Suppression of Crystallization in Polydimethylsiloxanes and Chain

Article pubs.acs.org/Macromolecules

Suppression of Crystallization in Polydimethylsiloxanes and Chain Branching in Their Phenyl-Containing Copolymers Alisa Zlatanic,† Dragana Radojcic,† Xianmei Wan,† Jamie M. Messman,§ and Petar R. Dvornic*,‡ †

Kansas Polymer Research Center and ‡Department of Chemistry, Pittsburg State University, Pittsburg, Kansas 66762, United States § Honeywell Federal Manufacturing & Technologies LLC, Kansas City, Missouri 64147, United States ABSTRACT: While incorporation of only a few mol % of diphenylsiloxy-, DiPhS, repeat units into polydimethylsiloxane, PDMS, chains is enough to completely suppress their crystallization, it also leads, in polymers obtained by silanolate-initiated ring-opening polymerization, to a puzzling and yet unexplained chain branching which significantly distorts polymer molecular weight distributions and affects their chain conformation. In contrast to this, we found that introduction of comparable amounts (ca. 5 mol %) of diethylsiloxy-, DiEtS, units into the same type of polymers also suppresses polymer crystallization but does not lead to any detectable branching, yielding polymers with the most probable molecular weight distribution and typical random coil conformation in a thermodynamically good solvent, such as toluene. On the basis of the results of a 29Si NMR and SEC-MALS study, we propose that branching in DiPhS-containing polymers is caused by a nucleophilic attack of initiating silanolate anions on their Si−CAr side bonds and a resulting formation of phenyltrisiloxysilane T-branches. We also propose that because branching may seriously disrupt mechanical properties of elastomers if such DiPhS-containing PDMSs are used for cross-linking, the DiEtS-modified derivatives represent polymers of choice if applications of such elastomers at extremely low temperatures are desired.

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INTRODUCTION Polysiloxanes, the long chain molecules with alternating arrangement of silicon and oxygen atoms in their repeat units, −[SiR2O]n−, are well-known for having some of the lowest glass transition temperatures, Tg, known to polymer science.1−7 For example, polydimethylsiloxane, PDMS, −[Si(CH3)2O]n−, the “parent” polymer of this family, has Tg of ca. −125 °C, while its closest relative, polydiethylsiloxane, PDES, −[Si(CH2CH3)2O]n−, has the lowest Tg of all polymers known of around −140 °C.7−10 As a consequence, polysiloxanes are excellent precursors for exceptional low-temperature elastomers, with a major deficiency resulting from their unfavorable crystallization behavior, since PDMS is prone to crystallization at ca. −40 to −50 °C,11−14 while PDES shows several different solid state transitions starting at as low as −70 °C.15−22 In both cases, these transitions limit the low-temperature usefulness of their elastomers to about 10 °C above the highest temperature transition, and it is therefore highly desirable to suppress crystallization in order to extend elastomeric properties of the resulting rubbers to well below these temperatures. Toward this end, it has been well documented that complete prevention of crystallization in linear PDMSs can be achieved by incorporating small amounts (often less than 5 mol %) of crystallization disruptors, such as large and bulky phenyl units, into the polymer side groups.4,10,23−26 On the other hand, for many applications of siloxane rubbers, a well-defined and complete cross-linking is often critical, since unconnected, dangling chains in the resulting networks would not perform as the load-bearing elements of structure. Often, © XXXX American Chemical Society

polysiloxane precursors for such networks are synthesized by ring-opening polymerizations (ROPs) of cyclic siloxane monomers, conveniently using the so-called “transient catalysts” which can be decomposed after polymerization by heating to somewhat higher temperatures where decomposition products are easily eliminated from the reaction mixtures.10,27,28 Among such “catalysts”, tetramethylammonium silanolates (TMAS) (see Scheme 1) have been found quite effective and practical, as either monofunctional or difunctional initiators, enabling the polymer growth to occur in either one or two linear directions.29−31 For phenyl-substituted PDMSs, designed to eliminate crystallization, such ROPs, can be represented as shown in Scheme 1 for an example of terpolymers containing dimethylsiloxy, −[Si(CH3)2O]−, DiMeS, diphenylsiloxy, −[SiPh2O]−, DiPhS, and methylvinylsiloxy, −[Si(CH3)(CHCH2)O]−, MeViS, repeat units, where x, y, and z represent relative molar contents of the respective ones. One problem with this approach, however, arises from the fact that in many cases of polymerizations in which phenylsiloxane-containing copolymers are made, some uncontrolled amounts of chain branching can be observed,32 and these branches were often found to have nonfunctional chain ends, freely dangling within the elastomer networks, unable to bear load during subsequent applications. Consequently, it is highly desirable to eliminate such branching while preventing polymer Received: March 3, 2017 Revised: April 6, 2017

A

DOI: 10.1021/acs.macromol.7b00474 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1

Scheme 2

scission of the phenyl radical from Si−Ph side groups,32 so that it can be reasonably expected that in the TMAS-initiated ROPs of cyclic siloxanes of Scheme 1 the −O−Si(Ph)2−O− units may also be prone to such desilylation by initiating silanolate anions, leading to the formation of trisiloxy, −[Si(CH3)2−O]3− Si−Ph, Ph-T-branches, or even tetrasiloxy, quarternary, [Si(CH3)2−O]4−Si, Q-branches, as illustrated in Scheme 2. If so, this would explain the formation of observed branching in (DiMeS)(DiPhS) copolymers, the appearance of dangling chains in their subsequently cross-linked networks, and a

crystallization in order to provide for desired rubber elasticity at very low application temperatures. In the 1970s and 1980s, many different silarylene−siloxane elastomers with various aromatic groups incorporated into the polysiloxane main chain backbones have been described,10 and it was clearly shown that even when embedded into the main polymer chains, Si−CAr bonds are quite prone to desilylation in the presence of ionic species, such as acids or bases.33 Furthermore, it was also reported that in polymerization of mixed methylarylcyclosiloxanes with various silanolate initiators chain transfer to another molecule occurred as a result of the B


