Polymerization of TS-12 diacetylene crystals: Crystal structures of monomer and polymer and spectroscopy of reaction intermediates

Chemical Physics 72 (1982) 201-212 North-Xoknd Publishing Company

POLYMERlZATiON

OF TS-12 DIACETYLENE

CRYSTAL STRUCTURES AND SPECTROSCOPY

CRYSTALS:

OF MONOMER AND POLYMER

OF REACTION

INTERhlEDIATES

D. SIEGEL, H. &XL Physikalisches Institu t, Teil3,

Universifit Stuttgart, D-7000 Stzcttgort SO, West Germany

and V. ENKELMANN and G. WENZ InsrirutFr~~a~7omolekulare

Chemie. Universit& F&burg. D- 7800 Freiburg, West Germany

Received 13 May 1982

Information concerning the mechanisms of the solid-state polymerization in TS-12 (dodeca-5,7diin-1,12diyl-bis-ptoluolsulphonate) is deduced from optical absorption spectra and gel-permeation chromatography. The monomer and polymer crystal data are reported. The spectroscopic properties of the polymer chains and of the short-chain (dimer, trimer, . ..) intermediates of the low-temperature photoreaction are described in this paper. The photochemiul and thermal generation and decay reactions are discussed.

In most cases the reaction is initiated photochemically (by UV or y irradiation) or thermally (by heating the crystal). Among the many different reactive diacetylene crystals with different substituents R, TS-6 (with R = CH,S0,C6H,CH,) has been the most extensively studied [S-9]. In this paper we report on a related diacetylene, hereafter referred to as TX-1 2 (with R = (CH2),S03C6 H4CH3), which has more extended sub-

larged rest group R a change in the topochemical reactivity of TS-12 is expected, which may be reflected in the properties of the low-temperature reaction intermediates. The reaction in TS-12 is of special interest because, due to their solubility, the polymer chain lengths obtained in the polymerization reaction now become measurable by gel-permeation chromatography GPC and light-scattering methods [IO]. GPC is a very sensitive method, which allows a reliable determination of the chain-lengths distribution within the photopolymerized TS-12 single crystal. In a recent paper the polymerization of TS-12 crystals by y irradiation at 300 K has been reported [lo]. The polymer obtained in this solid-state reaction is easily soluble in many organic soivents, such as chloroform. The absorption maximum of the resulting yellow-orange solutions lies at. a wavelength of 470 nm. The weighted average of the polymer chain lengths,~,,~ = 1470 monomer units, was obtained via the light-scattering intensity of the polymer solution. A GPC apparatus could be calibrated

stituents and soluble polymer filaments_ From the specific changes in the crystal structure due to the en-

by light-scattering methods. The most probable chain lengths p (that is the maximum of the distribution

1. Introduction The mechanism of the polymerization reaction in diacetylene crystals has been the subject of several investigations [l--4]. This interest is a consequence of the unusual polymerization chemistry in the crystalline phase. In the polymerization reaction the monomer diacetylene single crystals are transformed continuously into the polymer crystals following the reaction equarion ”

[R-CGCGR]”

~_,~,-_c@=,+~ . ”

0301-0104/82~0000-0000/S

”

02.75 0 1982 North-Holland

D. Siegel et al/PoIpnerization

of TS-I-7 diacerylene cpslds Table 1 Crystal dais

aw

KNOMER

TRIMER

DIMER

FHOTG!NITIATICN

ADDITICN

TETRAVER

POLYMEiilZATlON

13:. 1. Initial steps ol‘ the polymeriation reactions showing dimcr. rrimer. .._ radical structures. For the s&r of&&y the cxbon atoms ax symbolized by fill circles. the Iqe substiments by R. function)

was found to be 1100 units. Now it was interat single crystals polymerized by W irradiation (248 nm) at room temperature and at liquid-h-hum temperature. The insight into the mechanisms of the polymerization reaction has increased considerably since different reaction intermediates have been anaIyzed by ESR [I l.l?I] and optical spectroscopy [13,14]. The formation of the diradical dimer, trimer, ietramer, _._intermediates within the monomer cIysta1 is shown in fig. 1. Two photons are required in the dimer initiation reaction. Subsequent addition polymerization reactions are possible by further LJV irradiation (/zv) or by thermal annealing (HJ of the crystal. In the present paper we report new results of the opticai absorption spectra of the diradical (DR) intermediates, of the asymmetric carbene (AC) intermediates, of reactive polymer chains (P*) and of the unreactive polymer (P) in TS-I 2 crystals. Their physical and chemical properties are compared with those of the TS-6 c.ystaIs. From the photochemical and thermal reactions valuable infcrmation concerning the polymerization mechanisms in diacetylene crystals is obtained. esting

to make GPC measurements

2. Crystallization

and structure

determination

The preparation of TS-12 is described ekewhere [to]. The best single crystals were obtained by slow evaporation of a 2% solution of TS-12 in tetrachloro-

b (a cw a (dq) P (dfa I (de&)

q,(alg

PA-~)

space group

3fonomrr

Polymer

(110 R)

