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On the Shape of a DielsЦAlder Ladder Polymer.

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On the Shape of a Diels-Alder Ladder Polymer**
By Brittu L. Schurmann,* Volkev Enkelmann,*
Mattliiiis Liiffler, and Arnulf-Dieter Schluter*
We have developed a new generation of double-stranded
(ladder) polymers, which have poly-Diels-Alder addition reactions as a basis for their synthesis.['l Polymers f and 2 were
studied extensively as representatives of this group of polymers and found to be extremely soluble in common organic
solvents. Not only was their double-strandedness unambiguously proven.['] but for the first time in the history of ladder
polymers. comprehensive information on the molecular
weights,[31the nature of end groups,[31and configurational
and conformational
was also obtained. In addition. ;I model for the shape of polymer 1 was proposed,
according to which. the polymer attains the structure of a
three-dimensional
In this paper. we mostly provide
1
n
2
n
theoretical evidence that certain ladder polymers in solution
may attain the unprecedented shape of a two-dimensional
coil, as is illustrated by the computer-generated structure of
polymer 2 (Fig. 1).
The formulas of I and 2 suggest that both double-stranded
polymers have a symmetry plane ((TJ perpendicular to the
plane sketched here, which cuts the bonds that connect the
two strands. However, for 1. model studies clearly show that
this is not the case. In constrast to the endo-configurated unit
(see 3). the cyclohexene ring of the exo-configurdted repeat
units exists in one of the two possible staggered conformations,lsl which are both nonsymmetrical about (T,~. Figure 2 a shows the (published[41)molecular structure of model
compound 3 in the crystal to visualize the consequences for
the shape of 1 . Whereas the endo-unit has plane symmetry,
the em-unit does not. It is this unit that forces the backbone
of I into the third dimension.
[*] Dr. 8. Schurmann. Dr habil. V. Enkelmann,
Prof. Dr. A.-D. Schluter.'" Dip1.-Chem. M. Loffler[+'
Max-Planck-Institut fur Polymerforschung
Postfxh 31 48. D-W-6500 Mainz (FRG)
[ + ] Neh Address: lnstitut fur Organische Chemie der Freien Universitiit,
Taku.;trasse 3. D-W-1000 Berlin 33 (FRG)
[**I
Fig. 1. Top and sidc view of a computer-generated structure of 2 (iilkyl sabstituents omitted) with a statistical sequence of \iw and mici-oriented oxygen
bridges.
This work was supported by the Bundesministerium fur Forschung und
Technologie and the Fonds der Chemischen Industrie. We are grateful to
Prof. G Wegner. Maim. for his interest in and support of this work.
Fig. 2. ORTEP representations of the structures In the crystal o f 3 (a). aIlLsiw6
(b). and all-uwi-6 and cv7:uniii~yn-6(mixed crystal) (c).
The structural characteristics of polymer 2 differ distinctly
from those of 1, even though its backbone also consists of
linearly annulated, a l k d r b o n , six-membered rings. However, all these rings are now conformationally rigid and have
the proper symmetry elements for the polymer to show plane
symmetry. Compound 6, a model for 2, was selected to
demonstrate this point. The three diastereoisomers, all-mti6. all-syn-6, and syn/anti/syn-6were synthesized by reacting
the bisdienophile 4[61with two equivalents of a precursor for
isobenzofuran (5);'' single crystals were grown, and the
structures in the crystal determined. All-anti-6 and syw/cinti/
R
Ph
R
Ph
5
4
R
I
7
1
5
R
selected as a model for 2.['6* The M D studies were carried
out a) with two different starting conformations of 7 (A and
B) to avoid a dependence of the result upon the initial structure and b) in the presence of solvent molecules (benzene),
since polymer/solvent effects have considerable influence on
the conformation of a polymer chain.["] The starting structure A of model 7 was constructed by using bond lengths and
angles from the X-ray structures of 6 and the AMBER data
bank which gives it plane ~ymmetry."~.