Article

Macromolecules

Cr(acac)3 was added. 1H NMR spectra were acquired using 16 scans, π/2 pulse (10 μs), and 10 s relaxation delay. Basic 29Si NMR screening was done using 1000 scans, π/12 pulse, and 2 s relaxation delay. For quantitative analysis, 29Si NMR spectra were obtained using π/2 pulse (8 μs) with 120 s relaxation delay and 1024 scans and gated 1H decoupling. External calibration was performed using TMS/CDCl3 25 vol % solution. All tests were carried out at room temperature. A differential scanning calorimeter (DSC) model Q100, from TA Instruments, New Castle, DE, was used for studying polymer melting transitions. Measurements were performed at a heating rate of 10 °C/ min, from −80 to 50 °C. Viscosity of polymers was measured in a constant shear stress mode (100 Pa) on an AR2000 EX rotational rheometer (TA Instruments) at 25 °C using cone/plate geometry (plate diameter 40 mm). The content of vinyl groups in the prepared polymers and oligomers was determined by iodine value (IV), using the Hanus method, which is based on reaction of double bonds with the molar excess of iodine monobromide, determination of excess halogen by addition of KI, and titration of the liberated iodine with a standardized sodium thiosulfate solution. Preparation of α,ω-Bistetramethylammonium-oligodimethylsiloxydisilanolate Initiator (TMAS). An apparatus consisting of 250 mL three-necked, round-bottom glass reactor equipped with a Dean−Stark trap with condenser and bubbler, nitrogen inlet, and magnetic stirring bar was used. Tetramethylammonium hydroxide pentahydrate (50 g, 0.28 mol) and equimolar amount of D4 (83 g, 0.28 mol) were added into the reactor followed by the addition of cyclohexane (113 g, 145 mL). The reaction mixture was stirred and heated in a silicone oil bath at 80 °C for 24 h. Continuous strong purge of nitrogen was applied as the reaction proceeded to facilitate the elimination of crystalline water (from tetramethylammonium hydroxide pentahydrate) and water liberated from silanol condensation via cyclohexane azeotrope. The progress of the reaction was monitored by recording the amounts of water collected in a Dean− Stark trap with time. After quantitative removal of water, the reaction mixture was vigorously stirred and purged with nitrogen for about 1 h at 85 °C to allow the residual cyclohexane to evaporate. The product was transferred to a glass jar and kept under nitrogen prior to use. The TMAS initiator, initially a clear, colorless liquid that very slowly turned into waxy white solid upon cooling, was obtained in a very high yield (99%). From relative intensities of 1H NMR signals (in d-dimethyl sulfoxide) for N−CH3 protons at 3.14 ppm and Si−CH3 protons around 0 ppm, the number-average molecular weight of TMAS was calculated. For different batches these values ranged from ca. 700 (DP ca. 9) to 1250 (DP ca. 12). Preparation of α,ω-Telechelic Dimethylvinylsiloxy EndBlocker (DiViEB). DiViEB was synthesized by equilibration reaction of D4 with 1,3-divinyltetramethyldisiloxane, [CH2=CH-Si(CH3)2]2O. The reaction was carried out in a 250 mL three-necked, round-bottom flask, which was equipped with a thermometer and magnetic Teflon stirring bar and immersed in a paraffin oil bath at constant temperature. A mixture of D4 (100 g, 0.34 mol) and 1,3divinyltetramethyldisiloxane (41.9 g, 0.22 mol) and strongly acidic cation exchange resin Amberlyst15(H) (3 g, 2 wt % of the reactants mass) was placed in the reactor and vigorously stirred at 40 °C for 24 h. Amberlyst15(H) was, after the completion of the reaction, conveniently separated from the product by vacuum filtration. DiViEB was low-viscosity clear colorless liquid. Typical number-average MW of DiViEB was 650 as determined by iodometry (iodine value (IV) = 78 g of I2/100 g (theoretical IV = 79 g of I2/100 g)), viscosity at 25 °C was 3 mPa s, and acid value (AV) = 0.2 mg of KOH/g. Ring-Opening Polymerization (ROP). Compositions of all reaction mixtures used for preparations performed in this work are shown in Table 1. The following was a typical procedure used. Predetermined amounts (as shown in Table 1) of D4, D4Ph2, D3Et2, D4Et2 or DiEtS “hydrolyzate”, D4MeVi, and DiViEB were placed in a 250 mL three-necked, round-bottom flask equipped with a mechanical stirrer, thermometer, and nitrogen inlet. A small amount of D4 (1−2 mL) was reserved for quantitative transfer of initiator. The reaction mixture was heated to 105 °C in a silicone oil bath, and polymerization

variety of negative effects on their mechanical properties associated with it. In an attempt to better understand these mutually interconnected factors, in this paper we describe some results obtained from our recent study of a series of difunctional α,ωbistetramethylammonium-oligo-dimethylsiloxydisilanolate, TMAS, initiated ROPs of different mixtures of cyclic siloxanes (see Scheme 1), including octamethylcyclotetrasiloxane, −[Si(Me)2−O]4−, D4, octaphenylcyclotetrasiloxane, −[Si(Ph)2− O]4−, D4Ph2, octaethylcyclotetrasiloxane, −[Si(Et)2−O]4−, D4Et2, hexaethylcyclotrisiloxane, −[Si(Et)2−O]3−, D3Et2, and tetramethyltetravinylcyclotetrasiloxane, −[Si(MeVi)−O]4−, D 4 MeVi , 34 in the presence of α,ω-telechelic dimethylvinylsiloxy-terminated oligodimethylsiloxane end-blocker (DiViEB). The working hypothesis was that replacement of Ph side groups in these copolymers with large and bulky Et units will equally well suppress their crystallization while at the same time preventing chain branching by reducing the electrophilicity of the Ph-bearing silicons and thus also their susceptibility to nucleophilic attacks by the polymerization initiating silanolate anions. We focus here on the results obtained by using size exclusion chromatography (SEC) and nuclear magnetic resonance (29Si NMR) as analytical tools and evaluation of the effects of synthetic parameters on the structural and physical properties of the resulting polymers. In subsequent papers, we will describe in more detail fine structure of the resulting polymers and monitoring methods that were found useful for their controlled preparation.

