Intramolecular Cyclization in A2 + B3 Polymers via Step-Wise

Article pubs.acs.org/Macromolecules

Intramolecular Cyclization in A2 + B3 Polymers via Step-Wise Polymerization Resulting in a Highly Branched Topology: Quantitative Determination of Cycles by Combined NMR and SEC Analytics Heng Chen, Jie Kong,* Wei Tian, and Xiao-Dong Fan Department of Applied Chemistry, Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an, 710072, P. R. China S Supporting Information *

ABSTRACT: In this contribution, we report a convenient expression of average number of cyclic structures (ANC) and cyclic-average molecular weight (MC) to quantificationally describe the topological defect of intramolecular cyclization in highly branched polymers synthesized via A2 + Bn (n ≥ 3) stepwise polymerization strategy by a combination of nuclear magnetic resonance spectrometry (NMR) and size exclusion chromatography (SEC). The ANC and MC depend on the number ratio of dendritic, linear, terminal units and numberaverage molecular weight of hyperbranched polymers, which can be derived from NMR and SEC, respectively. The analysis of hyperbranched polycarbosilanes with silicon hydrogen bonds (A) or vinyl groups (B) termini from A2 + B3 approach indicates that the quantificational description of ANC and MC make it easy to well understand intramolecular cyclic structures resulting in a highly branched topology. Regulating the flexibility and rigidness of internal units in A2 monomers is an effective way to control the extent of intramolecular cyclization. Because of the general and convenient nature, the ANC and MC have potential for the quantificational description of intramolecular cyclization, i.e. one type of topological defect, in a variety of hyperbranched polymers synthesized via A2 + Bn strategy.

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INTRODUCTION Hyperbranched polymers possessing unusual properties, such as high solubility, low viscosity, multifunctional end groups and intramolecular topologic cavity, have received considerable attention because of their potential in functional materials,1 processing aids,2 gene delivery,3 drug selective encapsulation and controlled release.4 Numerous hyperbranched polymers, such as polyamides,5 polyesters,6 poly(ether sulfones),7 polyurethanes,8 and polycarbosilanes,9 were prepared through condensation reaction, addition reaction, ring-opening reaction or click chemistry.10 The synthetic strategies can be mainly divided into the following categories.11 The first one is the “ABn” (n ≥ 2) route, in which hyperbranched polymers are synthesized by polymerization of an ABn or a latent ABn monomer.5,6a,b,9b Besides the copper catalyzed alkyne−azide cycloaddition (CuAAC) and thiol−ene click reaction,12 selfcondensing vinyl polymerization (SCVP),13 self-condensing ring-opening polymerization (SCROP)14 and proton-transfer polymerization (PTP)15 are also accepted in the last decades. The second one is well-known as “A2 + Bn” (n ≥ 3) strategy, based on two monomers containing multifunctional groups, namely A2 and Bn, generating highly branched polymers.6c,7,9c The direct synthesis of hyperbranched polymers via core outward or periphery inward can also be employed either through iterative chain growth/branching reactions from well© 2012 American Chemical Society

defined and reactive building blocks, where novel hyperbranched polymers with well-defined linear chains between branching points (HyperMacs) have been constructed recently.16 However, regardless of the condensation or addition polymerization of ABn or “A2 + Bn” monomers, cyclization always occurs resulting in topological defect. It competes with chain growth at any concentration (even in bulk) and any stage of a polymerization.17 Unlike linear polymerization, in which cyclization only gives rise to the termination of chain growth by removal of the two reactive chain ends,18 hyperbranched polymers containing cyclic structures should exist at each degree of polymerization. According to the Kricheldorf’s viewpoint,19 cyclization generates cyclic and multicyclic structures, and further presents a strong influence on all important aspects, i.e., changing the architecture in the direction of cyclic or multicyclic structures and reducing the number of end groups, preventing gelation, and reducing the molecular weight and polydispersity. The extent of cyclization may depend on synthetic methods employed and affect the relationship between structure and property of products. For Received: April 3, 2012 Revised: July 10, 2012 Published: July 23, 2012 6185

dx.doi.org/10.1021/ma300686b | Macromolecules 2012, 45, 6185−6195

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example, due to the topography cavity inner hyperbranched architecture and molecular cavity of β-cyclodextrin units, an amphiphilic hyperbranched β-cyclodextrin-based polymer showed selective encapsulation behavior for double-guest drugs (levofloxacin lactate and phenolphthalein).20 Unlike the intermolecular cyclic structures that can be removed by separation, the intramolecular cyclic structures inner hyperbranched architecture do play an important role in the encapsulation behavior compared to perfect hyperbranched architecture without any cyclic structures. Thus, the conformation and extent of cyclic structures, i.e., perfect multicycles free of functional groups or multicycles having A or B groups,19 are key factors to explore the topology of hyperbranched polymers besides degree of branching, molecular weight and distribution, and three-dimensional scale. Unlike the determination of degree of branching by the wellknown NMR spectrometry, the extent of intramolecular cyclic structures is difficult to be characterized because the chemical linkages constituting the cyclic structures are almost the same as those structures forming the polymer backbone linkages. Actually, the molecular weight is not influenced to a large extent by intramolecular cyclic structures in comparison to intermolecular ones.21 Some groups have reported the extent of cyclization by indirect comparison of the molecular weight of hyperbranched polymers calculated from end groups with their absolute molecular weight,22 or subtracting the theoretical amount of A2 monomers needed at the gel point from the amount of A2 monomers used to reach the gel point experimentally.23 Up to date, matrix assisted laser desorption/ ionization time-of-flight (MALDI−TOF) mass spectroscopy is one effective tool to reveal the existence of the cyclic structures in some specialized cases.7b,17,21,24 However, the reorganization of macrocyclic structures or imbricate cyclic structures is difficult and the analysis of mass peaks of MALDI−TOF spectrum is not facile. Furthermore, it is not suitable for hyperbranched polymers without ionization feature, such as hyperbranched polycarbosilanes. So far, there is no convenient way yet to well determine the extent and conformation of intramolecular cyclic structures in hyperbranched polymers synthesized via either “A2 + Bn” or “ABn” strategy. Herein, we report a convenient expression of extent of intramolecular cyclic structures in hyperbranched polymers by a combination of nuclear magnetic resonance spectrometry (NMR) and size exclusion chromatography (SEC). The average number of cyclic structures (ANC) and the cyclicaverage molecular weight (MC) are defined and deduced for the highly branched polymers with A or B termini from a versatile “A2 + Bn” (n ≥ 3) stepwise polymerization strategy. Hyperbranched polycarbosilanes with silicon hydrogen bonds (A) and vinyl groups (B) termini synthesized via “A2 + B3” strategy (Scheme 1) were employed as model polymers to achieve the key parameters of ANC and MC by a combination of 29Si NMR and SEC. The strategy to control intramolecular cyclization and decrease topological defect in highly branched architecture was discussed by varying the flexible and rigid interval units of A2 monomers.

