Poly(Oxazoline)s with Tapered Minidendritic Side Groups as Models

Biomacromolecules 2001, 2, 729-740

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Poly(Oxazoline)s with Tapered Minidendritic Side Groups as Models for the Design of Synthetic Macromolecules with Tertiary Structure. A Demonstration of the Limitations of Living Polymerization in the Design of 3-D Structures Based on Single Polymer Chains Virgil Percec,* Marian N. Holerca, and Satoshi Uchida Roy&Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323

Duncan J. P. Yeardley and Goran Ungar Department of Engineering Materials and Center for Molecular Materials, The University of Sheffield, Sheffield, S1 4DU, U.K. Received May 9, 2001; Revised Manuscript Received July 10, 2001

The synthesis and living cationic ring-opening polymerization of 2-[3,4-bis(n-alkan-1-yloxy)phenyl]-2oxazolines with alkan being tetradecan and pentadecan, i.e., (3,4)nG1-Oxz with n ) 14 and 15, is described. The structural analysis of the resulting polymers with well-defined molecular weights and narrow molecular weight distribution was carried out by a combination of techniques, including differential scanning calorimetry (DSC), thermal optical polarized microscopy (TOPM), and X-ray diffraction (XRD). At low molecular weights both polymers self-assemble into spherical supramolecules that self-organize into a Pm3hn 3-D lattice while at high molecular weights they form cylindrical macromolecules that self-organize into a p6mm 2-D hexagonal columnar lattice. Both polymers exhibit a 3-D shape change as a function of their degree of polymerization as was reported for the first time in a previous publication from our laboratory (Percec, V.; Ahn, C.-H; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature (London) 1998, 391, 161). Since these polymers can be obtained via a living polymerization, a detailed mechanistic investigation of the influence of the degree of polymerization and molecular weight distribution on the formation of a 3-D spherical macromolecule from a single polymer chain, i.e., a tertiary structure, was possible. The experimental results have demonstrated that the synthesis of nonbiological macromolecules exhibiting tertiary structure is possible in at most a few percent of all macromolecules via living polymerization. This is the case even when macromolecules with very narrow molecular weight distributions and well-defined molecular weights are used. Therefore, the design of synthetic macromolecules with tertiary structure requires not only chains with well-defined molecular weight but also, in particular, macromolecules with no distribution of their chain length. Introduction The functions of biological macromolecules are determined by the three-dimensional (3-D) structure of the entire biomacromolecule chain, i.e., tertiary structure and by the spacial arrangement of complete biomacromolecule chains forming a complex, i.e., a quaternary structure.1 The creation of nonbiological macromolecules with biological functions requires the understanding of the principles involved in the design of synthetic macromolecules that are able to adopt well-defined tertiary and quaternary structures via the engineering of their primary and secondary structures. In a previous publication, we have reported a class of synthetic macromolecules able to fold their backbone in the bulk state into a random-coil or helical conformation.2 This folding process provides a mechanism for the formation of welldefined spherical and cylindrical tertiary structures.2 In addition, these macromolecules change their tertiary structure

from spherical to cylindrical by increasing their degree of polymerization. The driving force for the formation of the secondary or tertiary structure of these synthetic macromolecules and of related structures is determined by the intramolecular self-assembly of their quasi-equivalent dendritic side groups.3 The design and the mechanism of the 3-D structure formation of this class of macromolecules, both in bulk and in solution, is under active investigation.4-6 Nevertheless, the elucidation of the simplest principles involved in the formation of their tertiary structure requires the development of models that address each of the individual problems in the simplest possible way. A question addressed by our previous publication2 refers to the role of the backbone molecular weight and molecular weight distribution on the formation of synthetic macromolecules with well-defined tertiary structures and their role during the transition between various 3-D structures.

10.1021/bm015559l CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001

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This manuscript reports the synthesis of 2-[3,4-bis(n-alkan1-yloxy)phenyl]-2-oxazolines with alkan being tetradecan and pentadecan, i.e., (3,4)nG1-Oxz with n ) 14 and 15, their living cationic ring-opening polymerization, and the structural analysis of the resulting macromolecules by a combination of techniques that includes differential scanning calorimetry (DSC), thermal optical polarized microscopy (TOPM), and X-ray diffraction (XRD). These macromolecules represent the simplest models that can be used to investigate the formation of spherical and cylindrical tertiary structures and the transition from spherical to cylindrical 3-D structure induced by different degrees of polymerization. These experiments will elucidate the role of the degree of polymerization in the formation of the 3-D structure from single synthetic macromolecules and at the same time will demonstrate the limitations of the most advanced current synthetic methods, such as living polymerization, on the formation of the tertiary structure in nonbiological macromolecules.

Percec et al. Scheme 1. Synthesis of Monodendritic 2-oxazolines (3,4)nG1-Oxz, N ) 14, 15

Results and Discussion The synthesis of (3,4)14G1-Oxz and (3,4)15G1-Oxz followed a method elaborated previously in our laboratory for the preparation of related monomers and is outlined in Scheme 1.7-9 Living cationic ring-opening polymerization of (3,4)14G1-Oxz and (3,4)15G1-Oxz initiated with methyl triflate (MeOTf) (Scheme 2) was carried out in bulk in the isotropic state at 160 °C to 100% monomer conversion in order to prepare macromolecules with well-defined molecular weights and narrow molecular weight distributions. The kinetics of polymerization in the isotropic state demonstrated that this polymerization is first order in the concentration of growing species (Figure 1), and therefore, the theoretical molecular weight and the correspondent degree of polymerization (DP) of each polymer are available from kinetic data. At high molecular weights, the theoretical molecular weights are higher than the values obtained by GPC analysis with polystyrene standards (Tables 1 and 2). This difference is expected and is due to the difference in the hydrodynamic volume of these two polymers. Nevertheless, the determination of the absolute polymer molecular weight by the 1H NMR analysis of its chain ends provides values that agree with the theoretical molecular weights calculated from kinetic data. Figure 2 shows the heating scans of the DSC traces for poly[(3,4)14G1-Oxz] and poly[(3,4)15G1-Oxz]. The theoretical degrees of polymerization (DP) of these polymers correspond to the [M]0/[I]0 ratios marked on the figure. The sequence of phase transitions exhibited by these polymers was determined by XRD and is also marked in Figure 2 and summarized in Tables 1 and 2. All polymers exhibit melting and crystallization transitions below their glass transition. This demonstrates an ordered intramolecular microphase separated structure.2,3,5 The polymer with DPn ) 5 displays a columnar hexagonal (φh) p6mm phase followed by a cubic Pm3hn phase. These two phases are enantiotropic. Poly[(3,4)14G1-Oxz] with DPn )

