Ring-Opening Copolymerization of Mixed Cyclic Monomers: A Facile

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Ring opening copolymerization of mixed cyclic monomers - a novel structure-controllable approach to prepare polymethylphenylsiloxane with excellent thermal behavior Cheng Li, Deqi Zhang, Linbo Wu, Hong Fan, De-Yi Wang, and Bo-Geng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01279 • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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Industrial & Engineering Chemistry Research

Ring opening copolymerization of mixed cyclic monomers - a novel structure-controllable approach to prepare polymethylphenylsiloxane with excellent thermal behavior Cheng Li1, Deqi Zhang1, Linbo Wu1, Hong Fan1*, De-yi Wang2* and Bo-Geng Li1 1. State Key Laboratory of Chemical Engineering, Institute of polymer and polymer engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China 2. IMDEA Materials Institute, C/Eric Kandel, 2, 28906 Getafe, Madrid, Spain. Corresponding author: *Hong Fan (E-mail: [email protected], Tel: 86-571-87957371) * De-yi Wang (E-mail: [email protected], Tel: 0034-91-5503047) Abstract: Restricted by molecular structure defects, polymethylphenylsiloxanes usually exhibit a limited thermal stability. This paper introduces a cost-effective synthesis method to prepare polymethylphenylsiloxanes (PPMS-M) with methyl-phenyl mixed cyclic monomers as raw materials. The molecular structure characterization shows the PPMS-M contain abundant phenyl groups, and the phenyl siloxane units are singly distributed among methyl siloxane segments. The thermal degradation kinetics are systematically studied by the Flynn-Wall-Ozawa method. It shows the

PPMS-M

exhibits

polymethylphenylsiloxanes triphenylcyclotrisiloxane

much

higher

(PPMS-PD), (P3)

and

degradation which

is

activation prepared

octamethylcyclotetrasiloxane

energy by

than

ordinary

2,4,6-trimethyl-2,4,6-

(D4).

The

TG-FTIR

characterization shows the degradation process of phenyl group in PPMS-M occurs at the temperature of 100~200 ℃ higher than PPMS-PD. The PPMS-M exhibit good thermal stability while keep low glass transition temperature. Our method is hopefully to be applied to develop other high performance functional polisiloxanes. Keywords: polymethylphenylsiloxane, ring-opening copolymerization, methyl-phenyl mixed cyclic monomer, thermal stability

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1. Introduction

Polysiloxanes have been widely applied in textile, leather, construction, coating, cosmetic and medicine industries due to their unique properties1-4, such as excellent air permeability5, and hydrophobicity6. They are produced mainly by two routes7, 8: hydrolytic polycondensation of dichlorosubstituted silanes and ring opening polymerization (ROP) of cyclic monomers. Considering the ROP method’s higher productivity, and the products higher molecular weight than hydrolytic polycondensation method9, it becomes one of the most promising methods for preparing polysiloxanes since its invention from 1959. Conventional anionic catalysts such as tetramethylammonium hydroxide and potassium hydroxide are taken as catalysts in ring-opening copolymerization

10.

Hurd et al.11

discussed the mechanism of the polymerization of polysiloxanes with strong bases. It involves two steps: (a) the catalyst connect with the siloxane molecular chain, and (b) the continued action of the catalyst molecule in the reshuffling of siloxane linkages. His research showed that the base strength of the catalyst is the key factor in reaction rate control. In terms of cationically catalyzed ROP, a variety of protonic acids such as trifluoroacetic acid, concentrated sulfuric acid and trifluoromethanesulfonic acid are used10, 12. However, after reaction, these catalysts are left in the products and can hardly be removed. The solid state acid can solve this problem. Xiongfa Yang13 synthesized fluorine-containing polysiloxanes catalyzed by rare earth solid superacid SO42−/ TiO2/ Ln3+. This type solid acids have high reactivity, easy to remove from liquid polysiloxanes due to their solid status and low corrosion to reactors. The polysiloxanes have the potential to exert good thermal stability due to the higher energy of Si-O bond (460.5 kJ/mol) than the C-O bond (358.0 kJ/mol) and C-C bond (304.0 kJ/mol)14.


