Synthesis, Characterization, and Thermal Properties of Fluoropyridyl-Functionalized Siloxanes of Diverse Polymeric Architectures

Kevin A. Stewart, Dylan Shuster, Maria Leising, Isaac Coolidge, Erica Lee, Charles Stevens,

Article

Synthesis, Characterization, and Thermal Properties of

Fluoropyridyl-Functionalized Siloxanes of Diverse Polymeric

Architectures

Andrew J. Peloquin, Daniel Kure, Abby R. Jennings,* and Scott T. Iacono*

Cite This: https://doi.org/10.1021/acs.macromol.1c00333

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sı Supporting Information

ABSTRACT: High-temperature linear ﬂuoropyridyl silicone-based oils and network elastomers were prepared via hydrosilylation

with multifunctional perﬂuoropyridine (PFP)-based monomers possessing terminally reactive alkenes. Monomers with varying

degrees of functionalization were prepared in a scalable manner and in high purity via the facile, regio-selective, nucleophilic aromatic

substitution (S Ar) of PFP in good isolated yields. These multi-reactive monomers were polymerized via Pt-catalyzed

hydrosilylation with hydride-terminated polydimethylsiloxanes (H-PDMSs) possessing varying degrees of polymerization and cross-

linked with the highly functionalized octadimethylhydrosilyl cubic siloxane. These resulting polymers of varying architecture

possessed exceptional thermal stability with no onset of degradation up to 430 °C and char yields as high as 62%, under inert

pyrolysis conditions when modiﬁed with cubic siloxane. Furthermore, by nature of the aliphatic or aromatic content, programmable

glass transition temperatures were achieved from these elastomeric materials. Finally, the linear 3,5,6-ﬂuoropyridine PDMS systems

demonstrated the ability to undergo regio-controlled post-functionalization via S Ar with 4-bromophenol, allowing access to silicone

oils with potentially tailorable properties for desired applications.

INTRODUCTION

high-performance functionality. Additionally, these materials

can be accessed via facile procedures, including metal-free

■

High-performance materials, heavily composed of elastomers

and thermosetting resins, serve a critical role in a variety of

applications, especially within the ﬁelds of commercial and

military aerospace technologies. One such application that

high-performance elastomers have greatly underpinned is for

−7

azide−alkyne and thiol-ene click chemistries.

However, one

of the most common and rapid methods remains the platinum-

catalyzed hydrosilylation reaction. The incorporation of

siloxane segments into polymers provides a number of

enhanced properties to these materials, such as low surface

heat-shielding materials (HSMs) in rocket motor casings.

energy, chemical resistance, low T , resistance to impact

Material reliability in HSMs focuses strongly on a wide range

of service temperatures and high carbonaceous char following

degradation. Consequently, typical long-term low-temperature

8−12

damage or fracture, and thermal stability.

Common

siloxane additives include polydimethylsiloxane (PDMS) and

polyhedral oligomeric silsequioxane (POSS) macromers, the

latter receiving substantial recognition in recent years for its

diverse features. The robust POSS cage architecture is an

attractive platform due to its high degree of tunability and ease

of incorporation, demonstrably augmenting their use in

(

often cryogenic) storage must not jeopardize the material’s

performance at ultimately elevated operational temperatures,

whereupon eventual pyrolysis ideally leaves suﬃcient levels of a

sacriﬁcial, protective char. Thus, polymer candidates should

possess suﬃciently low glass transition temperatures (T ’s) so

that the material remains ﬂexible at sub-freezing temperatures

Received: February 11, 2021

Revised: March 19, 2021

but boasts thermal/thermo-oxidative dependability at or above

00−350 °C.

Due to the markedly high bond strength and ﬂexibility of the

silicon−oxygen bond, siloxane-containing polymers have

unprecedented thermo-oxidative stability and low T ’s.

Therefore, silicone elastomers are attractive materials for

Not subject to U.S. Copyright. Published

XXXX by American Chemical Society

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Scheme 1. Accessing Pyridyl-Functionalized Fluorosilicones of Linear and Networked Architecture by Regio-Controlled

Functionalization of PFP (1) with Alcohols Possessing Terminal Alkenes that Undergo Hydrosilylation with H-PDMS

The resulting linear polymers (via diene monomer) aﬀord viscous oils that can be pre- or post-functionalized and networks (via triene monomer)

that have programmable elasticity.

thermoplastic elastomers, polymer composites, and reprocess-

to a more diverse pool of highly tailorable, high-performance

ﬂuorosilicone materials.

13−21

able thermosetting networks (vitrimers).

Hence, the

incorporation of POSS components is a distinct and expedient

option for the fabrication of novel high-performance

thermosets and elastomers.

EXPERIMENTAL SECTION

■

Methods. HPLC-grade tetrahydrofuran was dried and deoxy-

genated by passage through a Pure-Solv solvent puriﬁcation system

equipped with Cu/Al columns from Innovative Technologies.

Anhydrous dimethylformamide was further dried by storage over

anhydrous magnesium sulfate under a nitrogen atmosphere. Glass-

ware and syringes were ﬂamed-dried and allowed to cool in a

desiccator prior to use. Syringes fastened with syringe needles were

ﬂushed with nitrogen prior to use. All reactions and solvent transfers

were carried out under an atmosphere of nitrogen.

