Novel poly(p-xylylenes): Thin films with tailored chemical and optical properties

Article abstract of DOI:10.1021/ma011769e

Chemical vapor deposition polymerization is used to prepare submicron thin films of poly-(p-xylylenes) with distinct chemical and optical properties. The polymers are prepared from 13 different [2.2]paracyclophanes with variable degrees of substitution and functionalities including hydroxy, methoxy, amino, triflate, or trifluoroacetyl groups. The chemical composition of the poly(p-xylylenes) is in good accordance with expected chemical structures as confirmed by X-ray photoelectron spectroscopy, microanalysis, and reflection-absorption infrared spectroscopy. For all polymers, basic optical properties, such as extraordinary and ordinary indices of refraction, are reported and reveal optical anisotropy of the coatings. Experimental data correlate well with data generated by a uniaxial Cauchy dispersion model. Furthermore, a distinct correlation between refractive indices and the electronic properties of the functional groups is observed. Similarly, optical birefringence depends on the nature of the functional group as significant variation in optical birefringence was found among the reported poly(p-xylylenes).

Full text of DOI:10.1021/ma011769e

4
380
Macromolecules 2002, 35, 4380-4386
Novel Poly(p-xylylenes): Thin Films with Tailored Chemical and
Optical Properties
J . La h a n n a n d R. La n ger *
Department of Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received October 10, 2001
ABSTRACT: Chemical vapor deposition polymerization is used to prepare submicron thin films of poly-
(p-xylylenes) with distinct chemical and optical properties. The polymers are prepared from 13 different
[2.2]paracyclophanes with variable degrees of substitution and functionalities including hydroxy, methoxy,
amino, triflate, or trifluoroacetyl groups. The chemical composition of the poly(p-xylylenes) is in good
accordance with expected chemical structures as confirmed by X-ray photoelectron spectroscopy,
microanalysis, and reflection-absorption infrared spectroscopy. For all polymers, basic optical properties,
such as extraordinary and ordinary indices of refraction, are reported and reveal optical anisotropy of
the coatings. Experimental data correlate well with data generated by a uniaxial Cauchy dispersion model.
Furthermore, a distinct correlation between refractive indices and the electronic properties of the functional
groups is observed. Similarly, optical birefringence depends on the nature of the functional group as
significant variation in optical birefringence was found among the reported poly(p-xylylenes).
In tr od u ction
rapid reaction with biological ligands or proteins and
was used for surface modification using microcontact
Thin-film polymer coatings are currently studied as
templates for several high-tech applications including
2
6
printing. However, the overall impact of this technol-
ogy in surface engineering may depend on how variable
poly(p-xylylenes) with different functional groups can
be prepared, enabling different binding modes for at-
tachment of biomolecules. The motivation of this paper
is to examine and broaden the variety of polymer films
prepared by CVD polymerization and to study their
basic optical properties.
1
2
biomaterials, insulating layers in integrated circuits
3
4,5
and thin-film transistors, light-emitting diodes, opti-
6
cal or microelectrical-mechanical systems (MEMS),
lasers,7 waveguides,9 and photodiodes. Poly(p-xy-
lylenes) belong to a group of special polymers that are
under investigation for thin-film applications. Some
members of this polymer family have gained commercial
acceptance due to their high solvent resistance, low
,8
10
Ma ter ia ls a n d Meth od s
1
1
dielectrical constants, and good barrier properties.
Although poly(p-xylylenes) can be prepared in solution,
Ma ter ia ls. If not indicated differently, chemicals were
purchased from Aldrich, Milwaukee, WI, and were used
without further purification. Gold substrates were prepared
from silicon wafers (Silicon Quest Int., Santa Clara, CA) by
deposition of a sequence of silicon nitride (200 nm), titanium
e.g., electrochemically,12 chemical vapor deposition (CVD)
polymerization is the preferred preparation method for
thin-film applications.13 CVD polymerization is a room
temperature process that requires no catalyst, solvent,
or initiator and can generally be carried out without
production of byproducts. Well-defined and chemically
robust polymer films can be deposited by this method;
(
10 nm), and gold (100 nm).
2.2]P a r a cyclop h a n e-4-ca r boxylic Acid Meth yl Ester
1d ): 950 mg of 1c was dissolved in 20 mL of methanol, and 2
[
(
mL of a solution of hydrogen chloride in ether (2 M) was added.
The solution was stirred for 12 h under reflux, and the crude
product was isolated by filtration. Drying under vacuum at
50 °C and subsequent column chromatography (silica gel,
1
4
suitable monomers include R,R′-dihydroxy-p-xylylenes,
16
1
5
R,R′-dibromo-p-xylylenes, R,R′-diacetoxy-p-xylylenes,
and [2.2]paracyclophanes.17
chloroform/hexane, 1:1 (v:v)) delivered 608 mg of product 1d .
Thin-film coatings that provide free functional groups
for surface modification may represent attractive tem-
plates for surface engineering. In an attempt to prepare
functionalized coatings, CVD polymerization was used
1
H NMR (300 MHz, CDCl
3
, TMS): δ ) 2.85-2.95 (m, 1H, CH
), 3.93 (s, 3H, CH ), 4.13 (t, 2H, CH O),
.55 (m, 4H, CH), 6.68 (d/d, 2H, CH), 7.28 (s, 1H, CH).
, TMS): δ ) 35.37, 35.51, 36.67, 36.54,
2.19, 129.21, 131.96, 132.68, 133.13, 133.51, 135.71, 136.54,
2
),
3
6
.00-3.23 (m, 7H, CH
2
3
2
1
3
C
NMR (75 MHz, CDCl
3
1
7
to prepare alkylated and acetylated poly(p-xylylenes)
5
1
8
as well as germanium- and silicon-substituted poly-
36.84, 139.78, 140.69, 142.50, 168. IR (KBr): ν ) 508, 723,
mers.1
8,19
We synthesized amino- and hydroxy-substi-
71, 1004, 1112, 1127, 1199, 1271, 1327, 1440, 1721, 2955,
2
0,21
-1
+
+
tuted poly(p-xylylenes)
and used these polymer films
2996 cm . MS (70 eV): m/z ) 266 (M ), 251 (M -CH ), 235
(C16
CH ), 104 (main, C H ), 103 (C H ), 78 (C H ), 77 (C H
3
as interfaces for protein attachment2 or for patterning
2
+
+
H
15CO ), 162 (C
8
H
7
COOCH
3
3 8 8
), 147 (162-CH ), 119 (C H -
2
3
+ + +
+
6
+).
of polymer brushes onto a CVD-coated substrate.