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EXPERIMENTAL SECTION

Materials. Octamethylcyclotetrasiloxane (D4), octaphenylcyclotetrasiloxane (D4 Ph2), 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (D4MeVi), hexaethylcyclotrisiloxane (D3Et2), octaethylcyclotetrasiloxane (D4Et2), mixture of cyclic and linear diethylsiloxanes (DiEt-“hydrolyzate”), 1,3-divinyltetramethyldisiloxane, and phenyltris(trimethylsiloxy)silane were obtained from Gelest (Morrisville, PA). With the exception of solid D4Ph2, which was dried at 110 °C for 2 h prior to use, all reagents were used as received. The solvents cyclohexane, toluene, methylene chloride, tetrahydrofuran, isopropanol, and methanol were purchased from Fisher Scientific (Pittsburgh, PA). Tetramethylammonium hydroxide pentahydrate was purchased from Sigma-Aldrich (St. Louis, MO). Ion exchange resin Amberlyst 15(H) was supplied from Alfa Aesar (Ward Hill, MA) and was used as received. The NMR solvents chloroform-d and dimethyl-d6 sulfoxide, as well as relaxation agent chromium(III) acetylacetonate, Cr(acac)3, were all purchased from Acros Organics (Morris Plains, NJ). Methods. A SEC/MALS/VIS/RI system consisting of an Agilent 1260 Infinity isocratic pump (Agilent Technologies, Santa Clara, CA), an Agilent 1260 Infinity Thermostated Column Compartment with a set of five Phenogel columns from Phenomenex (Torrance, CA) covering a MW range of 100−5 × 105, an Agilent 1260 Infinity autosampler, a DAWN HELEOS-II, multiangle (18 angles from 22.5° to 147°) laser-light scattering (MALS) detector with the He−Ne light wavelength at 658.0 nm, a ViscoStar-II online viscosity detector, and an Optilab T-rEX differential refractometer (all three detectors from Wyatt Technology Corp., Santa Barbara, CA) was used for assessing molecular weights (MW), molecular weight distribution (MWD), molecular sizes, and architecture of the obtained polymers. Toluene was used as eluent at 30 °C, and the flow rate was kept at 1.0 mL/min. Data were collected and processed by use of ASTRA 6 software from Wyatt Technology. NMR analysis was performed using a BRUKER DPX 300 instrument with 5 mm multinuclear probe at resonance frequency of 59.62 and 300.13 MHz for 29Si and 1H nuclei, respectively. In a typical procedure, approximately 300 mg of the sample was dissolved in 0.6 mL of CDCl3. In the case of 29Si NMR, 6 mg of relaxation agent C


D

(132) (132) (165) (165)

(99) (99) (99) (99)

(99) (99) (99) (99)

(132) (99) (99) (99) (99)

40 40 50 50

30 30 30 30

30 30 30 30

40 30 30 30 30

D4, g (mmol) 4.0 4.0 5.0 5.0

(4.9) (4.9) (6.2) (6.2)

D4Ph2, g (mmol)

2.1 2.1 2.1 2.1

(7.0) (7.0) (7.0) (7.0)

D3Et2, g (mmol)

2.9 2.1 2.1 2.1 2.1

2.1 2.1 2.1 2.1 (7.0) (5.2) (5.2) (5.2) (5.2)

(5.2) (5.2) (5.2) (5.2)

D4Et2, g (mmol)

0.14 0.11 0.11 0.11 0.11

0.11 0.11 0.11 0.11

0.10 0.10 0.10 0.10

0.14 0.14 0.17 0.17

(0.39) (0.29) (0.29) (0.29) (0.29)

(0.29) (0.29) (0.29) (0.29)

(0.28) (0.28) (0.28) (0.28)

(0.38) (0.38) (0.48) (0.48)

D4MeVi, g (mmol)

(2.09) (1.7) (0.87) (0.70)

(2.1) (1.7) (0.87) (0.70)

(6.9) (3.4) (1.4) (1.1)

2.30 (3.5) 1.39 (2.1) 1.16 (1.7) 0.87 (1.3) 0.57 (0.870

1.39 1.16 0.58 0.46

1.4 1.2 0.58 0.46

4.5 2.3 0.93 0.75

DiViEB, g (mmol)

1.19 0.72 0.60 0.45 0.30

1.31 1.09 0.55 0.44

1.3 1.1 0.55 0.44

2.4 1.2 0.90 0.72

(1.7) (1.05) (0.87) (0.65) (0.44)

(1.05) (0.87) (0.44) (0.35)

(1.05) (0.87) (0.44) (0.35)

(3.4) (1.7) (0.72) (0.57)

TMAS, g (mmol)

13 000 16 100 19 100 25 100 37 200

16 300 19 400 37 500 46 600

16 300 19 400 37 500 46 600

7 200 13 500 39 000 48 400

Mtb

160 200 240 320 480

200 240 480 600

200 240 480 600

80 160 480 600

DPtb

6.3 5.0 4.2 3.1 2.1

5.0 4.2 2.1 1.7

5.0 4.2 2.1 1.7

12.1 6.3 2.1 1.7

3.15 2.5 2.1 1.55 1.05

2.5 2.1 1.05 0.85

2.5 2.1 1.05 0.85

6.25 3.15 1.05 0.85

[DiViEB]c [TMAS]d

2 2 2 2 2

2 2 2 2

2 2 2 2

2 2 2 2

[DiViEB]/ [TMAS]

a MeViS content in all polymers was 0.3 mol %. DiMeS content was 96.1 mol % in DiPhS-containing polymers and 94.7 mol % in DiEtS-containing polymers. bMt: targeted molecular weight; DPt: targeted degree of polymerization. c[DiViEB]: mmol DiViEB/mol (SiR2O); SiR2O: sum of all −SiR2O− units introduced with cyclic monomers. d[TMAS]: (mmol silanolate/mol (SiR2O))/2; SiR2O: sum of all −SiR2O− units introduced with cyclic monomers.

group A with D4Ph2 1 2 3 4 group B with D3Et2 5 6 7 8 group C with D4Et2 9 10 11 12 group D with DiEtS “hydrolyzate” 13 14 15 16 17

polymer ID no.