A subsidiary of siloxane materials is ﬂuorosilicone rubbers/

resins (FSRs). FSRs possess qualities that are enhanced

compared to traditional silicones such as hydrophobicity,

chemical and thermal robustness, and resistance to weath-

2−24

ering.

However, FSR manufacturing and, therefore, all-

purpose use, is often hampered by the high cost of production.

Additionally, FSRs are generally limited to a narrow monomer

scopetypically composed of ﬂuoroalkyl substituents. Never-

theless, FSRs oﬀer a route to materials that can withstand the

harsh conditions required for high-performance applications by

combining the exceptional characteristics of silicones with

those of ﬂuoropolymers.

Materials. Chemicals and solvents, which were of reagent grade,

were purchased through commercial suppliers and used as received,

unless speciﬁcally noted. The synthesis of 5, 7, and 8 was carried out

1,34

from previously published work.

Instrumentation. H, C, and ¹⁹F NMR spectra were recorded

on a JEOL 500 MHz spectrometer. Chemical shifts were reported in

parts per million (ppm), and the residual solvent peak was used as an

internal reference: proton (chloroform δ 7.26), carbon (chloroform,

Perﬂuoroaromatics (PFAr’s) have proven to be highly

advantageous synthetic building blocks for diversifying

polymeric materials, attributable to predictable nucleophilic

C{D} triplet, δ 77.0 ppm), and ﬂuorine (CFCl δ 0.00) was used as a

reference. Data are reported as follows: chemical shift, multiplicity (s

singlet, m = multiplet), coupling constants (Hz), and integration.

Gas chromatography−mass spectrometry (GC−MS) analyses were

performed on an Agilent 7890 gas chromatograph coupled to an

Agilent 5975C electron impact mass spectrometer with a temperature

gradient of 80−250 °C at 15 °C min after an initial 2 min

temperature hold at 80 °C.

Diﬀerential scanning calorimetry (DSC) analyses were performed

on a TA Q20 or Q200 DSC utilizing aluminum hermetic pans. The

analyses were carried out using a 5 °C min temperature gradient

under nitrogen. Thermogravimetric analyses (TGAs) were performed

on a TA Q500 utilizing platinum pans at a 10 °C min temperature

gradient under argon.

Gel permeation chromatography (GPC) data in tetrahydrofuran

(HPLC-grade) were collected using polystyrene as a standard

Polymer Labs Easical PS-2) using a Agilent PL-GPC 220 System,

whereby samples were eluted in series through Agilent PLgel Mixed-D

and Mixed-E columns at 35 °C.

Synthesis of 2,3,5-Triﬂuoro-4,6-bis(pent-4-en-1-yloxy)-

pyridine (2). 4-Penten-1-ol (2.5 equiv, 7.14 mL, 165.3 mmol),

PFP (1 equiv., 7.28 mL, 66.37 mmol), and cesium carbonate (4 equiv,

86.5 g, 265 mmol) were added to acetonitrile (500 mL) and allowed

to stir at room temperature for 3 d. The reaction is typically

substitution patterns and a range of nucleophile tolerability.

Additionally, the high ﬂuorine content in conjunction with

aromaticity imparted to polymers derived from PFAr’s endows

these materials with exceptional thermal stability and

6,27

processability.

Of the many PFAr’s, perﬂuoropyridine

−

(

PFP) has proven to be an auspicious scaﬀold for

ﬂuoropolymer design, owing to its predictable and discrete

regio-selective substitution pattern at the 2-, 4-, and 6-positions

on the ring, its predilection for chemo-selective nucleophilic

aromatic substitution, and its wide scope of nucleophile

−

8−31

permissibility.

These properties permit facile incorpo-

−1

ration of PFP into novel polymer architectures, allowing for

32−34

diverse and tailorable material properties.

Scheme 1, PFP (1) oﬀers unique opportunities to design linear

via diene monomers) and network (from triene monomers)

As shown in

(

polymers in addition to tailoring pre-functionalized monomers

and post-polymerization modiﬁcation. The research herein

describes these applications of the modular PFP scaﬀold for

use in semi-ﬂuorinated silicone oils and elastomers utilizing

hydrosilylation chemistry. The thermal properties of the

resultant polymers and elastomers oﬀer an alternative route

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monitored by GC−MS until 100% conversion of the desired product