3 8 8 8 7 6 6 6
Recently, this technological platform was extended
(Meth ylca r bon yloxy)[2.2]p a r a cyclop h a n e (1g): 500 mg
of 1f was added to a solution of 30 mL of anhydrous acetic
acid anhydride and 3 mL of anhydrous pyridine. The solution
was stirred at 130 °C for 12 h. The reaction mixture was
quenched with aqueous sodium bicarbonate solution (1 N).
Extraction with ethyl acetate and subsequent chromatographic
further when reactive coatings (i.e., poly(p-xylylenecar-
2
4
boxylic acid pentafluorophenol ester-co-p-xylylene) and
2
5
poly(p-xylylene-2,3-dicarboxylic acid anhydride) ) were
prepared. Their intrinsic chemical reactivity supports
purification (silica gel, chloroform/hexane, 4:1 (v:v)) delivered
1
*
To whom correspondence should be addressed: e-mail
555 mg of 1g. H NMR (300 MHz, CDCl
3
, TMS): δ ) 2.13 (s,
rlanger@mit.edu; Fax 617-258-8827.
3H, CH
3
), 2.96-3.21 (m, 7H, CH
2
), 3.22-3.36 (m, 1H, CH ),
2
1
0.1021/ma011769e CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/17/2002

Macromolecules, Vol. 35, No. 11, 2002
Novel Poly(p-xylylenes) 4381
Ta ble 1. Rep r esen ta tive P a r a m eter s Used for CVD P olym er iza tion of [2.2]P a r a cyclop h a n es
purity of
precursor
(%)
amount of
precursor
(mg)
system
substrate pressure
sublimation
temp (°C)
pyrolysis
temp (°C) temp (°C)
argon mass
flow (sccm)
a
functionality
alcohol
methyl ether
acetate
methyl ester
tetramethyl ester
anhydride
lactone
amine
diamine
pentafluorophenol
ester
polymer
precursor
(mbar)
2a
2b
2c
2d
2e
2f
2g
2h
2i
1f
1j
100
99
100
100
100
95
98
100
95
56
30
55
50
50
36
30
59
39
50
220
240
260
275
325
340
355
270
280
230
750
720
610
670
670
620
750
700
620
600
20
20
15
15
15
12
12
20
15
12
0.08
0.1
0.2
0.11
0.12
0.2
0.05
0.2
10.0
0.5
1.2
2.0
2.0
4.5
5.0
15.0
2.0
2.0
1g
1d
1o
1p
1q
1m
1n
1e
0.12
0.12
2j
99
trifluoroacetate
triflluoromethyl
ketone
2k
2l
1h
1b
95
99
70
60
248
240
720
650
15
20
0.10
0.12
8.0
2.0
triflate
unsubstituted
2m
2n
1i
1a
100
100
60
20
340
270
650
750
12
20
0.07
0.07
0.9
7.0
a
sccm ) standard cubic centimeter.20
1
5
(
3
1
1
1
.13 (d, 1H, CH
2
O), 5.30 (d, 1H, CH
2
O), 6.42 (2, 2H, CH), 6.55
m, 5H, CH). C NMR (75 MHz, CDCl , TMS): δ ) 33.35,
4.89, 35.39, 35.69, 65.94, 130.74, 132.65, 133.32, 133.59,
v)) delivered 350 mg of 1j. H NMR (300 MHz, CDCl
δ ) 2.80-2.94 (m, 1H, CH ), 2.97-3.21 (m, 7H, CH ), 3.38 (s,
3H, CH ), 4.18 (d, 1H, CH O), 4.53 (d, 1H, CH2O), 6.43 (m,
2H, CH), 6,50 (s, 1H, CH), 6.53 (d/d, 2H, CH), 6.58 (d/d, 1H,
CH), 6.68 (d/d, 1H, CH). 13C NMR (75 MHz, CDCl
, TMS): δ
3
, TMS):
1
3
3
2
2
3
2
33.78, 134.30, 134.74, 135.46, 137.43, 139.62, 139.95, 140.59,
71.49. IR (KBr): ν ) 608, 615, 718, 871, 1025, 1235, 1250,
368, 1414, 1634, 1731, 1900, 2858, 2940, 3007 cm . MS (70
3
-
1
) 33.29, 34.83, 35.46, 35.73, 58.77, 74.49, 129.50, 132.54,
132.66, 133.58, 133.62, 133.68, 135.27, 137.34, 138.36, 139.72,
139.97, 140.26. IR (KBr): ν ) 513, 651, 723, 902, 758, 799,
876, 1120, 1097, 1189, 1260, 1352, 1409, 1496, 1593, 1731,
+
+
+
3
eV): m/z ) 280 (M ), 220 (C17
1
H
17 ), 176 (C
8
H
7
CH
2
OCOCH
),
+
+
48 (C
8
H
7
CH
2
CH
2
OH), 115 (C
9
H
7
), 104 (main, C
8
H
8
), 103
+
+
+
6
(C
8
H
7
), 78 (C
6
H
6
), 77 (C
6
H
).
