Sequence-Controlled Radical Copolymerization for the Design of High

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Chapter 20

Sequence-Controlled Radical Copolymerization for the Design of High-Performanced Transparent Polymer Materials Akikazu Matsumoto* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan *E-mail: [email protected]

The precise control of polymer sequence structures for the design of high-performance transparent polymer materials with excellent thermal, optical, and mechanical properties is described in this article. We investigated the 1:1 alternating and 2:1 sequence-controlled copolymerizations of N-substituted maleimides (RMIs) with various olefin and styrene derivatives. The monomer reactivity ratios were determined for the copolymerizations of the RMIs with various electrondonating olefins and styrenes based on the analysis of the comonomer-copolymer composition curves using terminal and penultimate unit models. The penultimate unit and solvent effects on the precise chain structure control during the sequence-controlled radical copolymerization were discussed. The thermal, optical, and mechanical properties of the alternating and 2:1 sequence-controlled copolymers of the RMIs with the olefins and styrenes were also investigated. We demonstrated the rational design of the thermally stable and transparent maleimide copolymers with tunable glass transition temperatures varying over the wide temperature range.

Introduction Sequence-controlled polymerization is one of the most challenging topics for polymer synthesis during recent years because naturally-occurring polymers © 2014 American Chemical Society In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


with complicated and well-defined sequences exhibit smart performances (1, 2). Many attempts to fabricate artificial polymers with highly controlled sequence structures have been made using solid-state synthesis (3), genetic engineering (4), template polymerization (5, 6), topochemical polymerization (7), regio-specific polymerization (8, 9), alternating copolymerization (10), preorganized oligomers (11), and post-polymerization approaches (12). The N-substituted maleimides (RMIs) polymerize in the presence of a radical initiator to give a polymer with excellent thermal stability (13–15). A high glass transition temperature (Tg) originated in the rigid poly(substituted methylene) structure of the poly(RMI)s, which has no methylene spacer as a flexible joint in their main chain (16–18), and a high onset temperature of decomposition arose from a robust imide-ring structure included in the repeating units of the polymers (15, 19, 20). The RMIs also copolymerize with electron-donating monomers, such as styrene, vinyl ethers, and olefins, to give alternating copolymers with excellent thermal stability and high Tg values (21, 22). Especially, the copolymerization of the RMIs with isobutene provided an alternating copolymer in a high yield with excellent thermal stability, high transparency, and high modulus and strength (23–25). The introduction of polar groups and cyclic structures into the olefin repeating units led to further increases in the thermally stability and the Tg values of the copolymers (26, 27). Recently, it was demonstrated that the RMIs were useful for the sequence regulated radical copolymerization with various olefin and styrene derivatives in 1:1 alternating and 2:1 sequence-controlled fashions (Scheme 1) (28–30). The mechanism of the sequence-controlled radical copolymerization has attracted significant attention in research fields of polymer synthesis and radical polymerization (31–38). In this chapter, the synthesis of the RMI copolymers with controlled sequence structures and their thermal, optical, and mechanical properties are reviewed.

Scheme 1. Radical copolymerization of N-substituted maleimides (RMIs) with various olefin and styrene derivatives.

1:1 Alternating and 2:1 Sequence-Controlled Radical Copolymerization The radical copolymerization of the RMIs with 1-methylenebenzocycloalkanes (BCms, m = 4–7) was carried out in the presence of a radical initiator 302 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


(Scheme 1) (28). The BCm monomers are α-substituted styrene derivatives with a reactive exomethylene group (39, 40). During the course of studies on the copolymerization of the RMIs with the BCms, we found that the sequence-controlled radical copolymerization occurred and the copolymers consisting of the AB- and AAB-repeating units (1:1 alternating and 2:1 sequence controls, respectively) were produced depending on the m number of the BCms (28). As shown in Table 1, the yield and the Mn value of the obtained copolymers with N-methylmaleimide (MMI) varied in the ranges of 0.5‒90% and 1‒25 × 103, respectively, and the both values drastically decreased with an increase in the ring size of the BCms. Similar results were observed for the homopolymeriza-tion. The obtained copolymers exhibited excellent thermal properties. The onset and maximum decomposition temperatures (Td5 and Tmax) were over 345 and 365 °C, respectively. The Tg value was 143 °C for the copolymer with BC4 and over 200 °C for the copolymers with BC5 and BC6 in spite of their decreased molecular weights. The effect of the α-substituents on the copolymerization reactivity was also investigated for the alternating copolymerization of the RMIs with noncyclic α-substituted styrene derivatives (29). Similar suppressed copolymerization process was observed by the introduction of bulky α-substituents.