is observed. The reaction is then vacuum-ﬁltered and concentrated

using rotary evaporation. The crude mixture is then reconstituted with

diethyl ether (250 mL), and then, saturated ammonium chloride (250

mL) is added in order to perform liquid−liquid extraction. The

saturated ammonium chloride is extracted with diethyl ether (3 × 50

mL), and the combined diethyl ether fractions are washed with

deionized water (250 mL), saturated brine (250 mL) and dried with

magnesium sulfate. The crude reaction mixture is then vacuum-

ﬁltered, dried using rotary evaporation, and vacuum-ﬁltered through a

silica plug (100 g) using 95:5 (hexanes/ethyl acetate) as the mobile

phase. The ﬁltrate was concentrated by rotary evaporation and dried

extracted with diethyl ether (3 × 25 mL) and the combined organic

fractions were washed with saturated brine (5 × 25 mL), dried with

magnesium sulfate, vacuum-ﬁltered, concentrated using rotary

evaporation, and dried under high vacuum, aﬀording an oﬀ-white,

waxy semi-crystalline solid (2.60 g, 77%). X-ray diﬀraction-quality

single crystals were harvested from the waxy semi-crystalline solid

after allowing one month of undisturbed settling in a round-bottom

ﬂask at ambient temperature (∼20 °C). H NMR (CDCl

, 500

MHz): δ 7.58−7.46 (m, 6H), 6.88−7.86 (m, 2H), 6.60−6.56 (s, 3H),

5.98−5.92 (m, 2H), 5.11−5.08 (m, 4H), 3.50 (br s, 6H), 3.34−3.25

(m, 4H), 5.96−5.88 (m, 2H), 5.07−5.05 (t, 4H), 3.53 (s, 6H), 3.30

(d, J = 6.59 Hz, 4H), 1.5 (s, 9H); C NMR (CDCl , 126 MHz): δ

under high vacuum, aﬀording a clear, colorless oil (17.2 g, 86%). H

151.6, 144.3, 144.2, 140.4, 139.6, 137.6, 137.5, 137.4, 130.1, 129.9,

NMR (CDCl , 500 MHz): δ 5.83−5.80 (m, 2H), 5.07−4.99 (m, 4H),

129.5, 128.6, 127.9, 122.3, 120.3, 116.0, 112.5, 55.7, 40.7; F NMR

(CDCl , 471 MHz): δ −150 (s, 2F); GC−EIMS (70 eV) m/z (%

.41−4.26 (m, 4H), 2.23−2.19 (m, 4H), 1.89−1.85 (m, 4H);

NMR (CDCl , 471 MHz): δ −93.78 (s, 1F), −160.0 (s, 1F), −166.9

relative intensity) 515 (M , 20), 454 (25), 281 (50), 207 (100), 147

(20), 73 (50).

(

s, 1F); GC−EIMS (70 eV) m/z (% relative intensity) 301 (M , 5),

Synthesis of poly2. H-PDMS (1000 g mol⁻avg molecular

weight, 500 mg, 0.5 mmol) was added to 2 (150.3 mg, 0.5 mmol),

and then, 1 drop of Pt catalyst (5 wt % platinum divinyltetrame-

thyldisiloxane complex in xylene) was added. The solution was stirred

at room temperature for 24 h. The product was used without further

33 (30), 205 (25), 165 (35), 68 (85), 53 (5), 41 (100).

Synthesis of 2-(4-Bromophenoxy)-3,5-diﬂuoro-4,6-bis-

pent-4-en-1-yloxy)pyridine (3). Cesium carbonate (7.56 g, 23.2

(

mmol), 2,3,5-triﬂuoro-4,6-bis(pent-4-en-1-yloxy)pyridine (2) (3.49 g,

1.6 mmol), and 4-bromophenol (2.05 g, 11.8 mmol) were added in

dimethylformamide (50 mL) and allowed to stir for 24 h at 65 °C.

puriﬁcation, aﬀording a brown liquid.

The reaction is typically monitored by F until 100% conversion of

the desired product is observed. The solution was vacuum-ﬁltered and

ﬂushed with chloroform (100 mL). The ﬁltrate was added to a

separatory funnel with saturated ammonium chloride (100 mL). The

aqueous layer was extracted with chloroform (3 × 50 mL), and the

combined organic fractions were washed with saturated brine (5 × 50

mL), dried with magnesium sulfate, vacuum-ﬁltered, concentrated

H NMR (CDCl , 500 MHz): δ 4.50−4.48 (m), 4.39−4.73 (m),

4.27−4.24 (m), 2.23−2.20 (m), 1.78−1.76 (m), 1.54−1.38 (m),

0.57−0.54 (m), 0.20−0.00 (m); F NMR (CDCl , 471 MHz): δ

−90.8 (br s), −93.9 (br s), −159.1 (br s), −159.9 (br s), −167.137

(br s).

Synthesis of poly3 from Pre-modiﬁed Monomer 3. H-PDMS

−

(1000 g mol avg molecular weight, 500 mg, 0.5 mmol) was added to

3 (227 mg, 0.5 mmol), and then, 1 drop of Pt catalyst (5 wt %

platinum divinyltetramethyldisiloxane complex in xylene) was added.

The solution was stirred at room temperature for 24 h. The product

using rotary evaporation, and dried under high vacuum, aﬀording an

pale yellow oil (3.03 g, 70%). H NMR (CDCl , 500 MHz): δ 7.45−

.43 (d, 2H), 6.99−6.96 (m, 2H), 5.82−5.73 (m, 2H), 5.01−4.96 (m,

H), 4.40−4.38 (s, 2H), 4.08−4.05 (s, 2H), 2.26−2.23 (m, 4H),

was used without further puriﬁcation, aﬀording a brown liquid.