-
1
+
(
Tr iflu or om e t h ylca r b on yloxy)[2.2]p a r a cyclop h a n e
1890, 2735, 2849, 2918 cm ; MS (70 eV): m/z ) 252 (M ),
+
+
+
(
1h ): 500 mg of 1f was dissolved in 30 mL of tetrahydro-
237 (M -CH
3
), 222 (M -CH
2
O), 207 (C16
CH
H
15 ), 165 (C
8
H
H
7
-
-
+
+
+
furan, and 5 mL of trifluoroacetic anhydride was added at 0
C. After 10 min, the solvent was removed in a vacuum, and
COOCH
3
), 147 (C
9
H
8
OCH
CH
3
), 134 (C
8
H
7
2
OH ), 133 (C
9
8
+
+
+
+
6
°
OH ), 118 (main, C
8
H
7
3
), 104 (C
8
H
8
), 78 (C
6
H
), 77
+
the residue was dissolved in diethyl ether. The solution was
extracted with aqueous sodium bicarbonate (1 M) and sodium
chloride (1 M) solutions. Column chromatography (silica gel,
chloroform/hexane, 1:1 (v:v)) delivered 683 mg of the pure
product 1h . H NMR (300 MHz, CDCl
(C
6
H
6
), 51.
[2.2]P a r a cyclop h a n e-4,5,12,13-t et r a ca r b oxylic γ-Bu -
tyr yl La cton e (1q): 500 mg of 1p was suspended in 45 mL of
tetrahydrofuran. 65 mg of sodium borohydride dissolved in 10
mL of tetrahydrofuran was added, and the reaction mixture
was stirred for 19 h at room temperature. The reaction mixture
was quenched with aqueous hydrogen chloride (6 N). Extrac-
tion with ethyl acetate and subsequent chromatographic
purification (silica gel, methyl sulfoxide/methylene chloride,
1
3
, TMS): δ ) 2.97-3.16
(m, 7H, CH
2
), 3.20-3.36 (m, 1H, CH
.32 (d, 1H, CH
H, CH), 6.63 (d, 1H, CH). C NMR (75 MHz, CDCl
2
), 5.13 (d, 1H, CH O),
2
5
1
2
O), 6.41 (s, 2H, CH), 6.55 (t, 3H, CH), 6.58 (d,
1
3
3
, TMS):
δ ) 33.20, 34.90, 35.76, 35.59, 69.07, 129.95, 131.96, 132.42,
1
1
1
33.42, 133.63, 134.14, 134.33, 135.57, 138.87, 139.23, 139.85,
40.68, 160. IR (KBr): ν ) 646, 733, 774, 907, 1163, 1214 1342,
399, 1496, 1593, 1778, 1911, 2858, 2925, 2955 cm . MS (70
2:1 (v:v)) yielded 72% of 1q. 1H NMR (300 MHz, CDCl
TMS): δ ) 2.83-3.18 (m, 6H, CH ), 3.74 (m, 2H, CH ), 5.13
(m, 4H, CH O), 6.44 (m, 2H, CH), 7.10 (m, 2H, CH). C NMR
(75 MHz, CDCl , TMS): δ ) 29.05, 30.30, 69.91, 131.68,
133.39, 134.45, 135.17, 135.98, 139.97, 167.82. IR (KBr): ν )
687, 794, 979, 1025, 1066, 1143, 1240, 1363, 1450, 1491, 1578,
3
,
2
2
-
1
13
2
+
+
+
eV): m/z ) 334 (M ), 230 (C
1
8
H
8
7
CH
2
OCOCF
3
), 133 (C
).
9
H
8
OH ),
3
+
+
15 (133-H O), 104 (main, C
2
H
8
), 78 (C
6
H
6
(
Tr iflu or om e t h ylsu lfoxyloxy)[2.2]p a r a cyclop h a n e
1i): 500 mg of 1f was dissolved in 30 mL of methylene
chloride, and 5 mL of triflic anhydride was added at 0 °C. After
0 min, solvent was removed in a vacuum, and the residue
-1
+
+
(
1747, 2858, 2930 cm . MS (70 eV): m/z ) 320 (M ), 302 (M -
1
+
+
+
+
7
H
78 (C
2
O), 160 ( /
2
M ), 131, 115 (C
9
H
7
), 104 (C
8
H
8
), 103 (C
8
H
),
+
1
6
H
6
+), 77 (C
6
H
6
).
was dissolved in diethyl ether and extracted with aqueous
sodium bicarbonate (1 M) and sodium chloride (1 M) solutions.
Column chromatography (silica gel, chloroform/hexane, 1:1 (v:
Synthesis of [2.2]paracyclophanes 1b, 1c, 1e, 1f, and 1k -
1p is described in detail elsewhere.2
characterized by infrared, nuclear magnetic resonance spec-
0,24,25,27
All products were
1
1
13
v)) delivered 710 mg of the pure product 1i. H NMR (300 MHz,
troscopy ( H and C), and mass spectrometry. The spectro-
2
0,24,25,27
CDCl
3
, TMS): δ ) 2.80-3.20 (m, 7H, CH
2
), 3.25-3.40 (m, 1H,
2
scopic data were in accordance with literature data.
CH ), 5.60 (d, 1H, CH
2
2
O), 5.90 (d, 1H, CH
O), 6.40 (s, 1H, CH),
The purity of [2.2]paracyclophanes used for CVD polymeriza-
tion was above 95% as determined by gas chromatography.
CVD P olym er iza tion . Polymers were obtained from the
corresponding [2.2]paracyclophanes by CVD polymerization
using an installation consisting of a sublimation zone, a
pyrolysis zone, and a deposition chamber.22 Polymerization
parameters are summarized in Table 1. The pyrolysis of the
[2.2]paracyclophanes was carried out in a furnace with three
independently regulated heating zones to ensure a nearly
constant temperature profile. The polymerization chamber is
equipped with a rotating, cooled sample holder, an on-line
thickness monitor (Sycon Instruments, New York), and a
temperature and vacuum gauge. Its walls were kept above 110
°C to avoid undirected deposition. For polymerization, a
defined amount of starting material was placed in the subli-
mation zone, and a substrate was fixed on the sample holder
6
8
.45-6.63 (m, 2H, CH), 6.75 (d, 1H, CH), 8.10 (t, 1H, CH),
13
3
.57 (t, 1H, CH), 9.00 (d, 1H, CH). C NMR (75 MHz, CDCl ,
TMS): δ ) 31.53, 32.81, 33.20, 33.57, 61.17, 127.50, 128.63,
1
1
7
cm
31.18, 131.84, 132.03, 132.74, 133.99, 134.49, 137.58, 138.01,
39.83, 143.80, 144.58, 149.99. IR (KBr): ν ) 518, 636, 687,
53, 1030, 1143, 1230, 1265, 1496, 1639, 2858, 2925, 3068
-
1
.