Scheme 1. A2 + B3 Strategy to Synthesize Highly Branched Polycarbosilanes via Hydrosilylation

at time t2, a small amount g2 of reactant is added, and at time t3, a small amount g3 of reactant is added. By using the offstoichiometric addition, i.e., controlling the feed ratio of A2 and B3 monomer, the highly branched polymer terminated with A or B functional groups could be obtained.9c Normally, the number of cycles formed will be a function of t1, t2, t3, g1, g2, g3, and of f1, f 2, f 3, f4 which represent respectively, the concentration f1 of the solution with first monomer, the concentration f 2 of the solution with second monomer, the concentration f 3 of the catalyst, and the propensity f4 of the first monomer to react with the second monomer. This dependence on 10 variables is too complex and, in any case, beyond the scope of the present investigation. Here, we limit ourselves to derive interesting formulae which connect the number of cycles formed with other parameters which describe the polymer. The visual expression of average number of cyclic structures (ANC) and cyclic-average molecular weight (MC) was deduced based on the microscosmic relationship of number of A units (NA), B units (NB), and number-average molecular weight (Mn) in these highly branched polymers. As will be presented, they can be directly calculated from population of dendritic, linear, terminal units and Mn, which can be checked by NMR, SEC, or MALDI−TOF analyses. The deduction is based on the following logic. From a microcosmic viewpoint, the proportion of NA and NB in hyperbranched topology is given according to the type of termini, i.e., A or B terminal groups. When the intramolecular cycles are formed, the proportion of NA and NB must be changed in manner of the number of cycles. By comparison of the original proportion and the changed one, we can obtain the relationship of ANC and number of NA and NB. If the parameters in the expression of ANC are all measurable, this deduction will be feasible. First, for the polymers terminated with A functional groups, the perfect hyperbranched architecture without any cyclic structures is composed of two components, i.e., one unit of “A” and the repetitive units of “BA(n‑1)” as shown in Chart 1. If the number of B units in one polymer is set as N′B, the number of A units (N′A) can be expressed as:

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RESULTS AND DISCUSSION Theoretical Deduction of ANC and MC in the A2 + Bn System. The synthesis of highly branched polymers via a stepwise polymerization strategy is a batch reaction, i.e., A2 added over B3. In a batch reaction with three additions, at time t1, a small amount g1 of reactant of A2 monomer is added, and

N ′A = 1 + (n − 1)N ′B

(1)

Obviously, if the total number of polymers is N, the number of A units (N′A) can be expressed as follows. N ′A = N + (n − 1)N ′B 6186

(2)


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NA + n × NC = N + (n − 1) × (NB + NC)

Chart 1. Perfect Architecture of Hyperbranched Polymer Terminated with A Functional Groups via “A2 + B3” Strategy

(5)

Although the equation above is derived from the model of perfect hyperbranched architecture, it is also an expression of its parent highly branched polymer with intramolecular cyclic structures, which contains the relations of the total number of A units, B units and intramolecular cyclic structures. If the average number of cyclic structures (ANC) is defined as the ratio of NC to N, it can be obtained from eq 5. ANC =

NC N N = (n − 1) × B − A + 1 N N N

(6)

Similarly, for the highly branched polymer terminated with B functional groups, the perfect hyperbranched architecture without any cyclic structures is also composed of two components, i.e., one unit of “B” and the repetitive units of “−AB” as shown in Chart 3. If the number of A units in one polymer is set as N′A, the number of B units, i.e., N′B can be expressed as: As mentioned above, the cyclization is actually difficult to be avoided in the growth of polymers. Because of the threedimensional growth feature and topology defect,19 the cyclic structure may be subdivided into three types: perfect multicycles free of functional groups, multicycles having B groups, and multicycles containing A groups.19 The complex of cyclic types makes it difficult to conclude the relationship between the A and B group number of polymer with intramolecular cyclic structures. So we deduce that in a converse way. If the intramolecular cyclic structure in a highly branched polymer with a number of A groups (NA) and a number of B groups (NB) was assumed to be opened from the broken bond of “A−B”, the perfect hyperbranched architecture without any cyclic structures could be generated. The hypothesis scheme for highly branched polymer terminated with A groups is shown in Chart 2. Because the highly branched polymers are terminated

N ′Β′ = 1 + N ′A

(7)

Chart 3. Perfect Architecture of Hyperbranched Polymer Terminated with B Functional Groups via “A2 + B3” Strategy

Chart 2. Hypothesis Scheme from a Hyperbranched Architecture with Intramolecular Cyclic to Perfect One (A Terminal Groups) If the total number of polymer is N, the number of A units (N′B) can be obviously expressed as follows. N ′B = N + N ′A

Similarly, if the intramolecular cyclic structure in a hyperbranched architecture with a number of B groups (NA) and a number of B groups (NB) was assumed to be opened by breaking the “B−A” bond, the perfect hyperbranched architecture without any cyclic structure could be generated. The hypothesis scheme is described in Chart 4. Since the highly branched polymers are terminated with B groups, a broken “−

with A groups, a broken “−B” group must be linked with “−A” group and a broken “−A” group can be linked with “−BA(n‑1)” group, respectively. If the number of intramolecular cyclic structures in highly branched polymers with the number of N is defined as NC, the total number of N′A and N′B of that perfect hyperbranched architecture after the hypothesis of “ringopening”can be expressed as follows. N ′A = NA + n × NC

(3)

N ′B = NB + NC

(4)

(8)

Chart 4. Hypothesis Scheme from a Hyperbranched Architecture with Intramolecular Cyclic to Perfect One (B Terminal Groups)

Considering that the generated hyperbranched architecture is perfect without any cyclic structures, the relationship between N′A and N′B is in accordance with eq 2. Even more than one “− A” group or “−BA(n‑1)” group, such as (n − 1), are considered for eq 3 and eq 4; eq 5 can also be deduced. 6187


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A” group must be linked with “−B” group and a broken “−B” group can be linked with “−AB” group, respectively. If the number of intramolecular cyclic structures in hyperbranched architecture with the number of N is defined as NC, the total number of N′A and N′B of perfect hyperbranched architectures can be expressed as follows. N ′A = NA + NC

(9)

N ′B = NB + 2NC

(10)

calculated from their relative integrations in NMR spectrum. For a highly branched polymer containing intramolecular cyclic structures terminated with A groups, we define the parameters of k0 and k1 as follows, which can be calculated in the NMR spectrum. NB0: NA 0: NA1 = 1: k 0: k1

With the combination of eq 13 and eq 15, the value of NB0/ N, NA0/N, and NA1/N is deduced, respectively.