.08−2.06 (m, 4H), 1.90−1.87 (m, 4H), 1.75−1.71 (m, 4H);

H NMR (CDCl , 500 MHz): δ 7.44−7.43 (m), 6.98−6.59 (m),

NMR (100 MHz): δ 137.6, 137.4, 122.0, 115.7, 115.2, 73.5, 66.4,

4.37−4.34 (m), 4.06−4.02 (m), 2.30−2.25 (m), 1.78−1.30 (m),

0.1, 29.7, 29.1, 29.7; F NMR (CDCl , 471 MHz): δ −161.8 (s,

0.56−0.53 (m), 0.15−0.00 (m); F NMR (CDCl

−161.7 to −161.3 (m), −163.2 (br, s).

, 471 MHz): δ

H), 163.2 (s, 1H); GC−EIMS (70 eV) m/z (% relative intensity)

55, 453 (M , 20, 20), 319, 317 (80, 80), 238 (80), 214 (30), 69

Synthesis of poly3 from Post-modiﬁcation of poly2. 4-

Bromophenol (1.1 mmol), cesium carbonate (3 mmol), and poly2 (1

(

100).

Synthesis of 2,4-Bis(4-allyl-2-methoxyphenoxy)-3,5,6-tri-

ﬂuoropyridine (4). Cesium carbonate (6.05 g, 18.6 mmol), PFP

0.912 mL, 8.31 mmol), and eugenol (2.641 mL, 17.1 mmol) were

added in dimethylformamide (100 mL) and allowed to stir for 24 h at

mmol) dissolved in d -dimethylformamide were added in an NMR

tube and placed in a sonication bath at 60 °C for 3−5 d until

(

conversion to the desired post-modiﬁed polymer was complete.

NMR (CDCl , 471 MHz): δ −159.0 to −161.3 (br, s), 168.8−169.9

room temperature. The reaction is typically monitored by F until

00% conversion of the desired product is observed. The solution was

(br, s).

Synthesis of poly4. H-PDMS (1000 g mol⁻avg molecular

weight, 500 mg, 0.5 mmol) was added to 4 (229 mg, 0.5 mmol) in

toluene (0.83 mL). The solution was sonicated, and then, 1 drop of

Karstedt’s catalyst (5 wt % platinum divinyltetramethyldisiloxane

complex in xylene) was added and the solution was stirred at room

vacuum-ﬁltered and ﬂushed with diethyl ether (100 mL). The ﬁltrate

was added to a separatory funnel with saturated ammonium chloride

(

100 mL). The aqueous layer was extracted with diethyl ether (3 × 50

mL), and the combined organic fractions were washed with saturated

brine (5 × 50 mL), dried with magnesium sulfate, vacuum-ﬁltered,

temperature for 24 h. The solution was dried under high vacuum at

concentrated using rotary evaporation, and dried under high vacuum,

80 °C, aﬀording a viscous brown liquid. H NMR (CDCl , 500

aﬀording an oﬀ-white, waxy solid (1.53 g, 39%). H NMR (CDCl ,

MHz): δ 7.02−6.98 (m), 6.78−6.15 (m), 3.83 (br s), 3.75 (br s),

00 MHz): δ 7.05−7.00 (m, 2H), 6.80−6.73 (m, 4H), 6.00−6.85 (m,

H), 5.13−5.07 (m, 4H), 3.83 (s, 3H), 3.74 (s, 3H), 3.39−3.67 (m,

H); ¹³C NMR (126 MHz): δ 151.2, 150.0, 143.5, 139.7, 138.8,

38.1, 137.1, 137.0, 122.5, 120.9, 120.7, 118.8, 116.8, 113.2, 113.1,

2.64−2.59 (m), 1.90−1.85 (m), 1.69−1.54 (m), 0.62−0.57 (m),

0.40−0.00 (m); F NMR (CDCl

−156.0 (br s), −162.7 (br s).

, 471 MHz): δ −91.4 (br s),

−

Synthesis of poly6. H-PDMS (1000 g mol avg molecular

weight, 500 mg, 0.5 mmol) was added to 6 (256 mg, 0.5 mmol) in

toluene (0.83 mL). The solution was sonicated, and then, 1 drop of Pt

catalyst (5 wt % platinum divinyltetramethyldisiloxane complex in

xylene) was added and the solution was stirred at room temperature

6.01, 55.9, 40.1, 40.0; F NMR (CDCl , 471 MHz): δ −91.2 (s,

F), −156.0 (s, 1H), −162.7 (s, 1H); GC−EIMS (70 eV) m/z (%

relative intensity) 457 (M , 15), 426 (10), 255 (10), 281 (50), 207

(

100), 147 (30), 131 (20), 115 (20), 91 (30), 73 (80).