(300 K)

Polymer (110 Ic)

20.60(2) 11.79(1> 5.19(l) 53.0 (3) 89.2 (3) 92.7 (3)

20.13(2) 6.11(l) 4.91(l) 95.1 (3) 93.7 (3) 85.7 (3)

ZO.Ol(2j 6.02(l) 4.91(l) 95.1 (3) 93.7 (3) 88.7 (3)

1.36

Pi

1.39

Pi

1.42

Pi

methane in a nitrogen atmosphere at room temperature. The crystals obtained by this way were platelets systematically twinned about the (011) twin plane. Single crystals were obtained by cleaving the crystals which usually consisted of two differently oriented parts. Pertinent crystal data are summarized in table I. The unit-cell dimensions urere determined from doubleradius Straumanis-type Weissenberg photographs (R = 57.3 mm). Polymerization of the monomer crystals in the X-ray beam was inhibited by collecting the data at 110 K. The low temperature was maintained by a low-temperature attachment to the Weissenberg camera. Intensities were recorded by multiple-film equi-inclination Weissenberg photographs using Ni-filtered Cu Kol radiation. The intensities were estimated visually by comparison with a series of timed exposures of a selected reflection. 2042 and 1176 unique reflections were recorded for the monomer and polymer, respectively. Of these 109 (129 for the polymer) were considered having intensities less than a threshold value. They were assigned half this value and were given zero weighr in the refuiement. The structures were solved by direct methods using the program MULTAN [IS]. Refinement was done by full-matrix least-squares analyses. The hydrcgen atoms were assigned physically reasonabIe parameters and their contribution was Muded into the structure-factor calculations but their parameters were not refined. When anisotropic temperature factors were introduced in the refinement of the monomer the R index showed no significant improvement considering the added number of parameters. In the refmement of the polymer structure the phenyl ring and the methyl group were refined as rigid groups

a

F&z. 2. (a) Projection of the StRiCNre of TS-12 monomer on the ah pIme. (b) Projection of the ‘IS-17 polymer structure on the ab plant. (c) Projection of the TS-12 monomer aucture on the plane of tbc diacetylene stz&s. (d) Projection of the pofymcr structure on the plane of the pofymer backbone. (e) Projection of the monomer and polymer structures or! a common plane.

Table 2 (conthued) X

CIi

1

C!?) (‘6) C-i-l)

C'(5) Clh)

C’(7) C( 8) C(9) C(lO) C(ll) C(l2l C(l3) S(1) O(I) O(1) O(3) C(Z1) C(Z) c(x) C(U) C(ZS) C(26) C(27) c(x) C(29) C(30) C(31) C(32) C(33) S(Z) O(4) O(5) O(6) H(l) H(2) H(3) H(J) ti(5) H(6) II(i)

H(S) Ii(9) HilO) H(11) H(lZ) H(13! H!l?) H(lj)

s

3

5271(j) 5518(j)

1910(9) 1441(9) 662(g) -446(g) -202(9) -1197(9)

5531(5) 6049(5) 6599(6) 6792(j) S]&(6) 7963(6) S371(6) S97S(6) 9167(6) 876-t(6) 9418(7) 7631(l)

7189(J) SOOS(3) 7143(d) 4951(S) 4676(S) 1324(S) 3920(5) 3603(5! 3278(6) 1813(6) 1229(6) 650(6) 10’2(6) 15S9(6) 1992(6) 59’(i) x45(:) 2750(4) 2778(-l) 1975(J) 5481 6255 5615 6100 6314 6999 6973 6366 74% 8’26 9659 5916 9ss1 9101 9539

-2468(10) -3474(10) -3930(11) -3434(11) -2410(11) -1933(11) -3920(12) -1S71(2) -920(7) -1239(7) -2723(7) 16 l-1(9) 3172(9) 3850(g) 3751(9) 2x19(10) 65 14(11) 7399(11) 6S33(11) 7316(11) 8398(11) 8953( 11) S363(10) S919(12) 67?6(‘7) 3982(7) 7675(7) 6031(7) 411 1063 -843 -1OJ8 367 143 -191-l -1713 -3566 -4713 -2019 -1173 -3430 -3973 -4797