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There have been many studies on the thermal decomposition behavior of polydimethylsiloxane (PDMS): Camino et al.15 investigated the thermal degradation mechanisms of PDMS, and proposed a hypothetical mechanism for the polymerization containing two competing mechanisms: 1. a molecular mechanism occurs along with formation of cyclic oligomers, 2. a radical mechanism takes place through homolytic Si-CH3 bonds scission. However, the polysiloxanes constituted by purely methylsiloxane chain is limited by their thermal stability due to Si-CH3 bond’s weak thermal stability 10. Their defects can be overcomed by attaching carbofunctional groups in the ends or on the sides of the main chain, to form functionalized polysiloxanes. Sauvet16, 17 took functional M2type molecules as chain terminator, and successfully introduced functionalional groups such as vinyl, carboxylic, or hydroxyl groups into polysiloxanes. However, the researches on improving the functional polysiloxanes’ thermal stability by adjusting their molecular structure is still limited

18, 19.

Take phenyl polysiloxane as example, the

phenyl polysiloxanes with different molecular structure, especially the amount and distribution will result in different thermal resistance behavior. The molecular structures are affected by different raw material and synthetic methods. Simple mixing and hydrolyzing of phenyl and methyl siloxane cyclic monomer will produce polysiloxanes with low phenyl content and heterogeneous and block distributed phenyl group, attributing to the different hydrolysis rate between phenyl and methyl cyclic monomers 10. In order to solve this problem, we take phenyl-methyl mixed cyclosiloxane as monomer to prepared phenyl contained functional polysiloxanes. The relationship between their molecular structure and thermal stability are studied.



2. Experimental section

2.1 Materials

Dimethyldichlorosilane (DDS, >99% purity), methylphenyldichlorosilane (PMS, >99% purity), hexamethyldisiloxane (MM, >99% purity), octamethylcyclotetrasiloxane (D4, >99% purity), zinc oxide, hexane, methanol, and tetramethylammoniun hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. 2,4,6-trimethyl-2,4,6-triphenylcyclotrisiloxane (P3) was kindly provided by Jiaxing United Chemicals. Co., Ltd. All the chemicals are used as received.

2.2 Preparation of mixed cyclic monomers

In a 250 ml four-necked round bottom flask equipped with a reflux condenser, a nitrogen inlet tube, a thermometer, and a dropping funnel, H2O (54.00 g, 3.00 mol), ZnO (33.21 g, 0.40 mol), and hexane (60 ml) were added. The mixture was heated to 40 ℃ under magnetic stirring. DDS (40.00g, 0.30 mol) and PMS (19.10 g, 0.10 mol) were added into the mixture with a drop rate of 0.5 ml/min. After two hours’ reaction, the organic phase was separated and washed to be neutral (pH=7). The crude product was purified by distillation to obtain the final methyl-phenyl mixed cyclosiloxanes. A series of methyl-phenyl mixed cyclosiloxanes with different molecular structure were prepared according to different recipe.

2.3 Synthesis and characterization

2.3.1 Ring-opening polymerization of methyl-phenyl mixed cyclic monomers

To a three-necked flask equipped with a reflux condenser, a nitrogen inlet tube, and a thermometer, methyl-phenyl mixed cyclic monomers, (CH3)4NOH and hexamethyldisiloxane (MM)


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were added. The reaction was conducted at 85 ℃ for 8 h, and additionally heated at 150 ℃ for 1 h to decompose the catalyst (CH3)4NOH. The crude product was washed by methanol, and dried under reduced pressure to obtain polysiloxanes (PPMS-M series). By adjusting the type and mol ratio of cyclosiloxanes, a series of polysiloxanes with different molecular weight and phenyl content were produced. To compare with PPMS-M, PPMS-PD polysiloxane series were prepared by ring opening reaction of traditional monomer: P3 (2,4,6-trimethyl-2,4,6-triphenylcyclotrisiloxane) and D4 (octamethylcyclotetrasiloxane). To a three-necked flask equipped with a reflux condenser, a nitrogen inlet tube, and a thermometer, P3, D4, MM and (CH3)4NOH were added. The reaction was conducted at 85 ℃ for 8 h, and heated to 150 ℃ for 1 h to decompose the catalyst (CH3)4NOH. The crude product was washed by methanol, and dried under reduced pressure to obtain PPMS-PD. A series of PPMS-PD with different molecular weight and phenyl content were synthesized by adjusting the reaction mol ratio of P3 and D4.