Scheme 2. Polymerization of (3,4)nG1-Oxz

5 to DPn ) 30 exhibits a Pm3hn phase. From DPn ) 30 to DPn ) 75, poly[(3,4)14G1-Oxz] shows a biphase that consists of a mixture of p6mm and Pm3hn phases. This mixture was demonstrated by XRD and TOPM experiments. An example of XRD experiment produced by synchroton that demonstrates the coexistence of these two phases is presented in Figure 3. These samples do not show the

Biomacromolecules, Vol. 2, No. 3, 2001 731

Poly(oxazoline)s with Side Groups

Table 1. Theoretical and Experimental Molecular Weights and Thermal Transitions and Corresponding Enthalpy Changes of Poly[(3,4)14G1-Oxz] thermal transitions (°C) and corresponding enthalpy changes (kcal/mol)a [M]0/[I]0

Mth

M hn

M h w/M hn

heating 30.06(4.66) 48.15 83.34 (0.15) Pm3 h n 97.73 (0.07) k 28.33 (4.48) g 50.12 φh 83.60 (0.13) Pm3 h n 98.00 (0.08) i k 25.66 (4.75) g 52.14 Pm3 h n 109.22 (0.13) i k 23.86 (5.04) g 51.12 Pm3 h n 108.24 (0.11) i k 14.26 (4.05) g 53.79 Pm3 h n 112.20 (0.17) i k 13.93 (5.43) g 52.81 Pm3 h n 112.77 (0.15) i k 14.44 (5.42) g 58.52 Pm3 h n 112.35 (0.14) i k 14.02 (5.71) g 58.14 Pm3 h n 113.76 (0.13) i k 14.60 (6.16) g 61.26 i 93.50 (-0.01) Pm3 h n 114.20 (0.13) i k 14.46 (5.94) g 61.01 i 94.40 (-0.01) Pm3 h n 114.13 (0.13) i k 14.12 (6.24) g 61.55 i 91.62 (-0.02) Pm3 h n 113.11 (0.09) i k 13.25 (6.33) g 62.13 i 92.00 (-0.02) Pm3 h n 112.65 (0.08) i k 12.86 (6.38) g 62.74 i 98.80 (-0.05) Pm3 h n 109.40 (0.05) i k 15.20 (6.40) g 63.33 i 99.20 (-0.05) Pm3 h n 107.80 (0.06) i k 15.32 (6.31) g 62.53 φh, Pm3 h n 85.56 (0) i k 14.76 (6.54) g 64.47 φh, Pm3 h n 85.13 (0) i k 11.69 (6.14) g 65.42 φh, Pm3 h n 87.10 (0) i k 15.40 (6.25) g 64.24 φh, Pm3 h n 86.86 (0) i k 19.66 (5.95) g 65.22 φh, Pm3 h n 88.30 (0) i k 21.99 (6.05) g 65.22 φh, Pm3 h n 88.40 (0) i k 15.48 (6.42) g 62.44 i 71.80 (-0.02) φh, Pm3 h n 91.05 (0.06) i k 18.80 (6.10) g 63.67 i 70.40 (-0.02) φh, Pm3 h n 90.00 (0.07) i k 14.40 (6.43) g 61.27 i 79.25(-0.04) φh, Pm3 h n 96.15 (0.14) i k 14.86 (6.49) g 62.80 i 80.00 (-0.04) φh, Pm3 h n 96.60 (0.13) i k 15.06 (6.56) g 62.47 i 77.85 (-0.06) φh, Pm3 h n 99.66 (0.18) i k 16.86 (6.44) g 63.47 i 77.60 (-0.06) φh, Pm3 h n 98.06 (0.18) i k 17.60 (6.51) g 66.47 φh 108.86 (0.23) i k 23.53 (6.38) g 65.80 φh 108.73 (0.22) i k 26.60 (6.11) g 65.13 φh 114.22 (0.23) i k 27.56 (6.37) g 67.88 φh 113.25 (0.23) i k 31.24 (6.37) g 67.67 φh 115.53 (0.23) i k 31.10 (6.35) g 69.26 φh 115.40 (0.24) i

5

3019

3966

1.22

7

4161

4312

1.22

10

5874

5673

1.21

15

8729

6244

1.18

20

11584

8904

1.13

25

14439

9112

1.13

27

15581

9821

1.13

30

17294

10560

1.12

35

20149

11456

1.12

40

23004

12303

1.12

45

25859

13669

1.11

50

28864

14340

1.11

75

43214

19560

1.09

100

57564

23125

1.09

150

85814

24052

1.10

200

114364

20043

1.11

kb

gc

φhd

cooling ie

i 62.56 (0.08) Pm3 h n 53.39 (0.08) φh 17.81 (4.27) k i 85.64 (0.07) Pm3 h n 14.36 (4.73) k i 91.99 (0.08) Pm3 h n 7.49 (5.23) k i 92.12 (0.07) Pm3 h n 7.53 (5.60) k i 92.19 (0.03) Pm3 h n 7.13 (5.75) k i 91.88 (0) Pm3 h n 7.72 (6.03) k i 88.26 (0) Pm3 h n 8.39 (6.15) k i 59.14 (0) φh, Pm3 h n 7.03 (6.22) k i 60.32 (0) φh, Pm3 h n 8.73 (5.90)k i 61.10 (0) φh, Pm3 h n 13.75 (5.78) k i 62.19(0) φh, Pm3 h n 11.13 (6.16) k i 64.03 (0) φh, Pm3 h n 8.39 (6.23) k i 69.76 (0.09) φh, Pm3 h n 10.19 (6.28) k i 85.59 (0.17) φh 14.30 (6.33) k i 92.26 (0.17) φh 18.22 (6.31) k i 95.92 (0.18) φh 21.11 (6.30) k

a Data from the first heating and cooling scans are on the first line and data from the second heating are on the second line. b k ) crystalline. c g ) glass transition temperature. d φh ) p6mm hexagonal columnar lattice. e i ) isotropic

Figure 4 displays the dependence of all phase transition temperatures collected from the heating and cooling scans (Figure 4, parts a and c) and of the corresponding enthalpy changes (Figure 4, parts b and d). Table 3 summarizes the XRD data obtained for selected samples of both polymers with different DPn in their p6mm phase. The experimental densities obtained at 20 °C (F20), the lattice dimension (a, in Å), and the number of tapered groups per column stratum (µ) with a height of 4.7 Å are also summarized in Table 3. Table 4 reports the d spacings of the Pm3hn cubic lattice of both series of polymers while Table 5 summarizes the analysis of these data.

Figure 1. Dependence of conversion and ln([M]0/[M]) on time for the polymerization of (3,4)14G1-Oxz initiated with MeOTf, [M]0/[I]0 ) 500.

sequence of the first-order transitions exhibited by the polymer with DPn ) 5 since these two phases coexist. At DPn > 75, poly[(3,4)14G1-Oxz] shows only the p6mm phase (Figure 2a). The DSC traces of poly[(3,4)15G1-Oxz] (Figure 2b) follow a similar trend with those of poly[(3,4)14G1-Oxz] (Figure 1a and Tables 1 and 2).