Considering that the generated hyperbranched architecture is perfect without any cyclic structures after the hypothesis of ring-opening, the relationship between N′A and N′B is in accordance with eq 8. Thus, the following equation can be obtained: NB + 2NC = N + (NA + NC)

⎧ NB Mn ⎪ 0 = M B0 + M A 0 × k 0 + M A1 × k1 ⎪ N ⎪ ⎪ NA 0 Mn × k0 = ⎨ M B0 + M A 0 × k 0 + M A1 × k1 ⎪ N ⎪ M n × k1 ⎪ NA1 ⎪ N = M +M ×k +M ×k B0 A0 0 A1 1 ⎩

(11)

Since the average number of cyclic structures (ANC) is defined as the ratio of NC to N, it can be obtained from eq 11. ANC =

NC N N = A − B +1 N N N

NB0 N

× M B0 +

NA 0 N

× MA0 +

NA1 N

× M A1

⎧N NB0 Mn ⎪ B = = N M B0 + M A 0 × k 0 + M A1 × k1 ⎪N ⎪ ⎪ NA 0 + NA1 ⎨ NA ⎪N = N ⎪ M n × (k 0 + k1) = ⎪ ⎪ + M M A 0 × k 0 + M A1 × k1 B0 ⎩

NA 0 N

× MA0 +

+ ···+

NB(n−1) N

NB0 N

× M B0 +

× M B(n−1)

NB1 N

(17)

With the combination of eq 17 and eq 6, the value of ANC can be expressed as follows. ANC =

M n × (n − 1 − k 0 − k1) +1 M B0 + M A 0 × k 0 + M A1 × k1

(18)

Similarly, for the hyperbranched polymer containing intramolecular cyclic structures terminated with B groups, the value of ANC is deduced from eq 12 and eq 14.

(13)

On the other hand, for the highly branched polymer containing intramolecular cyclic structures terminated with B groups, it is composed of −A− bridge units with a number of NA0, B0 dendritic units with a number of NB0, B1 units (one functional group left) with a number of NB1, B2 units with a number of NB2, B3 units with a number of NB3 ...and B(n‑1) units ((n − 1) functional group left) with a number of NB(n‑1). Thus, the number-average molecular weight of the above polymer can be expressed as the following equation Mn =

(16)

For a highly branched polymer containing intramolecular cyclic structures terminated with A groups (Chart 1), thus NB/ N and NA/N are deduced as follows.

(12)

Expression of ANC and MC by a Combination of NMR and SEC. As presented in eq 6 and eq 12, we deduced the relationship of ANC and A units, B units from a microcosmic viewpoint. However, the parameters of NA/N and NB/N are not measurable. Since NA/N and NB/N refer to the average number of A units and B units in each polymer, we try to employ the relationship of number of A and B units and Mn to calculate them. For highly branched polymer containing intramolecular cyclic structures terminated with A groups, as shown in Chart 1, the architecture is composed of B dendritic units (B0) with a number of NB0, −A− bridge units (A0) with a number of NA0 and −A terminal units (A1) with a number of NA1. Thus, the number-average molecular weight (Mn) of the above polymer can be expressed as follows, where MB0, MA0, and MA1 refer to the molar mass of B0 unit, A0 unit and A1 unit, respectively. Mn =

(15)

ANC = M n × (1 − k′0 − k′11 ···−k′(n − 1) M A 0 + M B0 × k′0 + M B1 × k′1 ···+M B(n−1) × k′(n − 1)

+1 (19)

Where the parameters of k0′, k1′..., k(n‑1)′ refer to the value of NB0/NA0, NB1/NA0..., NB(n‑1)/NA0 respectively. In eq 18 and eq 19, the value of MA0, MA1, MB0, MB1, MB(n‑1) is specialized according to the A2 and Bn monomers and the value of Mn and k0, k1, k0′, k1′ ..., k(n‑1)′ can be determined using SEC and NMR, respectively. Meanwhile, in order to avoid the influence of molecular weight, a parameter of MC can be defined as the ratio of Mn to ANC as shown in eq 20. Because ANC is equal to NC/N, the equation of NCMC = NMn can be obtained. So MC, i.e., the average molecular weight in accordance with one cyclic, can be defined as cyclic-average molecular weight. Normally, the value of MC is increased as the decrease of degree of cyclization. A higher value of MC indicates a lower cyclization degree.

× M B1 (14)

where MA0, MB0, MB1, ...... and MB(n‑1) refer to the molar mass of A0, B0, B1, ..., and B(n‑1) unit, respectively. Although the absolute value of NA0/N, NA1/N, NB0/N, ... or NB(n‑1)/N is difficult to be obtained, the ratio of A unit number to B unit number can be 6188


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Table 1. Main Polymerization Results of Highly Branched Polycarbosilanes from SEC−MALLS

a

samplea

A2 monomer

feed ratio A2/B3(mol/mol)

10−3Mw

10−3Mn

Mw/Mn

HP-But-SiH-1 HP-But-SiH-2 HP-Ph-SiH-1 HP-Ph-SiH-2 HP-But-Vi-1 HP-But-Vi-2 HP-Ph-Vi-1 HP-Ph-Vi-2 L-Ph-PCS

H(CH3)2Si−(CH2)4−Si(CH3)2H H(CH3)2Si−(CH2)4−Si(CH3)2H H(CH3)2Si−(CH2)4−Si(CH3)2H H(CH3)2Si−(CH2)4−Si(CH3)2H H(CH3)2Si−(C6H4)−Si(CH3)2H H(CH3)2Si−(C6H4)−Si(CH3)2H H(CH3)2Si−(C6H4)−Si(CH3)2H H(CH3)2Si−(C6H4)−Si(CH3)2H H(CH3)2Si−(C6H4)−Si(CH3)2H

2:1 2:1 2:1 2:1 1:1 1:1 1:1 1:1 1:1(B2b)

17.6 43.6 13.5 137.1 8.9 38.9 27.7 182.8 10.3

5.4 12.6 6.9 50.4 3.0 12.4 6.7 26.9 7.5

3.26 3.46 1.96 2.72 2.97 3.14 4.13 6.80 1.37

Samples were synthesized via a batch reaction of A2 added over B3. bB2 monomer is dimethyldivinylsilane.

