Mechanism of the radiation-induced crosslinking of copolymers of chloromethylstyrene and methylstyrene

Eur. Polym.J. Vol. 25, No. 7/8, pp. 701-707, 1989

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MECHANISM OF THE RADIATION-INDUCED CROSSLINKING OF COPOLYMERS OF CHLOROMETHYLSTYRENE AND METHYLSTYRENE RICHARD G. JONESand YOSHIAKIMATSUBAYASHI University Chemical Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NH, England NEVILLEJ. HASKINS Smith Kline and French Research, The Frythe, Welwyn, Hertfordshire AL6 9AR, England

(Received 13 February 1989) Abstract--Mixtures of benzyl chloride with o- and p-cymene (CY) have been used as low molecular weight models of copolymers of chloromethylstyrene with o- and p-methylstryrene. The products of u.v. photolysis have been analysed using NMR and gas chromatography/mass spectroscopic techniques. From the benzyl chloride/pCY system, products arising from benzyl-benzyl, benzyl-alpha and alpha-alpha radical combinations were indentified together with aromatic substitution products, whilst for the corresponding oCY system, markedly reduced yields of products arising from alpha radicals were observed. The sensitivity of the modelled copolymers to radiation induced crosslinking accords with these observations. The conclusion that o-methyl substituents sterically hinder the radical coupling reactions that afford crosslinks is discussed with due regard given to polymer chain flexibility.

INTRODUCTION Materials that are essentially chlorinated polymethylstyrenes (CPMS) have attracted interest in recent years for their application as negative-working electron beam resists in microlithography [1-3]. As a group, they are polymers with moderate to high electron beam sensitivities, with the potential for the achievement, reproducibly, of submicron resolution. More specifically, the copolymers of chloromethylstyrene (vinyl benzyl chloride, VBC) and methylstyrene (MS) prepared by radical initiation represent a subgroup whose structures can be tailored to offer either higher e-beam sensitivity at lower contrast, or vice versa [4]. This is most readily achieved by changing the isomer of the MS component. The sensitivity to radiation-induced crosslinking of the copolymers containing 2-methylstyrene (oMS) is significantly lower than that of the corresponding 4-methylstyrene (pMS) copolymers [5]. As to why this should be so is the subject of this paper. Pulse radiolysis studies of the crosslinking reaction in partially chloromethylated polystyrene (CMS) have been conducted by Tabata et al. [6]. Of most concern were the primary radiation chemical processes, but the overall mechanism proposed is shown in Scheme 1 where P~ and P~ represent the radicals depicted in Fig. 1. Thus, they conclude that crosslinking arises through benzyl-benzyl, alpha-alpha and benzyl-alpha radical combination reactions. Tanigaki et al. [7], using electron spin resonance spectroscopy and gas chromatography-mass spectroscopic analysis (GC-MS), subsequently studied the low temperature radiolysis of mixtures of isopropylbenzene (cumene; CU) and 4-methylisopropylbenzene (p-cymene;

pCY) with 4-chloromethylisopropyibenzene (4CMIPBZ), as low molecular weight model compounds for the polymers, reached the same conclusion but asserted that benzyl-alpha combinations predominate. The same crosslinking reactions might be presumed for copolymers of chloromethylstyrene and oMS but confirmation has not been sought. 2-methylisopropylbenzene (oCY) would be the appropriate low molecular weight model compound for oMS units in copolymers containing this monomer. The present work is mainly concerned with a

701

CMS ~CMS" + + e -

(1)

CMS -~ CMS*

(2)

CMS' ÷ + e- ~ CMS*

(3)

CMS + e- -~ P~ + C1-

(4)

C1- + CMS" + -~ complex

(5)

complex --, P~ + HCI

(6)

CMS* ~ PI + CI'

(7)

CMS + Cl' -* P~ + HCI CMS + Cl' ~ complex CMS + CMS* ~ P~ + complex Cl" + Cl" ~ C l : P~ + PI "-*Pt-PI

(8) (9) (10) (ll) (12)

p~ + P~ ~ PI-P2

(13)

P~ + P2 ~ P2-P2

(14)

Scheme 1

RICHARDG. JONESet al.

702

(o)

RESULTS

(b)

Final products from poly-( VBC-co - M S ) model corn pounds

Fig. 1. (a) Benzyl radical, P;, (b) alpha radical, P~. comparison of the products arising from the u.v. photolysis of benzyl chloride in oCY and pCY, and otherwise addresses the question of radical mobility in the various polymers. Tanigaki et al. [7] state that the deep-u.v, induced reactions in the systems studied are almost the same as the high energy radiation induced reactions. EXPERIMENTAL PROCEDURES

Materials

Benzyl chloride (BZCL) and CU (Fisons SLR) and oCY and pCY (Aldrich) were further purified by distillation under reduced pressure. The preparation and molecular weight data for VBC/oMS and VBC/pMS copolymers have been reported earlier [5].