2.3.2 Characterization

Gaseous chromatography-Mass spectra (GC-MS) Analysis were conducted to characterize the molecular structure of methyl-phenyl mixed cyclic monomers. They were performed on an Agilent 7890 A GC connected with an Agilent 5977 A MS in scan mode (carrier gas: Helium, flow rate: 1.0 mL/min, split rate: 80:1). Molecular structures of polysiloxanes were characterized by 1H NMR and 29Si NMR spectra. They were conducted on a Bruker Avance 600 spectrometer, with CDCl3 as solvent. Molecular weight of polysiloxanes were characterized with a gel permeation chromatography



Page 6 of 26

(GPC) instrument equipped with a reflective index detector (eluent: tetrahydrofuran, flow rate: 1 ml/min, column temperature: 35 ℃). The calibration curves for GPC analysis were obtained by polymethyl methacrylate standards (Standards’ molar mass: 162, 575, 1260, 2700, 4800, 19600, 28000, 43900, 72450 g/mol). Thermal stability were tested on on a TA-Q500 TGA apparatus. Before test, the samples were placed in vacuum oven at 80 ℃ for 2 h to remove moisture. The tests were conducted from room temperature to 800 ℃, with the heating rate of 10 ℃/min in nitrogen atmosphere. FTIR spectra were performed on a Nicolet 560 spectrometer. The scan rate of the FTIR spectrometer was 1 scan/ 6 s at the range of 500 ~ 4000 cm-1, with a resolution of 2 cm-1. Low temperature property of polysiloxanes were characterized by a TA Q200 differential scanning calorimeter (DSC) instrument. All samples were cooled from room temperature to -150 ℃, with a cooling rate of 10 ℃ /min, to observe their glass transition behavior in low temperature.

3. Results and discussion

3.1 Structure characterization

3.1.1 Characterization of methyl-phenyl mixed cyclic monomers

The

phenyl-methyl

mixed

cyclic

monomers

were

obtained

by

hydrolysis

of

dimethyldichlorosilane (DDS) with methylphenyldichlorosilane (PMS). Their molecular structures were confirmed by GC-MS. According to the spectra (see Fig.1 and Tab.1), the D4 (tetracyclosiloxane with no phenyl group) and D41Ph (tetracyclosiloxane with one phenyl group) are the highest content components in this system, being 22.8 and 19.5 mol %, respectively. All the cyclic monomers contain 3 ~ 5 siloxane units. Around 58 mol % cyclic monomers contain phenyl


Page 7 of 26

siloxane unit (Tab.1).

m/z

343.0

D4

1Ph

D4

4.509

163.9 73.0

80

7.890

252.9

135.0

120

313.0

160

200

240

280

320

360

400

Relative Abundance 281.0

D3

6.932 3.015 1Ph

D3

m/z

Relative Abundance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1Ph

D5 5.787

8.828

2Ph

D4

10.036

73.0 103.0

10.980

50

100

133.0

150

191.0

200

249.0

250

Relative Abundance

4

6

8

10

12

14

Time (min)

Fig. 1. Gas chromatogram of mixed cyclic monomers. Table 1. Selected observed peaks for mixed cyclic monomers. Retention time/min

Intensity (I/Itotal×100%)

Compound

3.012 4.504 5.785 6.927 7.060 7.888 8.824 9.900 10.036 10.980 12.649

11.151 22.768 5.747 14.027 1.789 19.515 6.020 1.464 5.191 8.752 3.610

D3 D4 D5 D31Ph D6 D41Ph D51Ph D61Ph D32Ph D42Ph D52Ph

3.1.2 Synthesis and molecular structure characterization of polysiloxanes (PPMS-M and PPMS-PD)

With the above mentioned methyl-phenyl mixed cyclosiloxanes, P3 and D4, we prepared two series of samples: PPMS-PD and PPMS-M. The PPMS-M series were synthesized from methyl-


300


phenyl mixed cyclic monomers (Scheme 1), while the PPMS-PD were synthesized with P3 and D4 as raw material (Scheme 2).

Scheme 1. Main reaction in ring-opening polymerization of cyclic monomers (PPMS-M).