It is instructive to inspect the plots of the isotropization temperatures as a function of [M]0/[I]0 ratios from parts a and b of Figure 4. At low DPs, i.e., low [M]0/[I]0 ratios, both polymers form a Pm3hn lattice while at high DPs they form a p6mm lattice. The Pm3hn lattice is generated from spherical objects while the p6mm lattice is generated from cylindrical objects. Therefore, at low DPs, both polymers self-assemble into spherical objects, while at high DPs they self-assemble into cylindrical objects. This trend is in agreement with the previously reported results.2 It is interesting to observe that the isotropization temperature of the Pm3hn


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Table 2. Theoretical and Experimental Molecular Weights and Thermal Transitions and Corresponding Enthalpy Changes of Poly[(3,4)15G1-Oxz] thermal transitions (°C) and corresponding enthalpy changes (kcal/mol)a [M]0/[I]0

Mth

M hn

M h w/M hn

5

3159

3085

1.37

10

6154

4299

1.19

20

12144

10162

1.09

30

18134

12095

1.08

40

24124

16899

1.08

50

30114

14481

1.07

75

45089

19655

1.07

100

60004

22158

1.07

heating

cooling

46.63 (2.92) 51.22 75.84 (0.13) Pm3 h n 106.63 (0.05) k 44.88 (2.01) g 53.82 φh 75.16 (0.12) Pm3 h n 106.77 (0.04) i k 37.91 (2.07) g 57.55 Pm3 h n 108.68 (0.06) i k 30.73 (2.59) g 56.23 Pm3 h n 109.52 (0.09) i k 25.26 (2.60) g 63.37 Pm3 h n 119.23 (0.05) i k 25.00 (3.00) g 67.28 Pm3 h n 119.04 (0.06) i k 26.20 (2.74) g 62.12 φh, Pm3 h n 85.23 (0) i k 25.00 (2.97) g 66.49 φh, Pm3 h n 86.33 (0) i k 35.34 (2.58) g 67.24 φh, Pm3 h n 92.35 (0.02) i k 25.66 (3.09) g 68.69 φh, Pm3 h n 94.32 (0.01) i k 43.34 (2.66) g 71.41 φh, Pm3 h n 94.50 (1.26) i k 21.00 (3.33) g 56.25 φh, Pm3 h n 103.46 (0.16) i k 41.86 (3.16) g 59.97 φh 109.66 (0.19) i k 21.60 (3.24) g 65.52 φh 109.22 (0.16) i k 38.96 (4.00) g 63.55 φh 114.58 (0.21) i k 32.12 (3.85) g 67.52 φh 114.22 (0.21) i

kb

gc

φhd

ie

i 92.8 (0.01) Pm3 h n 50.24 (0.36) φh 29.61 (1.65) k i 89.32 (0.05) Pm3 h n 21.63 (2.14) k i 104.23 (0.02) Pm3 h n 18.79 (2.72) k i 56.22 (0) φh, Pm3 h n 18.15 (2.77) k i 66.25 (0.01) φh, Pm3 h n 17.95 (2.95) k i 79.80 (0.12) φh, Pm3 h n 11.99 (2.92) k i 84.95 (0.15) φh 11.83 (3.11) k i 94.23 (0.20) φh 19.96 (3.75) k

a Data from the first heating and cooling scans are on the first line and data from the second heating are on the second line. b k ) crystalline. c g ) glass transition temperature. d φh ) p6mm hexagonal columnar lattice. e i ) isotropic

Figure 2. The second DSC heating scans for (a) poly[(3,4)14G1Oxz)] and (b) poly[(3,4)15G1-Oxz)]

lattice has a maximum value at a certain DP while that of the p6mm lattice increases with the DP and then reaches a plateau. We will discuss in more detail the behavior of poly[(3,4)14G1-Oxz]. The maximum of the Pm3hn isotropization temperature occurs at DPn ) 20. IntuitiVely we would consider that this DPn corresponds to the polymer that would generate a sphere from a single polymer chain since this would be the most stable sphere that should produce the lattice with the highest isotropization temperature. If this

is the case, the spheres generated from DPn < 20 would be produced via the self-assembly of fragments of sphere and therefore would be less stable and should undergo isotropization at higher temperatures. The spheres at DPn > 20 would be impurified with short cylinders and therefore their lattice would also exhibit a lower isotropization temperature. An inspection of the X-ray analysis summarized in Table 5 demonstrates that this is not the case. Within experimental error, the diameter of the spheres generated from poly[(3,4)14G1-Oxz] is almost constant and is produced from approximately 70 polymer repeat units as determined from XRD experiments. This value is in agreement with a DP ) 70. Therefore, within experimental error, a sphere generated from a single chain, i.e., tertiary structure, should be obtained when the polymer has a DP ) 70. However, Figures 2a and 4a demonstrate that poly[(3,4)14G1-Oxz] with DPn ) 45, 50, and 75 exhibit on their DSC scans (Figure 2a) a first-order transition that corresponds to the isotropization of a p6mm phase rather than a Pm3hn phase. The presence of Pm3hn lattice mixed with the p6mm lattice could be detected only by XRD (Figure 3 and Tables 3 and 4). Poly[(3,4)15G1-Oxz] has a similar phase diagram (Figures 2b and 4c). What is the rational for this behavior? Let us follow the mechanism of 3-D structure formation that was suggested previously (Scheme 3).2 The shape of the polymer changes continuously by increasing its DP from a cone shape to onefourth of a sphere shape to a half of a sphere shape to threefourths of a sphere shape and finally to a sphere. The transition from a sphere or a fragment of a sphere shapes to a cylinder is facilitated by the quasi-equivalence of the tapered side groups of the polymer.2 This shape transition is not understood and is currently under investigation. What is responsible for the unexpected dependence between the maximum of the Pm3hn isotropization temperature and the DPn of the polymer? The only parameter that has not been considered in this discussion is the molecular weight distribution. The molecular weight distribution of these



Figure 3. XRD diffractograms at variable temperature for poly[(3,4)14G1-Oxz)] DPn ) 45 and the corresponding temperatures and d spacings.