MC =

Mn ANC

monomer to B3 monomer, highly branched polycarbosilanes (HP-But-Vi and HP-Ph-Vi) terminated with B functional groups (vinyl) and highly branched polycarbosilanes (HP-ButSiH and HP-Ph-SiH) terminated with A functional groups (silicon hydrogen bond) could be obtained. In all the cases, the hydrosilylation reaction between A2 and B3 monomers under the suitable reaction conditions generated highly branched polycarbosilanes, which were soluble in aliphatic and aromatic solvents and stable to air and moisture. For SEC measurements, macromolecules are fractionated according to hydrodynamic volume. The size of highly branched polymers is known to be smaller than that of linear ones with the same molecular weight. So, the true molecular weight cannot be obtained through general SEC procedure. Since simultaneous measurement of light scattering intensity and concentration allows direct determination of the weightaverage molecular weight for each eluted fraction without calibration by using standards materials, the SEC coupled with multiangle laser light scattering detector (SEC-MALLS) was employed to determine molecular weights of soluble highly branched polycarbosilanes as shown in Figure S11, Supporting Information. It should be pointed out that Mn of branched and cyclized polymer is not absolutely reliable although the above method is a little better than general SEC. When determining highly branched polymers, the effluent at a certain elution volume may contain molecules of the same hydrodynamic radius but of different molecular weight, they differ in number and arrangement of branch points and number and topology of cycles. It will cause same deviations between the determined molar weight distribution and actual molar weight distributions. MALDI−TOF MS is a powerful method for detection of molar weight distribution, 11b but it is not fully suitable for hyperbranched polymers without ionization feature, narrow molecular weight distribution, such as these highly branched polycarbosilanes. Considering the deviation between the values obtained by MALDI−TOF MS and SEC is acceptable,29 we present the SEC−MALLS results as summarized in Table 1. The weight-average molecular weight (Mw) was in the range from 8900 to 182 800 g/mol, and the PDI (Mw/Mn, polydispersity index) was in the range from 1.96 to 6.80. The structure of highly branched polycarbosilane was confirmed by using NMR and FT-IR spectroscopies. Figure 1 shows the 1H and 13C NMR spectra of HP-But-Vi-1 in CDCl3. Compared to the starting monomer 1 (Figure S1, Supporting Information), the characteristic signals of proton on the Si−H bond completely disappeared. At the same time, the broad peaks in the region of 0.37−0.50 ppm in correspondence with the protons of formed alkyl-bridges appeared. Compared to the starting TVMS, the decreased integral area of vinyl protons at

(20)

For the synthesis of highly branched polymers, the “A2 + B3” route is an important type of “A2 + Bn”(n ≥ 3) approaches first reported by Fréchet6c and Kakimoto.25 Thus, the expression of ANC in eq 18 and eq 19 can be simplified as follows. ANC =

ANC =

M n × (2 − k 0 − k1) +1 M B0 + M A 0 × k 0 + M A1 × k1

MA0

(21)

M n × (1 − k′0 − k′11 − k′2 ) +1 + M B0 × k′0 + M B1 × k′1 + M B2 × k′2 (22)

If an addition reaction was employed in “A2 + B3” route, the ANC can be further simplified because the molar mass of A or B unit equals the molar mass of their original monomer. If we define the parameters of k and k′ as the value of NA/NB and N′B/N′A for highly branched polymers terminated with A groups and with B groups, the eq 21 and eq 22 can be simplified into eq 23 and eq 24, respectively. ANC =

NC M × (2 − k) = n +1 N MB + k × MA

(23)

ANC =

NC M × (1 − k′) = n +1 N MB + k′ × MA

(24)

The expression of ANC and MC is simple and measurable, where the parameters of k (k′) and Mn can be conveniently determined by a combination of NMR spectrum and SEC analytics. Synthesis of Model Polymers for A2 + B3 System. To quantificationally describe the intramolecular cyclization, the novel highly branched polycarbosilanes were employed as model polymers. They were synthesized via “A2 + B3” strategy based on hydrosilylation addition reaction where 1,4-bis(dimethylsilyl)butane (1) and 1,4-bis(dimethylsilyl)benzene (2) were used as A2 monomers with flexible or rigid interval unit between A groups. At the same time, trivinylmethylsilane (TVMS) was selected as B3 monomer as presented in Scheme 1. To avoid the gelation and to increase the molecular weight,26 a dilute solution of A2 monomer was added into B3 monomer by applying batch reaction model. After the polymerization, the crude product was dissolved in diethyl ether and subsequently precipitated in methanol for three times to remove residual monomers, oligomers or some intermolecular cyclic products with low molecular weight. According to the feed ratio of A2 6189


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Figure 1. NMR spectra of HP-But-Vi-1 in CDCl3: (a) 1H NMR; (b) 13 C NMR.

Figure 2. NMR spectra of HP-But-SiH-1 in CDCl3: (a) 1H NMR; (b) 13 C NMR.

