Synthesis, Thermal Properties, and Thermoresponsive...

CommunicationMacromolecularRapid Communications

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1. Introduction

The development and enrichment of complex macro-molecular architectures have revolutionized polymer sci-ence in the past few decades. The signifi cant progress in living/controlled polymerization techniques allows facile construction of a variety of precisely controlled topological polymers. [ 1–5 ] Among them, cyclic polymers prepared via ring-closure [ 6,7 ] and ring-expansion [ 8,9 ] techniques have attracted increasing attention due to the intriguing topo-logical constraint and its impact on comprehensive proper-ties. [ 10–16 ] Because of reduced hydrodynamic volume, ring polymers lacking chain ends have a more compact struc-ture as compared to their linear analogs with the same molecular weight (MW). The infl uence of “end-to-end”

This study aims at physicochemical properties of thermo- and pH/CO 2 -responsive cyclic homopolymers. Three examples of cyclic poly(2-(dimethylamino)ethyl methacrylate)s (PDMAs) are synthesized by combining the reversible addition–fragmentation chain transfer process and the Diels–Alder ring-closure reaction. After cyclization, the glass transition temperature signifi cantly increases (Δ T g = 51.8–59.7 °C) due to the different confi gurational entropy and end groups, and the maximum decomposition temperature to lose the pendent groups is dras-tically decreased from 309 to 278 °C. Effects of polymerization degree, polymer concentration, additive of NaCl, and pH/CO 2 on lower critical solution temperature behaviors of PDMA aqueous solutions are investigated. The cloud points ( T c ) of ring PDMAs are usually higher than their linear precursors, and the Δ T c values obtained under a fi xed condition can reach up to 20.7 °C, revealing the crucial role of the topology effect. This study paves the way for unique properties and applica-tions of smart cyclic polymers and their derivatives.

Synthesis, Thermal Properties, and Thermo-responsive Behaviors of Cyclic Poly(2-(dimethylamino)ethyl Methacrylate)s

Xiaonan An , Qingquan Tang , Wen Zhu , Ke Zhang , * Youliang Zhao *

X. N. An, Prof. Y. L. Zhao Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry Chemical Engineering and Materials Science Soochow University Suzhou 215123 , China E-mail: [email protected] Dr. Q. Q. Tang, Dr. W. Zhu, Prof. K. Zhang State Key Laboratory of Polymer Physics and Chemistry Institute of Chemistry The Chinese Academy of Sciences Beijing 100190 , China E-mail: [email protected]

Macromol. Rapid Commun. 2016, 37, 980−986

MacromolecularRapid CommunicationsSynthesis, Thermal Properties, and Thermoresponsive Behaviors of Cyclic Poly(2-(dimethylamino)ethyl Methacrylate)s

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linkage and the unique conformational properties have prompted studies on bulk and solution properties involving glass transition temperature, [ 17–20 ] self-assembly, [ 21,22 ] and thermo-dependent phase transition. [ 23–25 ] The expansion of functional ring polymers can not only inspire mod-eling/simulation studies to reveal the topology effect but also realize the unprecedented functions and properties for their eventual applications. Moreover, the introduc-tion of intelligent segments and moieties can further endow cyclic polymers with on-demand functions and applications. [ 24–28 ]

On the other hand, much emphasis has been paid on stimuli-responsive polymers due to the overwhelming ability to adapt to surrounding environments and great potential in smart materials. [ 29–31 ] Owing to the vital role as signaling factors in the physiological environment, the intelligent systems bearing thermo and pH dual stimuli have been widely investigated. [ 29–34 ] In theory, such sys-tems can be constructed via either introduction of dif-ferent segments/components or utilization of a multi-functional segment, and both are promising for practical applications. At present, an important trend lies in con-struction of dual- and multiresponsive homopolymers due to the facile synthesis and rapid response to external stimuli. [ 32–34 ] Among them, poly(2-(dimethylamino)ethyl methacrylate) (PDMA) sensitive to temperature, pH/CO 2 ,