Table 1. Synthesis and Thermal Properties of Poly(MMI-co-BCm)s and Poly(BCm)sa BCm

Comonomer

Yield (%)

Mn/103

Mw/Mn

BCm mol%

Td5 (°C)

Tmax (°C)

Tg (°C)

BC4

MMI

89.9

24.9

2.7

48.2

346

368

143

BC5

MMI

36.6

10.3

1.6

51.3

345

385

212

47.3

–

–

200

25.6

–

–

–

BC6

MMI

1.4

2.6

1.3

BC7

MMI

0.5

1.0

1.7

BC4

None

10.3

4.6

1.4

305

397

133

BC5

None

4.0

9.5

2.1

228

339

119

BC6

None

0

BC7

None

0

a

Copolymerization conditions: [MMI] = [BCm] = 0.2 mol/L, [A IBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C for 20 h. Homopolymerization conditions: [BCm] = 1.0 mol/L, [A IBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C for 5 h.

The copolymerization of the RMIs with olefins was carried out over various compositions in the feed, and the monomer reactivity ratios were determined based on the analysis of the obtained comonomer-copolymer composition curves (Figure 1) (30). For the copolymerizations of N-n-butyl-maleimide (BMI) with BC5 and BC6, the monomer reactivity ratios, r1 and r2, were successfully determined by the terminal unit model. Both the determined r1 and r2 values were close to zero; r1 = 0.046 and r2 = 0.0043 for BC5 and r1 = 0.019 and r2 = 0.047 for BC6, as 303 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


summarized in Table 2. On the other hand, the monomer reactivity ratios were determined using the penultimate unit model for the BMI–BC7 copolymerization, while appropriate values were not obtained using the terminal model (28); r11 = r21 = 0, r12 = 2.92, and r22 = 0.252. The radical copolymerization of the RMIs was also carried out using various olefins (O1–O4) and the isobutene oligomers (B2–B5 and PIB22) with a well-defined structure (Scheme 1). The bulkier olefins provided 2:1 sequence-controlled copolymers under the penultimate unit control (30).

Figure 1. Comonomer-copolymer composition curves for the radical copolymerization of (a) BCms and BMI in 1,2-dichloroethane, (b) B2 and EMI or EHMI in 1,2-dichloroethane, and (c) β-pinene and PhMI in various solvents.

The olefin and styrene monomers can be classified into three types according to their conjugated structure and the bulkiness of the substituents on the vinyl moiety (Figure 2) (30). The 1:1 alternating copolymers are produced during the copolymerization with the A-type monomers as planar and conjugated styrene monomers, as well as the copolymerization with less-hindered and non-conjugated olefins such as isobutene (type B). The olefin O1 served as the type B. On the other hand, hindered and non-conjugated olefins with the saturated or unsaturated substituents are classified as the C-type and exhibited the 2:1 sequence-controlled copolymerizations by the penultimate unit effect. The sequence-controlled copolymerization of the RMIs with the BCms occurred in the 1:1 alternating and 2:1 sequence-controlled fashions according to the BCm monomer reactivity, which significantly depended on the coplanarity of the exomethylene moiety and the benzene ring (28). For the olefins with a non-cyclic structure, the penultimate unit effects were observed during the copolymerization with O3 and O4 (30). The inverse of the r12 (1/r12) values, which represents the reactivity of the RMI radical to the olefin monomers, were 2.0 and 0.25 for the O3 and O4, respectively. This suggested that the O4 structure played a greater role as the penultimate unit than that of O3. The bulkier and stiff substituents tend to induce more significant penultimate unit effects, resulting in the greater r12 values, as shown in the results for the copolymerization of the olefins (Table 2).

