Synthesis of Isotactic-block-Syndiotactic Poly(methyl Methacrylate) via Stereospecific Living Anionic Polymerizations in Combination with Metal-Halogen Exchange, Halogenation, and Click Reactions

Isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) form unique crystalline stereocomplexes, which are attractive from both fundamental and application viewpoints. This study is directed at the efficient synthesis of it- and st-stereoblock (it-b-st-) PMMAs via stereospecific living anionic polymerizations in combination with metal-halogen exchange, halogenation, and click reactions. The azide-capped it-PMMA was prepared by living anionic polymerization of MMA, which was initiated with t-BuMgBr in toluene at –78 °C, and was followed by termination using CCl4 as the halogenating agent in the presence of a strong Lewis base and subsequent azidation with NaN3. The alkyne-capped st-PMMA was obtained by living anionic polymerization of MMA, which was initiated via an in situ metal-halogen exchange reaction between 1,1-diphenylhexyl lithium and an α-bromoester bearing a pendent silyl-protected alkyne group. Finally, copper-catalyzed azide-alkyne cycloaddition (CuAAC) between these complimentary pairs of polymers resulted in a high yield of it-b-st-PMMAs, with controlled molecular weights and narrow molecular weight distributions. The stereocomplexation was evaluated in CH3CN and was affected by the block lengths and ratios.


Introduction
Recent advances in controlled polymerizations, including living and stereospecific polymerizations, as well as highly efficient reactions, such as click reactions, enable the synthesis of various architecturally controlled polymers [1]. In addition, an alternative and more efficient synthetic route for polymer structures can be created by these developments, which is the case in the synthesis of organic molecules, such as medicines and natural products due to the development of new reactions.

Synthesis of Azide-ω-Capped it-PMMA via Isotactic Living Anionic Polymerization of MMA Followed by Termination with Halogenation and Subsequent Azidation
The reactions were carried out by syringe techniques under dry argon in a 100 mL baked glass tube equipped with a three-way stopcock. The following is a typical example for it-50-PMMA. The polymerization was initiated by adding MMA (5.2 mL, 49 mmol) slowly via a dry syringe into the prechilled initiator solution (31 mL), containing t-BuMgBr (0.97 mmol, 3.5 mL, 276 mM in Et2O) and toluene (27 mL), at −78 °C. The total volume of the reaction mixture was thus 36 mL. After stirring for 26 h, the THF solution (56 mL) of CCl4 (44 mmol, 4.3 mL) and DPHLi (3.4 mmol, 8.4 mL, 400 mM in THF) was added to the reaction mixture. After 1 h, into the reaction mixture was then added DBU (9.7 mmol, 1.5 mL) and the reaction temperature was gradually raised to 0 °C. After additional 24 h, the reaction was quenched by 6.8 mL of argon-bubbled methanol. The quenched solution was diluted with 100 mL of toluene and was washed with diluted hydrochloric acid and water, evaporated to dryness under reduced pressure, and then vacuum-dried to give the product (Mn =

Synthesis of Azide-ω-Capped it-PMMA via Isotactic Living Anionic Polymerization of MMA Followed by Termination with Halogenation and Subsequent Azidation
The reactions were carried out by syringe techniques under dry argon in a 100 mL baked glass tube equipped with a three-way stopcock. The following is a typical example for it-50-PMMA. The polymerization was initiated by adding MMA (5.2 mL, 49 mmol) slowly via a dry syringe into the prechilled initiator solution (31 mL), containing t-BuMgBr (0.97 mmol, 3.5 mL, 276 mM in Et 2 O) and toluene (27 mL), at −78 • C. The total volume of the reaction mixture was thus 36 mL. After stirring for 26 h, the THF solution (56 mL) of CCl 4 (44 mmol, 4.3 mL) and DPHLi (3.4 mmol, 8.4 mL, 400 mM in THF) was added to the reaction mixture. After 1 h, into the reaction mixture was then added DBU (9.7 mmol, 1.5 mL) and the reaction temperature was gradually raised to 0 • C. After additional 24 h, the reaction was quenched by 6.8 mL of argon-bubbled methanol. The quenched solution was diluted with 100 mL of toluene and was washed with diluted hydrochloric acid and water, Polymers 2017, 9, 723 4 of 14 evaporated to dryness under reduced pressure, and then vacuum-dried to give the product (M n = 5000, M w /M n = 1.50) containing low molecular weight residues. The obtained product was dissolved with toluene and reprecipitated into hexane three times to result in the chlorine-capped it-PMMA (M n = 5100, M w /M n = 1.61, mm/mr/rr = 92/7/1).