Scheme 2. Ring-opening polymerization of P3 and D4 (PPMS-PD). In order to study the relationship between phenyl distribution in main chain and thermal behavior, the PPMS-M and PPMS-PD with similar Si/Phenyl mol ratio (nSi/nPh), and similar molecular weight were prepared (see Tab.2). Comparing the mol ratio between phenyl groups and silicon atoms in raw material (nSi / nPh *), it can be observed that to achieve the same phenyl content in final product (nSi / nPh **), the PPMS-PD consume more phenyl siloxane segments in raw materials than PPMS-M. Comparing the phenyl group retaining percent, the PPMS-M-2, PPMS-M3 and PPMS-M-4 are 94.80, 92.23 and 93.28, respectively, much higher than PPMS-PD-2, PPMSPD-3 and PPMS-PD-4 (68.65, 74.34 and 75.18, respectively). It means in PPMS-M, more phenyl groups are reacted into polysiloxane main chain than PPMS-D. Table 2. The ratio and number-average molecular weight of polysiloxanes.


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Mn(104g/mol)

Samples

nph /nSi*

nph /nSi**

Phenyl group retaining percent (%)

PPMS-M-1

0.1000

0.0974

97.40

0.95

PPMS-M-2

0.2500

0.2370

94.80

1.02

PPMS-M-3 PPMS-M-4 PPMS-M-5

0.4000 0.4000. 0.6667

0.3689 0.3731 0.6536

92.23 93.28 98.04

1.04 3.13 1.03

PPMS-PD-1

0.1000

0.0971

97.10

1.01

PPMS-PD-2

0.3333

0.2288.

1.02

PPMS-PD-3

0.5000

0.3717

68.65 74.34

PPMS-PD-4 PPMS-PD-5

0.5000 0.9189

0.3759 0.6541.

75.18

3.18 1.02

71.18

1.03

Attention: The PPMS-M series represent for methyl-phenyl mixed cyclosiloxanes, the PPMS-PD series were prepared by P3 with D4. nph /nSi *: The mol ratio of phenyl versus silicon in reactants (raw material). nph /nSi **: The mol ratio of phenyl versus silicon in final products. Phenyl group retaining percent = (nph /nSi**) / (nph /nSi*) × 100 %, represents for the percent of phenyl groups remained after reaction. Beside the phenyl retaining percent, the PPMS-PD and PPMS-M exhibit discrepancy in distribution of phenyl group on siloxane chain as well. It can be observed from their molecular structures. The peak positions and assignment of 29Si NMR were shown in Fig.2 and Tab.3

20-22.

According to the integrated area ratio of each signal, the corresponding molecular structure can be determined. Compared with methyl in siloxane segment, the phenyl group has larger molecular size, and is more electronegative. Considering the cationic ring opening polymerization mechanism, the phenyl siloxane is more inclined to homopolymerize than copolymerize with methyl siloxane segment. In another word, P3 exhibits a higher reactivity ratio than D4. It influences the composition and sequencing of the polysiloxane 23. With P3 and D4 as raw material, for PPMS-PDs the phenyl groups mostly exists as dimer (PP) and trimer (PPP) of methyl-phenyl siloxane, such as PPDDD, DPPDD and PPPDD. For PPMS-M, the phenyl groups are mostly existed among dimethyl siloxane



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segments, such as DPDDD, DDPDD, and DPDPD (Tab. 3 and Fig. 3). It is of great interest that the difference in raw material lead to huge descrepancy in phenyl distribution in final polysiloxanes.

PPMS-PD-2

-20

-22

-32

-34

-36

PPMS-M-2

-16

-20

-24 -32

-34

-36

-38

ppm (a)

P=Ph(CH3)SiO

PPMS-PD-2

D=(CH3)2SiO

DPPDD PPPDP

PDPDD

-34.2

DDPDD

PDPDP

DPPDP -34.4

-34.6

-34.8

-35.0

-35.2

ppm

DPDDD PPDDD PPDPD DPDPD DPDDP PPDPP -20.0

-20.4

-20.8

DDDDD PDDDD PDDDP

-21.2

-21.6

-22.0

ppm (b)