19] Conformation B
is an equilibrated structure (after 30 ps) which was created
from A by performing a M D calculation in vacua. During
the calculation (without solvent) the initially two-dimensional coil starts to deviate significantly from planarity, which is
initiated by higher fluctuations of the termini. This can be
seen from the values of the moments of inertia ( I x , I,.. I,)
about the .Y, y , and 2 axes which remain similar and constant
(over 65 ps) after equilibrium is reached (Fig. 3a).
Both initial structures were placed in a box of constant
volume filled with benzene molecules of realistic density at
300 K, and the molecular dynamics were simulated under
periodic boundary conditions.[201The results are as follows:
Conformation A keeps its two-dimensional shape over the
entire period of simulation (1 30 ps). This was derived from
the dependence of I,. I,. and Iz on time. As is required for a
nonspherical structure, the values of I, and I, are significantly larger than those of I , (Fig. 3 b). Visualization of the
molecular dynamics in a movie showed that the structure, on
a time average, retains the symmetry plane onr. To avoid the
local minimum problem, the cakulations were repeated at
4
6
4 , 6 : R = (CH&-CH,
sy7-6 are unique cases because they form mixed crystals with
statistical disorder."] As can be expected, all isomers have a
symmetry plane in which the oxygen atoms lie (Fig. 2b.
c ) . [ ~ -' 'I These structures, however. and the vague description of the fragments of 2 as being rigid are not sufficient to
lead to the conclusion that 2 attains the shape of a (snakelike) two-dimensional coil. This would depend upon the degree of anisotropy of the flexibility of 2 in the plane and out
of the plane.[121All-atom molecular dynamics (MD) is an
appropriate method to gain information on the fluctuations
of the atoms' positions in molecules. It takes into account the
chemical structure as well as all the interactions between
each two particles of a given ensemble. Hence, we applied
this method (AMBER program['31) to help solve the problem of the shape of polymer 2 in solution. An assessment of
the reliability of this method has been published, in which
structural predictions based on M D are compared with results obtained from, for example, 2D N M R or X-ray invesitgations.[l4]
Polymer 7. which consists of seven repeat units carrying
no alkyl chains" and terminated by benzene rings, was
b)
2.5~10~~
5.0xlOL
0
0
10
20
-
50 60 70
30 LO
f Ips1
0
20 LO
-
80 100 120 110
60
f [PSI
z
1.0~10~
Ikg A218.0x10'
6.0~10~
5.0~10~
0
0
20
7
10
f
-
60
[PSI
80
100
0
20
60
LO
f
Ips1
80
100
120
Fig. 3. Dependence of the moments of inertia *, I , (0).and 1: ( 0 ) about the Y. and z axes on time (i) of the two different starting
conformations A and B (see text) of compound 7. a) B obtained from A in vacuo, and b) A in benzene (both runs at 300 K alter
equilibrium ia reached). c) B at 700 K during equilibration. and d) B at 300 K after heating to 700 K (d).
/A
400. 500, 600, and 700 K["] followed by cooling to 300 K,
which yielded results in full agreement with those obtained at
300 K .
For starting structure B, the simulation in benzene at
300 K did not lead to any significant change of the gross
conformation even at extended simulation time (200 ps).
Therefore, the system was heated to 700 K in steps of 100 K.
After each run had equilibrated, the system was cooled to
300 K . Finally, in the 700 K case for this conformation, a
change in a plane-symmetrical geometry takes place. Figure 3 c shows that the moments of inertia about the three
axes split into two sets of values (Z~y,
I,., and Zz), which are
retained upon cooling (Fig. 3d).12Z1
As in the case of conformation A. the visualization of the molecular motion revealed
that conformation B was not spherical anymore but, in fact,
two-dimensional and disc-like (on a time average). However.
the end-to-end distance was now smaller by a factor of approximately 2 and did not converge within 100 ps to that of
the equilibrated conformation A. Undoubtedly. this is due to
the large time scale within which the backbone is stretched
out.