and ionic strength is an ideal choice to reveal the advan-tages of multiple responsivenesses. [ 32,35–37 ] PDMA is liable to exhibit pH-dependent polyelectrolyte behavior and lower critical solution temperature (LCST) ranging between 32 and 53 °C, leading to widespread thera-peutic and biomedical applications. [ 32,38–40 ] Thus far, the examples of cyclic PDMAs and their derivatives are very scarce, [ 32 ] although abundant PDMA-bearing poly mers with distinct architectures have been prepared. Consid-ering that the researches on their functions and proper-ties can underlie the multipurpose applications in func-tional biomedical and surface materials, it is extremely urgent to systematically explore the architecture–property correlations of cyclic PDMAs.

This study aims at synthesis and properties of cyclic PDMAs with low molecular weights via UV-induced Diels–Alder click reaction. [ 13–16 ] To this end, 3-(2-formyl-3-methy lphenoxy)propyl 4-(benzodithioyl)-4-cyanopentanoate (FBCP) mediated reversible addition–fragmentation chain transfer (RAFT) polymerization of DMA was employed to generate α-orthoquinodimethane-ω-dithiobenzoate heterofunctionalized PDMA, and then followed by Diels–Alder reaction between two end groups to form cyclic PDMA (Scheme 1 ). Owing to its many advan-tages involving lack of catalyst, low temperature, and tol-erance to air, the UV-induced ring-closure method could


FBCPDMA

RAFT

linear PDMA (L1-L3)

O O

O

O

CN

S

OO

S

N

n

cyclic PDMA(C1-C3)

S

OH

S

OO

O

OO

N

n CN

FBCP

O OS

O

O

CN S

UV

Scheme 1. Synthesis of cyclic PDMAs via successive RAFT polymerization and UV-induced Diels–Alder reaction.

X. N. An et al.MacromolecularRapid Communications

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afford well-defi ned ring polymers, as evidenced from 1 H NMR and gel permeation chromatography (GPC) analyses. On this basis, thermal properties and thermo-dependent phase transition of cyclic PDMAs were investigated, and the properties were compared with their linear precur-sors. To the best of our knowledge, this is the fi rst report on physicochemical properties of multisensitive cyclic homopolymers, which is benefi cial to reveal the relation-ships among degree of polymerization (DP), topology, and functions. The progress in this study further paves the way for synthesis, properties, and applications of PDMA-related cyclic topologies involving cyclic block copoly-mers, sunfl ower-like starlike/graft copolymers, and their quaternized derivatives.

2. Results and Discussion

2.1. Synthesis of Cyclic PDMAs and Their Linear Precursors

First, linear PDMAs (L1–L3) were synthesized by RAFT polymerization of DMA mediated by FBCP in dioxane. Homopolymerization conducted at 60 °C for 8 h afforded linear PDMAs in 39.6%–57.5% conversion. In 1 H NMR spectra (Figure 1 a and Figure S1, Supporting Informa-tion), typical signals originating from the RAFT agent were noted at 10.67 (ArC H O), 7.87, 7.52, 7.38, 6.83 (Ph H and Ar H ), 2.57 (ArC H 3 ), 4.31 (ArC H 2 O), and 4.14 ppm (COOC H 2 ), and characteristic signals in PDMA segment appeared at 4.06 (C H 2 O), 2.57 (C H 2 N), and 2.28 ppm (C H 3 ). By comparing the integrated signals at 6.83 and 2.28 ppm, the degree of poly-merization (DP PDMA = I 2.28 /(3 I 6.83 )) was calculated as 11.2 (L1), 30.0 (L2), and 50.6 (L3), respectively. The molecular weight determined by 1 H NMR analysis ( M n,NMR ) was close to the theoretical values ( M n,th ), and the GPC traces exhibited monomodal distribution (Figure 2 ), with poly-dispersity index (PDI) in the range of 1.10–1.16 (Table S1, Supporting Information).