5.62−6.30 ppm (−SiCH3 as an internal standard) reveals the hydrosilylation between monomer 1 and TVMS. In addition, the 13C NMR spectrum in Figure 1 identifies vinyl functional groups of HP-But-Vi-1, whose signal is in the region of 131.42−137.56 ppm. The disappearance of Si−H bond and the reservation of vinyl groups suggest that highly branched polycarbosilane terminated with vinyl groups was successfully synthesized. Different from HP-But-Vi-1, the 1H NMR spectrum of HPBut-SiH-1 in Figure 2 indicates that the characteristic signals of vinyl protons from B3 monomer completely disappeared, resulting in the broad signal peaks in the region of 0.31−0.44 ppm and 0.85−0.93 ppm for the protons of alkyl bridges. The 13 C NMR spectrum in Figure 2 further confirms the alkyl bridges. In addition, a small signal of proton from Si−H bond at 3.82−3.90 ppm on 1H NMR is observed, which is also confirmed by its FT-IR resonance at 2110 cm−1 (Figure S2, Supporting Information). Thus, the termini of HP-But-SiH can be surely considered as Si−H bonds. As expected, the NMR analysis demonstrates that the terminals of highly branched polycarbosilanes were conveniently synthesized by regulating feed ratio of A2 and B3 monomer. Since the dendritic, linear, and terminal units of highly branched polycarbosilanes can be identified by 29Si NMR analysis, we employ 1H−29Si HMBC to analyze their hyperbranched architectures. For HP-But-Vi-1, the signal at 7.58−8.19 ppm in the 29Si NMR spectrum (Figure 3) is attributed to silicon atom correlated with one methyl (−0.05−

Figure 3. 1H−29Si HMBC NMR spectrum of HP-But-Vi-1 in CDCl3.

0.21 ppm in 1H NMR) and three alkyl bridges (0.37−0.50 ppm in 1H NMR), so it is defined as dendritic unit (Si(I)). In addition, the silicon atom on terminal unit (Si(II)) linked with one alkyl bridge (0.37−0.50 ppm in 1H NMR) and two vinyl groups (5.62−6.30 ppm in 1H NMR) shows the signals at −11.36 ppm on 29Si NMR spectrum. Similarly, the silicon atom of linear unit (Si(III)) linked with one vinyl (5.62−6.30 ppm in 1 H NMR) and two alkyl bridges (0.37−0.50 ppm in 1H NMR) shows the signals at −1.82 ppm on 29Si NMR spectrum. At last, the silicon atom on interval unit (Si(IV)) linked with two methyls (−0.05−0.21 ppm in 1H NMR), −CH2−CH2− 6190


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(0.37−0.50 ppm in 1H NMR) and −(CH2)4− (1.27−1.43 ppm in 1H NMR) shows the signals at 3.57−4.32 ppm on 29Si NMR spectrum. For HP-But-SiH-1, the signal around −11.36 ppm (Figure S3, Supporting Information) is attributed to the silicon atom on terminals or linear units because of the nearly same chemical environment, i.e., linked with one hydrogen atom, one alkyl-bridge and two methyl groups. In order to study the compact conformation of highly branched polycarbosilanes, their intrinsic viscosity values, [η], were measured in THF at 25 °C by triple-detection size exclusion chromatography (Triple-SEC). The intrinsic viscosity of polymers is related with their molecular weight by the Mark−Houwink−Sakurada (MHS) equation, [η] = KMα. The MHS exponent, α, is a parameter corresponding to the topology of a polymer in a good solvent. For hyperbranched polymers, the exponent typically varies between 0.5 and 0.2, depending on their degree of branching. In contrast, the MHS exponent is typically in the region of 0.6−0.8 for a linear polymer in a good solvent with a random coil conformation.27 The Triple-SEC results are summarized in Table 2 and the

Chart 5. Constituted Units and Silicon Atom Chemical Environments in HP-But-SiH

Table 2. Conformation Parameters of Highly Branched Polycarbosilanes from Triple-SEC and 29Si NMR sample

10−3Mw,triplea

[η]w (mL/g)a

HP-But-SiH-1 HP-But-SiH-2 HP-Ph-SiH-1 HP-Ph-SiH-2 HP-But-Vi-1 HP-But-Vi-2 HP-Ph-Vi-1 HP-Ph-Vi-2 L-Ph-PCS

17.6 43.6 13.5 137.1 8.9 38.9 27.7 182.8 10.3

10.3 19.0 8.8 19.8 7.0 16.6 16.2 23.2 19.0

K (mL/g)a

αa

DBb

1.246 0.218 1.152 1.224 0.390 0.742 1.614 1.152 0.068

0.25 0.37 0.26 0.25 0.35 0.31 0.25 0.26 0.58

− − − − 0.60 0.48 0.45 0.51 −

Figure 4. Quantitative CDCl3.

29

Si NMR spectrum of HP-But-SiH-1 in

⎧ ∑ Si(IV) − ∑ Si(V) ⎪ NA 0 2 = ⎪ ⎪ NB0 ∑ Si(I) ⎨ ⎪ NA ∑ Si(V) ⎪ 1 = ⎪ NB ∑ Si(I) ⎩ 0 NA 0 + NA1 N k= A = = NB NB0

a

The refractive index increment (dn/dc) value of sample in THF was determined at 25 °C. bThe parameters were determined by 29Si NMR as shown in Figure S5−S10, Supporting Information.

∑ Si(IV) − ∑ Si(V) 2

+ ∑ Si(V)

∑ Si(I)

For highly branched polycarbosilanes terminated with vinyl groups (HP-But-Vi), the molecular structure is constituted of four units, i.e. dendritic unit (B0), linear unit (B1), terminal unit (B2), and interval unit (A0). The possible silicon atom chemical environments are shown in Chart 6, which have been distinguished by the 1H−29Si HMBC spectrum (Figure 3). Here, Si(I), Si(II), Si(III) and Si(IV) only belong to B0 unit, B1 unit, B2 unit, and A0 unit, respectively. On the basis of the quantitative 29Si NMR spectrum in Figure 5, the value of NB0/ NA0, NB1/NA0 and NB2/NA0 can be calculated as follows. Thus, the values of MC and ANC of HP-But-Vi and HP-Ph-Vi were calculated on the basis of eq 20 and eq 24 as presented in Table 4.