polymers is as narrow as it can be obtained by the current capabilities of the living polymerization processes (Tables 1 and 2). NeVertheless, unexpectedly, its influence on the “composition” of each indiVidual molar weight fraction has enormous implications for the formation of its 3-D structure. To demonstrate this influence with the aid of the XRD results from Table 5 and of the molecular mass data from Table 1, we have constructed Figure 5. Figure 5a shows the GPC traces of the polymers with the most significant DP values required to explain the experimental phase diagram of poly[(3,4)14G1-Oxz] from Figure 4a. In Figure 5b, the scale from the top shows the percentage of one sphere occupied by the polymer chain with the theoretical DP ) [M]0/[I]0 shown from the bottom of the figure. The XRD results from Table 4 demonstrate that regardless of the DP, poly[(3,4)14G1-Oxz] requires approximately 70 repeat units to form a single sphere. Therefore, a single sphere can be generated from a single polymer backbone only if its DP is exactly 70. This sphere generated from a single polymer chain represents the synthetic homologue of the tertiary structure encountered in proteins. Let us compare the GPC traces corresponding to various significant DPn with the distribution of the polymer sizes and shapes available in that particular sample (Figure 5b). While a polymer with DP ) 20 represents 28% of a sphere and therefore adopts a shape that is slightly larger than that of one-fourth of a sphere, due to its molecular weight distribution (M h w/M h n ) 1.13) poly[(3,4)14G1-Oxz] with DPn ) 20 contains chains ranging from DP ) 4 to DP ) 56. Consequently, the shapes of these polymers range from that corresponding to DP ) 4, i.e., a cone equivalent to 6% of a sphere, to that correspond-

ing to DP ) 56, i.e., a shape equivalent to 80% of a sphere. Various combinations of all these fragments would lead to the formation of a sphere via co-assembly. Polymers with DPn < 20, e.g. DPn ) 15, 10, and 7 (Figures 2a and 4a), will also generate fragments that are able to co-assemble in a sphere. However, the spheres generated from the mixture of polymers with DPn ) 20 are more stable than the sphere generated from a mixture of polymers with DPn < 20 since a larger number of smaller fragments of sphere are forming a sphere in the case of lower DPn. This is due to the higher entropy of the spheres created from a larger number of fragments and is supported by the experimental thermodynamic parameter associated with the isotropization temperature (Figures 2a and 4b). The polymers with DPn > 20 cover a range of shapes that are larger than half of a sphere and do not contain the entire range of polymer fragments that are complementary to it. For example, the polymer with DPn ) 35 (Figure 5) presents 44% of the chains larger than DP ) 35, out of which 2% are larger than DP ) 70. These chains could contribute to the formation of spherical objects provided they co-assemble with chains that have complementary DPs. The excess of polymer fragments that do not contribute to the formation of spherical objects will most probably transform, due to their quasi-equivalence, into fragments that are able to co-assemble into cylinders. This may explain why, as observed by XRD, cylindrical structures are formed before the requested DP for the transition from sphere to cylinder is reached. The driving force for the formation of cylinders is provided by the minimization of the free energy of the system via a 2-D hexagonal lattice

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Percec et al.

Figure 4. Phase behavior of polymers poly[(3,4)14G1-Oxz)] and poly[(3,4)15G1-Oxz)]: (a and c) dependence of transition temperatures on the [M]0/I]0; (b and d) dependence of enthalpy changes at transition on [M]0/I]0. Hollow symbols refer to the first cooling scan, filled symbols refer to the second heating scan as follows: k, crystalline (2); g, glassy (9); φh, columnar hexagonal (b); Pm3 h n, cubic (b).

that has a higher free energy than the 3-D cubic lattice but a much lower free energy than a completely disordered amorphous state that would result from the fragments that have no complementary shapes. For the polymer with DPn ) 50, the range of DPs are even more unfavorable to the formation of spherical objects, with only 12% of the chains presenting DP < 35. At DP ) 70, a single polymer backbone should be enclosed in the sphere. However, a polymer with DPn ) 75 presents M h w/M h n 1.09 and consequently encompasses a range of DPs between DP ) 13 and DP ) 178, presenting less than 2% of the chains with exactly DP ) 70. The rest of the chains are either smaller (46%) or larger (52%) than DP ) 70. Moreover, out of the 46% of the chains smaller than DP ) 70, only 7% can adopt shapes that are less than half of a sphere (DP < 35) and consequently can co-assemble into spherical objects provided they find polymer chains that are complementary shaped. Only a very minor amount of chains (2%) that have DP ) 70 will form spherical objects via intramolecular self-assembly and therefore exhibit a tertiary structure. This small fraction of spheres generated from a single macromolecule cannot be separated from the mixture of macromolecules. The rest of the chains are too

large to co-assemble or self-assemble into a sphere and therefore, will form cylindrical objects. The above discussion considers the molecular weight distribution as the most important parameter that determines the distribution of shapes for a polymer with a certain DPn at a certain temperature. Further complications arise from the variation of the temperature, due to the quasi-equivalence of the repeat units. In general, a decrease in the diameter of the supramolecular object was observed when the temperature was increased (Figure 6). For a cylindrical macromolecule, this implies a rearrangement of the repeat units within the cylinders strata, accompanied by an elongation of the columnar supramolecule. The increase in the γ-gauche conformer population with the increase in the temperature leads to the deformation and eventually to the disruption of the columns. This process is different for different polymer chains, according to their DPs. For the polymers with DP ) 70, the increase in the γ-gauche conformer population at higher temperatures leads to the formation of spheres by a similar mechanism with that encountered in supramolecules formed from low molecular weight monodendrons.3j For polymers with DP > 70, such a transition is not possible due to the limited number of units that can be accommodated


Poly(oxazoline)s with Side Groups Table 3. Structural Analysis by X-ray Diffraction Experiments of the p6mm Phase Formed by Poly[(3,4)nG1-Oxz] (n ) 14, 15)

n

DP

T (°C)

d100 (Å)

d110 (Å)

d200 (Å)

d210 (Å)

d300 (Å)

14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 15 15

5 5 5 10 35 35 35 35 35 35 35 45 45 45 45 45 45 45 45 50 50 50 50 50 50 75 75 75 75 100 100 100 100 100 100 100 5 75

50 70 80 65 70 75 77 80 82 84 85 70 75 80 85 88 91 94 97 90 93 96 99 102 105 95 100 103 106 98 101 104 107 110 116 119 62 87

39.8 37.7 36.5 39.0 36.8 36.4 36.1 35.8 35.7 35.4 35.3 36.9 36.5 36.0 35.5 35.2 34.9 34.6 34.4 35.3 35.1 34.8 34.5 34.2 34.0 34.8 34.4 34.1 33.9 34.5 34.4 34.1 33.8 33.6 33.2 33.1 40.2 34.6

19.9 18.9 18.3 19.6 14.5 12.8 21.4 18.4 13.6 12.0 21.1 18.2 13.5 11.9 20.6 17.9 13.3 11.8

20.4 17.7 13.1 11.6

20.3 17.6 20.2 17.5 20.0

20.0 17.4 19.8 17.1 19.6 17.0 19.8 17.2 19.7 17.1 19.6 17.0

F20 (g/mL)

a (Å)

µ

1.01 1.01 1.01 1.00 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

45.95 43.58 42.20 43.01 42.47 41.98 41.72 41.33 41.19 40.87 40.75 42.60 42.14 41.56 40.99 40.64 40.29 39.95 39.72 40.66 40.44 40.09 39.83 39.49 39.25 40.12 39.6 39.27 39.14 39.71 39.53 39.27 39.02 38.79 38.33 38.22 46.41 39.95