Synthesis of Alkyne-α-Capped st-PMMA via Syndiotactic Living Anionic Polymerization of MMA Initiated via a Metal-Halogen Exchange Reaction
The reactions were carried out by syringe techniques under dry argon in a 100 mL baked glass tube equipped with a three-way stopcock. The following is a typical example for st-50-PMMA. Prior to the polymerization, the functionalized enolate initiator was in situ prepared by adding 6.4 mL of 1 (1.7 mmol, 272 mM in THF) into the THF solution (40 mL) of DPHLi (2.6 mmol) at −100 • C. Into the mixture, MMA (48 mmol, 5.1 mL) was added dropwise, and the total volume of the reaction mixture was thus 52 mL. After stirring for 30 min, the polymerization was terminated with acidic methanol (4.5 mL). The solution was diluted with 100 mL toluene, and was washed with diluted hydrochloric acid and water, evaporated to dryness under reduced pressure, and then vacuum-dried to give the product (M n = 6300, M w /M n = 1.11, mm/mr/rr = 1/22/77). The

Measurements
The number-average molecular weight (M n ) and molecular weight distribution (M w /M n ) of the polymers were measured by size-exclusion chromatography (SEC) using THF at a flow rate 1.0 mL/min at 40 • C on two polystyrene gel columns (Shodex KF-805L (Showa Denko K.K., Tokyo, Japan, pore size: 20-1000 Å; 8.0 mm i.d. × 30 cm)) that were connected to a JASCO PU-2080 precision pump and a JASCO RI-2031 detector (JASCO, Tokyo, Japan). The columns were calibrated against seven standard PMMA samples (Agilent Technologies, Santa Clara, CA, USA; M p = 202-1677000, M w /M n = 1.02-1.23). 1 H and 13 C NMR spectra were recorded on a JEOL ESC-400 spectrometer (JEOL, Tokyo, Japan) operating at 400 and 100 MHz, respectively. The triad tacticity of the polymer was Polymers 2017, 9, 723 5 of 14 determined by the peak intensity of carbonyl C=O carbons that were observed at 175-180 ppm in the 13 C NMR spectrum. The transmittance of polymer solutions was recorded in CH 3 CN using a JASCO V-550 spectrophotometer (JASCO, Tokyo, Japan) (cooling rate = 1 • C/min; wavelength 500 nm). The glass transition (T g ) and melting (T m ) temperatures were recorded on Q200 differential scanning calorimeter (DSC) (TA Instruments Inc., New Castle, DE, USA). T g was obtained form the second scan, where the samples were heated to 180 • C at 10 • C/min, equilibrated at 180 • C for 5 min, cooled to 0 • C at 5 • C/min, held at 0 • C for 5 min, and then reheated to 200 • C at 10 • C/min. T m was obtained from the first scan, where the samples were heated from 40 to 250 • C at 10 • C/min. The samples for T m were obtained by evaporating CH 3 CN from the solution of stereoblock PMMA (50 mg/mL) and dried under vacuo overnight.