4
-Meth oxym eth yl[2.2]p a r a cyclop h a n e (1j): 478 mg of 1f
was dissolved in 100 mL of methylene chloride, and 2.143 g of
bis(dimethylamino)naphthalene and 1.479 g of trimethyloxo-
nium tetrafluoroborate were added. The solution was stirred
for 3 h at room temperature and subsequently quenched by
adding 20 mL of aqueous sodium carbonate (1 M). Extraction
with methylene chloride, drying over sodium sulfate, and
column chromatography (silica gel, chloroform/hexane, 3:1 (v:

4
382 Lahann and Langer
Macromolecules, Vol. 35, No. 11, 2002
at a particular temperature. A defined argon flow was used
as carrier gas, the pressure was adjusted, and the pyrolysis
zone was heated. Subsequently, starting material was slowly
sublimed by increasing the temperature of the sublimation
zone. Polymerization resulted in transparent polymer films on
the substrate.
Sch em e 1. Ch em ica l Syn th esis of F u n ction a lized
[2.2]P a r a cyclop h a n es Used a s P r ecu r sor for CVD
P olym er iza tion
Su r fa ce Ch a r a cter iza tion . X-ray photoelectron spectros-
copy (XPS) data were recorded on an Axis Ultra X-ray
photoelectron spectrometer (Kratos Analyticals) equipped with
an Al KR X-ray source. The pass energy was 160 eV for survey
spectra and 10 eV for high-resolution spectra. All spectra were
calibrated in reference to the unfunctionalized aliphatic carbon
at a binding energy of 285.0 eV. The emission angle of electrons
was set at 55°, which results in an information depth of about
6
nm. Reflection-absorption infrared spectroscopy (RAIRS)
was done on a Magna 550 spectrometer (Nicolet) equipped with
a grazing angle accessory (Spectra-Tech) under a grazing angle
of 85°. Microanalysis was conducted on polymers deposited as
thick films (thickness > 1 µm) on glass slides. The polymers
were carefully separated from the substrate using a razor
plate. Spectroscopic ellipsometry was carried out on a variable
angle spectroscopic ellipsometer (J .A. Woollam Inc.) using a
uniaxial Cauchy model for curve fitting. Wavelengths between
3
50 and 800 nm and three angles (65°, 70°, and 75° with
respect to sample normal) were examined. Polymer films were
deposited on silicon wafers for examination. The silicon model
was modeled by assuming a 1 mm silicon bulk with a thin
2
8
native oxide layer (17 nm). Root-mean-square errors (RSE)
were below 20 for all samples included in this study.
Resu lts a n d Discu ssion
Syn t h esis of F u n ct ion a lized [2.2]P a r a cyclo-
p h a n es. Friedel-Crafts acylization of [2.2]paracyclo-
phane (1a ) with trifluoroacetic acid anhydride using an
excess of AlCl3 resulted in 4-trifluoroacetyl[2.2]-
paracyclophane (1b) in 92% yields (Scheme 1). Subse-
quent hydrolysis of 1b under basic aqueous conditions
led to 4-carboxy[2.2]paracyclophane (1c) (93% yields).
Acid-catalyzed esterification with methanol resulted in
[
2.2]paracyclophane-4-carboxylic acid methyl ester (1d )
in yields of 60%. [2.2]Paracyclophane-4-carboxylic acid
pentafluorophenolester (1e) was then synthesized by
conversion of compound 1c with pentafluorophenol
trifluoroacetate.24 As shown in Scheme 1, compound 1c
served as starting material in the synthesis of 4-hy-
droxymethyl[2.2]paracyclophane (1f) and its derivatives.
Reduction of 1c with lithium aluminum hydride in
tetrahydrofuran yielded compound 1f in 95%. Esterifi-
cation of compound 1f under mild conditions with either
acetic acid anhydride, trifluoroacetic acid anhydride (0
°
C), or triflic acid anhydride (0 °C) resulted in (meth-
ylcarbonyloxy)[2.2]paracyclophane (1g), (trifluorometh-
ylcarbonyloxy)[2.2]paracyclophane (1h ), and (trifluo-
romethylsulfoxyloxy)[2.2]paracyclophane (1i), respec-
tively. The yields of these reactions were between 94
and 97%. 4-Methoxymethyl[2.2]paracyclophane (1j) was
synthesized from 1f by etherification. Trimethyloxonium
tetrafluoroborate in combination with the bulky tertiary
base bis(dimethylamino)naphthalene as catalyst proved
to be an efficient etherification agent under gentle
reaction conditions (yield: 69%). Mono- or dinitro[2.2]-
paracyclophane (1k -l) was synthesized at low temper-
2
7
atures by nitration of 1a under superacidic conditions.
Using the superacidic ion-exchange resin Nafion/nitric
acid, compound 1k is obtained in 95% yield. Synthesis
of compound 1l is best carried out with stirring for 30
min at -78 °C and an additional 2 h at -20 °C. The
direct nitration suffers from the poor resistance of [2.2]-
paracyclophanes to oxidation and tendency to polym-
erization.29 In contrast, the approach used herein via
nitration under mild conditions in superacidic media
was found to be an effective pathway toward compounds
1k and 1l as yields between 93 and 95% were detected.