Second, UV-induced ring-closure reaction was used to generate ring PDMAs (C1–C3). The dilute solution of linear precursors in acetonitrile ( c = 0.05 mg mL −1 ) was subjected to UV irradiation for 12 h at room temperature, and fol-lowed by evaporation of solvent to give ring polymers in quantitative yield. Careful inspection of 1 H NMR spec-trum of isolated C3 (Figure 1 b) revealed that new signals generated by the cyclization appeared at 6.77 (Ar H ), 5.26 (ArC H OH), and 3.64 ppm (ArC H 2 ), and the characteristic signals corresponding to ArC H O (10.67 ppm), Ph H con-necting with the RAFT moiety (i.e., 7.87 ppm), and ArC H 3 (2.57 ppm) were wholly disappeared. In comparison with their linear precursors, the GPC traces of cyclic PDMAs completely shifted to lower molecular weight side because of reduced hydrodynamic volume (Figure 2 ). The apparent molecular weights estimated by GPC ( M n,GPC ) of C1–C3

ranged between 1660 and 6200 g mol −1 , with PDI values similar to their linear precursors. The M n,GPC (C x )/ M n,GPC (L x ) ( x = 1–3) values within 0.82–0.84 were roughly compa-rable to those reported for other cyclic polymers. [ 13–16 ] To further reveal the formation of ring topology, typical samples L2 and C2 were chosen to perform MALDI-TOF MS measurements (Figure S3, Supporting Information). As shown in the full spectra (left), the absolute molec-ular weights were similar for both cases expanding from 3000 to 8000. Comparing to the apparent M n,GPC ratio of


12 11 10 9 8 7 6 5 4 3 2 1 0

bO O

O

O

CN

S

OO

S

N

n

Chemical shift (ppm)

a

bc

de

f

g

h

ij

k l

mn

o

p

bc,p

d e

f,h,o

g,m

i,j,k,l

a,n

(a) L3

12 11 10 9 8 7 6 5 4 3 2 1 0

h

b

S

OH

S

OO

O

OO

N

n CN

Chemical shift (ppm)

ab

c

d

e

f

g

a,e

b cd

f,g,h

(b) C3

Figure 1. 1 H NMR spectra of typical a) linear and b) cyclic PDMA polymers in CDCl 3 .


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0.82 between C2 and L2, the similar absolute molecular weight indicated a more compact molecular structure for C2, strongly indicating the successful formation of cyclic topology. From the expanded spectra (right), two main peak distributions of L2 could be accurately assigned to L2 ionized with H + (a) and Na + (b), and two main peak distributions of C2 were possibly ascribed to C2 ionized with H + (c) and its corresponding derivative to lose HCN ionized with K + (d). A regular m/z interval of cal. 157.1 was observed between neighboring peaks in the main distribution for both cases, corresponding to the molar mass of the DMA monomer unit. No intermolecular cou-pling was noted in this study, and the possible reasons lay in some factors such as relatively low concentration, medium steric hindrance during orthoquinodimethane–dithiobenzoate coupling, and suitable chain length and fl exibility of PDMA segment.

2.2. Thermal Properties

The glass transition temperatures ( T g s) of cyclic PSt [ 17,18 ] and PMMA [ 19,20 ] usually go up with increasing MW. Dif-ferential scanning calorimetry (DSC) measurements were initially performed to describe the effects of MW and topology on chain relaxation of PDMA segments. Both linear and cyclic PDMAs exhibited signifi cant MW dependence of T g (Figure 3 ). With rising molecular weight, T g (L x ) slightly varied from 15.0 to 20.7 °C, while T g (C x ) substantially jumped from 66.8 to 80.4 °C. The PDMAs obtained after cyclization exhibited much higher T g s, possibly originating from the end-to-end linkage, rela-tively low confi gurational entropy, [ 17,18 ] and the presence of hydroxyl moiety in the rigid linker. The ring closure