Table 1. Compositions of Reaction Mixtures and Targeted Molecular Weights of the Prepared DiPhS- and DiEtS-Containing Polymersa

Macromolecules Article


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Macromolecules Scheme 3

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was initiated by the addition by Pasteur pipet of a 50 % solution of TMAS in THF. To provide quantitative transfer of the initiator, a vial in which the TMAS solution was prepared was rinsed with the reserved amount of D4. The reaction was allowed to proceed for 24 h at 105 °C with strong mechanical stirring, when TMAS was thermally decomposed by heating the vigorously stirred reactor content at 135 °C for an additional 2 h. The low molecular weight fraction (cyclic products) was partially removed from the system by nitrogen purge at 170 °C, and the remaining cyclics were then separated from the polymer product by repeated precipitation from a 10 wt % methylene chloride solution into a 6-fold volume of methanol. Traces of solvents in isolated polymer were removed by drying under vacuum at 80 °C for 2 h. Preparation of Model Phenyltris(polydimethylsiloxane) (PhT-PDMS) 3-Armed Star Polymer. The Ph-T-PDMS model star polymer of Scheme 3 was prepared by equilibration of D4 with phenyltris(trimethylsiloxy)silane, added in the molar ratio of 7.5:1, respectively, to yield an average of 10 dimethylsiloxy repeat units per arm. The reaction was conducted in a 125 mL three-necked, roundbottom flask, equipped with thermometer and magnetic Teflon-coated stirring bar and immersed in a paraffin oil bath at constant temperature. D4 (50 g, 0.17 mol) and phenyltris(trimethylsiloxy)silane (8.4 g, 0.022 mol) and strongly acidic cation exchange resin Amberlyst15(H) (1.2 g, 2 wt % of the total mass) was placed in the reactor and vigorously stirred at 40 °C for 24 h. After the reaction was completed, the catalyst was separated from the product by vacuum filtration. Removal of cyclic oligomers was accomplished by vacuum distillation at 130 °C for 3 h followed by multiple (6 times) precipitation from 10 wt % polymer solution in toluene into a 3-fold volume excess of methanol until all low-MW species were removed. Progress of low-MW species removal was monitored by SEC in toluene. Ph-T-PDMS was isolated in 42% yield. 29Si NMR (CDCl3): δ (ppm) −80.8 to −78.8 (−Si(CH3)2O]3−Si−Ph), −22.6 to −21.0 (−OSi(CH3)2O−), 7.2 to 9.1 ((CH3)3SiO−). SEC-MALS (toluene): Mw = 5060, Mn = 3500, polydispersity = 1.45. Viscosity at 25 °C = 47 mPa s.

RESULTS AND DISCUSSION

Polymers. Four series of α,ω-telechelic dimethylvinylsiloxy, CH2CH−Si(CH3)2−O−, terminated terpolymers containing different relative amounts of Me, Ph, Et, and Vi side groups (as listed in Table 1) were synthesized, using the TMAS-initiated anionic ROP, shown in Scheme 1. In the first series (group A of Table 1), all prepared polymers contained 3.6 mol % of DiPhS units, 0.3 mol % of MeViS, and 96.1 mol % of DiMeS and were targeted to have degrees of polymerization (DP) in the range from 80 to 600. In the other three series, the DiPhS repeat units were replaced with 5 mol % DiEtS while keeping the MeViS content at the 0.3 mol % level and DiMeS at 94.7 mol % and targeting similar DP values. It was found that relative compositions of the polymers obtained closely corresponded to the relative contents of the monomer mixtures used in respective polymerization reactions (x, y, and z in Scheme 1), as indicated in Table 2. DiPhS-Containing Polymers. As expected,7,23,24 in the series of polymers with 3.6 mol % of DiPhS units (polymers 1− 4 of group A of Table 2), crystallization was completely suppressed (as indicated by DSC thermograms from −80 to +40 °C shown in Figure 1) in all polymers ranging in DP from 80 to 600. 29 Si NMR spectra of these polymers showed all expected signals for Si atoms, including those from DiMeS units at −22 ppm, DiPhS units at −46 ppm, MeViS units at −35 ppm, terminal DiMeViS end-groups at −4.1 ppm, and dimethylmethoxysilyl terminal units at −12 ppm,35 but also in all cases an additional signal at around −80 ppm. By its position, this signal could be assigned to the trisiloxy, Ph−Si−[Si(CH3)2− O]3−, Ph-T-branches, and in support of such assignment its intensity increased with the decrease in polymer DP caused by E


F

80 160 480 600

200 240 480 600

200 240 480 600

160 200 240 320 480

16 300 19 400 37 500 46 600

16 300 19 400 37 500 46 600

13 000 16 100 19 100 25 100 37 200

DPta

7200 13 500 39 000 48 400

Mta

4.9 4.0 3.6 3.0 2.3

4.1 3.6 2.3 2.1

4.1 3.6 2.3 2.1

7.8 4.7 2.2 2.0

IVt, g of I2/ 100 g

5.0 4.0 4.0 3.5 2.5

4.3 4.0 2.6 2.4

4.3 3.8 2.5 2.4

7.4 5.0 2.8 2.4

IVexp, g of I2/ 100 g

b

37 200 89 600 234 800 260 500 24 600 27 100 54 600 59 300 24 500 27 400 56 400 65 300

20 700 26 300 28 600 37 500 55 600

−0.085 −0.087 −0.083 −0.084 −0.085 −0.084 −0.082 −0.084 −0.085 −0.085 −0.085 −0.084 −0.084