304 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Table 2. Monomer Reactivity Ratios for Radical Copolymerization of Various Olefins (M1) and the RMIs (M2) at 60 °C under Terminal and Penultimate Unit Controla Olefin

RMIb

Solventc

r1

r2

r11

r12

r21

r22

BC5

BMI

DCE

0.046

0.0043

BC6

BMI

DCE

0.019

0.047

BC7

BMI

DCE

0

2.92

0

0.252

O1

BMI

DCE

O3

PhMI

DCE

0

0.49

0

0.49

O4

PhMI

DCE

0

3.94

0

0.17

B2

PhMI

DCE

B2

EMI

DCE

0

1.33

0

0.26

B2

EHMI

DCE

0

1.95

0

0.15

CH1d

PhMI

PhC(CF3)2OH

0

1.9

0

0.21

0.013

0.0076

0.17

0.16

CH2d

PhMI

PhC(CF3)2OH

0

56

0

0.47

CH3d

PhMI

PhC(CF3)2OH

0

2.8

0

0.030

CH4d

PhMI

PhC(CF3)2OH

0

30

0

0.25

β-Pinene

PhMI

THF

0

15

0

0.53

β-Pinene

PhMI

DCE

0

10

0

0.24

β-Pinene

PhMI

TFE

0

2.5

0

0.032

β-Pinene

PhMI

PFTB

0

3.2

0

0.0058

a

r1 = k11/k12 and r2 = k22/k21 for the terminal control. r11 = k111/k112, r12 = k122/k121, r21 = k211/k212, and r22 = k222/k221 for the penultimate unit control. b BMI: N-n-butylmaleimide, PhMI: N-phenylmaleimide, EMI: N-ethylmaleimide, and EHMI: N-(2-ethylhexyl)maleimide. c DCE: 1,2-dichloroethane, THF: tetrahydrofuran, TFE: 2,2,2-trifluoroethanol, PFTB: perfluoro-tert-butanol. d Ref. (35). See Figure 2 for the structures of CH1 to CH4.



Figure 2. Classification of the olefins and styrenes used for 1:1 alternating and 2:1 sequence-controlled copolymerizations of the RMIs.

In order to more discuss the copolymerization behavior of the BCms, we estimated the activation energies (Eact) and energy differences between the reactant and the product (ΔE) by the DFT calculations using model reactions for the homo- and cross-propagations of MMI and the BCms (Scheme 2) (28). The results shown in Table 3 indicated that the cross-propagation of the MMI radical predominantly occurred rather than the homo-propagation of MMI for the cases of BCms (m = 4–6). In contrast, the MMI homo-propagation was preferred to the cross-propagation for the BC7 case. The calculated Eact and ΔE values were closely related to each other; i.e., the lower Eact values, the negatively larger the ΔE values. The formation of the copolymers with the 2:1 sequence was based on the reduced reactivity of the BC7 radical during the cross-propagation. We also investigated the molecular conformations in the transition state during the cross-propagations and found that no steric interaction was observed during the reaction of the RMI radical to the BCms. This was consistent with the low Eact-21 values and the fast addition of the RMI radical to the BCms. During the reactions of the BCm radicals to the RMI monomer, the distances between the closest hydrogen atoms on the maleimide carbon-to-carbon double bond and the cyclic methylene moieties of the BCm radicals decreased according to the increased ring size. The enhanced steric repulsion during the transition state led to an increase in the Eact values with an increase in the m number.



Scheme 2. Model reactions for homo- and cross-propagations observed during radical copolymerization of the RMIs with the BCms.

Table 3. Activation Energies (Eact) and Energy Differences in Reactant and Product (ΔE) for the Model Reactions of Propagations Observed during Copolymerization of MMI with BCmsa BCm

Cross-propagations (kJ/mol)

Homo-propagation (kJ/mol)

Eact-12

ΔE12

Eact-21

ΔE21

ΔE11

BC4

18.3

–61.3

9.2

–85.4

–60.9

BC5

29.4

–27.7

8.2

–97.9

–35.5

BC6

38.8

–14.4

7.1

–97.3

24.4

BC7

74.5

–2.5

33.4

–71.6

19.7

a

Calculated by the DFT method at the B3LYP/6-31G*//B3LYP/6-31G* and B3LYP/6311+G**//B3LYP/ 6-31G* levels of theory for the Eact and ΔE values, respectively, using the model reactions in Scheme 2. For the MMI homo-propagation, the Eact-22 and ΔE22 values were calculated to be 20.9 and –79.1 kcal/mol, respectively.



Penultimate Unit Effect on Sequence-Controlled Copolymerization The penultimate unit effects observed during the propagation include the electronic and steric interactions between the chain end unit of the propagating radical and the reacting monomer. The effects of the bulkiness and rigidity of the substituents of comonomers and the RMIs are significant, as shown in the copolymerization parameters in Table 2. For example, the greater penultimate effect was observed for the following cases; the BCMs with a larger ring size, O4 with a bulkier substituent than O3, and the combination of B2 with the RMIs containing large N-alkyl substituents. Similar effect was also observed for the monomer reactivity ratios of the olefins with saturated cyclohexyl and the unsaturated cyclohexenyl groups reported in literature (35). The cyclohexenyl substituent is expected to exhibit greater steric repulsion due to the fixed conformation. The propagation rate of a maleimide radical to an olefin (i.e., the k121 and k221 values) is reduced (Figure 3), while no greater effect by the steric bulkiness is expected for other propagations. As a result, the reduced k121 and k221 values led to an increase in the r12 and r22 value for the copolymerizations with the olefins including the cyclohexenyl moiety (See the results for CH2 and CH4 in Table 2).