B

7458(27) 6100(26) 4385(25) 6013(25) 7996(27) 9589126) 11442(26) 10‘?43(27) S727(29) S141(2S) 9167(29) 10835(28) 6377(31) 13446(7) 11496(17) 15277(17) 14406( 17) 9107(Z) 10547(26) 12159(17) 10393(26) 12137(37) 10324(29) 8755(29) 9395(B) 1 lljf(24) 1199lQ9) 11190(28) 9528(27) 13827(31) 6773(i) S777(18) 5474(17) 5lll(lS) 2931 3349 7162 4744 9130 6826 8453 10798 10929 7949 8668 11657 6049 4455 7180

(ii’)

0.87(19) O.S4(19) O.Sl(19) 0.88(10) 1.36(23) O.iO(20) 0.75(23) 0.9Of23) 1.27(Z) 0.9X(24) 1.17(24) 1.14(24) 2.04(30) 0.5716)

1.03(6) l.OS(16) 1.17(17) 0.75(19) 0.94(20) 1.26(21) 1.14t21) 1.13(21) 1.56(25) 1.36(26) 1.29(23) 1.21(2-t) 1.25(26) 1.13(25) 0.75(22) 1.91(B) 0.67(6) 0.96(16) 1.06(16} 1.27(17) 3.0 3.0 3.0 3.0 3.0 3.0 :.!I 3.0 3.0 3.0 0_. ‘ 3.0 3.0 3.0 3.0

4’

z

B (A*)

H(21) H(Z) H(23) H(24) H(25)

3971 4632 3569 4162. 3243

3343 4357 4250 5196 5041

13542 13395 9334 8903 13294

3.0 2.0 3.c 3.0 3.0

W6) H(Z7) H(78) H(29) H(30) H(31) H(2) H(33) H(34) H(35)

3972 306s 3634 1091 361 1734 2465 144 453 843

5891 7069 7000 6024 6913 9764 8875 8379 9756 8987

132ii 11624 9044 8888 11843 11782 8830 14354 13051 15605

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

with one common anisotropic temperature factor for each group. On tennination of the refinements the I7 indices

were

0.101

and 0.098

ior

the monomer

and

poIymer respectively. The programs used were those of the XF?AY76 system [ 161 and the SHELX system [ 171. Scattering factors for C, 0 and S were taken from ref. [ 1S] and for H from Stewart et aI. [19]. Projections of the monomer and polymer crystal structures are shown in fig. 2. The final atomic paramerers are listed in tabIes 2 and 3 *.

3. Phase change during the polymerization The polymerization of TS-12 is connected with a phase change of higher order which seems to be correIated with the increase of polymerization velocity. In the monomer the b axis is doubled. The transition can be followed by a continuous decrease of the intensities of reflections having odd k indices with increasing conversion. It should be emphasized that we are not dealing here wit\ a low-temperature phase which has been observed in TS-6 monomer and polymer crystals [7] since the doubling of the unit ceil is aIso present in TS-12 monomer at room temperature. The dependence of the lattice parameters determined at 110 K ’ Lists of observed ;Ind cllcukxted structure factors and of bond lcrqths ar.d nn$s arc avaihble upon request from V. EnkcInunn.

Table 3 Final atomic parameters of TS-12 polymer. The valuesgiven are fractional coordinates and anisotropic temperature factors Uii X 104.ESDSare given in parentheses X

Y

z

C(2) C(3) C(4) C(5) C(6) C(7) C(3)

5047(4) 5163(4) 5669(5) 59:4(s) 6351(5) 6382(5) 8094(3) S684(3)

C(Y)

9105(3)

14S41(18) 144W16) 12x74(17) 1179:16) 9 194(17) 7533(18) 5618(12) 6504(12) 5241(12) 3094(12) 2208(12) 3470(12) 1907(27) 6836(63 X488(13) SZSS(l5)

3861!17) 1064(19) 660(17) 3060(17) 2223(20) 4321(20) 6254(14) 5526(14) 3842(14) 2857(14) 36X(14) 5298(14) 1033(27) 8083(6) 59 15(14) 9632(13)

W)

C(lQ

Gill)

C(12) W3) S(1) D(l) O(2) O(3) H(1) D(2) X(3) H(4) H(5) H(6) H(7)

H(8) H(9) H(lOj W 1) H(12) H(13) H(14) H(15)

893‘S(3) 8348(3) 7927(j) 9403(7) X84(1) 7X1(3) 796X4) ?143(4) 5446(5) 6104(5) 5486(S) 6333(5) 6075(5) 6797(S) 6720(S) 6189(5) 8813(3) 9561(3) 8218(3) 7470(3) 9 352(7) YS51(7) 9408(7)

5286(15)