MHS plots are presented in Figure S4, Supporting Information. As a comparison, the linear analogue (L-Ph-PCS) was prepared via the hydrosilylation of monomer 2 and dimethyldivinylsilane (B2 monomer). The MHS exponent α = 0.58 indicates the random coil conformation of linear analogue, whereas the MHS exponents of hyperbranched polycarbosilanes are significantly lower (α = 0.25−0.37). With the combination of DB values from 0.45 to 0.60, it evidently demonstrates their highly branched architectures. Experimental Validation of ANC and MC in A2 + B3 System. For the highly branched polycarbosilanes terminated with Si−H groups (HP-But-SiH), the molecular structure is constituted of dendritic unit (B0), interval unit (A0), linear and terminal units (A1) as shown in Chart 5. They were clearly distinguished by 1H−29Si HMBC NMR spectrum in Figure S5, Supporting Information. Here Si(I) and Si(V) only belong to B0 units and A1 units, while Si(IV) is shared by A0 units and A1 units. On the basis of the quantitative 29Si NMR spectrum in Figure 4, the value of NA0/NB0 and NA1/NB0 can be calculated as follows. So, the values of MC and ANC of HP-But-SiH and HPPh-SiH were calculated on the basis of eq 20 and eq 23 as presented in Table 3.

⎧ NB ∑ Si(I) ⎪ 0 = ∑ Si(IV) N ⎪ A0 2 ⎪ ⎪ ∑ Si(II) ⎪ NB1 = ∑ Si(IV) ⎨ ⎪ NA 0 2 ⎪ ⎪ NB2 ∑ Si(III) = ∑ Si(IV) ⎪ ⎪ NA 0 ⎩ 2 6191


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Table 3. ANC and MC of Highly Branched Polycarbosilanes Terminated with Si−H Groups Synthesized via a Batch Reaction of A2 Added over B3 sample

NA0/NB0

NA1/NB0

k

Mn

MA

MB

ANC

MC

HP-But-SiH-1 HP-But-SiH-2 HP-Ph-SiH-1 HP-Ph-SiH-2

1.432 1.500 1.460 1.315

0.140 0.000 0.079 0.372

1.572 1.500 1.539 1.687

5400 12 600 6900 50 400

174.43 174.43 194.43 194.43

124.26 124.26 124.26 124.26

6.8 17.3 8.5 36

800 730 910 1400

analogues, respectively. It can be attributed to the A or B groups are consumed to form cyclic structures during the growth process of hyperbranched polymers. The obvious ANC with a range of 1.6−36 further confirms the existence of intramolecular cyclization in hyperbranched polycarbosilanes. Compared with the hyperbranched polycarbosilane terminated with vinyl groups, the high ANC and low MC of hyperbranched polycarbosilanes terminated with Si−H groups indicates their high extent of cyclization in a wide molecular weight range. It is mainly contributed to the high feed ratio of A2 to B3 monomer, which gives the high potential and possibility to intramolecular cyclization reaction in competition with chain growth. For highly branched polycarbosilanes terminated with either Si−H groups or vinyl groups, the high ANC is observed in high molecular weight due to the scale effect of hyperbranched architecture. It is in correspondence with the modeling and experiments by Dušek et al.,28 where the fraction of cyclics in hyperbranched polyester via BA2 stretegy increases with increasing molecular weight and increasing overall conversion of B groups. As to the ratio of the cyclic-average molecular weight, the MC of highly branched polycarbosilanes with rigid interval unit of phenyl (HP-Ph-SiH or HP-Ph-Vi) is higher than that of hyperbranched polycarbosilanes with flexible interval unit of butyl in a wide molecular weight range. Since a higher value of MC represents a lower degree of cyclization, it indicates that the highly branched polycarbosilanes with rigid interval units possess low extent of intramolecular cyclization compared with highly branched polycarbosilanes with flexible interval units. So the regulation of flexibility and rigidness of A2 or Bn monomers supplies a convenient method to control intramolecular cyclization degree and to decrease topology defects in hyperbranched architecture.

Chart 6. Constituted Units and Silicon Atom Chemical Environments in HP-But-Vi

Figure 5. Quantitative 29Si NMR spectrum of HP-But-Vi-1 in CDCl3.

k′ = = =

NB NA NB0 + NB1 + NB2 NA 0 ∑ Si(I) + ∑ Si(II) + ∑ Si(III) ∑ Si(IV) 2

■

CONCLUSIONS The average number of cyclic structures and cyclic-average molecular weight for highly branched polymers can be defined and obtained by a combination of NMR and SEC. They depend on the ratio of dendritic, linear and terminal units as well as number-average molecular weight. The quantificational description of ANC and MC make it easy to well understand the intramolecular cyclic structures resulting in a highly branched topology. The highly branched polycarbosilanes with rigid interval units possess low extent of intramolecular cyclization compared with those with flexible interval units. Regulating the

Besides the 29Si NMR, the ANC and MC can be also calculated from a combination of SEC and another quantitative NMR, such as 1H, 11B, 15N NMR if they can identify the dendritic, linear, and terminal units at the same time. So this strategy is very general and convenient to a variety of highly branched polymers synthesized via a versatile A2 + Bn strategy. Intramolecular Cyclization in A2 + B3 System. From Table 3 and Table 4, the k and k′ values in correspondence with NA/NB for hyperbranched polycarbosilanes terminated with A groups and N′B/N′A for polymers terminated with B groups are lower than the ratio of 2 and 1 of their perfect hyperbranched

Table 4. ANC and MC of Highly Branched Polycarbosilanes Terminated with Vinyl Groups Synthesized via a Batch Reaction of A2 Added over B3 sample