Figure 3. Elemental reactions for the copolymerization of the RMIs with olefins and steric repulsion between the bulky side groups of the olefin unit as the penultimate unit and the reacting monomer. M and O represent the RMI and olefin units, respectively. The magnitude of the penultimate unit effect varied depending on the copolymerization solvent and temperature [Figure 1(c)], because the solvent-monomer and solvent polymer interactions during the copolymerization were determined by the Lewis acidity of the used solvent, i.e., the magnitude as the electron-pair acceptor toward the carbonyl moiety of the maleimide groups (38). The Lewis acid effect is closely related to a change in the monomer reactivity by both the polar and steric interactions. In general, the propagating rate constants of polar monomers, such as acrylates, methacrylates, and the RMIs, tend to increase by the introduction of an electron-withdrawing group in the side group and the use of a solvent with great Lewis acidity. Especially, the cross-propagation of the RMI radical to an olefin was accelerated by the use of the highly electron-pair accepting solvents, resulting in the decrease in the r12 and r22 values, as shown in 308 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

Table 2. Thus, the alternating tendency increased according to the increase in the Lewis acidity of the used solvent (38).


Optical and Mechanical Properties of the Copolymers The RMI copolymers synthesized in this study were soluble in many organic solvents and transparent thin films were obtained by casting the solution and drying. The film strength, i.e., brittleness, was significantly dependent on the N-alkyl groups and the olefin substituents (28–30). For example, the flexibility of the poly(RMI-alt-BCm)s films increased in the order of MMI < EMI < BMI < EHMI, due to the increased free volume. In the UV-Vis spectra of the copolymer films (60–70 μm thickness), no absorption was observed and the transparency was greater than 90% in the visible light region. The thermal and optical properties of the RMI copolymers are summarized in Table 4. The all copolymers exhibited excellent thermal stability; Td5 > 330 °C and Tmax > 370 °C. The Tg values were tunable depending on the structures of the olefins and the N-substituents, being valuable over the wide temperature range of –68 to 203 °C. The refractive index values and the Abbe numbers (νD) were 1.54–1.56 and 38–43, respectively, for the poly(RMI-co-BC5)s, and 1.50–1.51 and 46–52, respectively, for the poly(RMI-alt-olefin)s. These values were comparable to those for a commodity transparent polymer, e.g., nD = 1.49–1.51 and νD = 42–53 for poly(methyl methacrylate) [poly(MMA)] and other polymethacrylates (41). The larger the N-substituents of the RMIs and the substituents of the olefins, the lower the nD values of the copolymers. Very recently, the optical property of methacrylate polymers including an MMI repeating unit was investigated (42). The positive orientational and photoelastic birefringence of poly(MMI), which is different from those for poly(MMA) and other polymethacrylates previously reported in the literature, is useful for the design of zero-zero-birefringence polymers for optical devices (43). The viscoelastic properties of poly(RMI-co-B2)s were also investigated at the frequencies of 0.5–10 Hz in the range of –150 °C to a temperature over each Tg (25, 28–30). The storage modulus (E’) values were determined to be 920, 500, 540, and 250 MPa at 30 °C for poly(BMI-co-B2), poly(EHMI-co-B2), poly(BMI), and poly(EHMI), respectively. The flexural moduli of both copolymers were higher than those of the corresponding homopolymers, and the poly(BMI-coB2) showed the highest E’ value. The loss modulus (E”) values of the copolymers showed a peak at 155 and 103 °C for poly(BMI-co-B2) and poly(EHMI-co-B2), respectively. Similarly, the peak temperatures of tan δ were also determined as 180 and 143 °C, respectively. Based on the plots of the tan δ values as a function of the temperature determined by the flexural experiments at various frequencies, the apparent activation energies (Eα) were revealed to increase as the enlarged chain rigidity in the order of poly(EHMI-co-B2) < poly(EHMI) < poly(BMI-co-B2) and poly(BMI). This order agreed with the orders observed for the Tg values and viscoelastic data, such as E” and tan δ values. All the polymers exhibited broad and weak β-dispersions due to the side-chain dynamics over the temperature range of -100 to 0 °C, indicating that these high-Tg polymers have frozen main chain 309 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