Regardless of the starting conformation, 7 in benzene solution equilibrates to a two-dimensional conformation, which
indicates a unique structural feature of 2 and other ladder
polymers of comparable rigidity and symmetry. It is important, however, to realize that this calculation does not account for other effects which might play a roleduring synthesis of the polymer. It may well be the case that. as the
polymer chain grows. larger fragments of the Same polymer
chain overlap each other irreversibly o r that iiitermolecular
effect5 (for example: formation of entanglements) prevent
the polymer from relaxing into a two-dimensional geometry.
Small-angle X-ray scattering is an appropriate method with
which one can approach this problem
Esper-iritmtcil Procedure
A suspension ofa niixture ofrinti-4 and .vri7-4 [6] (3.0 g, 7.9 mmol) and 5 (8.38 g.
15.X nimol) in toluene (80 m L ) is heated at reflux for 24 h under nitrogen.
whereupon the solvent is removed. The crude material obtained is chroinatogriiphed through silica gel with petroleum ether (60:40):ethyl acetate (9. I )
iis eluent t o remove 1 .'.3.4-tetraphenylbenzene.
A mixture of at least four o f the
six possible diartercomeric adducts ( N M R ) is then recovered from the column
hy using T H F as eluent. Yield' 4.37 g (90%). Correct elemental analysis. Fracilli~ationfrom chloroform affords pure all-mri-6. whose solubilits is the lowest of d l diastereoiners obtained. pure all-.\w-6. as well as mixed
crystals or ;ill-un/i-6 and . \ ~ . i r . a r i r r ' . s ~ n - 6' H
. N M R (300 MHz. CDC1,): all-unii6: (5 = 2.3X. 2.49 (2m. 4 H : n-CH?), 2.64 (dd. J , = 3.2, Jz = 1.7 Hz: 4 H . H-5a.
Xa. 1421. 1 7 ~ )4.61
.
( a . 4 H . H-6.8. 15. 17). 5.25 (dd. J , = 3.2..1, =1.7 Hz:4H.
H-5. -9. -14. -1X); all-.\~w6:6 =1.87 (s, 4 H . H-5a. 8 a . 14a. 17a). 2.57 (t.
J =1.6 HI. 4 H . z - C H ~ ) .5.39 (s; 4 H . H-5, 9. 14. 18). 5.40 ( s : 4 H : H-6. 8. 15,
17). .si'ir unii 5rii-6: d =1.95 (s: 4 H . H-52. Xa. 143. 17a). 2.45-2.67 (m: 4 H .
7-CHJ. 5.44. 5.46 ( I s : 8 H : H-5. 6. 8. 9, 14. 15. 17. 18).
Received. July 4. 1992 [Z 5446 IE]
German version: A n g a r . Clioir. 1993. 105, 107
[I] A,-D Schliiter. A r k . Muter. 1991.3.284:for more recent. related work. see
U . Schcrf. K . Mullen, Swr1ie.vi.s 1992. 23.
[2] K. Blatter. A,-D. Schluter. Mu~ror,iok,cirir.s 1989, 22. 3506: T. Vogel. K.
Blatter, A.-D. Schluter. Mrrkrornol. CIii~r77.Rupid Coinii~rrr~.
1989, 10. 427:
A. Godt. A.-D. Schliiter. A n g a r . C/iern. 1989. I O I . 1704. Ang[w C % P I I I .
I n / . Ed. Engl. 1989, 78, 1680.
[3] A. Godt. A.-D. Schluter, Mukrontol. C/i<wi.1992. /Y3. 501.
[4] A. Godt, V. Enkelmann. A.-D. Schluter. ('hem E P ~1992.
.
125. 433.
[S] E. L. Eliel. Siereochemrstr~ of Curhon Con7pou17d5 McCruw-Hill. New
York. 1962.
[6] K. Blatter, A,-D. Schluter, Cl7rm. Ber 1989. 172. 1351.
[7] In analogy to L. F. Fieser, M. J. Haddadin, Cm. J. Clin,? 1965. 43. 1599
[8] As a consequence, corresponding segments in a polymer cham can replace
each other without altering the shape of the polymer.