Fig. 2. (a) 29Si NMR spectra of PPMS-PD-2 and PPMS-M-2 (b) Analysis of 29Si NMR spectra of PPMS-PD-2. Table.3. Peak positions, assignments and relative intensity of 29Si NMR spectrum for phenyl containing siloxane copolymers. Peak positions

Assignment

8.76 7.37 -20.19 -20.31 -20.43 -20.91 -21.02 -21.14 -21.53 -21.62 -21.77 -34.27

MDP MDD PPDPP PPDPD DPDPD DPDDP PPDDD DPDDD PDDDP PDDDD DDDDD DPPPD

Intensity (I/Itotal×100%) PPMS-PD-2 0.09 1.07 0.38 2.56 3.13 2.20 11.28 19.07 3.32 14.11 21.55 0.96


PPMS-M-2 0.97

0.89

29.34

50.40

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-34.35 -34.47 -34.69 -34.8 -34.91 -35.03

DPPDP PPPDD DPPDD PDPDP PDPDD DDPDD

0.57 3.51 4.75 1.12 4.78 5.92

6.48

12.90

*P represents for -PhCH3SiO- unit, D represents for -(CH3)2SiO- unit, and M represents for (CH3)3SiO- unit.

Fig. 3. Schematics of molecular structure during PPMS-PD and PPMS-M preparation 3.2 Thermal behavior in high temperature

3.2.1 Thermal behavior of polysiloxanes prepared with different monomers

For the PPMS-M and PPMS-PD with the same nph /nSi** value (mol ratio of phenyl groups versus silicon atoms), the PPMS-M exhibit an initial decomposition temperature (degradation at 5 wt.% weight loss, Td5) of over 30 ℃ higher than PPMS-PDs (Tab. 4, Fig.4). When the nph /nSi** value increases to 0.65, the PPMS-PD-5 exhibit a much lower initial decomposition temperature than the PPMS-M-5: 251 ℃ versus 391 ℃. Even compared with PPMS-PD-1 to PPMS-PD-3, the PPMS-PD-5’s initial decomposition temperature sharply decreases. In its degradation curve, two



degradation stages can be clearly observed (Fig.4d). It is attributed to the low reactivity ratio of P3 and D4. They tend to form polydimethylsiloxane-block-polymethylphenylsiloxane copolymer structure, while the polymethylphenylsiloxane segment content achieve a certain content, its thermal stability will be seriously affected. The degradation mechanism will be further studied and analyzed in section 3.2.4.

Fig. 4. TG and DTG curves of PPMS-M and PPMS-PD: (a) PPMS-PD-1, PPMS-M-1; (b) PPMSPD-2, PPMS-M-2; (c) PPMS-PD-3, PPMS-M-3; (d) PPMS-PD-5, PPMS-M-5. All the samples have a number average molecular weight of around 1×104g/mol Table 4. Initial degradation temperature of PPMS-M and PPMS-PD. Sample

Td5(℃) *

PPMS-M-1 PPMS-M-2 PPMS-M-3 PPMS-M-5 PPMS-PD-1 PPMS-PD-2 PPMS-PD-3 PPMS-PD-5

386.00 394.72 396.28 391.01 348.50 359.57 363.64 250.87


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*Td5% represents the onset of degradation at 5% weight loss.

3.2.2 Thermal behavior of polysiloxanes with different molecular weight

In order to study the relationship between the thermal stability and molecular weight, a series of PPMS-Ms with the same nPh /nSi** value (mol ratio of phenyl groups versus silicon atoms) and varified number average molecular weight (Mn) were prepared. Their relationship between degradation temperature (temperature at 10 % weight loss) and molecular weight is shown in the Fig.5. The degradation temperature increases with molecular weight. At this stage, the degradation reaction mainly release cyclosiloxane15. However, when the molecular weight is higher than 3.0×104 g/mol, the decomposition temperature keeps almost unchanged. Interestingly, lnT and -1/Mn forms a linear relationship according to linear curve fit, as shown in Fig.5b. We will further study the relationship between degradation mechanism and molecular weight in near future24.