9 8 8 8 8 8 8 7 7 7 7 8 8 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 6 6 6 9 7

a ) lattice parameter (Å), a ) 2(d100 + d110x3 + 2d200 + d210x7 + d300x12/5x3, F20 ) experimental density at 20 °C, µ ) number of units per disk, µ ) (x3a2 FtNA)/2 M, t ) thickness of stratum (4.7 Å), NA ) 6.022 × 1023(mol-1), and M ) molecular weight.

in a sphere, and consequently the increase in the γ-gauche conformer population leads eventually to isotropization. The polymers with 35 < DP < 70 form short cylinders, as we discussed above. An increase in temperature leads to the deformation of these cylinders, yet the formation of spheres is conditional to the existence of complementary shaped macromolecules in the system, otherwise the resulting phase is isotropic. The polymers with DP < 35 present a cubic phase up to isotropization. This is due to their ability to coassemble in spheres via a mix-and-match process that can involve many combinations of fragments. Conclusions We have employed the living cationic ring-opening polymerization of minidendritic 2-oxazolines for the synthesis of model dendritic poly(ethyleneinine)s with controlled

Figure 5. (a) Selected GPC traces for poly[(3,4)14G1-Oxz)]. (b) Weight fraction of poly[(3,4)14G1-Oxz)] polymers as a function of the theoretical DP ) [M]0/[I]0. The red line represents the threshold for spherical supramolecules, and the shaded area represents the range of DPs for which a biphase was observed by XRD.

molecular weight and narrow molecular weight distribution. The analysis of the bulk phase of the polymers showed the formation of either 2-D columnar hexagonal p6mm phase or 3-D cubic Pm3hn phase and thus indicated that these macromolecules adopted cylindrical or spherical 3-D shapes, respectively, via a self-assembly process. The shape, structure, and stability of the resulting supramolecular objects display complex dependencies on the DP, temperature, and molecular weight distribution. The formation of a spherical supramolecule involves the self-assembly of polymers with a combined DP equal to the equilibrium occupancy of the spherical object at a certain temperature. Beyond this value, the polymers self-assemble in cylindrical supramolecules. On the other hand, in polymer systems with M h w/M h n * 1.00, the

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Percec et al.

Table 4. X-ray Diffraction Data of the Pm3 h n Cubic Lattice Formed by Poly[(3,4)nG1-Oxz] (n ) 14, 15)

n

DP

T (°C)

d110 (Å)

d200 (Å)

d210 (Å)

d211 (Å)

d220 (Å)

d310 (Å)

d320 (Å)

d321 (Å)

d400 (Å)

d410 (Å)

d420 (Å)

d421 (Å)

d332 (Å)

14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 15 15

5 10 27 35 35 35 40 45 45 45 45 50 50 50 75 75 75 5 10

90 90 77 82 84 87 82 88 91 94 97 99 102 105 100 103 106 97 87

58.2 57.4 57.8 57.4 57.2 56.7 57.2

41.3 40.8 40.7 40.7 40.4 40.2 40.5 40.2 40.0 39.9 39.6 39.5 39.3 39.1 39.4 39.2 39.0 42.3 42.5

31.0 36.7 36.3 36.1 36.1 35.9 36.2 36.0 35.8 35.6 35.4

33.7 33.4 33.5 33.2 33.0 32.8 33.1 33.0 32.8 32.6 32.4 32.4 32.1 32.0 32.4 32.1 31.9 34.5 34.7

29.2 29.0 29.0 28.7 28.6 28.5 28.8 28.4 28.4 28.3 28.1

26.1 26.0 25.8 25.6 25.5 25.5 25.7 25.4 25.4 25.3 25.1

23.0 22.8

22.1 22.0

20.7 20.6

20.1 19.9

18.6 18.4

18.0 17.9

17.6 17.5

22.5 22.2 22.2

21.6 21.4 21.5

20.2 20.0 20.0

19.5

18.0

17.6

17.2

27.7

24.9

24.9 24.8 29.9 30.0

27.0 27.0

59.8 59.8

35.1 35.0 35.0 34.8 37.8 37.9

Table 5. Structural Analysis by X-ray Diffraction Experiments of the Pm3 h n Phase Formed by Poly[(3,4)nG1-Oxz] (n ) 14, 15)

n

DP

T (°C)

F20 (g/mL)

a (Å)

D (Å)

µ

z

14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 15 15

5 5 10 10 27 35 35 35 35 40 45 45 45 45 45 50 50 50 50 75 75 75 75 5 10

90 90 90 90 77 90 82 84 87 82 90 88 91 94 97 90 99 102 105 90 100 103 106 97 87

1.01 1.01 1.00 1.00 1.00 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

81.64 81.64 82.04 82.04 81.66 80.21 81.16 80.80 80.41 80.89 80.26 80.31 80.19 79.86 79.31 80.46 79.18 78.56 78.28 80.75 79.08 78.42 78.05 84.72 84.92

50.65 50.65 50.90 50.90 50.66 49.76 50.34 50.13 49.88 50.18 49.79 49.82 49.75 49.55 49.20 49.92 49.12 48.74 48.56 50.10 49.06 48.64 48.41 52.56 52.68

72.33 72.33 72.68 72.68 71.67 68.60 71.07 70.12 69.11 70.36 68.73 68.86 68.55 67.71 66.32 68.56 65.34 63.81 63.13 69.30 65.09 63.47 62.58 76.29 76.83

14.47 14.47 7.27 7.27 2.65 1.96 2.03 2.00 1.97 1.76 1.53 1.53 1.52 1.50 1.47 1.37 1.31 1.28 1.26 0.92 0.87 0.85 0.83 15.26 7.68

a ) lattice parameter, a ) (d110x2 + 2d200 + d210x5 + d211x6 + d220x8 + d310x10 + d320x13 + d321x14 + 4d400)/9, D ) diameter of

sphere, D ) 23x3a3/32π µ ) (a3FNA)/(8 M), µ ) number of units per sphere, F20 ) experimental density at 20 °C, NA ) 6.022 × 1023(mol-1), M ) molecular weight, z ) number of polymer chains per sphere, and z ) µ/DP. The data in italics is extrapolated to 90 °C.

formation of spherical supramolecules via the self-assembling process is accompanied by the formation of cylindrical supramolecules due to macromolecules with values of DP that are not self-complementary, i.e., are beyond values that correspond to a half of a sphere and are different from those for a whole sphere. Hence, an average even monodisperse

Figure 6. Dependence of the diameter of the supramolecular dendrimer formed by poly[(3,4)14G1-Oxz)] on temperature in (a) the p6mm columnar hexagonal phase and (b) the cubic Pm3h n phase.

molecular weight distribution prevents the construction of supramolecules with well-defined 3-D structures based on a single macromolecule, i.e., tertiary structure, and thus demonstrates the inability of current synthetic methods, i.e. living polymerizations, to construct nonbiological macro-