X-ray Structure data ofconipounds 6: Enraf-Nonius CAD-4 diffractometer, room temperature. Cu,, radiation. i = 1.5405 A. graphitc monochromator. The structure was solved by direct methods (MULTAN) and by
vector search methods (DIRDIF). Empirical absorption correction. anisotropic temperatur factors for O and C. refinement ofthe H atoms in the
"riding mode" with fixed isotropic temperature factors. All-un/1-6 (at
273 K ) : monoclinic. space group P2ir. ( I = 14.4470(9). h = 6.0272(8). 1' =
23.9443(15)A.p = 91.995(8). V = 2083.7A3.%= 4.0,,~,,,,= I 360gcm-'
3583 reflections. 2392 observed [ I > 3c(I)]. R = 0 05X. R, = 0.056. all51~6
(at 273 K): orthorhomhic. space group Pna2,. (/ = 14.2156(10).
h = 10.673(2). c = 29.358(1), V =7588 A3. Z = 8. (1',,, r J = 1.225 g c m - " .
6369 reflections. 3092 observed [ I > 3 u ( I ) ] .R = 0.083. R, = O.OX5: .\)'I?r i n / i - . s ~ r i - 6 from a mixed crystal (at 195 K):monoclinic. space group PI, n.
R = 13.671(1). h = 22.387(2), c = 5.6060(5). lj = 100.308(4). V = 16x8 k 3 .
Z = 4. pcA,cd=1.210gcm-3. 2177 reflections, 1158 observed [ I > 3 u ( l ) ] .
R = 0.068. R, = 0.070.
Further details of the crystal structure investigations may be obtained
from the Fachinformationszentrum Karlsruhe, Gesellschnlt fur x i s senschaftlich-technische Information mbH. W-7514 Eggenstein-Lcopoldshafen 2 on quoting the depository number CSD-56902. thc nameb of
the authors. and the journal citation.
All other stereoisomers should show plane symmetry.
For this specific type of double-stranded polymers, a new theiiretical delinition of persistence length is required.
S. J. Weiner, P. A. Kollman, D . A. Case, U. C . Singh. C . Ghio. G. Alagona. S. Profeta. P. Weiner. J. An?. Clirin. Soc. 1984. 106. 765.
B. L. Schurmann. M. Depner. Mol. PI??.\. 1991. 74. 715; B. L. Schurmann.
6 . .lung, Lipid Cr:vsi. Mol. Ci:~.vt.1990. 1x5. 141. For reviews. see W. F.
van Gunsteren. H. 1. C. Berendsen. Angeiir C / : P ~ .1990, 1 0 . 1020; Angviv.
C h m . In!. 0 1 . Engl. 1990, 29. 991: H. FruhbeiD, R. Klein. H. Wallmeicr.
i b i d 1987, 99, 413 and 1987. 26.403.
The alkyl chains containing six C atoms iire expected to have only minor
influence on the conformation of the backbone.
The sequence of the relative configurations of neighboring oxygen bridges
in 7 is unri.'untr;~jn'.vj.n.\~n!'.srniuntr.sy'unri.unri~antrs i x . s y i ( i n t i uiiii.
The structure shown in Figure 1. though having all features required Tor
the formation of a two-dimensional coil. could not be used for simulation\
in solution because of the large number of atoms.
For example see M. Depner. B. L Schiirmann. J. C o ~ n p i ~(%mi?.
i.
1992. 13.
1210.
The partial charges used to describe Coulombic interactioiis were obtained
from M N D O calculations and scaled by the empirical factor of 1.3. ii
factor generally used to obtain consistency with the AMBER force field.
[I 31
An appropriate distance (1.2 A) between the solvent and solute :nolecules
was chosen to ensure the correct physical density of the solvent. For a more
detailed description, see ref. [IS].
The heating was performcd to explore the conformational space in the
sense that kinetic energy pumped into the ensemble is used to overcome the
energy barriers separating the different local minima. Bond hreaking cannot occur because of the particular mathematical expression used for the
potential.
The I,. I,. and I= values of the two-dimensional coils obtained from stiirting conformations A and B. respectively (Fig. 3 b and d). var) due to
differences in the shape of these coils as reflected by the differcni cnd-toend distances.
G. Urban, M. Ballauff. Karlsruhe, private communication.
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