Fig. 5. Relationship between molecular weight and decomposition temperature of PPMS-M (nPh / nSi** = 0.1). 3.2.3 Thermal behavior of polysiloxanes with different phenyl content

The degradation of polysiloxanes usually start from 300 ℃24-26. Their thermal stability depend on groups attached in Si atoms27, 28. The methyl polysiloxanes’ initial degradation temperature are



Page 14 of 26

around 300 ~ 350 ℃29. The thermal degradation temperature of our polysiloxanes with various phenyl group contents are shown in Tab.5 and Fig.6. For all the PPMS-M, no obvious weight loss is observed below 300 ℃. Their decomposition temperature at 5 wt. % weight loss are above 380 ℃. It is attributed to phenyl groups’ high steric hindrance to hamper Si-O bond scission30. The significant DTG peak appeared at nearly 550 ℃, implying an intensive degradation, which mainly produce cyclosiloxanes15. Table 5. Summary of thermal properties for PDMS (polydimethylsiloxane), PPMS-M and PPMS-PD. Sample

Td5(℃)

Td10(℃)

Tmax(℃)

Cy (%)

PDMS-1 PDMS-2 PPMS-M-1 PPMS-M-2 PPMS-M-3 PPMS-M-4 PPMS-M-5 PPMS-PD-1 PPMS-PD-2 PPMS-PD-3 PPMS-PD-4 PPMS-PD-5

316.15 377.37 386.00 394.72 396.28 407.60 391.01 348.50 359.57 363.64 385.59 250.87

368.83 428.32 428.02 459.50 456.07 463.03 454.87 391.64 403.56 408.91 436.36 325.87

490.24 564.15 536.97 572.46 539.43 545.50 543.76 518.76 510.48 551.81 550.83 562.70

0.14 0.71 3.85 19.95 31.41 33.67 42.84 3.60 13.10 18.97 36.56 39.58

*The number- average molecular weight of PDMS-1 is 1 × 104 g/mol. The number-average molecular weight of PDMS-2 is 3 × 104 g/mol. The values of nSi / nph are included in Tab. 2. Td5% represents the onset of degradation at 5% weight loss. Tmax is the temperature at the maximum degradation rate.


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100

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

60

PDMS-1 (PPMS 0 %) PPMS-M-1 PPMS-M-2 PPMS-M-3 PPMS-M-5

PDMS-1 (PPMS 0%) PPMS-PD-1 PPMS-PD-2 PPMS-PD-3 PPMS-PD-5

40

20

0 100

200

300

400

500

600

700

800 100

200

Temperature (℃)

300

400

500

600

700

800

Temperature (℃)

Fig. 6. TG curves with different phenyl contents. (a) PPMS-PD; (b) PPMS-M. For PPMS-M, the relationships between phenyl content and decomposition temperature (at 5 wt.% weight loss) is shown in Fig.7. The thermal decomposition temperature increases obviously when the phenyl content is below 10 wt. %; and keep almost unchanged when the phenyl group content being higher than 10 wt%. An appropriate amount of 10 wt.% phenyl components are beneficial for thermal stability. If there are too much phenyl groups, some of them would be affected by the adjacent phenyl groups. The rotation of phenyl groups would be hindered, and inhibiting the dissipation of heat31, and lead to slightly decreased decomposition temperature. In additoin, the lnT and -1/Phenyl forms a linear relationship according to linear curve fit, as shown in Fig.7b. It is an interesting phenomenon worthy of further study.

Fig. 7. Phenyl content versus decomposition temperature of PPMS-M. The number- average molecular weight of PPMS-M is 1 × 104 g/mol.



3.2.4 Thermal degradation mechanism by TG-FTIR analysis

According to previous literatures, the thermal degradation of polysiloxanes occurs through two competing mechanism 15, 29: molecular degradation mechanism and radical degradation mechanism. Molecular degradation mechanism occurs by Si-O-Si bond scission to generate some low molecular weight cyclic siloxane oligomers, including trimers, tetramers, pentamers, etc.. Radical degradation mechanism occurs during the debonding at the side groups of the main chain, such as Si-CH3, SiC6H5, etc. To study the degradation mechanism, as well as discover the chain structure of prepared siloxane, the TG-FTIR and TG-MS techniques were applied to test the degradation process of PDMS (polydimethylsiloxane) and PPMS. The samples were placed in the TGA instrument and the evolved gases were led to the FTIR/Mass spectrometer, to identify the gaseous degradation products. The TG-FTIR spectra reveal that no volatile degraded product is detected below 300 ℃, indicating that polysiloxanes are stable at this stage (Fig.8). Between 300 and 350 ℃ several peaks appear for PDMS and PPMS-PD-1: The absorption peaks at 2967~3015 cm-1 are attributed to C-H stretching in CH2 and CH3, 1264 and 811 cm-1 correspond to Si-CH3 bonding, and 1025 ~ 1085 cm1