Synthesis of Azide-ω-Capped it-PMMA via Isotactic Living Anionic Polymerization of MMA Followed by Termination with Halogenation and Subsequent Azidation
Isotactic living anionic polymerization of MMA was conducted with t-BuMgBr in toluene at −78 • C [51,52] at different molar ratios ([M] 0 /[t-BuMgBr] 0 = 25 and 50) to synthesize it-PMMA with different molecular weights. The polymers obtained after quenching the reaction with acidic methanol had controlled molecular weights and narrow MWDs, as shown in the SEC curves ( Figure 1A,E). The 13 C NMR showed isotactic-rich enchainments (mm/mr/rr = 91/7/2 or 92/7/1), as reported [51,52]. The termination with CCl 4 as the chlorinating agent in the presence of DBU, a strong Lewis base, also resulted in polymers with similar molecular weights and narrow MWDs [50]. Although higher and lower molecular weight fractions were slightly observed, probably being due to high viscosity upon quenching ( Figure 1B,F), the SEC curves were nearly unimodal after the reprecipitations ( Figure 1C,G). methanol had controlled molecular weights and narrow MWDs, as shown in the SEC curves ( Figure  1A,E). The 13 C NMR showed isotactic-rich enchainments (mm/mr/rr = 91/7/2 or 92/7/1), as reported [51,52]. The termination with CCl4 as the chlorinating agent in the presence of DBU, a strong Lewis base, also resulted in polymers with similar molecular weights and narrow MWDs [50]. Although higher and lower molecular weight fractions were slightly observed, probably being due to high viscosity upon quenching ( Figure 1B,F), the SEC curves were nearly unimodal after the reprecipitations ( Figure 1C,G).  The 1 H NMR spectrum of the polymers obtained by termination with CCl 4 shows new peaks, a 2 and c 2 , which are assignable to the terminal methyl ester and methylene protons that are adjacent to the chloride chain end, respectively ( Figure 2B). The terminal chloride functionality (F n (Cl)) was determined by the ratio of M n (SEC), as measured by SEC based on the PMMA calibration to M n (NMR), which was obtained from the peak intensity ratio of c 2 to the methyl ester protons (c) of the repeating MMA units. The obtained F n (Cl) values were 1.00 and 0.81 for M n = 2700 and 5100, respectively. In addition, the F n (Cl) values that were calculated from the peak area ratio of the ω-end methyl ester (c 2 ) to the α-end t-butyl (d) protons were 1.00 and 0.78, respectively. These results indicate that most of the isotactic living chain ends of the PMMAs were capped with chlorine.
The chlorine ω-chain-end polymers were then reacted with NaN 3 in the presence of copper catalysts in CH 3 CN/H 2 O = 9/1 at 40 • C. The SEC curve of the polymers that were obtained after azidation showed similarly narrow MWDs ( Figure 1D,H). The terminal methylene peak (a 2 ) clearly shifted upfield (a 3 ) via the transformation from a chloride into an azide terminal ( Figure 2C). The chain-end functionality of the azide group (F n (N 3 )) was similarly calculated by the ratio of M n (SEC) to M n (NMR), which was obtained from the peak intensity ratio of the methyl ester protons, c 3 and c. The F n (N 3 ) values were 1.00 and 0.74 for M n = 2700 and 4800, respectively. In addition, similar values were obtained from the peak area ratio of the ω-end methyl ester (c 3 ) to the α-end t-butyl (d) protons were 0.96 and 0.75, respectively. They were close to those for F n (Cl) and indicated that the chloride terminal was almost quantitatively converted into the azide terminal. These results indicate that the it-PMMA with an azide group at the ω-chain end was successfully obtained by isotactic living anionic polymerization of MMA, followed by chlorination for quenching and subsequent azidation.
to Mn(NMR), which was obtained from the peak intensity ratio of the methyl ester protons, c3 and c. The Fn(N3) values were 1.00 and 0.74 for Mn = 2700 and 4800, respectively. In addition, similar values were obtained from the peak area ratio of the ω-end methyl ester (c3) to the α-end t-butyl (d) protons were 0.96 and 0.75, respectively. They were close to those for Fn(Cl) and indicated that the chloride terminal was almost quantitatively converted into the azide terminal. These results indicate that the it-PMMA with an azide group at the ω-chain end was successfully obtained by isotactic living anionic polymerization of MMA, followed by chlorination for quenching and subsequent azidation.

Synthesis of Alkyne-α-Capped st-PMMA via Syndiotactic Living Anionic Polymerization of MMA Initiated via a Metal-Halogen Exchange Reaction
To synthesize the alkyne-capped functionalized st-PMMA, syndiotactic living anionic polymerization of MMA was initiated via a metal-halogen exchange reaction between