Mw

−0.077 −0.072 −0.068 −0.071

dn/dc, mL/g

c

12 500 15 800 17 700 23 000 34 100

14 200 16 400 33 400 38 700

14 900 14 400 34 600 36 100

13 600 21 400 50 600 58 500

Mn

19 100 22 800 26 000 35 000 52 100

21 000 23 500 51 100 57 600

21 100 24 100 49 600 54 500

15 600 23 300 67 300 71 200

Mp

SEC-LS-Visc

1.66 1.67 1.61 1.63 1.63

1.73 1.68 1.69 1.69

1.65 1.88 1.58 1.64

2.74 4.19 4.64 4.45

PDId

11.8 13.7 14.9 17.5 23.2

12.5 13.4 21.1 24.4

12.8 14.0 21.9 23.8

11.9 19.0 35.4 41.0

[η], mL/g

e

4.2 4.8 5.0 5.8 7.3

4.6 4.8 7.2 8.0

4.6 5.0 7.0 7.5

3.7 5.6 9.3 10.2

Rh, nm

f

0.664 0.661 0.666 0.668 0.662

0.633 0.638 0.643 0.653

0.636 0.630 0.647 0.646

0.474 0.474 0.480 0.481

a

0.0172 0.0177 0.0170 0.0165 0.0178

0.0226 0.0213 0.0202 0.0190

0.0221 0.0246 0.0202 0.0211

0.096 0.108 0.124 0.135

K, mL/g

MHS parameters

0.47 0.55 0.7 2.8 4.4

0.58 0.70 3.50 6.01

0.6 0.8 3.8 5.6

0.3 1.2 19.5 15.0

η at 25 °C, Pa s g

5.0 5.0 5.0 5.0 5.0

5.0 5.0 5.0 5.0

5.0 5.0 5.0 5.0

3.6 3.6 3.6 3.6

feed, mol %

4.6 5.1 5.1 5.7 4.9

4.0 3.8 3.6 3.9

4.4 4.6 4.6 5.4

2.4 3.1 3.5 3.6

polymer (by 1H NMR), mol %

DiPhS or DiEtS content

Mt: targeted molecular weight; DPt: targeted degree of polymerization. bIVt and IVexp: iodine values theoretical and experimental, respectively. cdn/dc was determined online, using 100% mass recovery. PDI: polydispersity index; PDI = Mw/Mn. e[η]: intrinsic viscosity as determined by Visco Star II online detector. fRh: hydrodynamic radius. gη: dynamic viscosity of isolated polymer samples as determined by cone-and-plate viscometry.

d

a

group A with D4Ph2 1 2 3 4 group B with D3Et2 5 6 7 8 group C with D4Et2 9 10 11 12 group D with DiEtS “hydrolyzate” 13 14 15 16 17

polymer ID no.

b

Table 2. Characterization of Terpolymers Obtained from Reaction Systems Described in Table 1

Macromolecules Article


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Macromolecules Table 3. Relative Abundance of Linear, DiPhS, and Branched, Ph−Si−[Si(CH3)2O]3−, Units in DiPhSContaining Terpolymers of Group A polymer 1 polymer 2 rel int of DiPhS signal (%) rel int of Ph-Si−[Si(CH3)2O]3− signal (%)

39 61

73 27

polymer 3 94 6

was synthesized as shown in Scheme 3, and its 29Si NMR was compared with those of polymers from group A and a small molecular weight analogue, phenyltris(trimethylsiloxy)silane, Ph−Si(OSi(CH3)3)3, as shown in Figure 3. It can be seen from

Figure 1. DSC thermograms of group A polymers containing 3.6 mol % of DiPhS repeat units and a PDMS homopolymer. Crystallization of PDMS is completely suppressed by substitution of 3.6 mol % DiMeS units with DiPhS ones.

increase in concentration of the silanolate initiator used in the corresponding polymerization reactions (see Figure 2). Comparison of the intensities of 29Si NMR signals from DiPhS and Ph-T-branch units of polymers of group A (Table 3) showed that as the concentration of silanolate initiator increased from 2.1 mmol of SiO−/mol of siloxy for polymer 3, to 6.3 mmol of SiO−/mol of siloxy for polymer 2, to 12.1 mmol of SiO−/mol of siloxy for polymer 1, the relative amounts of Ph-T-branches in the sum of all Ph-containing units increased from 6 % to 27 % to 61 %, respectively. In the case of polymer 4, prepared with the lowest concentration of silanolate initiator (1.7 mmol of SiO−/mol of siloxy) the reliable estimation of the concentration of Ph-T-branch units was not possible due to a too low signal-to-noise ratio. To confirm this Ph-T-branch assignment, a model 3-armed PDMS star polymer with a built-in trisiloxyphenyl, −[Si(CH3)2−O]3−Si−Ph, Ph-T-branch as a key structural feature

Figure 3. 29Si NMRs of the model phenyltris(PDMS) 3-armed star polymer of Scheme 3 (A), phenyltris(trimethylsiloxy)silane (B), and DiPh-containing terpolymer, polymer 2 of Tables 1 and 2, with DP = 160 (C).

this figure that spectra of both model compounds contained clearly distinguished signals at −80 ppm, thus confirming the validity of the assignment for identical signals in the DiPhScontaining copolymers of group A (Figure 2).

Figure 2. 29Si NMR spectra of group A polymers containing 3.6 mol % of DiPhS units. G


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Macromolecules It is interesting to note that no indication of the presence of possible Q-branches could be found in NMR spectra. If present, their signal(s) would be expected at around −110 ppm. SEC-MALS-Vis (see Figure 4) analyses of DiPhS-containing terpolymers showed that in all cases their number-average

Figure 5. Mark−Houwink−Sakurada plots for DiMeViS-terminated polysiloxane terpolymers of group A [DiMeDiPh3.6MeVi].