Synthesis of 2,6-Bis(4-allyl-2-methoxyphenoxy)-3,5-di-

for 24 h. The solution was dried under high vacuum at 80 °C,

ﬂuoro-4-phenylpyridine (6). Cesium carbonate (3.80 g, 11.6

mmol), 2,3,5,6-tetraﬂuoro-4-phenylpyridine (5) (1.32 g, 5.8 mmol),

eugenol (1.90 g, 11.6 mmol), and dimethylformamide (20 mL) were

mixed and allowed to stir for 24 h at room temperature. The reaction

aﬀording a vicious brown liquid. H NMR (CDCl , 500 MHz): δ

7.57−7.47 (m), 6.85−6.83 (m), 6.58−6.54 (m), 3.55 (br s), 2.56−

2.53 (m), 1.89−1.80 (m), 1.63−1.53 (m), 0.60−0.57 (m), 0.21−0

(m); F NMR (CDCl , 471 MHz): δ −147.5−149.9 (m).

is typically monitored by F until 100% conversion of the desired

product is observed. The solution was vacuum-ﬁltered and ﬂushed

with diethyl ether (50 mL). The ﬁltrate was added to a separatory

funnel with saturated brine (100 mL). The aqueous layer was

General Synthesis of Networks net7a−7d and net8a−8d.

To compound 7 or 8 (1.0 mmol) was added H-PDMS (1.0 mmol).

The vial was then vortexed at room temperature for 1 min; 1 drop of

Pt catalyst (5 wt % platinum divinyltetramethyldisiloxane complex in

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Scheme 2. Synthesis of 2 and Subsequent Polymerization with H-PDMS to poly2

Pre-installation of 2 with 4-bromophenol produced 3 and subsequently poly3, which can also be prepared from the post-functionalization of

poly2.

Scheme 3. Synthesis of 4 and Subsequent Polymerization with H-PDMS to poly4

Attempts to pre-install 4 with phenol failed to produce the desired product Ia but aﬀorded a mixture of adducts, primarily of Ib.

xylene) was added and the reaction allowed to stir at room

temperature until gelation of the sample. The sample was then

placed in a vacuum oven and the elastomers were cured at 200 °C for

RESULTS AND DISCUSSION

■

Fluoropyridine siloxane-functionalized linear polymer systems

were initially prepared by synthesizing the difunctional

pentenyl ether-derivatized 3,5,6-ﬂuoropyridine monomer (2)

in multi-gram quantities from commercial PFP (1) by

controlled addition of 4-penten-1-ol, under mild carbonate

base conditions in excellent isolated yield (86%), as shown in

Scheme 2. 4-Bromophenol can be conveniently substituted on

the 6-position of monomer 2 under mild carbonate conditions

to aﬀord the 3,5-ﬂuoropyridine monomer 3, which demon-

strates the utility of pre-installing latent-reactive functionalities

on the monomeric unit. We have reported on expansive

examples of nucleophile additions to polymers poly2 and

poly3 prepared from monomers 2 and 3, respectively, by mild

Pt-catalyzed hydrosilylation with H-PDMS with a number-

48 h. The elastomers were puriﬁed by Soxhlet extraction with

reﬂuxing toluene for 16 h. The puriﬁed materials were dried in a

vacuum oven at 60 °C prior to thermal characterization.

General Synthesis of Networks net9 and net10. To

compound 7 or 8 (0.375 mmol) was added hydride terminated

octasilane Q M (1.0 mmol) in a minimal amount of toluene,

enough to promote dissolution. The vial was then vortexed at room

temperature for 1 min; 1 drop of Pt catalyst (5 wt % platinum

divinyltetramethyldisiloxane complex in xylene) was added and the

reaction allowed to stir at room temperature until gelation of the

sample. The sample was then placed in a vacuum oven and were

cured at 200 °C for 48 h. The network samples were puriﬁed by

Soxhlet extraction with reﬂuxing methanol for 16 h. The puriﬁed

materials were dried in a vacuum oven at 60 °C prior to thermal

characterization, whereby >95% of dried mass was recovered.

−

average molecular weight (M ) of 1000 g mol (n ∼ 13). H

NMR conﬁrmed the quantitative conversion of the AMX

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Scheme 4. Synthesis of Dieugenol Monomer 6 with ORTEP Projection with Atomic Displacement Ellipsoids Shown at the

0% Probability Level and Subsequent Polymerization with H-PDMS to poly6

pattern of the terminal alkenes with broad multiplets at 5.90−

monomer 4 with phenol (Ia) resulted in a mixture of products

with competitive formation of primarily Ib, as discovered by

F NMR analysis. These observations are consistent with

.80 ppm (CH) and 5.05−4.90 ppm (CH ) to 1.50−1.45

(

−CH −) ppm and 0.58−0.54 (−Si(CH ) CH −). The

integration ratio of PDMS (−Si(CH ) −), whereby n = 13,

with the aﬃxed aliphatic pentyl chain conﬁrmed 13:1

previous reports whereby reaction equilibria exist between

ﬂuoride and aromatic O-nucleophiles when substituted at the

4-position (as comparably facile leaving groups), resulting in

integration of the represented repeat structure shown in

Scheme 2 and H NMR spectra in the Supporting Information.

scrambling of the 2,4,6-position on the pyridine ring.