Synthesis of Alkyne-α-Capped st-PMMA via Syndiotactic Living Anionic Polymerization of MMA Initiated via a Metal-Halogen Exchange Reaction
To synthesize the alkyne-capped functionalized st-PMMA, syndiotactic living anionic polymerization of MMA was initiated via a metal-halogen exchange reaction between 1,1-diphenylhexyl lithium (DPHLi) and an α-bromoisobutyric acid ester bearing a Me 3 Si-protected alkyne (1) in THF at −100 • C [36]. The 50and 100-mer alkyne-end-functionalized st-PMMAs were synthesized by changing the feed ratios of the monomer to the initiator. The polymers that were obtained after quenching the reaction with acidic methanol showed controlled molecular weights, narrow MWDs ( Figure 3A,C), and syndiotactic-rich enchainments (mm/mr/rr = 1/22/77).  The 1 H NMR showed the presence of protected Me3Si-(h) and methylene protons (g1) adjacent to the alkyne function. The chain-end functionality of the protected alkyne group (Fn(C≡C-SiMe3)) was calculated from the ratio of Mn(SEC) to Mn(NMR), which was calculated from the peak intensity ratio of g1 to the methyl ester protons (c) of the repeating MMA units ( Figure 4A). They were close to unity for both of the polymers with different molecular weights (Fn = 0.98 and 1.12 for Mn(SEC) = 6300 and 12100, respectively), indicating that the alkyne-functionalized α-bromoisobutyric acid ester  The 1 H NMR showed the presence of protected Me 3 Si-(h) and methylene protons (g 1 ) adjacent to the alkyne function. The chain-end functionality of the protected alkyne group (F n (C≡C-SiMe 3 )) was calculated from the ratio of M n (SEC) to M n (NMR), which was calculated from the peak intensity ratio of g 1 to the methyl ester protons (c) of the repeating MMA units ( Figure 4A). They were close to unity for both of the polymers with different molecular weights (F n = 0.98 and 1.12 for M n (SEC) = 6300 and 12100, respectively), indicating that the alkyne-functionalized α-bromoisobutyric acid ester quantitatively initiates the syndiotactic living anionic polymerization via a metal-halogen exchange reaction in the presence of DPHLi to result in the well-defined alkyne-functionalized st-PMMA. The 1 H NMR showed the presence of protected Me3Si-(h) and methylene protons (g1) adjacent to the alkyne function. The chain-end functionality of the protected alkyne group (Fn(C≡C-SiMe3)) was calculated from the ratio of Mn(SEC) to Mn(NMR), which was calculated from the peak intensity ratio of g1 to the methyl ester protons (c) of the repeating MMA units ( Figure 4A). They were close to unity for both of the polymers with different molecular weights (Fn = 0.98 and 1.12 for Mn(SEC) = 6300 and 12100, respectively), indicating that the alkyne-functionalized α-bromoisobutyric acid ester quantitatively initiates the syndiotactic living anionic polymerization via a metal-halogen exchange reaction in the presence of DPHLi to result in the well-defined alkyne-functionalized st-PMMA.  The deprotection of the Me 3 Si-group at the α-end was conducted using tetrabutylammonium fluoride (n-Bu 4 NF) in THF at 0 • C. There were almost no changes in the SEC curves with narrow MWDs after the deprotection ( Figure 3B,D). The complete and successful deprotection was confirmed by the disappearance of the characteristic Me 3 Si-protons (h), and the appearance of a new peak that was assignable to the ethynyl proton (i 2 ). The chain-end functionalities of the deprotected alkyne group (F n (C≡C-H)) were similarly calculated from the methylene protons (g 2 ) and were close to unity (F n = 1.05 and 1.04 for M n (SEC) = 6100 and 12000, respectively). These results show that st-PMMA with an alkyne group at the α-chain end was successfully obtained by syndiotactic living anionic polymerization of MMA initiated via a metal-halogen exchange reaction between the protected alkyne-functionalized α-bromoester and DPHLi followed by deprotection.