NB0/NA0

NB1/NA0

NB2/NA0

k′

Mn

MA

MB

ANC

MC

HP-But-Vi-1 HP-But-Vi-2 HP-Ph-Vi-1 HP-Ph-Vi-2

0.425 0.331 0.247 0.407

0.326 0.454 0.535 0.390

0.073 0.099 0.189 0.000

0.824 0.884 0.971 0.797

3000 12 400 6700 26 900

174.43 174.43 194.43 194.43

124.26 124.26 124.26 124.26

3 6.2 1.6 21

1010 2010 4140 1300

6192


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−CH2CH2CH2CH2−). 29Si NMR (CDCl3, ppm): −13.24. FT-IR (KBr, cm−1): 2112 (ν Si−H), 1250 (ν Si−CH3), 836 (ν Si−CH2−). Synthesis of 1, 4-Bis(dimethylsilyl)benzene (2). The synthesis procedures of 1,4-bis(dimethylsilyl)benzene was similar to 1, but using 1,4-dibromobenzene (4.81 g, 20 mmol) as the reactant instead of 1,4dibromobutane. The product 2 is also a colorless liquid (2.83 g, 73% yield). 1H NMR (CDCl3, ppm): 0.38 (12H, −Si(CH3)2−H), 4.44 (2H, −Si(CH3)2−H), 7.45 (4H, −C6H4−). 13C NMR (CDCl3, ppm): −3.69 (4C, −Si(CH3)2−H), 133.85 and 138.68 (6C, −C6H4−). 29Si NMR (CDCl3, ppm): −17.85. FT-IR (KBr, cm−1): 3047 (ν −C6H4−), 2120 (ν Si−H), 1250 (ν Si−CH3). Preparation of Highly Branched Polycarbosilanes Terminated with Si−H Bonds from Monomer 1 (HP-But-SiH). The synthesis of highly branched polymers is a batch reaction, i.e. A2 added over B 3 . A dried flask was charged with B 3 monomer of trivinylmethylsilane (TVMS, 0.149 g, 1.2 mmol), 1.2 mg of karstedt’s catalyst, and 20 mL of toluene. At 50 °C, A2 monomer of 1 (0.419 g, 2.4 mmol) was added into the solution by three-batch addition with an interval of 30 min. In detail, at first, 0.14 g of the A2 monomer was added by a syringe, after 30 min another 0.14 g of reactant was added. And after 30 min, the last 0.139 g of reactant was added. The reaction mixture was stirred overnight at 50 °C and monitored by FTIR spectroscopy. Before the gel point, the reaction was stopped and the solvent was removed by rotary evaporation (40 mbar, 40 °C) yielding colorless liquids. The crude product was soluble in 5 mL of diethyl ether and precipitated into 50 mL of methanol for three times. Then the precipitate was dried in vacuum oven for 2 days (10 mbar, 60 °C). The HP-But-SiH-1 was obtained as a colorless viscous liquid (78.6% yield). 1H NMR (CDCl3, ppm): −0.10−0.09 (−Si−CH3), 0.31−0.44 (Si−CH2−CH2−Si), 0.48−0.55 (4H, Si−CH2CH2CH2CH2−Si), 0.85−0.93 (Si−CH(CH3)−Si), 1.28 (Si−CH(CH3)−Si), 1.29−1.35 (Si−CH2−CH2−CH2 −CH2−Si), 3.82−3.90 (Si-H). 13C NMR (CDCl3, ppm): −6.61, −3.82 (−Si−CH3), 4.59, 7.20 (Si−CH2− CH2−Si), 14.41 (−CH2−CH2−CH2−CH2−), 28.06 (−CH2−CH2− CH2−CH2−). 29Si NMR (CDCl 3, ppm): −13.31 to −13.06 [(−C4H8−)(CH3−)2Si−H], 3.16−4.53 [(−C4H8−)(CH3−)2Si− C2H4−], 7.22−8.38 [(CH3−)Si(−C2H4−)3]. FT-IR (KBr, cm−1): 2111 (Si−H), 1408, 1248 (Si−CH3), 832 (Si−C). Preparation of Highly Branched Polycarbosilanes with Si−H Terminal from Monomer 2 (HP-Ph-SiH). The preparation procedure of HP-Ph-SiH was similar to HP-But-SiH, but the feed amounts were changed: 0.6 mmol of TVMS, 1.2 mmol of monomer 2, 40 mL of toluene and 0.6 mg of catalyst. HP-Ph-SiH-1 was obtained as a colorless viscous liquid (77.5% yield). 1H NMR (CDCl3, ppm): −0.14−0.30 (−Si−CH3), 0.39−0.65, (Si−CH2−CH2−Si), 4.39−4.48 (Si-H), 7.41−7.57 (−C6H4−). 13C NMR (1, CDCl3, ppm): −6.61, −3.73 (Si−CH3), 4.49, 7.65 (Si−CH2−CH2−Si), 133.04, 139.98 (−C6H4−). 29Si NMR (CDCl3, ppm): −17.50 to −17.05 [(−C6H4−)(CH3−)2Si−H], −1.05 to −2.15 [(−C6H4−)(CH3−)2Si−C2H4−], 7.91−8.94 [(CH3−)Si(−C2H4−)3]. FT-IR (KBr, cm−1): 3046 (ν −C6H4−), 2119 (ν Si−H), 1248 (ν Si−CH3), 799 (ν Si−C). Preparation of Highly Branched Polycarbosilanes Terminated with Vinyl Groups Monomer 1 (HP-But-Vi). The preparation procedure of HP-But-Vi was similar to HP-But-SiH, but the feed amounts were changed: 2.4 mmol TVMS, 2.4 mmol monomer 1, 20 mL toluene and 2.4 mg catalyst. HP-But-Vi was obtained as a colorless viscous liquid (73.5% yield). 1H NMR (CDCl3, ppm): −0.05 to +0.21 (−Si−CH3), 0.37−0.50 (Si−CH2−CH2−Si), 0.52−0.64 (4H, Si−CH2CH2CH2CH2−Si), 1.27−1.43 (−CH2−CH2− CH2−CH2−), 5.62−5.88 (−CHCH2), 5.94−6.30 (−CHCH2). 13 C NMR (CDCl3, ppm): −6.32, −3.84 (Si−CH3), 4.59, 5.15, 7.15 (Si−CH 2 −CH 2 −Si), 14.43 (−CH 2 −CH 2 −CH 2 −CH 2 -), 28.03 (−CH2−CH2-CH2−CH2−), 131.42, 132.27 (−CHCH2), 136.47, 137.56 (−CHCH2). 29Si NMR (CDCl3, ppm): −11.36-(−11.10) [(CH3−)(CH2CH−)2Si−C2H4−], −2.02 to −1.41 [(CH3−)(CH2CH−)Si(−C2H4−)2], 3.57−4.32 [(−C4H8−)(CH3−)2Si(−C2 H4−)], 7.58−8.19 [(CH 3−Si(−C2 H4−) 3)]. FT-IR (KBr, cm−1): 3047 (−CHCH2), 1248 (Si−CH3), 809 (Si−C). Preparation of Highly Branched Polycarbosilanes Terminated with Vinyl Groups Monomer 2 (HP-Ph-Vi). The

flexibility and rigidness of A2 or Bn monomers supplies a convenient method to control intramolecular cyclization and to decrease topology defects in hyperbranched architecture. Because of the general and convenient nature, the ANC and MC can have potential for the quantificational description of intramolecular cyclization in a variety of highly branched polymers synthesized via a versatile “A2 + Bn” (n ≥ 3) strategy. It is very helpful to the comprehension of topological defect and to obtain the “defect-free” topological polymers with controlled methodologies.