Scheme 3. Schematic Representation of the Formation of Supramolecular Dendrimers via Self-assembly and Co-assembly of Macromolecules of Different Molecular Weights

molecules with a degree of perfection encountered in functional biological systems. Therefore, synthetic methods that produce polymers with no chain length distribution are required for the design of nonbiological macromolecules able to exhibit a tertiary structure and subsequently to display biological functions. Experimental Section Materials. THF and Et2O (Fisher, ACS reagent) were refluxed over sodium ketyl and freshly distilled before use. CH2Cl2 (Fisher, ACS reagent) was refluxed over CaH2 and freshly distilled before use. Benzene (Fisher, ACS reagent) was shaken with concentrated H2SO4, washed twice with water, dried over MgSO4, and finally distilled over sodium ketyl. Methanol (MeOH), ethanol (EtOH), H2SO4, pyridine,

dimethylformamide (DMF), KOH, K2CO3, and NaHCO3 (all Fisher, ACS reagents) were used as received. SOCl2 (97%), 3,4-dihydroxybenzoic acid (98%), ethanolamine (99+%), and LiAlH4 (95+%) (all from Aldrich) were used as received. 1-Bromotetradecane (97%, Lancaster) and 1-bromopentadecane (97%, Fluka) were used as received. Methyl trifluoromethanesulfonate (MeOTf) (Fluka, g97%) was vacuum distilled. General Methods. 1H NMR (200 MHz) and 13C NMR (50 MHz) spectra were recorded on a Varian Gemini 200 spectrometer. CDCl3 was used as solvent and TMS as internal standard unless otherwise noted. Chemical shifts are reported as δ, ppm. The purity of products was determined by a combination of techniques including thin-layer chromatography (TLC) on silica gel plates (Kodak) with fluorescent indicator and high-pressure liquid chromatography (HPLC)

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using a Perkin-Elmer Series 10 high-pressure liquid chromatograph equipped with an LC-100 column oven, Nelson Analytical 900 Series integrator data station, and two PerkinElmer PL gel columns of 5 × 102 and 1 × 104 Å. THF was used as solvent at an oven temperature of 40 °C. Detection was by UV absorbance at 254 nm. Weight-average (M h w) and number-average (M h n) molecular weights were determined with the same instrument from a calibration plot constructed from polystyrene standards. Thermal transitions of samples that were freeze-dried from benzene were measured on a Perkin-Elmer DSC-7 differential scanning calorimeter (DSC). In all cases, the heating and cooling rates were 10 °C min-1 unless otherwise noted. First-order transition temperatures were reported as the maxima and minima of their endothermic and exothermic peaks. Glass transition temperatures (Tg) were read at the middle of the change in heat capacity. Indium and zinc were used as calibration standards. An Olympus BX-40 optical polarized microscope (100× magnification) equipped with a Mettler FP 82 hot stage and a Mettler FP 80 central processor was used to verify thermal transitions and characterize anisotropic textures. X-ray diffraction experiments on liquid crystal phases were performed using an Image Plate area detector (MAR Research) with a graphite-monochromatized pinhole-collimated beam and a helium tent. The samples, in glass capillaries, were held in a temperature-controlled cell. XRD experiments were also performed at several small-angle stations of the Synchotron Radiation Source at Daresbury, U.K. A double-focused beam and a quadrant detector were used. In both cases the sample was held in a capillary within a custom-build temperature cell controlled to within (0.1 °C. Capillaries holding the dried samples were sealed under nitrogen. Densities, F20, were determined by flotation in glycerol/H2O or glycerol/MeOH at 20 °C. Synthesis. Methyl [3,4-Bis(n-tetradecan-1-yloxy)]benzoate (2/14). Compound 1 (11.76 g, 0.07 mmol) was dissolved in 300 mL of DMF under Ar, and K2CO3 (38.64 g, 0.28 mol) was added. The mixture was purged with Ar for 0.5 h at reflux, then the n-bromotetradecane (38.7 g, 0.14 mol) was added. The heterogeneous mixture was stirred at reflux under Ar for 8 h, and then the suspension was poured into ice-cooled water and acidified to pH ) 1 with HCl (10%). The precipitate was filtered, washed thoroughly with water on filter, and then recrystallized two times in acetone to yield 22.49 g (88.3%) of a white powder. Mp: 50-52 °C. 1H NMR (CDCl3, δ, TMS): 0.89 (t, 6H, J ) 6 Hz, CH3), 1.1-1.9 (m, 48H, CH3(CH2)12), 3.9 (s, 3H, COOCH3), 4.02 (t, 4H, J ) 6 Hz, CH2OAr), 4.8 (s, 1H, OH), 6.85 (d, 1H, J ) 8 Hz, meta to COO), 7.5 (s, 1H, ortho to COO), 7.6 (d, 1H, J ) 8 Hz, ortho to COO). 13C NMR (CDCl3, δ, ppm): 14.0 (CH3), 22.7 (CH3CH2), 26.0 (CH2CH2CH2O), 29.2, 29.3, 31.7 (CH3CH2(CH2)9), 29.2 (CH2CH2O), 51.91 (COOCH3), 68.15 (CH2OPh), 111.8 (meta to COO, ortho to O), 113.2 (ortho to COO, ortho to O), 122.8 (ortho to COO, meta to O), 126.7 (ipso to COO), 146.3, 156.4 (ipso to O), 165.2 (COO). Methyl [3,4-Bis(n-pentadecan-1-yloxy)]benzoate (2/15). Starting from 1 (11.76 g, 0.07 mol), K2CO3 (38.64 g, 0.28

Percec et al.