Synthesis of it-b-st-Stereoblock PMMA via Copper-Catalyzed Azide-Alkyne Cycloaddition and Stereocomplexation
Using two series of polymers with different molecular weights, i.e., it-PMMA-N 3 (DP n~2 5 and 50) and st-PMMA-C≡CH (DP n~5 0 and 100), CuAAC was conducted with four pairs of the polymers to produce it-b-st-stereoblock PMMAs with different block lengths (it-25-b-st-50, it-25-b-st-100, it-50-b-st-50, and it-50-b-st-100). The click reactions between it-PMMA-N 3 and st-PMMA-C≡CH were carried out in the presence of CuSO 4 /sodium ascorbate in DMSO/H 2 O (98/2) at 70 • C, and the two polymers were mixed at the same molar ratio of the two functional groups using each F n value. After the reaction, the SEC curves shifted to higher molecular weights ( Figure 5A-D) although lower molecular weight peaks slightly remained, most probably due to the presence of a small amount of unfunctionalized prepolymers. The coupling products were further purified by reprecipitation in n-hexane to result in narrow SEC curves with higher molecular weights than the starting prepolymers. the reaction, the SEC curves shifted to higher molecular weights ( Figure 5A-D) although lower molecular weight peaks slightly remained, most probably due to the presence of a small amount of unfunctionalized prepolymers. The coupling products were further purified by reprecipitation in n-hexane to result in narrow SEC curves with higher molecular weights than the starting prepolymers. In the 1 H NMR spectrum of the obtained polymers, new peaks for the triazole-ring methine (j) at 7.8 ppm and the adjacent methylene (g4) protons at 5.2 ppm appeared due to the reaction of the alkyne group at the α-end of st-PMMA with the azide ω-terminal of it-PMMA ( Figure 6C). Although In the 1 H NMR spectrum of the obtained polymers, new peaks for the triazole-ring methine (j) at 7.8 ppm and the adjacent methylene (g 4 ) protons at 5.2 ppm appeared due to the reaction of the alkyne group at the α-end of st-PMMA with the azide ω-terminal of it-PMMA ( Figure 6C). Although a small peak for methylene protons (g 2 ) was observed due to the unreacted remaining st-PMMA prepolymers, the purity of the block polymers was relatively high (90%). The purity was calculated from the peak intensity ratio, g 4 /(g 2 + g 4 ). The 13 C NMR spectrum of the carbonyl groups of the obtained coupling products ( Figure 6F) was a weighed superimposition of the two spectra of it-PMMA ( Figure 6D) and st-PMMA ( Figure 6E). The block length ratio was calculated from the tacticities of both the it-PMMA and st-PMMA prepolymers (mm total = x it × mm it + x st × mm st , rr total = x it × rr it + x st × rr st ) and was x it :x st = 0.28:0.72, which was close to the theoretical value, x it :x st = 0.31:0.69, assuming the formation of it-b-st-stereoblock PMMA with the expected block length. These results indicate that it-b-st-stereoblock PMMAs with various block lengths were efficiently synthesized via CuAAC between the azide-capped it-PMMA and alkyne-capped st-PMMA (Table 1).
it-PMMA ( Figure 6D) and st-PMMA ( Figure 6E). The block length ratio was calculated from the tacticities of both the it-PMMA and st-PMMA prepolymers (mmtotal = xit × mmit + xst × mmst, rrtotal = xit × rrit + xst × rrst) and was xit:xst = 0.28:0.72, which was close to the theoretical value, xit:xst = 0.31:0.69, assuming the formation of it-b-st-stereoblock PMMA with the expected block length. These results indicate that it-b-st-stereoblock PMMAs with various block lengths were efficiently synthesized via CuAAC between the azide-capped it-PMMA and alkyne-capped st-PMMA (Table 1).   The stereocomplexation of these stereoblock polymers was evaluated in CH3CN by measuring the transmittance of the solutions (100 mg/mL). Figure    The stereocomplexation of these stereoblock polymers was evaluated in CH 3 CN by measuring the transmittance of the solutions (100 mg/mL). Figure   According to the most recent studies on stereocomplexation, two it-PMMA chains intermolecularly aggregate to form a double helix, which is surrounded by a single helix of st-PMMA to form a unique triple helix-like structure [2,7]. In this model, the degree of polymerization of the outer st-PMMA chain is double that of the inner it-PMMA chain (it:st = 1:2), while the MMA unit ratio of the total it-PMMA chains and st-PMMA chain is 1:1 because of the two it-chains per st-chain in the stereocomplex.