Furthermore, F NMR proved a useful diagnostic tool to

validate the post-functionalization, whereby only the −93.7

ppm shift from the F in the 6-position is present in 2 and the

shifts −161.1 and −163.1 ppm are retained due to the de-

symmetrized F’s in the 3,5-positions of 3 with no indication of

Because monomer 4 caused scrambling at the 6-position,

phenyl magnesium bromide addition to 1 as a C-nucleophile in

the 4-position aﬀorded intermediate 5, whereby no com-

petitive equilibrium exists with ﬂuoride in the 2,6-positions, as

illustrated in Scheme 4. Therefore, monomer 6 was prepared in

good isolated yield (77%) as an alternative route. Polymer-

polymer degradation. The F NMR of the subsequent

polymers poly2 and poly3 demonstrated similar patterns to

those of the monomers with the presence of the multiple,

broadened peaks, representative of polymer backbone

dispersity. In only the case of poly3, incomplete conversion

−

ization of 6 with H-PDMS (M of 1000 g mol , n ∼ 13)

aﬀorded ﬂuorosilicone poly6 as characterized by H and

NMR. Although this route does not provide for post-

functionalization, the ability to pre-install substituents of

interest in the 4-position via C-nucleophile addition from

Grignard or lithiated intermediates demonstrates the potential

for an expanded polymer pool with functionalized pendant

groups.

(

less than 12%) of the terminal alkenes was observed by

NMR, possibly due to the steric encumbrance of the bulky 1,4-

arylbromoether moiety promoting polymer chain entangle-

ment.

In order to expand the scope of linear perﬂuoropyridine

The linear polymers prepared in this study produced

(

PFP)-based silicone polymers, aromatic functionalities were

acceptable number-average molecular weights (M ) with

introduced using eugenol, a naturally sourced feedstock from

clove oil, which possesses orthogonal reactivity as a

nucleophile (via phenol) and an electrophilic terminal alkene.

The addition of two equivalents of eugenol resulted in the

formation of monomer 4, in a non-optimized yield of 34% due

to the competitive tri-substituted addition of eugenol, which

was mimicked to aﬀord a similar strategy in the preparation of

poly2 and poly3 to demonstrate post-functionalization

molar mass dispersity indexes (D̵) of 2.20−3.55, as determined

by GPC, which is typical of step-growth polymerizations under

optimized, Pt-catalyzed conditions, aﬀording viscous oils

(Table 1). From the GPC data, poly2 and poly4 had similar

−

M values of 7000−8700 g mol , whereby functionalized

polymers poly3 and poly6 exhibited a higher M , 12,700−

−1

13,200 g mol , presumably due to larger hydrodynamic

volume due to the presence of the additional pendant aryl

groups. Thermal analysis using DSC showed consistent,

(

Scheme 3). Although the polymerization of 4 with H-

−

PDMS (M of 1000 g mol , n ∼ 13) resulted conclusively

with the formation of ﬂuorosilicone poly4 as evidenced by H

narrow T ’s in the range of −28 to −25 °C for poly2,

poly4, and poly6. The bulkiness of the 4-bromophenol side-

chain-functionalized polymer poly3 showed an expected

and F NMR, the ability to prepare the exclusive pre-installed

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Table 1. Selected Properties of Linear Polymers

functionalized network series net7a−7d demonstrated slight

improvement in thermal stability with post-Soxhlet extraction

compared with no extraction presumably due to the removal of

the residual Pt catalyst that is known to cause accelerated

_M_×₁₀−3 GPC

entry

D̵

(°C)

char yield (%)

poly2

7.00

12.7

8.70

13.2

2.20

2.50

3.51

2.89

−25

−12

−26

−28

360

404

388

382

thermal degradation with T exceeding 378 °C to as high as

poly3

poly4

poly6

18 °C with increasing PDMS chain-length (n) inclusion. A

similar trend is observed of higher char yield with increasing

PDMS chain length up to 43% for net7c that possesses higher

GPC in tetrahydrofuran using polystyrene as the standard. DSC (5

−1

silicon content, which oxidizes to SiC and SiO char under

°C min ) in nitrogen determined by the third heating cycle. TGA

−

inert polymer pyrolysis of the post-Soxhlet sample. Surpris-

onset (10 °C min ) of chain-extended polymers in argon. From

monomer 3 and post-modiﬁcation of poly2 aﬀorded the same values

within error and were reported as the average of the two.

ingly, the T remains relatively unaﬀected, ranging −42 to −49

C by chain length for net7a and net7b, whereby longer chain

length is expected to induce a plasticizing eﬀect and increasing

network chain mobility, thus lowering T . These results

marked increase in T of −12 °C, compared with the non-

indicate that networks net7a and net7b are highly densiﬁed

from both chemical and physical cross-linking from mobile

aliphatic ether linkages with PDMS. Only a slight decrease in

functionalized poly2 analogue. TGA showed all polymers

possessing an onset of thermal decomposition (T ) at >360 °C

in nitrogen, with the highest exhibited from the more thermally

stabilized aryl bromo-functionalized polymer, poly3. TGA also

produced char yields of 2−3% in nitrogen for the

predominately aliphatic polymers poly2 and poly3, whereby

higher aromatic content polymers poly4 and poly6 aﬀorded

expected increased char yields of 13−14%.