Apparatus and procedures Photolyses were carried out at room temperature under an argon atmosphere in 1 ml quartz cells using the unfiltered beam from a Hanovia 100 W mercury compact source. GC-MS analyses were carried out on a VG-analytical m a s s spectrometer fitted with an on-line Pye 104 gas chromatograph. The GC was equipped with a 12 m × 0.25 m m i.d. fused silica capillary column coated with BP-5 (OV-17 equivalent) supplied by SGE of Milton Keynes. The operating condition w a s at a helium flow rate of 1 ml min-' and with the column temperature programmed from 30° to 200° at 10 ° m i n - ' .

t3C and IH NMR spectra were obtained on a JEOL JNM-GX270 spectrometer operating at 67.8 MHz, and a JEOL JNM PMX-60 spectrometer operating at 60 MHz, respectively. Chemical shifts are measured relative to TMS. Polymer glass transition temperatures (TB)were obtained using a Perkin Elmer DSC-7 differential scanning calorimeter.

Tanigaki et al. [7] used 4-CMIPBZ and pCY to model CPMS systems. Although 4-CMIPBZ is also the logical choice to represent VBC in the copolymer systems, it suffers from the disadvantage that the 4-iso propylbenzyl radical formed in the primary radiation chemical process is the same as the secondary radical formed by hydrogen atom abstraction from the substituent methyl group ofpCY. Products containing this residue would therefore give no indication of its source. It is for this reason that BZCL w a s chosen as a model for VBC in the present work. Photolyses and GC-MS product analyses were carried out on equimolar mixtures of BZCL and pCY, and BZCL and oCY. Figure 2, a typical total ion current (TIC) chromatogram for the latter system, demonstrates the complexity of the product mixtures. To aid the interpretation of the chromatograms, similar photolyses and analyses were carried out on BZCL alone, Fig. 3, and on equimolar mixtures of BZCL and CU, Fig. 4. Figure 5 depicts a comparison of the regions eluting between 200 and 300 scans in the TIC chromatograms of the B Z C L / o C Y and BZCL/pCY systems. The mass spectrometric assignments of the structures shown for the BZCL system were confirmed by ~H and ~3C NMR spectroscopy. The dominant product is dibenzyl (1) formed by the combination of primary benzyl radicals. Two isomers (presumably the 2- and 4-) of benzyltoluene (2) and ~-chlorodibenzyl (3) are also identified. These products are consistent with the observations and inferences from Eberhardt's [8] study of the radiolysis of BZCL in the liquid phase. The mass spectroscopic assignments of the structures associated with the various peaks for the other systems are shown in Table 1, in which the major products of relevance to the modelled crosslinking reaction are classified into four main groups as follows: those arising from benzyl-benzyl radical combinations, benzyl-alpha radical combinations, alpha-alpha radical combinations, and aromatic ring substitution reactions. Two features are immediately

100

200

¢00

Scans

Fig. 2. The TIC chromatogram of the product mixture from the photolysis of an equimolar mixture of BZCL and pCY.

Radiation-induced crosslinking of copolymers of CMS and MS

703

100

E

.=

/

60

o

20

BZCL

I

2do

o

~6o Scans

Fig. 3. The TIC chromatogram of the product mixture from the photolysis of BZCL: 1, dibenzyl; 2, benzyltoluenes; 3, chlorodibenzyl.

100

BZCL

CU

60

n-

1"~4 *64 ~** .

20

200

~00

600

Scans

Fig. 4. The TIC chromatogram of the product mixture from the photolysis of an equimolar mixture of BZCL and CU (for peak identification see Table 1 and text). 9

evident: (i) the complete absence of alpha-alpha combination products from the BZCL/oCY system, and (ii) aromatic ring substitution products that were either not observed, or not reported by Tanigaki et al. No attempt was made to carry out quantitative analyses because of the unavailability of definitive samples of many of the products, however the aliphatic region of the ~3C N M R spectrum of the product mixture from the BZCL/CU system after removal of the bulk reactants is shown in Fig. 6. In this case, the major resonances have been assigned by literature reference [9]. Although the spectrum is not a quantitative representation, the dominance of the resonance at 25 ppm which corresponds to the methyl carbon atoms of dicumyl (the alpha-alpha combination product of secondary radicals formed by hydrogen atom abstraction from the alpha carbon atom of CU) is evident. The remaining resonances can readily be assigned to the aliphatic carbon atoms of the other product structures for this system that are shown in

Ai ;WWK 10 *

JL c

.......

(b)

14

.

.

.

.

.

.