terpolymers were prepared as shown in Table 2, using three different sources of DiEtS units: (a) hexaethylcyclotrisiloxane, D3Et2, for group B polymers; (b) octaethylcyclotetrasiloxane, D4Et2, for group C polymers; and (c) DiEtS “hydrolyzate”,40 a commercial mixture of D3Et2, D4Et2, and D5Et2 cyclic siloxanes, and a linear PDES of a moderately high molecular weight (ca. 17 000), for group D polymers. Within each group, targeted degrees of polymerization were similar to those for the DiPhScontaining polymers of group A. Composition of the commercial DiEtS “hydrolyzate” was determined using 29Si NMR and SEC in THF as shown in Figure 6. It contained about 30 wt % of linear fraction with an average degree of polymerization of ca. 170 and about 70 wt % of cyclics (predominantly D4Et2 and trace amounts of D3Et2). DSC clearly showed that relative content of DiEtS units was important for suppression of terpolymer crystallization and that its minimum required value was ca. 5 mol %. Thus, while lower amounts seemed to affect (i.e., reduce) the extent of crystallization (and shift the melting temperature to somewhat lower values, as judged by a significantly reduced melting peak observed for the polymer containing 3.6 mol % of DiEtS units from D3Et2 monomer shown in Figure 7) in all cases of group B and C polymers prepared from D3Et2 and D4Et2 cyclics, respectively, a minimum of 5 mol % of DiEtS units was necessary to completely prevent it (see Figure 8). With this composition, all of these polymers showed no crystallization peaks within the investigated DP range of 80−600, clearly suggesting that DiEtS units were bulky enough and properly distributed along the polymer chains to effectively disrupt their regularity of packing into well-ordered crystalline domains. However, an attempt to prevent crystallization in polymers of group D by using DiEtS “hydrolyzate” as a reagent failed in all cases in which the targeted polymer DP was higher than 200 (see Figure 9). 29 NMR spectra of DiEtS-containing polymers of groups B and C showed two major signals at −21 and −23 ppm from DiMeS and DiEtS units, respectively, and, like for polymers of group A, a small signal at −4.1 ppm from terminal DiMeViS end-groups. Expectedly, the last mentioned signals decreased in intensity with polymer DP, as shown in Figure 10. It should be particularly noted that there was no indication of any branching in any of the examined samples.

Figure 4. SEC-MALS chromatograms illustrating refractive index detector responses of DiMeViS-terminated polysiloxane terpolymers of group A, [DiMeDiPh3.6MeVi], prepared with [TMAS]/[ViEB] = 1/ 2.

molecular weights were considerably higher than the targeted values (calculated based on the end-blocker/monomer stoichiometry; see Table 2) and that all traces had pronounced shoulders at lower elution times, resulting in significant polydispersities which reached index values well over 4 in the cases of polymers 2−4. All of these parameters can be interpreted as indicating branching of polymer chains caused by desilylation of the side Si−CAr bonds and attachment of the long polymer branches from the initiator or from growing chains bearing silanolate anions. The lengths of these chains were comparable with those of the terpolymer backbones.36,37 Utilizing multiangle light scattering (MALS) in combination with an online viscometer for SEC detection, the Mark− Houwink−Sakurada (MHS) plots yielding the K and a constants of the well-known MHS equation [η] = KMa were obtained, providing further information about these polymers’ chain conformations. Generally, the value of the exponent a increases with rigidity of polymer chains, and in the ultimate case of rigid rods equals 2, while increasing compactness of polymer molecules, which occurs in cases of chain branching, is usually characterized by decreasing values of exponent a, reaching zero for the extreme case of compact spheres. Since the probability of branching also increases with polymer chain length, it is often found that experimental MHS plots of randomly branched polydisperse macromolecules exhibit curvature,36,37 i.e., evenly decreasing slope (exponent a) with molecular weight, exactly as we found in this work for polymers of group A (see Figure 5). DiEtS-Containing Polymers. Several earlier studies have shown that amorphous polysiloxanes with highly improved lowtemperature flexibilities can be obtained if diethylsiloxy, DiEtS, units are incorporated into PDMS chains in relative amounts ranging from rather low (ca. 5 mol %)38 to very high (ca. 50 mol %).39 Therefore, three series of DiEtS-containing H


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Figure 6. SEC chromatogram (A), 29Si NMR (B), and estimated composition (C) of DiEtS “hydrolyzate” (a name usually given to mixtures of linear and cyclic siloxanes obtained by hydrolysis of the corresponding dichlorosilanes).

Figure 7. DSC thermograms of DiEtS-containing polymers with DP = 600 prepared with various amounts of DiEtS units from cyclic trimer, D3Et2.

Figure 9. DSC thermograms of group D polymers prepared from DiEtS “hydrolyzate”.

Figure 10. 29Si NMR spectra of group B terpolymers of different DPs. Figure 8. DSC thermograms of group B and C polymers containing 5 mol % of DiEtS units.

In contrast to this, 29 Si NMR of DiEtS-containing terpolymers prepared from DiEtS “hydrolyzate” revealed the presence of longer polydiethylsiloxane, PDES, blocks (see I


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Macromolecules Figure 11, signals at −23 ppm), indicating that linear PDES fraction of the “hydrolyzate” did not effectively participate in

(with the curve of polymer 3 shown in Figure 12 for comparison). The MHS plots of these polymers further supported that they were free of branching, having MHS a values ranging from 0.63 to 0.67, typical for linear random coils in a thermodynamically good solvent, as shown in Figure 13.

Figure 11. 29Si NMR spectra of copolymers prepared from DiEtS “hydrolyzate” (A), D3Et2 (B), and polydiethylsiloxane, PDES, DiEt99.7MeVi, prepared from D3Et2 (C). Note that spectrum B did not contain signal at −23 ppm originating from the linear fraction of DiEtS “hydrolyzate” (compare with Figure 6B).

equilibration reactions with the forming terpolymer chains. This reduced effectiveness of incorporation of DiEtS units into the resulting polymer products also reduced their effectiveness in inhbiting terpolymer crystallization. SEC-MALS-Vis analysis (see example for polymers 3, 7, 11, and 17 with targeted DP values of 480 in Figure 12) showed

Figure 13. Mark−Houwink-Sakurada plots for DiEtS-containing polymers from D3Et2, D4Et2, and DiEtS “hydrolyzate” targeted for DP of 480 of Figure 12.