11384(17) 132xX17) 10623(16) 12255(16) 8268(17) 9917(17) 5985(18) 7278(18) 8166(12) 5927(12) 546(12) 2785(12) 264(27) 768S(27) k29(27)

9279(16)

-S92(17) -98(17) 4011(17) 4523(17) 565(20) 1474(20) 3253(20) 5649(?0) 6265614) 3279(14) 2575(14) 5862(14) 1668(27) 1944(27) -117X27)

II11

oru

u22

J733

131(37) 227(46) 172(48) 137(45) 318(58j

-76(56j

429(47)

665(37) 466(40) 842(60) 736(5S) 658(61) 632(54) 623(25) 623(25) 623(25) 623(25) 623(25) 623(25) 1056(98) 744(18) 660(40) 887(49) 862(45)

401(62) 4 14(B) 414(28) 414(28) 414(2S) 414(28) 414(B) 555(88) 223(13) 384(41) 578(53) 4 12(45)

1369(234) 1369(234) 1369(234) 1369(234) 13691234) 1369(234) 1369(234) 1369(234) 373(24) 373(24) 373(24) 373(24) 786(99) 786(99) 786(99)

623(25) 623(25) 623(25) 623(25) 1056(98) 1@56(98) 1056(98)

414(28) 414(25) 4L4(2X) 414(28) 55X88) 555(,98) 555(8&g

332(38) 219(50) 327(49) 301(55) 431168) 133(49) 373(24) 373(24) 373(24) 373(24) 373(24) 373(24) ?86(99) 329(lS) 239(3S) 53S(56)

on the polymer content is shown in fig. 3. Up to ~20% conversion the monomer phase which can be regarded as a solid solution of polymer chains in the monomer matrix is stable. The polymerization proceeds homogeneously without phase separation or abrupt changes of the lattice parameters in the transition region which have been observed for other diacetylene ]20,21]. The doubling of the unit ccl! in the monomer can be explained by two different models. The first model consists of alternating layers of molecules with different side-group conformation like in the low-temperature phase of TM [7]. Jn the second model which has been confirmed

by the structure analysis the

u23

VI3

u12

-131(64) -22(28) -22(28) -22(28) -22(28) -22:28) -22(X) 73(96) -27(17) -29(40) -449(53) 78(W

-SO(37) lO(40) 1W) -76(43) -32(53) -7(47) -43(21) -43(21) -43(X) -43(21) -43(21) -43(21) lY(S2) 19(12) 18(33) -227(46) llS(39)

-30(42) -5S(44) 99(60) 89(55) 144(6 1) -41(54) O(25) O(25) O(25) O(25) 005) O(2S) 160(98) 106(16) -5(36) lSS(49) 134(45)

-22(28) -22(2S) -22(28) -22(2S) 73(96) 73(96) 73(96)

-43(21) -43(21) -43(X) -43(X) 19(82) 19(82) lY(S2)

O(25) O(25) O(25) X25) 160(98) 160(98) 160(98)

80(47) -13(45) -138(54) 36(50)

monomer molecules are not centrosymmetric

and the

two differently oriented side groups are attached to the same diacetylene group. Both structures consist of molecular stacks extended in the (21@) plane forming continuous sheets. Polymerization c direction.

proceeds in the

Neighboring diacetylene groups are separated by 5.19 A making an angle of 47.9’ with the c axis. These packing parameters are well within the range where high reactivity is expected [2,22]. The polymer backbone and the methy;rlene side chains lie approximately in the same plane whereas in the monomer one of the side chains is rotated out of this plane (fig. 2). This gives rise to a packing arrangement where the centers of the reacting diacetylene

206

D. S&t-I et oL/Polwncnhrion

of I’SIZ

diacerylrne cryrrais

the center between the sulfur atoms is located at 4 = 0.4989, E = 0.2453 and c = 1 .Oll 1 (cf. table 2). Therefore, the additional centers of symmetry which appear at the phase transition are generated without much change of side-group packing by movements of the methylene spacer chains until they assume symmetrical conformations. It can also be seen in fig. 3e that this side-chain rearrangement involves a small movement of the phenyl rings toward the pofymer chain which accounts for the rather large continuous decrease of the a axis with increasing conversion.