Scans

Fig. 5. The 200-300 scan regions of the TIC chromatograms of the product mixtures from the photolysis of equimolar mixtures of BZCL and (a) pCY, (b) oCY, normalized to the magnitude of the dibenzyl peaks (for peak identification see Table 1 and text).

704

RICHARDG. JONESel al.

Table 1

BZCL/CU Benzylbenzyl ~

BZCL/pCY

CH,CH2~

l

BZCL/oCY

~CH2CH2~

I

~CH2CH, ~ CH3~

Q

~ /CHa CH,CH,/(~X~CH 7

c./L,

~

CH2CH2~

12

CH3~cH CH3~ ~

~

/CH3 Sll

CHI k ~ /

k~/\CH~

CH~ N

~=~CH2 CH2~P=~

1311

x._~ fiH, X~./ CH \ CHa

CH3

CHa~ >

alpha ~ /

CH2--C / ( ' ~ CH~

cn/'

( ( })CH~--C( ( I )

14

CH/~/ CH3

/H S~_.)/CH2--.C~ [ )>CH, ol;

~

~'~)CH2--C( ( } )

13b

cH/X~/ CH \ Ctt3

Alpha-~'~ alpha ~

CHsCH3 C/,L~C~---~

CH3CHs

CHaCHa

CHs CH3

\

10

/

~

Aromatic (( / f ~ \ ))CH2=(=-b /f'~,\ - })CH 6 substitution \ ~ / ~ ~CH3 Other products

,

.CHs

f-'lls~

/CHs

~

~/CH3

CH,"t"t- ))

Other products

Table 1. Similar spectra for the BZCL/CY systems are too complex to afford ready analysis.

Glass transition temperatures of VBC /MS copolymers Figure 7 depicts the variation of Tg with chlorine content for poly-(VBC-co-oMS) and poly-(VBC-copMS). In either case, increasing chlorine content is accompanied by a decrease in Tg. The opposite trend was observed for CPMS materials [10] and this is also represented in Fig. 7.

CH , ~'CH3

11

Other products

,

DISCUSSION A discussion of the primary photochemical processes of BZCL is not the intention of this paper, so it is sufficient to note that the products observed from the bulk liquid phase photolysis accord with Scheme 2 which is based on conclusions drawn from a study of the photolysis of BZCL in methanol-water mixtures [11]. Dibenzyl from the combination of primary radicals is the dominant product, but the formation

Radiation-induced crosslinking of copolymers of CMS and MS

705

PPM

Chemical

shift

Fig. 6. The aliphatic region of the ~3CNMR spectrum of the products from the photolysis of an equimolar mixture of BZCL and CU.

of benzyltoluenes and ct-chlOrodibenzyl indicates the readiness with which these radicals also undergo hydrogen atom abstraction reactions. Whilst it is proposed that the aromatic substitution reaction required for the eventual formation of the benzyltoluenes is through the intermediacy of benzyl carbonium ions resulting from the oxidation of benzyl radicals by chlorine atoms, it is accepted that benzyl radical substitution of toluene could also lead to these products. Significant concentrations of toluene could only be present after prolonged irradiation, so this sequence of reactions is not relevant to the modelled crosslinking process. The same intermediate species are sufficient to explain the observed products from the photolysis of BZCL/CU mixtures. Dibenzyl (1) from the combination of primary benzyl radicals is again observed, but the dominance of the alpha-alpha product dicumyl (5) is indicative of the ease of hydrogen atom abstraction from the alpha carbon atom of cumene by either,

140~

aop I

i

20

i

40

,

60

,

80

b

1[)O

MOL % VBC (or equivalent) in polymer

Fig. 7. The variation of T~ with chlorine content for

copolymers of vinyl BZCL with oMS (O), and pMS (x). Using data taken from Ref. [10], the corresponding variation for chlorinated polymethylstyrenes is also shown (---).

706

RICHARD G. JONES et al. hv

C6Hs" CH2 CI "-*C6Hs' CH2 CI*

(15)

C6H5•CH2 CI* --* C6H ~.CH 2 + CI'

(16)

C6Hs"CH2 + CI' -* C6H s 'CH~- + CI-

(17)

CI' + C6Hs.CH:CI --, C6Hs'CHCI' + HC1

(18)

C6H5"CH2 + C6H5"CH2 C1 ~ C6H5.CHCI' + C6H5. CH 3

(19)

C6H $ "CH2+ C6Hs"CH~ ~ C6Hs'CH2CH2"C6H5

C6Hs'CHf + C6H~'CH 3--*C6Hs. CH2.C6H4-CH 3+ H +

(20) (21)

