyalkyl radical of the growing chain is observed. R3 or R4+

H,C=CH

-

R3(R4)-CH,-6dH

(13)

I c=o

c=o I

OR

OR (RS)

In air-saturated solutions both the radical anion and the dianion react with the oxygen and the unspecific spectra of carbon-centered peroxyl radicals can be observed below 320 nm. Detailed experimental results and a comprehensive discussion will be published.

4. Conclusion Pulse radiolysis with optical detection has proved to be useful for the investigation of primary reactions in the electron induced polymerization of acrylates. It could be shown that cations as well as anions contribute to the short-lived transients found in neat liquid TPGDA. They transform quickly into radicals, which finally generate the same carboxyalkyl radical of the growing chain.

References [l] .I. Schmidt and U. Decker, Proc. 7th Tihany Symp. on Radiat. Chem., eds. J. Dob6 and R. Schiller (Akademiai Kiado, Budapest, 1991) p. 239. [2] L. WojnLrovits, E. Taklcs, J. Dobd and G. F6IdiBk, Radiat. Phys. Chem. 39 (1992) 59. [3] R. Mehnert and W. Knolle, Proc. Int. Conf. on Radiat. Technol., Mediterraneo, 1993, RadTech Europe, Fribourg (1993) p. 415. [4] W. Knolle and R. Mehnert, Proc. Int. Conf. on Radiat. Processing, Istanbul, 1994, to be published in Rad. Phys. Chem. [5] S. Arai, A. Kira and M. Imamura, J. Phys. Chem. 80 (1976) 1968. [6] R. Mehnert, in: Radical Ionic Systems, eds. A. Lund and M. Shiotani (KIuwer Academic Publishers, Dordrecht, 1991) p. 231. [7] S. Egusa, Y. Tabata, S. Arai, A. Kira and M. Imamura, J. Polym. Sci. Polym. Chem. 16 (1978) 729. [8] B. Badger and B. Brocklehurst, Trans. Faraday Sot. 65 (1969) 2576. [9] R. Mertens and C. v. Sonntag, Angew. Chemie 106 (1994)1323. [lo] T. Gilbro, H. Yamaoka and S. Okamura, J. Phys. Chem. 3 (1973) 1163.