LO

E

4. Optical absorption

spectroscopy

.9 * 4.1. E_Tperimen faZprocedure

Fig. 3. Dcprndcncr of the lattice parsmctcrs

detcrmincd

at

i 10 li an 11~2convrrsion.

groups in neighboring molecular sheets are ahernating11;above and below the site they assume in the polymer. This is quite surprising since large molecular motions arc connected with the polymerization. In other diacetylenes the reaction proceeds by a rotation of diacetylene groups about the center so that both the position of the side groups and the center of gravity of the molecuIe are retained [7]. In TS-17 monomer the center of the diacetylcne group moves from D = 0.5 111, .?I= 0.2782, c = 0.8298 to a = 0.5, b = 0.25 2nd c = 1 .O in the polymer, i.e. a distance of 0.97 a without any destruction or macroscopic deformation oi the crystals. This unexpecred behaviour can be understood ifit is assumed that the terminal fl-toluene sulfonate groups retain their position during the reaction. The movement of the diacetyiene groups is brought about by conformational changes of the methylrne spacer groups. This model is illustrated in 1+=. ?e where monomer and polymer structures are plotted on 3 common $ane. It can be seen that indeed the p-toluene sulfonate groups in the monomer arc already pseudo-centrosymmetrically related, e.g.

Optical absorption spectroscopy was performed with a variable-temperature cryostat (Oxford Instruments, CF 204) in a conventional optical absorption spectrometer (Pklips SPS-250). The monomer crystals were mounted with the (011) cleavage or natural growth plane perpendicular to the Incident light. The photoreaction was initiated by UV light using the 248 nm line of an excimer laser (Lambda Physics, EMG 100). Optical bleaching of the intermediates was performed with a xenon high-pressure arc (Varian VIX 300) and appropriate interference and edge filters. 4.2. UVpolymeriza~ion 0t room temperature Room-temperature and low-temperature unpolarized absorption spectra of the TS-12 monomer crystals are shown in fig. 4. OD is the optical density of the diacetyiene plates defined by OD = iogIo/l = 0.434 old. 1, and I are the incident and the transmitted light intensities. Consequently OD is proportional to the absorption coefficient (Yand the thickness d of the crystals. Polymer zbsorption is absent in the original monomer crystals. Consequently the monomer crystals are almost completely transparent in the visible spectral range (see (la) and (22) of fig. 4). The spectra (lb) and (2b) show an intense absorption due to the roomtemperature W photoreaction. Two principal species PO and PI of polymer molecules with marked phonon peaks are formed upon LJV irradiation. At higher irradiation doses Pt increases with respect to PO. Therefore PI is attributed to the highly concentrated poly-

D. Siegel ez al.fPolymc&atio?rof TS-12 WAVELENGTHS 600

h

Inm

201 hlnm

-

coo

SO0

dloceqletre crysrais

600 700

300

15000 20000 WAVENUMBERS

i

I

I

I

25ooO 15000 WAVENUMBERS V/cm-l

35000 -

Fiz. 4. Unpolarized absorption spectra of the TS-12 monomer crystal at low polymer conwxsion befoxz (a) and after (b) UV irradiation (245 nm) at room temperature. (1) Roomtemperature spectra, (2) low-temprature spectra. Par clvity the spectra (2) are shifted with respect to the spectra (1) by *O.-S in the OD scale. at the crystal surface, where most of the UV quanta are absorbed, whereas PO corresponds to the polymer chains at low concentration in the crystal volume. The shift of the PO and PI absorption lines due to temperature variation shown in fig. 5 stresses the sensitivity of the polymer line positions even on very slight changes in the monomer-crystal lattice parameters. mer formed

B/cm-1

-1 h *?

f

high

cont.

1gooo leooo

inm

_ 5:[!

* d .

“PO

6**

03

17000-

- 560

0

- 580

cont.

- EOO

4 0

00

0

low

-

600

25000 7 /cm-l -

Fig. 6. Optical absorption spectra of the low-temperature reaction intermediates. The TS-I2 monomer crystal has bcun irradiated with 500 excimcr laser pulses a1 248 nm (=I00 mJ). The TSd monomer crystal has been irrodiatcd with 5 cscimcr laser pulses at 308 nm (=lOO mJ).

4.3. Uvphatowction

cl1low temperahires

4.3.1. Formation of diradical intermediates A, B, C... By 248 nm UV irradiation of the TS-12 monomer crystals at low temperatures a series of absorption lines A-F is observed (see fig. 6). The spectra of the TS-I:! low-temperature photoproducts in the TS-6 monomer crystal - which have been correlated to dimer, trimer, tetramer, .. . diradical (DR) reaction intermediates j14] - are included. The TS-12 photoproduct absorption lines are split into doublets and in contrast to ‘E-6 there is an additional photoproduct F, which corresponds to the diradical heptamer. The pair partners in TS-6 are very weak in intensity and therefore they cannot be recognized in fig. 6. With increasing oligomer chain length in TS-12 the line splitting vanishes asymptotically, consistent with the observation of only one po!ymer absorption line (PO or PI). From the very different number of laser pulses used in the experiments we conclude that the photoreactivity in TS-l?I crystals is reduced by a factor of ==102 as compared to TS-6. 4.3.2. Formation of asymmetric carbejle intermediates b, c, d... The effect

of low-temperature

the DR intermediates Fig.