Scheme 2

or both, the primary chlorine atoms or benzyl radicals. The benzyl-alpha cross radical product ~,~t'-dimethyldibenzyl (4) is also observed. Products derived from the chlorobenzyi radical (including stilbene) are amongst the minor products alluded to at the foot of the columns of Table 1, as are the benzyitoluenes, but not surprisingly, in the presence of a large excess of CU the dominant aromatic substitution products are benzylcumenes (6). Though TIC chromatograms do not represent a quantitative analysis, the magnitude of these two peaks indicates that when taken together, they are probably the second major product. Whereas this kind of reaction appeared to be of little significance to the modelled crosslinking process from analysis of the bulk BZCL system, it assumes greater importance from this observation on the BZCL/CU system. The reactions of Scheme 3 in addition to those of Scheme 2 are sufficient to represent these observations. With the exception of reactions (17)-(19), (21) and (21a), all the reactions of Schemes 2 and 3 have their analogues in Scheme 1, either singly or in groups. In particular, reactions (20), (20a) and (20b) are analogous to the crosslinking reactions 0 2 ) - 0 4 ) of Scheme I. Taking account of the methyl groups of oCY and pCY being additional sites for hydrogen atom abstraction by primary radicals, most of the products from the photolyses of BZCL/oCY and BZCL/pCY mixtures can be accounted for in a similar scheme. Thus, in both of these systems there are two types of benzyl radical (benzyl and isopropylbenzyl) so one alpha-alpha, two benzyl-alpha and three benzyl-benzyl combination products would be predicted, along with benzylcymene aromatic substitution products. In the case of the BZCL/pCY system examples of each kind of product are observed, with the benzyl-alpha combination product again being prominant. Notable by its absence from the BZCL/oCY system however, is the alpha-alpha combination product. Furthermore, since the mass spectra of the

corresponding radical-radical combination products of the two systems display similar and quite simple fragmentation patterns, it is not unreasonable to normalize the TIC chromatograms to the magnitude of the dibenzyl peaks. It is then evident that all of the radical-radical combination products are formed in significantly lesser ratio to dibenzyl in the oCY system than in the pCY system. On the other hand, the corresponding ratios of the peaks for the benzylcymene aromatic substitution products remain broadly comparable. As in the case of the BZCL/CU system, the other peaks in the chromatograms can be rationalized as products derived from chlorobenzyl radicals. In the chromatograms of both systems however, there are not insignificant peaks for products that elute between dibenzyl and the benzyl-~cumyl combination product. These materials have as yet uncharacterized fragmentation patterns and their identity remains an enigma. These observations are broadly consistent with the conclusions of Tanigaki et al. that although benzyl-alpha combinations are predominant in the radiation chemistry of the CPMS systems, alpha-alpha and benzyl-benzyl combinations cannot be neglected due to the large diffusion length of primary chlorine atoms. It is additionally evident however, that because of steric hindrance, if the radicals concerned are centred on carbon atoms that are ortho-substituted, all such coupling reactions are suppressed in comparison with the corresponding reactions ofpara-substituted radicals. The implications of this conclusion in respect of the copolymer systems that are modelled is a reduction in the probability of formation of ring-tobackbone crosslinks, and a marked reduction in the probability of formation of backbone-to-backbone crosslinks, when the MS component is oMS. The acknowledged steric hindrance afforded by the o-methyl substituent in poly(VBC-co-o MS) has other implications concerning the radiation sensitivity of this material. The Tgs are some 15°-20 ° higher than

CI' + C6H~'CH(CH3) 2-* C6Hs'C(CH~) 2+ HCI

(18a)

C6Hs"CH~ + C6Hs"CH(CHa): ~ C6Hs"CH3 + C6Hs"C(CH3)2

(19a)

C6H~"CH~ + C6Hs" C(CH3)2 "* C6H5" CH2 C(CH3)2' C6H5

(20a)

C6H 5•C ( C H 3)-~ + C 6 H~' C(CH 3)i ~ C6Hs" C(CH3 )2 C(CH3)2" C6H~

(20b)

C6Hs' CH~" + C6H~' CH(CH 3): ~ C6Hs' CH2" C6H4' CH(CH3)2 + H + Scheme 3

(21a)

Radiation-induced crosslinking of copolymers of CMS and MS those of the corresponding poly(VBC-co-p MS) materials. The chain is correspondingly less flexible and this will suppress the short range conformational changes necessary for radical-radical reactions to take place readily. F o r both systems Tg decreases with increasing chlorine content but so also does the radiation sensitivity. Over the same range of chlorine contents however, the T s of C P M S materials increases and at greater than 40% exceeds those of either of the poly(VBC-co-MS) systems. The radiation sensitivity of C P M S materials nonetheless increases well beyond this chlorine content, so it is concluded that the effects of chain flexibility are subordinate to the influences of steric effects on radical reactivity.

Acknowledgements--We thank Smith Kline and French Research and specifically Dr Charles Brown, for the use of their GC/MS equipment. We also thank the management of the Oji Paper Co. of Japan for their financial support for Mr Matsubayashi.

707

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