27

Macromolecules, Vol. 35, No. 11, 2002
Novel Poly(p-xylylenes) 4383
Sch em e 2. CVD P olym er iza tion of F u n ction a lized [2.2]P a r a cyclop h a n es To Yield th e Cor r esp on d in g
F u n ction a lized P oly(p-xylylen es)
Subsequent reduction of the nitro[2.2]paracyclophanes
to mono- and diamino[2.2]paracyclophane (1m -1n ) was
carried out with triiron dodecacarbonyl under toluene/
aqueous KOH phase transfer conditions in the presence
of 18-crown-6 as phase transfer catalyst.27
at temperatures below 25 °C. Exploitation of function-
alized [2.2]paracyclophanes for CVD polymerization is
generally limited by the requirement to preserve the
integrity of the functional groups under the conditions
of p-quinodimethane creation in the pyrolysis zone. CVD
polymerization of a specific [2.2]paracyclophane re-
quired optimum adjustment of reaction conditions to
minimize decomposition. Thus, the experimental setup22
was designed to individually control several polymeri-
zation parameters, such as pyrolysis temperature, pres-
sure, gas flow, substrate temperature, and temperature
and heating rate of the sublimation zone. We found the
pyrolysis temperature to be critical for the quality of
the reactive coatings: For sensitive polymers such as
esters, ketones, or triflates, pyrolysis was best conducted
at temperatures under 670 °C as compared to 750 °C
used for the unfunctionalized [2.2]paracyclophane. On
the other hand, amine 1m or lactone 1q was deposited
as homogeneous films with excellent properties at
temperatures above 720 °C.
Argon was used as a carrier gas to dilute p-quin-
odimethane concentration in the gas phase. A delicate
balance of argon flow rate and pressure was maintained,
as an increase in flow rate at a constant pressure
reduced the concentration of the precursor in the gas
phase. This resulted in reduced intermolecular interac-
tions and prohibited side reactions. On the other hand,
higher pressure at a constant argon flow rate resulted
in decreased sublimation and lower monomer concen-
trations in the gas phase. In this case, contact times of
the [2.2]paracyclophanes were extended, resulting in
enhanced cleavage of the C-C single bonds and en-
hanced tendency to decompose. Generally, moderately
low pressures between 0.12 and 0.2 mbar were used to
guarantee complete formation of p-quinodimethanes
without favoring intermolecular interactions. However,
if pyrolysis temperatures above 720 °C were chosen,
4
,5,12,13-Tetrakis(methyloxycarbonyl)[2.2]paracy-
clophane (1o) was obtained by Diels-Alder reaction
of acetylenedicarboxylic acid dimethyl ester with hexatet-
raene which was previously generated by the conversion
of propargylic bromide with the Grignard compound
allenylic magnesium bromide.25 Dehydratization of 1o
with concentrated sulfuric acid resulted in [2.2]paracy-
clophane-4,5,12,13-tetracarboxylic acid dianydride (1p ).
[2.2]Paracyclophane-4,5,12,13-tetracarboxylic γ-butyryl
lactone (1q) was then synthesized from compound 1p
by reduction with sodium borohydride.
CVD P olym er iza tion . The CVD process followed a
procedure first described by Gorham that was previ-
ously used to produce commercial, pinhole-free coat-
3
0
ings. As shown in Scheme 2, functionalized [2.2]-
paracyclophanes were pyrolyzed in the vapor phase to
form the corresponding p-quinodimethanes, which con-
densed onto a substrate and spontaneously polymerized.
Optimized polymerization conditions are summarized
for each [2.2]paracyclophane in Table 1. Since the
temperature of the substrate is relatively low (<25 °C),
even temperature-sensitive substrates can be coated
without affecting the substrate material. Carefully
purified [2.2]paracyclophanes were sublimed under a
reduced pressure and temperatures between 220 and
3
55 °C depending on the individual [2.2]paracyclophane.
The sublimed material was then transferred to the
pyrolysis zone and was exposed to temperatures be-
tween 600 and 750 °C using pressures between 0.07 and
0
.2 mbar to ensure cleavage of the C-C bonds resulting
in the corresponding p-quinodimethanes (monomers). In
the last step, monomers polymerized on the substrate

4
384 Lahann and Langer
Macromolecules, Vol. 35, No. 11, 2002
Ta ble 2. Ch em ica l Com p osition of P oly(p-xylylen es) Dep osited by CVD P olym er iza tion on to Gold Su bstr a tes As
Exa m in ed by XP S a n d Gr a zin g An gle IR Sp ectr oscop y
atomic ratiosa
O/C: 5.9 (6.0)
XPS signals
IR bands
2
a
b
c
d
e
f
285.0 (C-C); 286.3 (C-O); 291.5 (πfπ*); 531.9
835, 892, 1025 (C-O), 1163, 1245, 1450, 2863,
-
1
(
C-O)
2928, 3013, 3308 (OH) cm
2
2
2
2
2
2
2
O/C: 6.3 (5.6)
285.0 (C-C); 286.7 (C-O); 291.5 (πfπ*); 531.0
C-O)
285.0 (C-C); 286.5 (C-O); 289.6 (CdO); 291.7
πfπ*); 530.0 (CdO); 531.3 (C-O)
285.0 (C-C); 286.0 (C-O); 289.5 (CdO); 291.7
πfπ*); 530.2 (CdO); 531.6 (C-O)
285.0 (C-C); 286.3 (C-O); 289.3 (CdO); 291.5
πfπ*); 529.9 (CdO); 531.3 (C-O)
285.0 (C-C); 286.2 (C-CO-O); 289.5 (CdO); 291.7
πfπ*); 532.5 (CdO); 533.9 (C-O)
285.0 (C-C); 285.7 (C-CO-O), 286.9 (C-O); 288.9
CdO); 290.7 (πfπ*); 531.5 (CdO); 533.0 (C-O)
285.0 (C-C); 286.0 (C-N); 291.3 (πfπ*); 397.1
C-N)
830, 1030 (CO), 1265, 1362, 1378, 1930, 2873,
2940, 3017 cm
830, 1034, 1260, 1363, 1378, 1746 (CdO), 2863,
2935, 3018 cm
789, 817, 1076, 1112, 1209, 1260, 1276, 1434,
1726 (CdO), 2858, 2914, 2950, 3018 cm
798, 840, 1004, 1122, 1214, 1296, 1440, 1737
-
1
(
O/C: 12.2 (10.5)
O/C: 15.9 (11.1)
O/C: 26.0 (33.3)
O/C: 26.4 (30.0)
O/C: 19.5 (20.0)
N/C: 6.3 (6.3)
-
1
(
-
1
(
-
1
(
(CdO), 2879, 2945, 3001 cm