surrounded by the dynamically moving side groups at room temperature. The coexistence of a rigid main-chain and the flexible side-groups is important for the design of the maleimide polymer materials with a high-Tg value and high toughness. On the other hand, the Tg values were –65 and –68 °C for poly(EMIco-PIB22) and poly(EHMI-co-PIB22), respectively. These low-Tg copolymers including the PIB22 repeating units exhibited a characteristic fluidity. The flowing rate was significantly reduced by the introduction of the RMI repeating unit in the main chain, although both Tg values of the polyisobutene macromonomers and the poly(ERMI-co-PIB22)s as the grafted copolymers were much lower than room temperature. The poly(RMI-co-PIB22)s were frozen without any fluidity at room temperature, due to the highly branched structure and the intermolecular interaction of the polar RMI repeating units of the copolymers.

Table 4. Thermal and Optical Properties of Poly(RMI-co-BCm)s and Poly(RMI-co-olefin)sa Mw/104

RMI mol%

Td5 (°C)

Tmax (°C)

Tg (°C)

nD

νD

Poly(EMI-alt-BC5)

8.5

49.4

344

381

203

1.559

38

Poly(BMI-alt-BC5)

16.6

49.7

341

381

158

1.554

40

Poly(EHMI-alt-BC5)

21.7

50.7

330

377

109

1.543

43

Poly(BMI-co-O1)

8.0

60.3

371

425

105

1.514

48

Poly(BMI-co-O2)

14.4

67.9

357

420

108

1.510

48

Poly(BMI-co-O4)

20.2

71.6

345

420

123

1.511

46

Poly(BMI-co-B2)

18.9

64.4

377

422

148

1.503

48

Poly(EHMI-co-B2)

20.6

64.1

377

428

95

1.498

52

Poly(EHMI-co-B3)

6.1

68.8

361

427

62

–

–

Poly(EHMI-co-B4)

4.4

62.8

368

430

38

–

–

Poly(EHMI-co-B5)

4.1

64.5

376

426

–13

–

–

Poly(EHMI-co-PIB22)

33.3

61.3

312

404

–68

–

–

Poly(MMI-alt-isobutene)b

22.0

50.0

396

157

1.53

51

Poly(MMA)b

303

100

1.49

53

Polycarbonateb

454

140

1.58

29

Polymera

a EMI: N-ethylmaleimide, BMI: N-n-butylmaleimide, EHMI: N-(2-ethylhexyl)male-imide, PhMI: N-phenylmaleimide, MMI: N-methylmaleimide, and MMA: methyl methacrylate. See Figure 2 for the structures of the olefins. b Ref. (24).

Conclusions The copolymers of the RMIs with the BCms and various olefins were synthesized by the radical copolymerization process. The yield and molecular 310 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


weight of the resulting copolymers decreased with an increase in the ring size of the BCm and the steric hindrance of the olefin substituents. We determined the monomer reactivity ratios for the copolymerization of the RMIs with the BCms and various olefins based on the propagation mechanism under the terminal and penultimate unit controls. The mechanism for highly 1:1 alternating and 2:1 sequence-controlled copolymer production depending on the steric bulkiness of the substituent of the comonomers was discussed. We also investigated the poly-mer properties, such as the thermal stability and transparency. We demonstrated the rational design of the thermally stable and transparent maleimide copoly-mers with tunable Tg values varying over the wide tempera-ture range. We concluded that the high-molecular-weight and high-Tg copoly-mers were produced during the copolymerization of MMI, EMI, and BMI with the BCms and various olefins containing an appropriate substituent and that the transparent and thermally stable films were readily obtained by casting the polymer solutions, while the copolymerization of EHMI with the isobutene oligomers and macromonomers produced low-Tg copolymers. The unique birefringence properties of the poly(RMI)s are expected to be powerful tool for the design of zero-zero-birefringence polymers for optical devices in the future. Thus, the sequence-controlled radical copolymerization of the RMIs with various olefins is valuable for the synthesis of high-performance transparent polymer materials. It is also noted that the radical copolymerization of the RMIs is useful not only as the method for high-performance polymer production, but also as the tool for the fundamental research of radical polymerizations, for example, mechanistic analysis of the penultimate unit effects on a radical polymerization process.

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