Polymerization of functionalizable triblock copolymers

For chemical LC gels it has been shown that the rubbery as well as the electro-optical properties depend on the crosslinking density.^[152,222] In ABA triblock copolymers the network density and network span is dependent on the polymer backbone. The critical gelator concentration, i.e. the minimum polymer concentration necessary to form a gel, is in turn dependent on network span. The length of the functionalized B-block largely determines the network span and a lager span should result in a lower critical gelator concentration.

Depending on the gelator concentration the block copolymers can adopt different conformations in the solvent as shown in Figure 5.11. For low concentrations the probability that a ABA triblock copolymer gelator will form loops is high. In this configuration both A-block self-assemble in the same polystyrene sphere. If more than one polymer chain loops back to one sphere the configuration can be called flower-like.

With increasing concentration the probability that a gelator chain forms a bridge configuration increases. In this case the functionalized backbone is connected to two different polystyrene spheres. With increasing concentrations this configuration leads to the formation of a gel network.^[223]

Kornfield et al. assumed that the tendency to form loops is dependent on the rigidity of the geator backbone.^[118,119] Flexible polymer backbones tend to form more loops with both A-blocks self-assembled in the same node. Thus, these loops do not contribute to the network formation. More rigid backbones are expected to form less loops and, hence, decrease the critical gelator concentration. Therefore, the polymer backbone of the B-block is an important factor for affecting the rheological and electro-optical properties of a LC gel.

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Figure 5.11: Configuration of the functionalized ABA triblock copolymer gelator in 5CB depending on the gelator concentration or rigidity of the polymer backbone. Left: ring or flower configuration at low gelator concentrations or with soft polymer backbone of the B-block;

right: partially bridge conformation at higher gelator concentrations or with gelators with a more rigid backbone.

In this work, poly(4-hydroxystyrene) was selected as the functionalizable backbone for the middle block. As already described in chapter 4 4-tert-butoxystyrene (tBS) was used as the protected monomer for the anionic polymerization. An advantage is the phenolic functionality that allows subsequent polymer analogous etherification that is used to attach the mesogenic moieties. Furthermore, it is known that poly(4-hydroxystyrene) result in relatively high glass transition temperature (Tg) even if mesogenic side-groups are attached.^[87,122]

To investigate the influence of the polymer backbone composition and length of the ABA block copolymers on the gel properties several functionalizable ABA block copolymers were synthesized. One aim was the preparation of a very long B- block with a high degree of polymerization (DPPtBS > 2000), that increase the network span and, therefore, lowers the critical concentration of the gelator in 5CB. For the synthesis of the polymer backbone two different routes have been employed that are presented in Figure 5.12.

For route A a bifunctional initiator, sodium naphthalene, was used for the anionic polymerization of tBS forming the B-block. Sodium naphthalene was one of the first initiators utilized in anionic polymerization and acts as an electron transfer agent.^[48,49]

The radical anion that is formed in the reaction of sodium with naphthalene transfers an electron onto the tert-butoxystyrene (tBS) monomer. Two of these styrenic anion radicals form a dianionic dimer yielding the effective bifunctional initiator. Prior to the addition of styrene, an aliquot of the PtBS (I) block was isolated and terminated. These PtBS homopolymers were required for characterization and were also used for the synthesis of the functionalized homopolymers IX-XII. Subsequently, styrene was added for the polymerization of the symmetric A-blocks.

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For the route B a monofunctional initiator for the sequential polymerization of (1) styrene, (2) tert-butoxystyrene and (3) styrene was employed (see Figure 5.12). For the preparation of ABA triblock copolymers containing a B-block with a high degree of polymerization the sequential polymerization a commercial monofunctional initiator (e.g.

s-BuLi, 1.3 M in pentane) was used. Here, practical issues have to be considered due to the restrictions imposed by the reactor size and equipment (see chapter 3). The degree of polymerization can be calculated using equation (5.1) for a given amount of monomer. If the first A-block, polystyrene, should feature a lower degree of polymerization (e.g.

DP_PS ≈ 500) and the following B-block, poly(tert-butoxystyrene), should be much longer (e.g. DP_PtBS > 2000) the required volume of initiator or the volume of the first monomer (styrene) must be very low because the overall amount of monomer is limited by the reactor size.

= ( )

( ) × ( ) (5.1)

Initiator volumes of less than 0.1 mL are difficult to handle and the error while measuring and transferring is significant. For instance the dead volume of the utilized syringe is in the range of 0.4 mL. This issue can be overcome by diluting the initiator with a dry, inert solvent that effectively increase the volume to be injected and, thus, reduces the risk of faulty initiator amounts. Another approach to circumvent this issue is to start the polymerization with considerable amount of styrene and initiator yielding the desired first block with the required low molecular weight but at higher concentration in the reactor.

Subsequent disposing a larger fraction of the living polystyrene solution and adding fresh dry solvent allows one to add an appropriate amount of tBS. That is required for the large middle block, without facing the issues associated with a non-manageable viscosity in the reactor.