763, 850, 917 (CO), 1132, 1188, 1260 (CC), 1452,
1501, 1778 (CdO), 1849 (CdO), 2863, 2930 cm
846, 1035, 1076, 1161, 1364, 1450, 1501, 1762
-
1
(
g
h
-
1
(
(CdO), 2858, 2935 cm
820, 866, 1117, 1286, 1440, 1521, 1583, 1624,
2858, 2914, 3001, 3237 (NH), 3360 (NH), 3426
(
-
1
(NH) cm
2
2
i
j
N/C: 11.1 (12.5)
285.0 (C-C); 286.0 (C-N); 291.3 (πfπ*); 397.1
C-N)
820, 861, 1434, 1521, 1582, 1629, 2853, 2928,
3005, 3232 (NH), 3355 (NH), 3426 (NH) cm
999 (CF), 1040 (CF), 1118 (CCO), 1182, 1250
-
1
(
O/C: 7.8 (8.7);
F/C: 23.9 (21.8)
285.0 (C-C); 285.8 (C-CO-O); 286.7 (C-O); 288.3
(CdO); 290.6 (C-F); 292.2 (πfπ*); 532.5 (CdO);
(OCC), 1470, 1491, 1521, 1761 (CdO), 2858,
-
1
5
33.9 (C-O); 686.9 (C-F)
2930 cm
822, 1048, 1145, 1178, 1225, 1414, 1467, 1498,
2
k
O/C: 7.3 (10.5);
F/C: 17.8 (15.8)
285.0 (C-C); 285.5 (C-CO-O); 288.4 (CdO); 292.9
-
1
(C-F); 292.0 (πfπ*); 530.9 (CdO); 534.0 (C-O);
1785 (CdO), 2866, 2923, 3023 cm
6
86.8 (C-F)
2
2
l
O/C: 6.0 (5.5);
F/C: 14.2 (16.6)
O/C: 16.9 (16.7);
F/C: 18.2 (16.7);
S/C: 3.4 (5.5)
285.0 (C-C); 288.3 (CdO); 293.6 (CF3); 530.5
822, 984, 1165, 1210, 1712 (CdO), 2862, 2926,
-
1
(CdO); 687.0 (C-F)
3015 cm
m
168.0 (S(O2)); 285.0 (C-C); 285.9 (C-O-S); 290.6
(C-F); 291.7 (πfπ*); 532.0 (SdO); 688.0 (C-F)
646, 692, 758, 1030, 1163, 1235 (SO2), 1286,
1496 (SO2), 1537, 1609, 2853, 2925, 3078 cm
-
1
2
n
285.0 (C-C)
820, 1022, 1140, 1278, 1514, 1690, 2855, 2922,
-
1
3
018 cm
a
Calculated XPS ratios in parentheses.
pressures could be as low as 0.05 mbar (for polymer 2g).
In addition, argon flow rates varied substantially be-
tween different polymers. Highest flow rates were used
when high pyrolysis temperatures and moderate pres-
sures were combined. For instance, alcohol 2a was
prepared with a pyrolysis temperature of 750 °C, a
pressure of 0.08 mbar, and an argon flow rate as high
as 10.0 sccm (standard cubic centimeter). In this case,
the dilution effect of the carrier gas inhibited side
reactions despite harsh pyrolysis conditions.
these results confirmed the integrity of the functional
groups during polymerization, no evidence was found
to support the occurrence of broad side reactions.
Occasionally, signals at binding energies around 291.5
eV were detected, which are typical for π f π* transi-
tions associated with aromatic polymers.
XPS results revealed the chemical composition of the
polymer films to be in good accordance with the theo-
retically expected values. Gold signals were not detected
in any of the polymer films. We concluded that all
polymer films were homogeneously deposited without
gaps or larger pinholes that would give raise to the
detection of gold signals. XPS signals were generally
acquired under a constant emission angle of 55°, which
results in an information depth of about 6 nm. However,
for polymer 2j, XPS composition was studied at variable
emission angles (15°-75°). Both O/C and F/C ratios did
not change over the entire range, indicating a homoge-
neous distribution of the polymer in the studied bulk
volume.
P olym er Ch a r a cter iza tion . Chemical compositions
of the poly(p-xylylenes) were determined by X-ray
photoelectron spectroscopy (XPS), microanalysis, and
reflection-absorption infrared spectroscopy (RAIRS). As
shown in Table 2, chemical compositions (XPS) ex-
pressed by molecular ratios O/C, N/C, F/C, and S/C were
generally in good accordance with expected theoretical
values. In some cases, however, both over- and under-
determination of oxygen was determined by XPS. Poly-
mers 2b-d showed an increased amount of oxygen that
may indicate oxidation during or after polymerization.
In addition, secondary reactions of free oxygen with
radicals of the chain ends may account at least partially
for this finding. Similar reactions were observed for
plasma-etched polymers, where free radicals were found
To support the XPS study, we determined the elemen-
tal bulk composition of the polymers 2a -2n by mi-
croanalysis (cf. Table 3). XPS and elemental analysis
are complementary methods in the sense that XPS
allows selective study of the outermost surface volume,
while microanalysis reveals information regarding the
bulk composition. Similar to the XPS results, elemental
compositions of polymers 2a -2n determined by mi-
croanalysis were in excellent agreement with the cal-
culated compositions. Furthermore, the results of the
elemental analysis allowed further evaluation of the
divergence in the elemental composition detected by
XPS. Microanalysis did not confirm differences from the
chemical composition determined by XPS. Conse-
3
1
to react with oxygen to yield hydroperoxides. On the
other hand, oxygen-rich polymers, such as esters 2e and
2
k or anhydride 2f, were characterized by slightly
decreased oxygen contents. Partial decomposition of
groups may account for these results. Similarly, a slight
decrease in fluorine and sulfur content indicated partial
loss of the triflate groups of polymer 2m under the
conditions of pyrolysis. For all polymers, the XPS
spectra revealed binding energies that are in accordance
with the expected chemical structures (Scheme 2). While

Macromolecules, Vol. 35, No. 11, 2002
Novel Poly(p-xylylenes) 4385
Ta ble 3. Elem en ta l Com p osition of P oly(p-xylylen es)
Dep osited by CVD P olym er iza tion on to Gla ss Su bstr a tes
As Exa m in ed by Micr oa n a lysis
as indicated by a characteristic signal at a wavelength
of 1761 cm (CdO) and supported by characteristic
-1
bands of C-F stretching vibrations at wavelengths
-1
experimental
theoretical
lit.