5.

Temperature line positions.

dependence

of the polymer

absorption

fig. 7. Analogous

A-F

optical

by visible

light

bleaching is shown

of in

to the photoreactions in TS-6 [ 141 the DR intermediates A, B, C decrease in intensi?y. Si-

208 ?.lnm

hlnm

-

800 70C1 @IJO

-

LOO

5w

A , 0.7 i

0.6

gi ml s 0.5 c

0.L

9

0.3

-t 2

02

B

@1 00

FEDC 0.00

1 WAVENUMBERS

Y/cm

by

1

1

!

15000

20000

25000

4.3.3. Formation of reactive [P *) and unreactive (P,J po(mer The effect of low-temperature 3 1 I nm W irradiation of the monomer crystal containing the reaction intermediates of fig. 6 is shown in fig. S. With 311 nm W quanta a further photoinitiation of the polymerization reaction is not possible, because the monomer absorption edge lies at 279 nm. Only photoaddition rtactions of the intermediates ace allowed. In spite of only slight changes of the intermediate absorptions

muItaneously the absorption lines b, c, d (attributed to asymmetric carbene intermediates [ 1 l]) are produced. In spite of the superposition of the different lines again a splitting of the new lines is observed. As before - in contrast TOTS-6 [ 141 - these transformation reactions A b, B c. C d are not very efficient. As with TS-6 weak AC-intermetiiate absorptions are also present in the initial spectra obtained by 245 nm W irradiation (see also figs. 6 and 8).

WAVELENGTHS

hfnm

-

dtfference

i= %

A

v/cm-l 311 MI UV irradiation (2), (3) Fe. 8. Low-temperature ;~frer 243 nm UV irradiation of the TS-I2 monomer crystal (1). The 311 nm irrzdi;ltion time is given aI rhe spectra.

--p

Fig. 7. Optical b!taching of the rcxtion intcrmedkes visible li+ht (-lXL8CO nm) at low temperntures.

E

spectra

g-L

cj 0.001&0

2&o

2-&o

WAVENUMBERS

2oMxl

15boo

”

ii I cm-’

250x

-

Fig. 9.OptisJ polymer transi’ornxtion reaction P: = Pz at 10 E. S~crnrm (1) is obtained in the same way as spectrum (3) in Ii:. 8. Spectrum (2) is obtticd from (1) upon irradiation with visible Light. Specrrtlm (3) is obtained from (2) upon 311 nm UV irradixion.

The difkrcnce

spccrrc (2)-C 1) xnd (3)-(l)

are &WI on the right side.

D. Siegel et al/Polymerization

if TS-12 diacefylene cgvstalr

209

appreciable polymer formation with maxima PO and Pg (described below) on top of a broad polymer absorption underground is observed. This is in contrast to the effective addition polymerization reaction of the AC and DR intermediates with final polymer formation in the TS-6 crystals [14]. 4.3.4. Transformationof the reactivepolymer P,” f Pb* Two absorptions, due to reactive polymer are observed at 646 nm (Pi*, and at 624 nm @‘{). Pg can be transformed to the polymer absorption Pi photochemically (upon irradiation with visible light) or thermally (see below). Reversible ch;mges of the polymer absorption 311 nm P* PC “vis.kT have been observed at low temperatures. This is shown in fig. 9. Spectrum (1) is obtained following the same irradiation procedure as with spectrum (3) of fig. 8. Irradiation of the crystal with visible light (400-800 nm) produces a new absorption line Pj_ @lus vibronic progression). This is shown more clearly in the difference spectrum (2)-(l). Successive 311 nm W irradiation reverses this effect and in addition polymer Po is produced as shown by the difference spectrum (3)-(l). Apart from polymer formation this reversible effect has been reproduced several times without any observation of further spectral changes especially of the intermediate states.

o.oo-

I 15000

2oaao

25OCO Olcm-l -

Fi& 10. Thermal reactions of the intermediates of the UV of the TS-12 crystals. The annc&n_e temperatures (anncoling time 30 min or longer) are given at the spectra. All spectra are taken at 90 Ii. photopolyme_izatio;1

5. Gel-permeation

chromatography

Gel-permeation chromatography (GPC) has been performed after annealing of the crystals at room temperature. Therefore, information is obtained only concerning the length of the final unreactive polymer, obtained after themral addition reactions of the metastable low-temperature photoproducts (A, B, C . . . b, c, d . ..). The chain-length distributions of two TS-12 crystals, which have been irradiated with W light (348 nm) at