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CONCLUSIONS Four groups of polydimethylsiloxane terpolymers containing small amounts (5 mol % or below) of methylvinylsiloxy-, MeViS, and diphenylsiloxy-, DiPhS, or diethylsiloxy-, DiEtS, repeat units, were reproducibly synthesized via silanolateinitiated anionic ring-opening polymerization, ROP, using an α,ω-bis(tetramethylammonium)-oligo-dimethylsiloxydisilanolate, TMAS, as a “transient” initiator, and an α,ωtelechelic dimethylvinylsiloxy end-blocker, DiViEB, as the polymer chain terminator. Within each of the groups, polymers with degrees of polymerization ranging from 80 to 600 were prepared. As reported previously,7,23,24 substitution of only 3.6 mol % of DiMeS units by DiPhS led to complete suppression of polymer crystallization (by DSC), indicating that even a relatively large separation of DiPhS “crystallization disruptors” (estimated at several nanometers between the neighboring units) along these polymer chains was effective enough to prevent regular PDMS segmental packing into crystalline domains.26 29 Si NMR and SEC results showed presence of trifunctional Ph-T-branch junctures in the phenyl-substituted polymers, while the same feature was clearly absent in all DiEtScontaining analogues. This branching in Ph-containing polymers was identified by signals at −80 ppm in 29Si NMR. It is interesting to note that substitution of one Ph group from a DiPhS unit by a polydimethylsiloxy branch appears to stabilize the remaining Ph group on the same silicon atom, since no indication of the formation of quarternary Q-branches could be detected. We propose that this branching was caused by the increased electrophilicity of two Ph groups bearing silicons in DiPhS

Figure 12. SEC-MALS chromatograms illustrating refractive index detector responses of DiEtS-containing polymers from D3Et2, D4Et2, and DiEtS “hydrolyzate” targeted for DP of 480, overlaid with SEC chromatogram of DiPhS-containing analogue. Refractive index detector.

that all DiEtS-containing polymers (7, 11, and 17) had simple molecular weight distribution curves with polydispersity indices ranging between 1.5 and 2, typical for fully equilibrated linear polysiloxanes. SEC traces of all these polymers overlapped almost perfectly with one another and were free of shoulders at high molecular weight sides (low elution time), in clear contrast to their phenyl-substituted, branched analogues of Figure 4 J


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(6) Mark, J. E. Physical Properties of Polymers Handbook; Mark, J. E., Ed.; Springer: New York, 2007. (7) Dvornic, P. R. Thermal Properties of Polysiloxanes. In SiliconContaining Polymers: The Science and Technology of Their Synthesis and Applications; Jones, R. G., Ando, W., Chojnowski, J., Eds.; Springer Netherlands: Dordrecht, 2000; pp 185−212. (8) Beatty, C. L.; Karasz, F. E. Transitions in Poly(diethyl siloxane). J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 971−975. (9) Beatty, C. L.; Pochan, J. M.; Froix, M. F.; Hinman, D. D. Liquid Crystalline Type Order in Polydiethylsiloxane. Macromolecules 1975, 8 (4), 547−551. (10) Dvornic, P. R.; Lenz, R. W. High Temperature Siloxane Elastomers; Huthig & Wepf Verlag: Heidelberg, 1990. (11) Helmer, J. D.; Polmanteer, K. E. Supercooling of polydimethylsiloxane. J. Appl. Polym. Sci. 1969, 13, 2113−2118. (12) Clarson, S. J.; Dodgson, K.; Semlyen, J. A. Studies of cyclic and linear poly(dimethylsiloxanes): Glass transition temperatures and crystallization behaviour. Polymer 1985, 26, 930−934. (13) Aranguren, M. I. Crystallization of polydimethylsiloxane: effect of silica filler and curing. Polymer 1998, 39 (20), 4897−4903. (14) Andrianov, K. A.; Slonimskii, G. L.; Zhdanov, A. A.; Levin, V. Y.; Godovskii, Y. K.; Moskalenko, V. A. Some Physical Properties Of Polyorganosiloxanes. I. Linear polyorganosiloxanes. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10, 1−22. (15) Andrianov, K. A.; Yakushkina, S.; Ye; Terenteva, N. N. The effects of substituents on the reactivity of organocyclosiloxanes in anionic polymerization. Polym. Sci. U.S.S.R. 1968, 10 (8), 1992−1998. (16) Godovskii, Y. K.; Papkov, V. S. Thermotropic Mesophases in Element-Organic Polymers. Adv. Polym. Sci. 1989, 88, 129−180. (17) Molenberg, A.; Siffrin, S.; Möller, M.; Boileau, S.; Teyssié, D. Well defined columnar liquid crystalline polydiethylsiloxane. Macromol. Symp. 1996, 102 (1), 199−207. (18) Molenberg, A.; Möller, M. Structure and Phase Transitions of Poly(diethylsiloxane). Macromolecules 1997, 30, 8332−8337. (19) Kogler, G.; Hasenhindl, A.; Möller, M. Solid-state NMR Studies on the Molecular Structure and Dynamics of Poly(diethylsiloxane) Polymorphs. Macromolecules 1989, 22, 4190−4197. (20) Kogler, G.; Loufakis, K.; Möller, M. Mesomorphism, molecular structure and dynamics of polydiethylsiloxane. Polymer 1990, 31, 1538−1545. (21) Tsvankin, D. Y.; Papkov, V. S.; Zhukov, V. P.; Godovsky, Y. K.; Svistunov, V. S.; Zhdanov, A. A. Structure and phase transitions in poly(diethylsiloxane). J. Polym. Sci., Polym. Chem. Ed. 1985, 23 (4), 1043−1056. (22) Miller, K. J.; Grebowicz, J.; Wesson, J. P.; Wunderlich, B. Conformations of poly(diethylsiloxane) and its mesophase transitions. Macromolecules 1990, 23 (3), 849−856. (23) Polmanteer, K. E.; Hunter, M. J. Polymer Composition Versus Low-Temperature Characteristics of Polysiloxane Elastomers. J. Appl. Polym. Sci. 1959, 1 (1), 3−10. (24) Babu, G. N.; Christopher, S. S.; Newmark, R. A. Poly(dimethylsiloxane-co-diphenylsiloxanes): Synthesis, Characterization, and Sequence Analysis. Macromolecules 1987, 20, 2654−2659. (25) Andrianov, K. A.; Slonimskiĭ, G. L.; Zhdanov, A. A.; Levin, V. Y.; Godovskiĭ, Y. K.; Moskalenko, V. A. Some physical properties of polyorganosiloxanes. II. Nonlinear and irregular polyelementorganosiloxanes. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10 (1), 23−43. (26) Chistov, S. F.; Levina, O. I.; Lebedeva, N. Y.; Skorokhodov, I. I. Features of the crystallization of polymethylsiloxanes and the related copolymers. Polym. Sci. U. S. S. R. 1984, 26 (12), 2911−2917. (27) Chojnowski, J.; Cypryk, M. Synthesis of Linear Polysiloxanes. In Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications; Jones, R. G., Ando, W., Chojnowski, J., Eds.; Springer Netherlands: Dordrecht, 2000; pp 3−41. (28) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley Sons Inc.: 1999; p 704. (29) Gilbert, A. R.; Kantor, S. W. Transient catalysts for the polymerization of organosiloxanes. J. Polym. Sci. 1959, 40 (136), 35− 58.

