The Mechanism of Stereoregulation in Free-Radical Polymerization of

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The Mechanism of Stereoregulation in Free-Radical Polymerization of Bulky Methacrylates Isa Degirmenci, Benjamin B. Noble, Ching Yeh Lin, and Michelle L. Coote* ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia *E-mail: [email protected]

Theoretical calculations are performed to explore the origin of inherent tacticity in bulky methacrylates. Geometries and conformer distributions of monomers and oligomeric propagating radicals are calculated to study the impact of steric bulk and π-stacking interactions on the preferences for meso versus isotactic propagation. Consistent with the previous qualitative analyses by Satoh and Kamigaito, we have demonstrated a correlation between the preference for meso propagation and the steric bulk of the ester side chain, where the latter is measured as the volume of the side chain. We have also confirmed that syndiotactic methacrylates prefer linear chains, isotactic methacrylates prefer helical chains and the increasing isotactic preference with chain length can thus be understood in terms of the increasing helical tendency as substituents become more bulky. We also demonstrated that, whilst π-stacking interactions in aryl methacrylates are significant, the extent to which they influence the tacticity depends on their bulkiness and associated helical tendency. We have also provided an explanation for their solvent dependence in terms of the disruption of π-stacking conformations by the formation of inclusion complexes.

© 2012 American Chemical Society In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Introduction While controlled radical polymerization regulates most aspects of the resulting polymer microstructure, a remaining and significant inadequacy of radical based techniques is the lack of stereochemical control in common radical polymerizations. Controlling the tacticity (stereochemistry) of a polymer is highly desirable because it influences its physical properties such as the melting point, solubility, density, crystallinity and mechanical strength (1, 2). For instance, the melting points of isotactic, syndiotactic and atactic polypropene are 165 °C, 130 °C and 0 °C, respectively (3). Given the industrial significance of radical polymerization, much current research is aimed at finding inexpensive stereocontrol agents that are usable with ordinary monomers under practical reaction conditions (4). Two notable approaches to stereocontrol are polar solvent mediated radical polymerisation yielding syndiotactic polymers and Lewis acid mediated radical polymerisation yielding isotactic polymers (see Scheme 1) (5). While both approaches have had a lot of success in influencing polymer tacticity, neither can replicate the high stereoregularity of polymers produced by ionic or coordination methods. Additionally these approaches can be expensive to implement, are only applicable to monomers with polar substituents such as carbonyl groups, and are often incompatible with successful controlled radical polymerization processes.

Scheme 1. Current stereocontrol strategies in radical polymerisation: Lewis acid mediated and polar solvent mediated. (Adapted from Ref. (4)). As a first step toward designing better stereocontrol strategies we have conducted a theoretical investigation into the mechanism of stereoregulation in free-radical polymerization in the absence of added control agents. For most monomers, the stereochemistry of the penultimate unit only weakly influences the terminal radical, primarily because of the planarity of the propagating radical and the relatively early position of the transition state. However, a number of exceptions have been documented in the experimental literature, 16 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


typically involving monomers with side chains that are very bulky (6), chiral (7), highly aromatic (8–10) or complexed with metal ions (11). In such cases highly stereoregular polymers have been successfully prepared by radical polymerization. In this article we draw on our theoretical studies, as well as experimental evidence from the literature, to identify the principal factors affecting the stereochemistry in the polymerization of bulky methacrylates. Understanding the origin of stereoregulation in these special cases, and in particular the role of non-covalent interactions between substituents, can help us to design better control strategies for more conventional monomers.

Theoretical Background The stereoregularity (or tacticity) of a polymer is determined by the relative orientation of substituents with respect to the C-C macromolecular backbone (see Scheme 2). Tacticity can be quantified by the relative fraction of racemo (r) and meso (m) diads or more precisely, by the fraction of syndiotactic (rr), isotactic (mm) and heterotactic triads (rm, mr). On the basis of diad structures, a syndiotactic polymer has r → 1, an isotactic polymer has m → 1 and a ‘purely’ atactic polymer has r = m = 0.5. On the basis of triad structures, a syndiotactic polymer has rr → 1, an isotactic polymer has mm → 1 and a ‘purely’ atactic polymer has rm = mr = 0.5. Polymers produced by radical polymerisation typically have a tacticity in the range of r = 0.7 to m = 0.7 depending on the type of monomer and the conditions used for the polymerisation reaction. In practical terms these polymers are considered atactic because they are neither syndiotactic nor isotactic.