T and a pronounced melt transition (T ) is observed for

net7c and net7d at −46 and −38 °C, respectively, due to an

onset of plasticizing and/or higher polydispersity present in the

H-PDMS with extended siloxanes (n = 60 to 230), inducing

bulk crystalline morphology in the conﬁned network.

Monomer 8 networked with H-PDMS aﬀorded thermally

cross-linked species net8a−8d with mixed results. It is clear

overall that the bulky eugenol units in the cross-linked

framework complicated cure kinetics and ultimately thermal

properties. Attempts to cure 8 with H-PMDS (n = 1) failed to

produce the intractable rubbery elastic solid, net8a, even with

increased Pt catalyst loading. However, highly densiﬁed

networks net8b−8d were successfully prepared, possessing

By derivatizing PFP at the 2,4,6-positions with 4-pentene-1-

ol and eugenol under mild basic conditions, tri-functionalized

monomers 7 and 8 were prepared, respectively (Scheme 5).

The synthesis of 7 and 8 has been previously reported and

were used for the preparation of elastomeric binders possessing

programmable thermal properties via thiol-ene chemistry. An

expansion of this work herein demonstrates the ability to

prepare networked siloxane-based elastomers, whereby the H-

PDMS units are introduced at varying molecular weights from

commercially available sources, with repeat units of n = 1, 5,

slightly higher T compared with the analogous net7b−7d

series due to increasing aromatic content. Char yields ranged

25−34% but with no clear trend with increasing repeat units of

H-PDMS. Curiously, net7b and net8b (both reacted with H-

PDMS, n = 5) showed a signiﬁcant increase in char yield with

post-Soxhlet networks, which may be due to unbound H-

PDMS due to the formation of cyclic structures. Finally, the T_g

produced consistent results, whereby the highest was observed

for net8b at 10 °C; net8c did not produce an observable T_g

and net8d aﬀorded −38 °C T_minﬂection with increasing

0, and 230. To promote highly densiﬁed cross-linked

elastomers, either monomer 7 or 8 was cured with the desired

H-PDMS at 200 °C for 48 h and, upon Soxhlet extraction with

toluene, aﬀorded tough, rubbery solids that only swelled in

reﬂuxing toluene or dimethylsulfoxide at room temperature.

Selected thermal properties of networked 3,5-ﬂuoropyridine

functionalized silicone elastomers net7 and net8 using DSC

and TGA are illustrated in Table 2. The pentyl ether

Scheme 5. Preparation of Network Polymers Monomer 7 and 8 with H-PDMS to Aﬀord net7a−7d and net8a−8d, Respectively

Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

The Supporting Information is available free of charge at

Article

Table 2. Selected Thermal Properties of Networks net7a−7d and net8a−8d

H-PDMS repeat unit

T_gpre-sox

T_gpost-sox

T_dpre-sox

T_dpost-sox

char yield pre-sox

(%)

char yield post-sox

(%)

entry

(n)

(°C)

net7a

net7b

net7c

net7d

net8a

net8b

net8c

net8d

net9

230

−45

−44

−45

−43

377

380

400

410

378

379

409

418

−50

−49

406

414

425

402

409

431

407

net10

410

−1

DSC (5 °C min ) in nitrogen determined by the third heating cycle. TGA onset (10 °C min ) of chain-extended polymers in nitrogen. Melt

transition observed at −46 °C. Melt transition observed at −38 °C.

Scheme 6. Cross-linking of Q M with Either 7 or 8, Aﬀording High Ceramic Char Upon Pyrolysis

PMDS units of n = 230, forming crystalline domains within the

cross-linked matrix.

electrophilic nature of PFP which is relatively unexplored. In

addition, the ability to decorate PFP with aliphatic/aromatic

and/or a combination of nucleophiles possessing latent

reactivity could provide access to tunable thermal, rheological,

electrical, and surface energy properties that warrant further

exploration for a myriad of material applications.

The comprehensive thermal study of networks net7a−7d

and net8a−8d led to the utilization of octadimethyl cubic

M Q silane (Q M ), commercially sourced as octasilane

polyhedral oligomeric silsesquioxane (OctaSilane POSS) as the

cross-linking agent possessing eight sites of functionality

(

Scheme 6). The additional sites of cross-linking aﬀorded

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.macromol.1c00333.

■

signiﬁcantly more dense networks using stoichiometric

equivalents as attributed by the higher T ’s observed from

the formation of net9 (Q M and 7) and net10 (Q M and

) of 28 and 83 °C, respectively (see also Table 2). Although

NMR spectra and single crystal X-ray diﬀraction

methods for elucidation of 6 (PDF)

thermal stability (T ) was comparable to that of net7a−7d and

net8b−8d, the recorded char yields at 900 °C were

signiﬁcantly higher at 49 and 62%, respectively, as attributed

to the Si−O ceramic cage thermal stability under an inert

atmosphere.