20
between 999 and 1040 cm . Furthermore, C-C-O and
O-C-C stretches were found at wavelengths of 1118
2
2
2
2
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
C: 85.74; H:6.65; O: 3.57
C: 85.67; H: 7.61;
O: 6.71
C: 85.67; H: 7.99;
O: 6.34
-1
and 1250 cm , respectively. Polymer 2f was character-
ized by the double signal of the CdO stretching vibra-
C: 85.47; H: 8.23; O: 6.12
-1
tion at 1778 and 1849 cm , which revealed the typical
C: 83.89; H: 6.87; O: 10.55 C: 81.40; H: 7.19;
intensity distribution of cyclic anhydrides with the
O: 11.41
C: 81.10; H: 6.78; O: 12.14 C: 81.17; H: 6.81;
O: 12.01
C: 65.45; H: 5.49;
O: 29.06
C: 68.97; H: 3.47; O: 26.57 C: 68.97; H: 3.47;
O: 27.56
C: 74.92; H: 5.11; O: 17.86 C: 74.99; H: 5.03;
O: 19.98
-1
stretch at 1778 cm being of higher intensity. Further-
more, characteristic bands of C-O and C-C stretching
C: 65.89; H: 5.45; O: 28.
-1
vibrations at wavelengths of 917 and 1260 cm accord-
ingly confirmed the presence of unsaturated cyclic
anhydride groups. The IR spectrum of polymer 2b
revealed aliphatic C-O stretches at wavelengths of 1030
g
h
i
-1
cm and confirmed the presence of the methoxy group,
while the IR spectrum of polymer 2m is characterized
by the symmetric and asymmetric SO2 stretches at
C: 81.37; H: 7.47; N: 8.34
C: 86.05; H: 7.67;
N: 6.27
C: 80.63; H: 7.61;
N: 11.75
20
20
C: 80.57 H: 7.52; N: 12.21
-
1
wavelengths of 1235 and 1496 cm
.
j
C: 65.70; H: 3.59; F: 24.01; C: 66.03; H: 3.61;
O: 7.89 F: 22.71; O: 7.65
C: 65.45; H: 4.32; F: 22.40; C: 71.04; H: 4.97;
O: 6.48 F: 18.73; O: 5.26
C: 69.11; H: 5.35; F: 16.03; C: 68.26; H: 5.13;
O: 7.98 F: 17.05; O: 9.57
C: 64.37; H: 4.43; F: 13.24; C: 58.37; H: 4.63;
In summary, the repertoire of complementary ana-
lytical methods used in this study allowed us to confirm
the chemical composition of all 13 poly(p-xylylenes).
CVD polymerization of [2.2]paracyclophanes resulted in
transparent and topologically uniform polymer films.
Film thickness varied substantially as films deposited
in this study were between 50 and 781 nm thick (cf.
Table 4). The film thickness is mainly determined by
the amount of starting material used for polymerization,
although the heating rate of the sublimation zone
influenced the film thickness as well.
k
l
m
O: 13.61; S: 6.37
F: 15.39; O: 12.96;
S: 8.66
C: 92.26; H: 7.44
2
n
C: 92.24; H: 7.46
quently, these superficial divergences must be restricted
to the outermost molecular layers.
To examine the integrity of functional groups unam-
biguously, RAIRS was used as complementary surface
analysis method. Especially when conducted under a
grazing angle of 85°, RAIRS revealed surface-sensitive
information regarding the presence or absence of func-
tional groups. IR data are summarized in Table 2. For
polymer 2a , OH stretches were found at a wavelength
Op tica l P r op er ties of th e P olym er F ilm s. Several
members of the poly(p-xylylene) family including poly-
(p-xylylene), poly(chloro-p-xylylene), or poly(tetrafluoro-
p-xylylene) are optically anisotropic polymers and show
3
2
optical birefringence. The polymer chains are aligned
in a preferred orientation and exhibit strong anisotropic
molecular polarizibility due to their main-chain phenyl
groups. Benzene is a planar molecule with a delocalized
π-electron system that has an anisotropic molecular
-1
of 3308 cm , while polymers 2h and 2i were character-
ized by NH stretches at wavelengths of 3237 (2i: 3232),
360 (2i: 3355), and 3426 cm-1. These stretches confirm
3 33
3
polarizibility of ∆R ) 5.62 Å . Substitution of the
the presence of hydroxyl or amino groups, and in the
benzene rings in the main chain of polymers 2a -q
alters the electronic properties and changes the aniso-
tropic molecular polarizibility. Therefore, the electronic
nature of the functional groups must be expected to
directly influence the optical properties of the polymer
films. Other critical parameters include the reflection
of chain orientation and the polymer crystallization. In
this study, polymer films were examined as-deposited
without optimizing deposition conditions with respect
to optical properties or annealing samples after deposi-
tion.