Scheme 2. Polymer tacticity nomenclature. 17 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Scheme 3. Racemo and meso propagation of a polymer radical. The concentrations of the various diads and triads are in turn determined by the relative orientation of the terminal substituent during the addition step (see Scheme 3). Thus the tacticity of a polymer merely reflects the kinetic selectivity of the propagating radical for different forms of monomer addition. Stereoregular polymers are formed when the stereochemistry at the penultimate unit of the polymer chain induces stereospecific monomer addition at the pro-chiral reactive center. It should be noted that the type of stereocenter formed in the propagation step will depend on both the conformation of the attacking the radical and the face from which the monomer attacks the (typically planar) radical center. Thus, in principle, a given conformation of the propagating radical can give rise to either type of stereocenter, according to the face from which the monomer attacks. The favoring of one propagating radical conformer over another is not sufficient to direct stereochemistry unless there is also an accompanying preference for the face of attack. However, theoretical studies of propagation reactions have found that attack usually occurs from the opposite face to the polymer chain (i.e. the first and fourth transition structures in Scheme 3) (12). For instance, Figure 1 shows our B3-LYP/6-31G(d) optimized racemo- and meso-like conformers of the methyl methacrylate propagating radical and their preferred transition structures. Each conformer in this case is expected to be selective for each stereoisomer and the favoring of one conformer over another provides a plausible mechanism of stereocontrol. 18 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. B3-LYP/6-31G(d) Optimized Geometries of the racemo and meso conformers of a trimeric poly(MMA) propagating radical and their preferred transition structures in each case.

Inherent Tacticity of Bulky Monomers In their excellent review, Satoh and Kamigaito (4) highlighted an intriguing qualitative relationship between the steric bulk of a monomer and the tacticity of the resulting polymer. On the one hand, they noted methyl methacrylate (MMA) and other aliphatic methacrylic esters had a tendency to form syndiotactic rich polymers, presumably due to steric repulsion between the α-methyl groups and the ester groups. Somewhat counterintuitively, however, the syndiotactic preference decreases, rather than increases, as the ester side chain becomes more bulky, to the extent that some exceptionally bulky monomers form highly isotactic polymers. They suggested that these differing impacts of steric repulsions are due to changes in the chain structure as the size of the bulky groups increase. Whereas moderately bulky polymers might be expected to form linear growing chains for which syndiotactic structures would minimize steric repulsion, more bulky chains tend to form helical structures where it is the isotactic structures that are best able to orient the groups with minimal steric repulsion. Superimposed on these general trends, they also noted that some (but not all) bulky methacrylates with aromatic side chains can form highly syndiotactic polymers instead, a tendancy was affected by the solvent type. This indicates that other interactions, such as π-stacking can also contribute though the effect is not general. In the present work, we have used theoretical calculations of monomer and propagating radical structures to test these ideas and place the relationship between steric bulk on inherent tacticity on a more quantitative footing. To this end, we considered the free-radical polymerization of a number of bulky methacrylate monomers as shown in Scheme 4. Table 1 summarizes the experimental tacticities of their resulting polymers, as produced under relatively consistent 19 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


conditions (temperatures in the range 50-70°C; toluene or benzene solution). All experimental data are drawn from the review by Satoh and Kamigaito (4), except for the silicon containing methacrylates (M17-23), which are drawn from their more recent work (13). These data are reported in the literature as measured percentages of the isotactic (mm), heterotactic (mr) or syndiotactic (rr) triads. To simplify the analysis, we have converted these into a kinetic preference for meso versus racemo propagation using a simple Bernoullian model of propagation in which the reactivity is assumed to be dependent only on the stereochemistry of the penultimate unit. Under this model, the probability of meso propagation is given by %m = mm + mr/2, the probability of racemo propagation is given by %r = rr + mr/2, and their sum adds to 100%. To test this model, in Figure 2, we plot the experimentally observed triad fractions, mm, mr and rr against %m, and compare the data with the theoretical lines for Bernoullian statistics. The plot shows that the Bernoullian model provides an excellent description of the data, with M14 the only significant outlier. Interestingly, this species was studied at a different monomer concentration compared with the others, implying that solvent effects might have been responsible for its non-Bernoullian behavior. For the rest of the test set, these results imply that quantum-chemical studies need only take into account the stereochemistry of the penultimate unit of the polymer chain into account when predicting the stereochemistry; further ab initio testing of this result is currently underway.

Figure 2. Isotactic (mm), heterotactic (mr), and syndiotactic (rr) triad fractions as a function of %m, the probability of meso propoagation, in the radical polymerisation of various methacrylates (in toluene or benzene solution, at 60±10 °C). %m was calculated according to the following equation, %m = mm + mr/2, by using the observed mm and mr values. The solid lines indicate the fitting of the data and the dashed lines indicate the theoretical lines for Bernoullian statistics [mm = %m2, mr = 2%m(1 – %m), rr = (1 – %m)2]. 20 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Table 1. Experimental triad fractions and probabilities of meso propagation (%m) of bulky methacrylates, and corresponding volumes and solids of revolution of their pendant groups Monomer

% (mm)

% (mr)

% (rr)

%m

Volume (Å3)

Solid of revolution (Å3)

M1

3

34

63

20.0

33

37

M2

3

37

60

21.5

88

119

M3

c

14

53

33

40.5

143

313

M4

d

10

35

55

27.5

112

201

M5

c

10

43

47

31.5

130

321

M6

11

35

54

28.5

184

394

M7

13

47

40

36.5

185

436

M8

9

41

50

29.5

98

162

M9

7

37

56

25.5

98

507

M10

12

45

43

34.5

117

284

M11

20

51

29

45.5

135

283

M12

17

53

30

43.5

169

540

M13

c

14

39

47

33.5

150

383

M14

e

64

24

12

76.0

283

563

M15

>99

The Mechanism of Stereoregulation in Free-Radical Polymerization of

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