attribute to Si-O-Si linkages [Fig.8(a) and (b)]. At this temperature range, the polysiloxanes

decompose through Si-O bond scission to form low-molecular-weight siloxane oligomers32, most of which are cyclosiloxanes15. For PPMS-M this Si-O debond process occurs when temperature is higher than 400 ℃. As the temperature keeps rise from 500 to 600 ℃, all the samples show strong absorption peaks at 1025 ~ 1085 cm-1, representing for further decomposition of Si-O-Si bonds. The characteristic absorptions of CO2 (2338 and 2362 cm-1) can be observed in the spectra of PPMS-PD-1 and PPMS-M-1, while there is no such peak in the spectra of PDMS. The CO2


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absorption peaks are attributed to the degradation of phenyl group. In PPMS-PD-1 [Fig. 8(b)], the peaks corresponding to CO2 are disappeared beyond 700 ℃, indicating that the phenyl groups are totally decomposed. For PPMS-M-1, the CO2 absorption peaks are observed from 500 ℃ to 800 ℃ [Fig. 8(c)]. The degradation process of phenyl group in PPMS-M-1 occurs at the temperature of 100~200 ℃ higher than PPMS-PD-1. It is resulted from their difference in molecular structure. According to previous results, in PPMS-PDs the phenyl groups exists as dimer (PP) and trimer (PPP) in siloxane chain. These phenyl groups exert a much more significant steric hindrance effect than methyl groups. For PPP component, the phenyl groups along the chain are significantly affected by adjacent phenyl groups. The rotation of phenyl groups along the Si-C axis is hindered, thereby inhibiting the dissipation of heat. Once heat accumulates to a certain level, Si-C bond will fracture, resulting in decreased thermal stability31. As for PPMS-M, siloxane units are mostly existed among dimethyl siloxane segments. At high temperature, these phenyl groups can easily rotate along the Si-C axis to dissipate heat by molecular movement. Thereby, the thermal stability of PPMS-M increase remarkably.

Fig 8. FTIR spectra of gas escaping from PDMS, PPMS-PD and PPMS-M at different



temperatures.

3.2.5 Thermal degradation kinetics

The thermal degradation curves of PPMS-PD-1 and PPMS-M-1 are shown in Fig. 9. In the DTG (derived TG) curves, both the PPMS-PD-1 and PPMS-M-1 show one main peak. The peak move toward high temperature region with enhanced heating rate.

Fig. 9. TG and DTG curves at different heating rates under nitrogen atmosphere: (a) PPMS-M-1; (b) PPMS-PD-1. In order to further study the decomposition mechanism, the thermal degradation kinetics of PPMS-PD-1 and PPMS-M-1 were calculated and compared by the Flynn-Wall-Ozawa method.33-35


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This method has been successfully applied in thermal degradation kinetics of various polymers, such as cyanate ester resins 36, novolac type phenolic resins 37, benzoxazine 38 and epoxy39. It can be employed to quantify Eα without mentioning specific reaction mechanisms, and produce results with high precision. According to this method, the relationships between degradation activation energy (Eα) and conversion (α) can be obtained from Eq.1. log 𝛽 = −0.457

𝐸α 𝐴𝐸α + {𝑙𝑜𝑔 [ ] − 2.315} (1) 𝑅𝑇 𝑔(α)

β represents for heating rate (K/min); T is the temperature (K) at corresponding conversion; Eα is degradation activation energy (kJ/mol); R is universal gas constant, being 8.314 J/K·mol). By plotting log β versus 1/T, a series of straight lines are acquired as Fig. 10. Eα is extracted from the slope at corresponding conversion (see Fig. 11 and Tab. 6).

Fig. 10. Plots of log β against (-1/T) at different heating rates under nitrogen atmosnphere: (a) PPMS-M-1; (b) PPMS-PD-1.

Initially, in low conversion (

Ring-Opening Copolymerization of Mixed Cyclic Monomers: A Facile

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