repeat units and consequent increase in sensitivity of their Si− Ph side bonds to desilylation by the nucleophilic attack of silanolate anions used for initiation of these ring-opening polymerizations reactions. Ethyl side groups in DiEtS repeat units of the prepared polymers were apparently large and bulky enough to disrupt PDMS crystallization just as effectively as DiPhS units did. Their minimum required molecular concentration to do so was found to be only slightly higher, i.e., about 5 mol % (relative to 3.6 mol % for DiPhS units), while on the weight basis it was actually lower (6.7 wt %) than the corresponding DiPhS concentration in phenyl-substituted analogues (9.0 wt %). Both DiEt cyclic trimer and tetramer were equally effective for incorporation in the polymer products by this silanolate initiated ROP route. In contrast to this, an attempt to utilize DiEtS “hydrolyzate” for this purpose led to products that apparently contained extended PDES blocks rather than randomly placed DiEtS units. Most importantly, we found that incorporation of DiEtS repeat units into PDMS chains (even at as low as 5 mol % content level) by this silanolate-initiated ROP, provided welldefined products which showed no crystallization and no chain branching. This combination of properties makes, in our opinion, these polymers the polymers of choice for application as precursors for cross-linked elastomers for extremely low temperature applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (P.R.D.). ORCID

Alisa Zlatanic: 0000-0002-4959-6189 Petar R. Dvornic: 0000-0003-4184-7323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support of this work from The Department of Energy’s Kansas City National Security Campus, which is operated and managed by Honeywell Federal Manufacturing & Technologies, LLC, under Contract DE-NA0002839. The use of facilities of Kansas Polymer Research Center, Department of Chemistry, and the Polymer Chemistry Initiative of the Pittsburg State University are also gratefully acknowledged. Last but not least, we particularly acknowledge Dr. Barry Arkles of Gelest Inc. for providing us with very high quality samples of hexaethylcyclotrisiloxane and octaethylcylotetrasiloxane used in this work.

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REFERENCES

(1) Noll, W. Chemistry and Technology of Silicones; Academic Press: New York, 1968. (2) Mark, J. E. Silicon-Containing Polymers. In Silicon-Based Polymer Science; American Chemical Society: 1989; Vol. 224, pp 47−68. (3) Mark, J. E. Overview of Siloxane Polymers. In Silicones and Silicone-Modified Materials; Clarson, S. J., Ed.; Oxford University Press: New York, 2000. (4) Warrick, E. L.; Pierce, O. R.; Polmanteer, K. E.; Saam, J. C. Silicone Elastomer Developments 1967−1977. Rubber Chem. Technol. 1979, 52 (3), 437−525. (5) Yilgor, E.; Yilgor, I. Silicone containing copolymers: Synthesis, properties and applications. Prog. Polym. Sci. 2014, 39, 1165−1195. K


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Macromolecules (30) Andrianov, K. A.; Nogaideli, A. I.; Khubulava, E. I.; Kopylov, V. M.; Khananashvili, L. M.; Kolchina, A. G.; Sycheva, N. G. Tetraorganoammonium Polydimethylsiloxanolate. Polym. Sci. U.S.S.R. 1976, 18 (12), 2987−2995. (31) Elsbernd, C. S.; Spinu, M.; Krukonis, V. J.; Gallagher, P. M.; Mohanty, D. K.; McGrath, J. E. Synthesis and Fractionation Studies of Functionalized Organosiloxanes. Adv. Chem. Ser. 1989, 224, 145−164. (32) Andrianov, K. A. Rearrangements and polymerization of cyclic organosilicon compounds. Polym. Sci. U. S. S. R. 1971, 13 (2), 284− 298. (33) Dvornic, P. R. Degradative side reactions in the syntheses of exactly alternating silarylene-siloxane polymers. Polym. Bull. 1992, 28, 339−334. (34) For notation of cyclic monomers and polymer branch junctures, the so-called “GE nomenclature” for polysiloxanes was addopted. In this nomenclature, letter D denotes a linear, diorgano-substituted, difunctional unit, −[SiR2O]−, T denotes a branched, organosubstituted, trifunctional unit, R−SiO1.5, and Q denotes a tetrafunctional =SiO2= unit, the structures of which are shown in Scheme 2. Subscripts and superscripts in D denote the numbers or repeat units in the cyclic compound and the type of organic substituent, respectively. Ph stands for phenyl, Et for ethyl, and Vi for vinyl. Only methyl groups in dimethylsiloxy cyclics are not indicated by convention, so D4 is used instead of perhaps more systematic D4Me2. (35) Kintzinger, J.-P.; Marsmann, H. Oxygen-17 and Silicon-29; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer Berlin Heidelberg: 1981; Vol. 17. (36) Podzimek, S. Light Scattering, Size Exclusion Chromatography and Asymmetric Flow Field Flow Fractionation; John Wiley & Sons, Inc.: Hoboken, NJ, 2011; p 372. (37) Burchard, W. Solution Properties of Branched Macromolecules. In Branched Polymers II; Roovers, J., Ed.; Springer: Berlin, 1999; pp 113−194. (38) Williams, T. C. Union Carbide Corporation. US Patent 3,234,174, 1966. (39) Liu, L.; Yang, S.; Zhang, Z.; Wang, Q.; Xie, Z. Synthesis and Characterization of Poly(diethylsiloxane) and Its Copolymers with Different Diorganosiloxane Units. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2722−2730. (40) The term “hydrolyzate” traditionally refers to an equilibrium mixture of cyclic siloxanes (D3, D4, D5, ...) and low molecular weight linear oligomers which is formed during the hydrolysis of dichlorosilanes, in excess water.

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Suppression of Crystallization in Polydimethylsiloxanes and Chain

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