4.4. nzemral reczctiow The spectral changes induced in the optical absorption spectra by thermal reactions are shown in fig. IG. The thermal reactions start at =I40 K. This is far above the corresponding reactions in TS-6, which start at ==90 K. As expected from the well-known thermal addition polymerization sequence A + B + C + . . . the short intermediates vanish first. Simultaneously reactive polymer P,* is formed at the very beginning of the reaction, which in a final reaction (starting at 2160 K) is transformed into the unreactive polymer Po. The relatively high intensity of the P,* and especially of the PO absorption is a clear indication of very long polymer chains produced out of one initiation step (that is by one DR dimer).

CHAIN

LEkWH

Fig. 11. Weight distribution of ch
room temperature and at 10 K are shown in fig. 1l_ The most-probable chain Ien@ j for the 280 # sample is 1600 units and for *he 10 K sampIe @ is Only 400 units. From the different distribution curves we suggest that different chain propagation mechanisms are dominant in the low-temperature and in the roomtemperature solid-state reactions. The average values of the polymer conversion have been calculated from the absorption of the solution at X = 470 nm. With a moiar extinction coefficient of E = ! 6700 cm2/kmol a polymer concentration between 0.5 and 3% has been obtained. But it is reasoaable that there are much higher conversions at the irradiated surface of the crystal. The quantum yield is about two orders of magnitude higher for the room-temperature photopolymerization reaction than for the Iow-temperature photoreaction, including the thermal-addition po!ymerization reactions of the Iow-temperature photoproducts.

6. Discussion 6.Z. Optical absorptiitn spectnaof the iiztennedintes and of rite polymer chins

The absorption lines of the low-temperature photoreaction products in the TS-I 3 monomer crystaIs are summarized in the diagram of fig. 12. Due to the almost identical line positions the correlation of the photoproducts corresponds to that of the TS-6 mo-

, /

,

0

2

CHAIN

I

L

a

a

LENGTH n -

!‘ig. 12. Lint positions of tk opticat sbsorptions of the diadial (DR) and ss~mmctric carbece (.-IQ ilt~rmediatcs.

Tilt: chain Icnpth n is giwn in monomc~ units. Curves are cLXlsrzd {141.

nomer crystals described by Gross et al. 1141. ESR experiments with TS-12 crystals have not been successful due to the expected extremely low concentration of the intermediates (see section 5.2). The correlation of the A, B, C, . . . photoproduct series to diradical Intermediates and of the b, c, d . .. photoproducts to asymmetric carbene intermediates is based on detailed ESR experiments [l l] of the corresponding radical species in TS-6. As shown in fig_ 12 the chain-length dependence of the optical absorption is excellently described by a simple theory of an electron in a one-dimensional potential box, which already has proved successful in its application to dye molecules [23] _Following Exarhos et al. [24] the explicit dependence is given by E,, = (~r”/smr’)<4n

i- 1) i&(1

- l/412) ,

0)

with I = aln + +, h and m are Planck’s constant and the eIectron mass. a1 is given by the length of the repetition unit and a2 describes the boundary condition at the ends of the oligomers. They are given by a,(DR) = 0.50. a2(DR) = 0.21(0.36) and by nl(AC) = 0.52 and a?(AC) = 0.14(fJ.l8). The energies En for ?I 4 - are g&n by E,(DR) = 6000 cm-1 and E,(AC) = 8.500 cm-l. In accordance with the theory of Brkdas et 21. [25] distinctly lower v&es of E, for the butatriene structure E,(DR) of the diradicals are expected as compared to the acetylene structure E_,(AC) of the asymmetric carbenes. Due to the fLved butatriene structure respectively acetylene structure in the diradical respectively asymmetric carbene intermediates, which are responsible for the optical absorptions, the line positions are only slightly infiuenced by the different side groups in TS-6 and TS-12. The chain-length-dependent line splitting in TS-12 is a clear indication of the specific monomer and polymer structure and the characteristic phase changes during polymerization discussed in sections 2 and 3. The specific displacements of the molecules obviously result in two inequivalent stacks of monomer molecules with different van der Waals’ interactions (site shifts). The discrimination between these two stacks is lost with increasing chain lengths of the intermediates and is completely absent in the spectra of the polymer absorptions Pl, P&P, and P, . This is in contrast to the TS-6 crystals, where a distortion of the side groups in the low-temperature phase is present in both the monomer crystals and in the