AUTHOR INFORMATION

Corresponding Authors

■

Abby R. Jennings − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

CONCLUSIONS

■

This work prepares both linear and network ﬂuoropyridine-

based siloxane polymers by utilizing the unique regio-

selectivity of PFP to prepare monomers with varying degrees

of functionalization by programming nucleophiles (O-

nucleophiles in addition to possibly C-, N-, S-, or P-

nucleophiles) at the 2,4,6-positions. Furthermore, the ability

to pre-install or post-functionalize moieties of interest by

substitution of the 6-position of difunctionalized aryl/alkyl

ether monomers or their subsequent polymers aﬀords

expanded utility of these polymeric systems. Although this

study demonstrates the general utility of monomer synthesis

and subsequent polymerization, one can conceive the

application of using additional main-group nucleophiles of

interest such as C-, N-, P-, S-nucleophiles, given the facile

6226, Colorado, United States; Email: abby.jennings@

usafa.edu

Scott T. Iacono − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

226, Colorado, United States; orcid.org/0000-0002-

2313-2618 ; Email: scott.iacono@usafa.edu

Authors

Kevin A. Stewart − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

6226, Colorado, United States

Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Zhou, J.; Ke, J.; Xu, X.; Xi, W. A Novel Synthetic UV-Curable

Article

Dylan Shuster − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

(8) Zhu, C.; Yang, H.; Liang, H.; Wang, Z.; Dong, J.; Xiong, L.;

Fluorinated Siloxane Resin for Low Surface Energy Coating. Polymers

9) Jennings, A. R.; Morey, A. M.; Guenthner, A. J.; Iacono, S. T.

018, 10, 979.

226, Colorado, United States

Maria Leising − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

Synthesis and Characterization of Siloxane-Based Cyanate Ester

Elastomers from Readily Available Materials: A Top-down Approach.

Polym. Int. 2017, 66, 540−547.

226, Colorado, United States

(10) Hamdani, S.; Longuet, C.; Perrin, D.; Lopez-cuesta, J.-M.;

Isaac Coolidge − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

Ganachaud, F. Flame Retardancy of Silicone-Based Materials. Polym.

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(11) Doede, C.; Panagrossi, A. Polysiloxane Elastomers. Ind. Eng.

226, Colorado, United States

Chem. 1947, 39, 1372−1375.

Erica Lee − Department of Chemistry & Chemistry Research

Center, Laboratories for Advanced Materials, United States

Air Force Academy, Colorado Springs 80840-6226,

Colorado, United States

Charles Stevens − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

(12) Ogliani, E.; Yu, L.; Mazurek, P.; Skov, A. L. Designing Reliable

Silicone Elastomers for High-Temperature Applications. Polym.

Degrad. Stab. 2018, 157 , 175 −180.

(13) Scott, D. W. Thermal Rearrangement of Branched-Chain

Methylpolysiloxanes. J. Am. Chem. Soc. 1946, 68, 356−358.

(14) Tong, C.; Tian, Z.; Chen, C.; Li, Z.; Modzelewski, T.; Allcock,

H. R. Synthesis and Characterization of Trifluoroethoxy Polyphos-

phazenes Containing Polyhedral Oligomeric Silsesquioxane (POSS)

Side Groups. Macromolecules 2016, 49, 1313−1320.

226, Colorado, United States

Andrew J. Peloquin − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

(15) Lichtenhan, J. D.; Pielichowski, K.; Blanco, I. POSS-Based

Polymers. Polymers 2019, 11, 1727.

16) Lichtenhan, J. D.; Vu, N. Q.; Carter, J. A.; Gilman, J. W.; Feher,

F. J. Silsesquioxane-Siloxane Copolymers from Polyhedral Silsesquiox-

226, Colorado, United States

anes. Macromolecules 1993 , 26 , 2141 −2142.

Daniel Kure − Department of Chemistry & Chemistry

Research Center, Laboratories for Advanced Materials,

United States Air Force Academy, Colorado Springs 80840-

(17) Liu, H.; Zheng, S. Polyurethane Networks Nanoreinforced by

Polyhedral Oligomeric Silsesquioxane. Macromol. Rapid Commun.

2005, 26, 196−200.

226, Colorado, United States

(18) McMullin, E.; Rebar, H. T.; Mather, P. T. Biodegradable

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.macromol.1c00333

Thermoplastic Elastomers Incorporating POSS: Synthesis, Micro-

structure, and Mechanical Properties. Macromolecules 2016, 49,

769−3779.

Notes

(19) Lee, K. M.; Knight, P. T.; Chung, T.; Mather, P. T.

Polycaprolactone-POSS Chemical/Physical Double Networks. Macro-

molecules 2008, 41, 4730−4738.

The authors declare no competing ﬁnancial interest.

20) Yang, H.; He, C.; Russell, T. P.; Wang, D. Epoxy-Polyhedral

ACKNOWLEDGMENTS

■

Oligomeric Silsesquioxanes (POSS) Nanocomposite Vitrimers with

High Strength, Toughness, and Efficient Relaxation. Giant 2020, 4,

The authors acknowledge ﬁnancial support from the Defense

Threat Reduction Agency (DTRA) and the Air Force Oﬃce of

Scientiﬁc Research (AFOSR).

21) Hajiali, F.; Tajbakhsh, S.; Maric,

00035.

Bio-Based Polymethacrylate Vitrimers and Nanocomposites. Polymer

021, 212, 123126.

M. Thermally Reprocessable

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