it-/st-
If this model is applied to it-b-st-stereoblock PMMA, the it-segments in two block polymer chains can form a double helix to result in a supramolecular structure, e.g., a Y-shape containing two independent single helixes of st-segments that are connected to each it-segment in the double helix of the it-segments. The it-double helix segment in the Y-shape supramolecule can be incorporated into a single st-helix of another Y-shape molecule to induce further intermolecular associations and finally result in a gel. Among the four it-b-st-stereoblock PMMAs with different block lengths, the it-50-b-st-100 PMMA has the most suitable ratio (it:st = 1:2) of block lengths for this stereocomplexation model, and can easily result in a gel, whereas the total chain length of it-25-b-st-50 PMMA is shorter and less efficient for gel formation, in spite of the suitable ratio (it:st = 1:2). Irrespective of the longer st-segment in it-25-b-st-100 PMMA, the solution was nearly transparent even at low temperatures, suggesting a low degree of association or intramolecular association due to the long st-segment in comparison to that with the short it-segment (it:st = 1:4).
To investigate the effects of the block structure on the stereocomplexation, the four precursor non-block polymers, i.e., it-25-or it-50-PMMA and st-50-or st-100-PMMA, were mixed together at the same molar ratio in CH3CN. All of the solutions became turbid upon decreasing the temperature. However, no gelation occurred for any of the it-and st-mixtures at 100 mg/mL. Only the mixture of it-50-and st-100-PMMAs formed a gel at a higher concentration, i.e., 200 mg/mL. These results indicate that connecting the it-and st-PMMA segments via a covalent linkage in it-b-st-PMMA enhances gelation via stereocomplexation.
To further confirm the stereocomplexation, the polymers were analyzed by DSC. The precursor linear it-50-and st-100-PMMA showed Tg around 35 and 110 °C, respectively. The sample of it-50-b-st-100 PMMA, which was obtained by evaporating CH3CN from the solution and dried in vacuo, showed melting temperatures at 166 and 185 °C, which were slightly higher than those that were reported to the stereocomplexes of linear it-and st-PMMAs [33]. These results also support that the it-b-st-steroblock PMMA forms the stereocomplex. According to the most recent studies on stereocomplexation, two it-PMMA chains intermolecularly aggregate to form a double helix, which is surrounded by a single helix of st-PMMA to form a unique triple helix-like structure [2,7]. In this model, the degree of polymerization of the outer st-PMMA chain is double that of the inner it-PMMA chain (it:st = 1:2), while the MMA unit ratio of the total it-PMMA chains and st-PMMA chain is 1:1 because of the two it-chains per st-chain in the stereocomplex.
If this model is applied to it-b-st-stereoblock PMMA, the it-segments in two block polymer chains can form a double helix to result in a supramolecular structure, e.g., a Y-shape containing two independent single helixes of st-segments that are connected to each it-segment in the double helix of the it-segments. The it-double helix segment in the Y-shape supramolecule can be incorporated into a single st-helix of another Y-shape molecule to induce further intermolecular associations and finally result in a gel. Among the four it-b-st-stereoblock PMMAs with different block lengths, the it-50-b-st-100 PMMA has the most suitable ratio (it:st = 1:2) of block lengths for this stereocomplexation model, and can easily result in a gel, whereas the total chain length of it-25-b-st-50 PMMA is shorter and less efficient for gel formation, in spite of the suitable ratio (it:st = 1:2). Irrespective of the longer st-segment in it-25-b-st-100 PMMA, the solution was nearly transparent even at low temperatures, suggesting a low degree of association or intramolecular association due to the long st-segment in comparison to that with the short it-segment (it:st = 1:4).
To investigate the effects of the block structure on the stereocomplexation, the four precursor non-block polymers, i.e., it-25or it-50-PMMA and st-50or st-100-PMMA, were mixed together at the same molar ratio in CH 3 CN. All of the solutions became turbid upon decreasing the temperature. However, no gelation occurred for any of the itand st-mixtures at 100 mg/mL. Only the mixture of it-50and st-100-PMMAs formed a gel at a higher concentration, i.e., 200 mg/mL. These results indicate that connecting the itand st-PMMA segments via a covalent linkage in it-b-st-PMMA enhances gelation via stereocomplexation.
To further confirm the stereocomplexation, the polymers were analyzed by DSC. The precursor linear it-50and st-100-PMMA showed T g around 35 and 110 • C, respectively. The sample of it-50-b-st-100 PMMA, which was obtained by evaporating CH 3 CN from the solution and dried in vacuo, showed melting temperatures at 166 and 185 • C, which were slightly higher than those that were reported to the stereocomplexes of linear itand st-PMMAs [33]. These results also support that the it-b-st-steroblock PMMA forms the stereocomplex.

Conclusions
The isotactic-syndiotactic stereoblock PMMAs were efficiently prepared via CuAAC between end-functionalized isotactic and syndiotactic PMMAs that were obtained by stereospecific living anionic polymerization of MMA in combination with a metal-halogen exchange reaction for initiation and a halogenation reaction for termination. The reversible transformation between the halide and living anionic chain end, which was used for initiation or termination, enables efficient end-functionalization of stereoregular polymethacrylates, which can be further utilized for the construction of structurally controlled polymers and supramolecular architectures.