mol), and n-bromopentadecane (40.74 g, 0.14 mol) in 300 mL of DMF at reflux for 8 h, 37.49 g (91.1%) of a white powder was obtained after two recrystallizations from acetone. Mp: 45-47 °C 1H NMR (CDCl3, δ, TMS): 0.89 (t, 6H, J ) 6 Hz, CH3), 1.2-1.9 (m, 52H, CH3(CH2)13), 3.9 (s, 3H, COOCH3), 4.0 (t, 4H, J ) 6 Hz, CH2OAr), 4.7 (s, 1H, OH), 6.8 (d, 1H, J ) 8 Hz, meta to COO), 7.5 (s, 1H, ortho to COO), 7.6 (d, 1H, J ) 8 Hz, ortho to COO). 13C NMR (CDCl3, δ, ppm): 14.1 (CH3), 22.6 (CH3CH2), 26.0 (CH2CH2CH2O), 29.1, 29.3, 29.5, 29.6, 31.7 (CH3CH2(CH2)10), 29.4 (CH2CH2O), 51.9 (COOCH3), 68.1 (CH2OPh), 111.8 (meta to COO, ortho to O), 113.2 (ortho to COO, ortho to O), 122.8 (ortho to COO, meta to O), 126.7 (ipso to COO), 146.3, 156.4 (ipso to O), 165.2 (COO). 3,4-Bis(n-tetradecan-1-yloxy)benzoic Acid (3/14). Compound 2/14 (16.8 g, 30 mmol) was dissolved in 150 mL of EtOH (95%), and KOH (12.42 g, 90 mmol) was added. The mixture was stirred at reflux for 12 h, and then cooled to 30 °C. The resulteding suspension was poured into ice-cooled water and acidified to pH ) 1 with HCl (10%). The precipitate was filtered, washed with water on the filter, and then recrystallized three times in EtOH (95%) to yield 16.83 g (92.5%) of a white powder. Mp: 88-90 °C. 1H NMR (CDCl3, δ, ppm, TMS): 0.88 (overlapped t, 6H, CH3), 1.231.62 (m, 44H, CH3(CH2)11), 1.70 (m, 4H, CH2CH2OAr), 4.06 (overlapped t, 4H, CH2OAr), 6.90 (d, 1H, J ) 9 Hz, meta to COOH), 7.59 (d, 1H, J ) 2 Hz, ortho to COOH), 7.72 (dd, 1H, J ) 9 Hz J ) 2 Hz, ortho to COOH). 13C NMR (CDCl3, δ, ppm): 14.1 (CH3), 22.6 (CH3CH2), 26.1 (CH2CH2CH2O), 29.2, 29.3, 31.5 (CH3CH2(CH2)3), 29.1 (CH2CH2O), 68.2 (CH2OPh), 113.4 (meta to COOH, ortho to O), 113.9 (ortho to COOH, ortho to O), 118.1 (ortho to COOH, meta to O), 131.5 (ipso to COOH), 147.9, 153.7 (ipso to O), 165.2 (COOH). 3,4-Bis(n-tetradecane-1-yloxy)benzoyl Chloride (4/14). Compound 3/14 (17.6 g, 32 mmol) was suspended in CH2Cl2 (200 mL), and a catalytic amount of DMF was added. SOCl2 (20 mL, excess) was added dropwise, and the mixture was heated to reflux for 0.5 h. After cooling to room temperature, the solvent and the excess SOCl2 were distilled off to yield 18.01 g (99.9%) of a white powder, which was used without further purification. 1H NMR (CDCl3, δ, ppm, TMS): 0.89 (overlapped t, 6H, CH3), 1.26-1.70 (m, 44H, CH3(CH2)11), 1.77 (m, 4H, CH2CH2OAr), 4.05 (overlapped t, 4H, CH2OAr), 6.91 (d, 1H, J ) 9.2 Hz, meta to COCl), 7.54 (d, 1H, J ) 1.8 Hz, ortho to COCl), 7.73 (dd, 1H, J ) 9.2 Hz J ) 1.8 Hz, ortho to COCl). N-[3,4-Bis(n-tetradecane-1-yloxy)benzoyl]-2-aminoethanol (5/14). Compound 4/14 (17.8 g, 31 mmol) was dissolved in CH2Cl2 (125 mL) and slowly added to ice-cooled ethanolamine (25 mL, excess) under vigorous stirring. The mixture was stirred at 0 °C for 1 h, and then the temperature was raised to 40 °C for another 4 h. The reaction mixture was poured into a separatory funnel and washed three times with H2O, then dried over MgSO4, and filtered. The solvent was removed by rotary evaporation, and the crude product was recrystallized twice from acetone at 0 °C to yield 16.1 g (86.3%) of a white powder. Purity (HPLC), 99+% TLC (1:1 hexanes:EtOAc): Rf ) 0.30. 1H NMR (CDCl3, δ, ppm,


TMS): 0.88 (t, 6H, CH3, J ) 6.3 Hz), 1.26-1.68 (overlapped peaks, 44H, CH3(CH2)11), 1.86 (m, 4H, CH2CH2OAr), 3.02 (bs, 1H, OH), 3.63 (q, 2H, CH2OH, J ) 5.0 Hz), 3.85 (t, 2H, NHCH2, J ) 5.1 Hz), 4.04 (overlapped t, 4H, CH2OAr), 6.55 (bs, 1H, NH), 6.88 (d, 1H, meta to CONH, J ) 8.2 Hz), 7.28 (dd, 1H, ortho to CONH, J ) 8.2 Hz, J ) 2.4 Hz), 7.40 (d, 1H, ortho to CONH, J ) 2.4 Hz). 13C NMR (CDCl3, δ, ppm): 14.0 (CH3), 22.6 (CH3CH2), 26.0 (CH2CH2CH2O), 29.1, 29.2, 29.3, 29.4, 29.6 (CH3CH2CH2(CH2)8), 30.2 (CH2CH2O), 31.9 (CH3CH2CH2), 42.9 (NHCH2), 62.3 (CH2OH), 69.1, 69.4 (CH2OAr), 112.3, 112.8 (meta to CONH, ortho to CONH and O), 119.8 (ortho to CONH), 126.4 (ipso to CONH), 148.9, 152.1 (meta to CONH, ipso to O and para to CONH), 168.4 (CONH). N-[3,4-Bis(n-pentadecane-1-yloxy)benzoyl]-2-aminoethanol (5/15). Compound 2/15 (10.58 g, 18 mmol) was mixed with ethanolamine (15 mL, 0.36 mol, 1:20 excess), and the temperature was raised to 140 °C under vigorous stirring. The mixture was stirred at 140 °C for 10 h, and then 50 mL of 85% ethanol was added to the hot mixture and allowed to cool at 23 °C. The suspension was thoroughly washed with water and the solid was filtered and recrystallized twice from acetone at 0 °C to yield 8.38 g (75.1%) of a white powder. Purity (HPLC), 99+% TLC (1:1 hexanes:EtOAc): Rf ) 0.28. 1H NMR (CDCl3, δ, ppm, TMS): 0.88 (t, 6H, CH3, J ) 6.2 Hz), 1.25-1.65 (overlapped peaks, 48 H, CH3(CH2)12), 1.86 (m, 4H, CH2CH2OAr), 2.75 (bs, 1H, OH) 3.62 (q, 2H, CH2OH, J ) 5.1 Hz), 3.84 (t, 2H, NHCH2, J ) 5.1 Hz), 4.04 (overlapped t, 4H, CH2OAr), 6.63 (bs, 1H, NH), 6.87 (d, 1H, meta to CONH, J ) 8.4 Hz), 7.25 (dd, 1H, ortho to CONH, J ) 8.4 Hz, J ) 2.3 Hz), 7.39 (d, 1H, ortho to CONH, J ) 2.3 Hz). 13C NMR (CDCl3, δ, ppm): 14.5 (CH3), 22.9 (CH3CH2), 26.5 (CH2CH2CH2O), 29.3, 29.4, 29.5, 29.6, 29.8 (CH3CH2CH2(CH2)9), 30.2 (CH2CH2O), 31.9 (CH3CH2CH2), 42.9 (NHCH2), 62.6 (CH2OH), 69.2, 69.5 (CH2OAr), 112.4, 112.9 (meta to CONH, ortho to CONH and O), 119.9 (ortho to CONH), 126.5 (ipso to CONH), 148.9, 152.2 (meta to CONH, ipso to O and para to CONH), 168.4 (CONH). General Procedure for the Synthesis of 2-[3,4-bis(nalkane-1-yloxy)phenyl]-2-oxazolines. 2-[3,4-Bis(n-tetradecane-1-yloxy)phenyl]-2-oxazoline [(3,4)14G1-Oxz)] (7/14). Compound 5/14 (17 g, 28 mmol) was dissolved in CH2Cl2 (700 mL), and SOCl2 (6.56 mL, 0.09 mol) was added dropwise at 23 °C. The mixture was stirred for 15 min, when the 1H NMR indicated complete conversion. The reaction was neutralized by addition of 700 mL of saturated NaHCO3 and vigorous stirring for 1 h. The aqueous layer was discarded and the organic layer was washed three times with 400 mL water, then dried over MgSO4, and filtered. The solvent was distilled and the product was recrystallized twice from hexanes, purified by column chromatography (SiO2, hexane:EtOAc 5:1), and recrystallized again in acetone to yield 13.11 g (82%) of a white solid. Mp: 58-60 °C. Purity (HPLC), 99+%; TLC (CHCl3), Rf ) 0.42. 1H NMR (CDCl3, δ, ppm, TMS): 0.89 (t, 6H, CH3, J ) 6.5 Hz), 1.23-1.69 (m, 44H, CH3(CH2)11), 1.77 (m, 4H, CH2CH2OAr), 4.00 (overlapped t, 6H, CH2OAr, OCH2CH2N), 4.44 (t, 2H, OCH2CH2N, J ) 9.0 Hz),