■

EXPERIMENTAL SECTION

Materials. 1,4-Dibromobenzene (98%), 1,4-dibromobutane (99%), chlorodimethylsilane (97%), magnesium powder and platinum(0)-1,3divinyl-1,1,3,3-tetramethyldisiloxane complex (karstedt’s catalyst) were purchased from Alfa Aesar China. Trivinylmethylsilane (95%) were purchased from Gelest, Inc. (USA). All the above reagents were used as received without further treatment. Anhydrous hexane, toluene, and tetrahydrofuran (THF) were freshly distilled under reflux using sodium/benzophenone. Characterization. Nuclear magnetic resonance (NMR) measurements were carried on a Bruker Avance 500 spectrometer (Bruker BioSpin, Switzerland) operating at 50.7 MHz in CDCl3. Chemical shifts are referenced to tetramethylsilane (TMS). Long-range 1H−29Si heteronuclear multiple-bond correlation (1H−29Si HMBC) spectra were acquired with pulse field gradients in absolute value mode. The multiple-bond delay was adjusted to a coupling constant of 5 Hz. The data were collected in an 8192 × 256 matrix with 8 transients per t1 increment. The recycle period was 1.5 s. Sine-bell window functions were applied before Fourier transformation in a 2048 × 1024 matrix. Fourier transform infrared spectroscopy (FT-IR) measurements were conducted on a FT-IR spectrophotometer (Perkin-Elmer, USA). Triple-detection Size Exclusion Chromatography (Triple-SEC) measurements were conducted on SEC system equipped a Waters 515 pump, an autosampler and two MZ gel columns (103 Å and 104 Å) with a flow rate of 0.5 mL/min in THF (HPLC grade) at 25 °C. Detectors were including differential refractometer (Optilab rEX,Wyatt), multiangle light scattering detector (MALS) equipped with a 632.8 nm He−Ne laser (DAWN EOS, Wyatt) and Viscometer (ViscoStar, Wyatt). The MALS detector was used to determine the molecular weight, whereas the viscometer provided Mark−Houwink− Sakurada relationships. The refractive index increments of polymers in THF were measured at 25 °C using an Optilab rEX differential refractometer. ASTRA software (Version 5.1.3.0) was utilized for acquisition and analysis of data. Synthesis of 1, 4-Bis(dimethylsilyl)butane (1). Under an insert argon atmosphere, a 500 mL flame-dried flask equipped with thermometer, stir bar, condenser and drop funnel was charged with magnesium powder (0.73 g, 30 mmol), iodine crystal (1 mg), and 100 mL of anhydrous THF at room temperature. 1,4-Bis(dimethylsilyl)butane (4.36 g, 20 mmol) in 50 mL of THF was added to the flask under ultrasonic condition via drop funnel. The starting of reaction was indicated by the decolorization of iodine and the reflux of THF. When the addition was completed, the mixture was stirred for 1 h under ultrasonic at 60 °C, and then the mixture was cooled to 5 °C without ultrasonic. Chlorodimethylsilane (2.15 g, 22 mmol) in 50 mL of THF was added to the mixture drop by drop. After that, the mixture was heated to 60 °C and stirred for 2 h. Upon the completion of reaction, the THF in filtrate was removed by rotary evaporation. The collected product is soluble in anhydrous hexane again. After the filtration and rotary evaporation, the crude product was purified by silica column chromatography using hexane as elute, yielding 1,4bis(dimethylsilyl)butane (1) as colorless liquid (2.61 g, 75% yield). 1H NMR (CDCl3, ppm): 0.06 (12H, −Si(CH3)2−H), 0.66 (4H, Si− CH2CH2CH2CH2−Si), 1.45 (4H, −CH2CH2CH2CH2−), 3.92 (2H, −Si(CH3)2−H). 13C NMR (CDCl3, ppm): −4.41 (4C, −Si(CH3)2− H), 14.22 (2C, S i−CH 2 CH 2 CH 2 CH 2 −Si), 28.22 (2C, 6193


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preparation procedure of HP-Ph-Vi was similar to HP-But-SiH, but the feed amounts are changed: 3.2 mmol TVMS, 3.2 mmol of monomer 2, 10 mL of toluene and 3.2 mg of catalyst. Ph-Vi-01 was obtained as a colorless viscous liquid (86.5% yield). 1H NMR (CDCl3, ppm): −0.17−0.33 (−Si−CH3), 0.37−0.72 (Si−CH2−CH2−Si), 0.76−1.06 (Si−CH(CH 3)−Si), 1.08−1.39 (Si−CH(CH 3 )−Si), 5.52−5.77 (−CHCH2), 5.87−6.22 (−CHCH2), 7.38−7.57 (−C6H4−). 13C NMR (CDCl3, ppm): −6.75, −4.04 (Si−CH3), 4.96 (Si−CH(CH3)−Si), 4.25, 7.19 (Si−CH2−CH2−Si), 29.25 (Si−CH(CH3)−Si), 132.44, 139.54 (−C6H4−), 131.78 (−CHCH2), 136.33, 137.20 (−CHCH2). 29Si NMR (CDCl3, ppm): −11.55-(−10.40) [(CH3−)(CH2CH−)2Si−C2H4−], −2.13-(−0.84) [(−C6H4−)(CH3−)2Si−C2H4−] and [(CH3−)(CH2CH−)Si(−C2H4−)2], 7.79−8.70 [(CH3-)Si(−C2H4−)3]. FT-IR (KBr, cm−1): 3047 (ν −CHCH2 and −C6H4−), 1248 (ν Si−CH3), 795 (ν Si−C).

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ASSOCIATED CONTENT

* Supporting Information S

NMR and FTIR results and Mark−Houwink−Sakurada plots of highly branched polycarbosilanes and a typical SEC−MALLS elution curve. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from NSFC of China (No.21174112 and 20874080) is acknowledged. J.K. thanks the grant from the Program for New Century Excellent Talents of Ministry of Education, New Faculty and Research Area Project, and Basic Scientific Research Foundation of NPU. The helpful discussion with Prof. Axel H. E. Müller (University of Bayreuth, Germany) and Dr. Jun Ling (Zhejiang University, China) is gratefully acknowledged.

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REFERENCES

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Intramolecular Cyclization in A2 + B3 Polymers via Step-Wise

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