211

D. Siegel et al./Pol~meri;ation of TS-I -7diacetylene crystals

polymer crystal. This difference in the low-temperature polymer structure is already reflected in the appearance of only one single dosorption line of the individual polymer fiaments in the partially polymerized monomer crystals in contrast to the situation in TS-6 crystals, where a doublet absorption line of the individual polymer filaments is observed [6,8,13]. It should be noted, however, that ‘the splitting of the absorption spectra of oligomer chains growing in the monomer matrix can only be explained by a temporary loss of symmetry between the two different molecular sheets resembling the formation of periodic stacking faults. In this way two kinds of polymer chains with somewhat different symmetry and molecular environment could e-x&t at intermediate conversions The absorptions of the reactive polymer P,” and Pg are attributed to asymmetric carbene or dicarbene (DC) oligomers with n > 8. These states are generated from the heptamer diradical intermediate F by further photoaddition or thermal addition of monomer molecules. As known from the photopolymerization reaction in TS-6 the diradical intermediates with butatriene structure become unstable at a definite chain length and are transferred to dicarbenes with acetylene structure. Rue to the structural change the position of the absorption line is shifted to shorter wavelength [ 14,23]_ However, at the moment we have no plausible explanation for the reversible reaction P; *P* The&$ intensities of the absorptions of the final unreactive polymer PO and PI indicate an appreciable increase in the chain length with II 2 50. The different line positions of PO and P, are explained by the different lattice parameters of the monomer crystal at low polymer conversion as compared to those at higher concentration (above =2-5%) analogous to the shift of the polymer absorption in TS6 [S]. 6.2. Polymerization mechanism Apart from the low photoreactivity and thermal reactivity the different steps of the photopolymerization in TS-I2 are parallei to those of the TS-6 diacetylene crystals. As shown in fig. 1 in the photoinitiation reaction dimer diradical @R2) molecules are produced from two monomer molecules following the reaction equation

2M 2

2M* + DR 2.

(22)

Presumably (amalogous to TS-6) with 2 lower rate also asymmetric carbene dimers (.4C2) are produced following (2b) The necessity of two light quanta is derived from the non-linear intensity dependence even at high temperatures. The photochemicaland thermalu&itiun polymerization reactions follow the equations Dq

1

+ M* + DR,,l

AC, -r-M* + ACn+l ,

with2GnG7, with2
(3)

In analogy to the TS-6 crystals in the photochemical trarlsfimlatiort reactiorx 2 monomer molecule is added following the reaction equations [4,14] with 2
,

DR; +M+AC,+I

(4)

The diradical intermediates F with ?z= 7 (therefore F = DR,) are the last in the DR sequence. Upon further (thermd- or photo-) addition reactive “polymer” molecules (P*) are formed, which - from the spectral

blue-shift 2nd from ESR experiments of TS-6 crystals [4,1 l] - are interpreted 2s long-chain dicarbene intermediates with acetylene chain structure. Due to the instability of the diradical structure at n > 7 we obtain the transformation reaction DR7 +M*4DCg,

(9

with subsequent addition polyr:zt=ti,zation reactions ,

DC, + hl* + DC,,,,

withna8.

(6)

The stationary number of the dimer intermediates&f; 2nd of the oligomer intermediate &fz with (rz > 2) is obtained using the rate equations corresponding to the processes of eqs. (2) 2nd (3) describing the photoinitiation (second-order rate constant kl) and photoaddition reactions (first-order rate constants k,,)_ Mj= il-,N,,,/&ko

,

&fi+I =(i~~/k,,+~)M~ ,

(7) withn >2.

(8)

The number of absorbed light quanta is given by Nabs. The deactivation processes of the monomer excitation competing with the chcmic2l photoinitiation process are characterized by X-o.

The number of dimer molecules initiated laser pulse is given by 111, =x-,&,/2x-,

by the first

_

(9)

Therefore the ratio Fc,/ko is a measure of the quantum yield of the bimo!ecular initiation reaction. A comparison of the dimer A absorption intensities obtained after one escimer laser pulse shows that the ratio k,/k, is a factor of -103 larger in the TS-5 crystals than in the TS-I:! crystals. A comparison of the station= intensities of the absorptions A, B, C, D... obtained after lone UV irradiation doses shows that the addition reaction constants k, (with 12> 2’) are a factor of =lO--100 larger in TS-6 crysta!s than in IS-13 crystals. From these estimates it is clear that the signal-to-noise ratio of the ESR spectra of the low-temperature intermediates in l-S-1 2 crystais is redxed by a factor of = IGG which is below the detection limit of our ESR spectrometer.

Acknowledgement The work was supported by the Stiftung Volkswagen werk. We thank H. Gross for valuable assistance in the experiments and calculations. Helpful discussions with W. Neumann. K. Ulrich and G. Wegner are gratefully acknowledged.

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