6.86 (d, 1H, meta to CON, J ) 9.2 Hz), 7.47 (d, 1H, ortho to CON, J ) 2.4 Hz) 7.48 (dd, 1H, ortho to CON, J ) 9.2 Hz, J ) 2.4 Hz). 13C NMR (CDCl3, δ, ppm): 14.3 (CH3), 22.4 (CH3CH2), 26.2 (CH2CH2CH2O), 29.3, 29.4, 29.7 (CH3CH2CH2(CH2)8), 30.2 (CH2CH2O), 31.9 (CH3CH2CH2), 54.8 (NCH2), 67.8 (OCNCH2), 69.2, 69.4 (CH2OAr), 112.6, 113.2 (meta to OCN, ortho to OCN), 119.6 (ipso to CON) 121.8 (ortho to OCN), 148.8 (meta and para to CON), 164.9 (OC ) N). Anal. Calcd for C37H65O3N: C, 77.70; H, 11.46. Found: C, 77.56; H, 11.36. 2-[3,4-Bis(n-pentadecane-1-yloxy)phenyl]-2-oxazoline [(3,4)15G1-Oxz] (7/15). Starting from 5/15 (8 g, 13 mmol) and SOCl2 (2.83 mL, 0.039 mol) in 600 mL of CH2Cl2 for 15 min at 23 °C, the reaction mixture was then neutralized by 600 mL of saturated NaHCO3 for 1 h, yielding 6.38 g (82%) of a white solid. Mp: 61-63 °C. Purity (HPLC), 99+%; TLC (CHCl3), Rf ) 0.40. 1H NMR (CDCl3, δ, ppm, TMS): 0.88 (t, 6H, CH3, J ) 6.5 Hz), 1.24-1.69 (m, 48H, CH3(CH2)12), 1.75 (m, 4H, CH2CH2OAr), 4.02 (overlapped t, 6H, CH2OAr, OCH2CH2N), 4.42 (t, 2H, OCH2CH2N, J ) 9.0 Hz), 6.88 (d, 1H, meta to CON, J ) 9.2 Hz), 7.45 (d, 1H, ortho to CON, J ) 2.4 Hz) 7.45 (dd, 1H, ortho to CON, J ) 9.2 Hz, J ) 2.4 Hz). 13C NMR (CDCl3, δ, ppm): 14.3 (CH3), 22.4 (CH3CH2), 26.2 (CH2CH2CH2O), 29.3, 29.4, 29.6 (CH3CH2CH2(CH2)9), 30.2 (CH2CH2O), 31.9 (CH3CH2CH2), 54.6 (NCH2), 67.5 (OCNCH2), 69.2, 69.6 (CH2OAr), 112.6, 113.4 (meta to OCN, ortho to OCN), 119.6 (ipso to CON) 121.8 (ortho to OCN), 148.7 (meta and para to CON), 164.7 (OC ) N). Anal. Calcd for C39H69O3N: C, 78.07; H, 11.59. Found: C, 78.12; H, 11.58. General Procedure for the Synthesis of Poly{N-[3,4Bis(n-alkane-1-yloxy)benzoyl]ethylenimine} {poly[(3,4)nG1-Oxz]}. A Schlenk tube equipped with a magnetic magnetic stirrer was dried at 200 °C overnight and charged with (3,4)nG1-Oxz. The monomer (typically 0.4 mmol) was dissolved in minimum amount of dry benzene and freezedried in the septum-sealed Schlenk tube. After freeze-drying, the Schlenk tube was flushed with Ar. The initiator, MeOTf, was added via syringe according to the desired DP, and the tube was brought on a preheated oil bath at 160 °C. The melt was gently stirred until 1H NMR indicated complete conversion of monomer. General Procedure for the Kinetic Determination of the Rate Constant of Propagation. Polymerization of (3,4)14G1-Oxz, [M]0/[I]0 ) 500. (3,4)14G1-Oxz (4 mmol, 2.284 g) was brought into a two-necked pear-shaped flask, which was sealed, and freeze-dried. The flask was filled with Ar and MeOTf (0.008 mmol, 0.88 µL) was added under Ar. The mixture was brought on a preheated oil bath at 160 °C. Sampling of the melt was done under a stream of Ar, and the conversion was followed by 1H NMR. A conversion of 99.2% is obtained in 380 min. Conversion was monitored by cross-integration of the aromatic protons in the unreacted oxazoline vs the aromatic protons in polymer (7.49 ppm, dd, 1H, Ar 6-position and 7.47 ppm, d, 1H, Ar 2-position in unreacted vs 6.87, d, 1H, 5-position in unreacted overlapped to 6.75, br, Ar 2,5,6-position in reacted oxazoline) and/or the unreacted oxazoline vs the backbone (4.42, t, 2H, OCH2-

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CH2N vs NCH2CH2N and CH3N in polymer overlapped to CH2OAr and OCH2CH2N in unreacted). Acknowledgment. Financial support from the National Science Foundation (DMR-9996288) and the Engineering and Physical Science Research Council (U.K.) are gratefully acknowledged. We thank Professor S. Z. D. Cheng from Akron University, Akron, OH, for the density measurements.

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