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Figure 5.12: Synthetic pathways to the functionalizable ABA and ABA’ triblock copolymers 17a-17e using two different routes. Route A uses a bifunctional initiator resulting ABA’ triblock copolymers and route B uses a monofunctional initiator.( THF: tetrahydrofuran, s-BuLi:

sec-butyl lithium).

The use of the route B also allowed the preparation of ABA triblock copolymers, which in part features a star-shaped architecture (PS-PtBS-PS)_y 24 (y = 1-3). This was achieved by using a coupling agent after the sequential polymerization of the triblock copolymer 17d that will be discussed in chapter 5.6.

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Both approaches have their distinct advantages. In case of route A, the use of a bifunctional initiator ensures that the lengths of both A-blocks are identical and, therefore, one might assume that phase separation occurs more easily. In addition, this initiator also allows the relatively easy preparation of high molecular weight B-blocks that cannot be obtained in the same fashion as with a monofunctional initiator due to the practical issue discussed above. Disadvantageously is the fact that the initiator solution necessitates the absence of excessively used sodium. If this solution contained two different initiating species the resulting molecular weight distribution would be broadened.

In case of route B where a monofunctional initiator is used, the required amount of styrene for the third block has to be carefully calculated since the previously disposed amount has to be taken into account. Advantageously, this approach typically yielded narrower molecular weight distributions compared to the bifunctional initiator. The resulting triblock copolymers are inherently ABA’ triblock copolymers because it is unlikely that the length of the end blocks will be identical.

Characteristic data of all ABA and ABA’ triblock copolymers 17 prepared by both routes as well as their respective precursor blocks are given in Figure 5.12. The triblock copolymers are listed according to the used routes and ordered with increasing B-block lengths. Block copolymers prepared by route A are 17a-c whereas 17d and 17e were synthesized using route B.

As described in chapter 4.3.1, the molecular weights of all PS and PtBS precursors were determined on a SEC setup with THF as eluent with respect to a polystyrene calibration.

The molecular weights determined by SEC were in good agreement with the theoretical molecular weight calculated from the ratio of initiator and tert-butoxystyrene. Thus, the molecular weights as determined with respect to the polystyrene calibration were used for all further calculations. The average number of repeating units (ruPtBS) was calculated from the number average molecular weight (Mn) determined by SEC and were rounded to 10 units.

The ABA triblock copolymers 17a contains the shortest B-block in the ABA series with ruPtBA ≈ 930. The A-blocks is about half the length (ru_PS ≈ 490). The overall molecular weight of M_n = 306 kg/mol with a PDI of 1.10. ABA’ triblock copolymer 17d features an overall molecular weight of M_n = 124 kg/mol with a B-block length (ru_PtBA = 680) comparable to 17a. In this case the A-blocks are not equal in length (ru_PS = 190, 120).

The B-block of ABA triblock copolymer 17b (ru_PtBA = 1860) is about two times the length of the preceding polymers 17a. The A-block lengths (ruPS = 430) are about the same length while the molecular weight (Mn = 403 kg/mol). 17c features a long B-block (ruPtBA = 2670) and A-blocks (ruPS = 360) comparable in length to 17b. The molecular weight of this ABA triblock copolymer is determined to Mn = 528 kg mol. The member

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of series 17 with the highest molecular weight (Mn = 1000 kg/mol) and the longest B-block (ruPtBA = 3740) is the ABA’ triblock copolymer 17e. The molecular weight distributions in series 17 are in the range of 1.03 (17d) to 1.10 (17a). Generally the molecular weight distribution was broader for the block copolymers that were prepared using route A.

Table 5.2: Characteristic data of PS-P(tBS)-PS triblock copolymer series 17

block copolymer series 17 route^a)

Mnb)

Mwb)

PDI^c) AaBbAa

kg/mol ru_A^d) ru_B^d) ru_A^d)

17a A 278 306 1.10 490 930 490

precursor Ij 163 171 1.05 - 930 -

17b A 403 436 1.08 430 1860 430

precursor Ik 327 345 1.07 - 1860 -

17c A 528 558 1.06 360 2670 360

precursor Il 470 499 1.06 - 2670 -

route^a) M_n^b) M_w^b) PDI^c) A_aB_bA’_a’

ru_A^d) ru_B^d) ru_A’^d)

17d B 124 125 1.03 190 680 120

A-block precursor 16a 112 115 1.03 190 680 -

AB-block precursor VIIIa 20.1 20.8 1.04 190 - -

17e B 1000 1120 1.06 530 3740 550

A-block precursor 16b 912 953 1.05 530 3740 -

AB-block precursor VIIIb 55.4 57.0 1.03 530 - -

a) route used for anionic polymerization: A: bifunctional initiator, B: monofunctional initiator; b) determined by SEC (eluent: THF + electrolyte); molecular weight with respect to polystyrene standards, UV-detection; c) polydispersity index, M_w/M_n; d) average number of repeating unit, rounded to 10 units, calculated from M_n for first block in polymerization sequence, calculated from block ratio determined by

1H-NMR for other block(s)

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5.5 Polymer analogous reaction to cyanobiphenyl-containing triblock copolymers