case of polymers 2h and 2i, the characteristic pattern
(
double band) of the stretches indicated the presence of
primary amino groups. Consequently, N-alkylation
under the condition of pyrolysis was ruled out. The
integrity of carboxyl or carbonyl groups was verified by
the presence of CdO valence stretches detected for
polymers 2c (1746 cm ), 2d (1726 cm ), 2e (1732
cm ), 2g (1762 cm ), 2k (1785 cm ), and 2l (1712
-
1
-1
-
1
-1
-1
-
1
cm ). Similarly, the IR spectrum of polymer 2j con-
firmed the integrity of the pentafluorophenol ester group
Ta ble 4. Op tica l P a r a m eter s of th e F u n ction a lized P oly(p-xylylen es) As Deter m in ed by Sp ectr oscop ic Ellip som etr y a t a
Wa velen gth of 633 n m
in-plane
out-of-plane
functionality
alcohol
methyl ether
acetate
methyl ester
tetramethyl ester
anhydride
lactone
amine
diamine
RSE
film thickness (Å)
n0
ne
n0
ne
B
2a
2b
2c
2d
2e
2f
2g
2h
2i
11.830
5.410
12.932
19.740
9.062
2.421
8.560
11.380
19.519
3.764
5121.4
418.4
1.4984
1.5161
1.5657
1.4900
1.5727
1.6585
1.6176
1.5539
1.4496
1.4927
1.6173
1.4544
1.5106
1.4512
1.4649
1.4293
1.4307
1.5065
1.4565
1.3726
1.4616
1.4162
1.4355
1.5039
1.4539
1.4515
1.4734
1.4627
1.4275
1.4418
1.5582
1.5363
1.5145
1.5356
1.4358
1.4893
1.5719
1.4841
1.7741
1.4168
1.4307
1.3651
1.4052
1.5328
1.5329
1.4414
1.5153
1.4038
1.4458
1.5003
1.4602
1.4802
0.0344
0.0342
0.0642
0.0255
-0.0263
-0.0764
-0.0688
-0.0537
0.0126
-0.0103
0.0036
2498.4
5181.5
5943.7
557.2
711.2
7808.0
7247.5
2806.4
497.3
pentafluorophenol ester
trifluoroacetate
triflluoromethyl ketone
triflate
2j
2k
2l
2.472
10.030
4.603
3895.5
208.2
-0.0063
-0.0451
2m

4
386 Lahann and Langer
Macromolecules, Vol. 35, No. 11, 2002
Refractive indices were determined using a variable
can be tailored to the specific requirements of a given
application.
angle spectroscopic ellipsometer. The ellipsometric con-
stants ψ and δ were measured vs wavelengths for three
angles of incidence (65°, 70°, 75°). Then, experimental
data were compared to data generated using a birefrin-
gent Cauchy dispersion model and showed excellent
correlation with low root-mean-square errors (RSE, see
Table 3). Since all polymer films were well described
by assuming uniaxial properties, both extraordinary and
ordinary indices of refraction are listed in Table 4 for a
wavelength of 633 nm. Refractive indices n0 varied
between 1.4544 (2l) and 1.6585 (2f), while the extra-
ordinary indices of refraction (ne) diverged from 1.3726
Ack n ow led gm en t. We thank Prof. H. H o¨ cker and
PD. Dr. D. Klee, RWTH Aachen, Germany, for the use
of their custom-built installation for CVD polymeriza-
tion and Prof. K. K Gleason and Dr. Q. Wu of MIT for
their kind support with ellipsometry. Prof. K. F. J ensen
supported this study with numerous valuable discus-
sions.
Refer en ces a n d Notes
(1) Lahann, J .; Klee, D.; Thelen, H.; Bienert, H.; Vorwerk, D.;
(
2g) to 1.5065 (2e). All polymer films were negatively
uniaxial (ne < n0) with the optic axis parallel to the
substrate surface.
H o¨ cker, H. J . Mater. Sci.: Mater. Med. 1999, 10, 443.
(
2) Majid, N.; Dabral, S.; McDonald, J . F. L. Electron. Mater.
988, 18, 301.
1
(3) Stirringhausen, H.; Tessler, N.; Friend, R. H. Science 1998,
280, 1741.
With respect to applications in optical devices or
waveguides, another critical property is the optical
birefringence of a material. Optical birefringence can
be immediately determined from refractive indices as
(4) Burroughes, J . H.; Bradley; D. D. C.; Brown, A. R.; Marks,
R. N.; Macay, K.; Friend, R. H.; Burns, P. L.; Holmes A. B.
Nature (London) 1990, 347, 539.
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3
4
follows:
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B ) ∆n ) n
in-plane
- nout-of-plane
(1)
e
(
7) Tessler, N.; Denton, G. J .; Friend, R. H. Nature (London)
991, 382, 695.
1
with nout-of-plane being the out-of-plane index of refrac-
tion (perpendicular to the plane of substrate) and
nin-plane being the in-plane index of refraction (defined
by the plane of substrate). The polymer films examined
in this study showed a relatively wide range of bire-
fringence due to the variable electronic properties of the
prepared polymers. Interestingly, some of the fluori-
nated polymers (2j-l) showed extremely low birefrin-
gence (abs(B) < 0.01). In contrast, highest values of
optical birefringence were found for polymers 2c, 2f, and
(
8) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J .; Anderson, M.
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(
(
(
(
2
g (abs(B) > 0.06). Although two members of this group,
anhydride 2f and lactone 2g were 4-fold-substituted, a
simple correlation between high optical birefringence
and high substitution degree is not evident since the
comparably substituted tetramethyl ester 2e showed
rather moderate birefringence (-0.0263). The optical
properties reported in Table 4 depended on the chemical
nature of the functional groups and confirmed the
assumption that design of functionalized polymers may
be an appropriate approach to control optical properties.
However, other critical parameters such as deposition
temperature or postdeposition annealing critically influ-
ence the optical properties of polymers as well and may
be used to enhance structure-based tendencies.
(
2
32.
(
17) Gorham, W. F. US Patent US 3342754, 1967.
(18) Hopf, H.; Gerasimov, G. N.; Chvalun, S. A.; Rozenberg, V.
L.; Popova, E. L.; Nikolaeva, E. V.; Grigoriev, E. I.; Zavjalov,
S. A.; Trakhtenberg, L. I. Adv. Mater., Chem. Vap. Deposition
1
997, 3, 197.
(
19) Gerasimov, G. N.; Popova, E. L.; Nikolaeva, E. V.; Chvalun,
S. N.; Grigoriev, E. I.; Trakhtenberg, L. I.; Rozenberg, V. I.;
Hopf, H. Macromol. Chem. Phys. 1998, 199, 2179.
(
(
(
(
(
(
20) Lahann, J .; Klee, D.; H o¨ cker, H. Macromol. Rapid Commun.
1
998, 19, 441.
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Con clu sion s
23) Lahann, J .; Langer, R. Macromol. Rapid Commun. 2001, 22,
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