introduced a bulky linkage bearing a 3,4-dihydro-3,3,8-trisubstituted-1 H -isothiochromen-1-ol moiety, which comprised a bicyclic moiety and a hydroxyl group. The constrained architecture of cyclic polymers restricted long-range segmental motions, the formation of more rigid coupling structure bearing bicyclic moiety led to less fl exibility as compared to C−C linkages, and the hydroxyl moiety may enhance intermolecular interactions via hydrogen bonds. To reveal the infl uence of the new func-tionality formed by DA reaction, the linear PDMA with terminal DA linkage (L3-OH) was synthesized via succes-sive radical-induced reaction to remove the RAFT end of L3 and a subsequent DA reaction with 2-(2-cyanopropyl) dithiobenzoate to generate the hydroxyl-bearing DA moiety (Scheme S1, Supporting Information). DSC analysis revealed T g (L3-OH) was higher than its precursor (Δ T g = 6.8 °C; Figure S4, Supporting Information). Since L3-OH was similar to the linear analog of C3, the introduction of new functionality could partly lead to the signifi cantly enhanced T g via cyclization besides the architectural role. As DP(PDMA) varied from 11.2 to 50.6, the T g differences between ring and linear polymers (Δ T g = T g (C x ) – T g (L x ), x = 1–3) increased from 51.8 to 59.7 °C.

Meanwhile, thermogravimetric analysis (TGA) meas-urements of typical samples L3 and C3 were performed to understand the different thermal stabilities between linear and cyclic PDMAs (Figure S5, Supporting Informa-tion). The decomposition of L3 could be roughly classifi ed into three stages corresponding to the loss of end groups, side groups, and the polymer backbone, and the max-imum decomposition temperatures ( T max ) at each stage were deduced to be 188, 309, and 434 °C by differential curves. Due to the lack of end group, only two obvious decomposition stages were noted in TGA curve of C3, and the T max values appeared at 278 and 427 °C. Careful


-20 0 20 40 60 80 100 120

)p

umre

hto

dn

E(w

olftae

H

Temperature (oC)

(a) L1 (15.0)

(b) C1 (66.8)

(c) L2 (18.2)

(d) C2 (73.5)

(e) L3 (20.7)

(f) C3 (80.4)

Figure 3. DSC curves of linear (L1–L3) and cyclic (C1–C3) PDMA samples, in which the T g s are identifi ed using solid lines.

18 20 22 24 26 28

Elution time (min)

L1 (1980, 1.10) C1 (1660, 1.12) L2 (4600, 1.13) C2 (3750, 1.12) L3 (7490, 1.16) C3 (6200, 1.17)

sample (Mn,GPC

, PDI)

Figure 2. GPC traces of various linear (L1–L3) and cyclic (C1–C3) PDMA polymers, in which C x was prepared by UV-induced Diels–Alder click reaction of L x .


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inspection of TGA curves revealed that the decomposition temperature to lose N,N -dimethylethanolamine in ring PDMA was remarkably reduced when compared with its linear precursor (Δ T max = −31 °C), possibly resulting from more expanding conformations of pendent groups in cyclic architecture.

2.3. Thermoresponse of DPMAs in Aqueous Solution

Some factors such as topology, DP, polymer concentration ( c p ), additive, and pH can affect phase transition of polymer solutions, and cyclic polymers usually exhibit enhanced cloud point ( T c ) than their linear counterparts. [ 23–25 ] To reveal the role of cyclic topology, the thermoresponsive behaviors of L x and C x ( x = 1–3) under different conditions were compared.

First, the infl uence of DP on thermoresponse was studied, and the polymer concentration was fi xed at 5.0 mg mL –1 due to relatively high cloud point of ring polymers. Figure 4 a shows the temperature-dependent transmittance, in which the LCST-type soluble-to-insoluble phase transition was observed in each case, and both linear and cyclic PDMAs tended to augment their T c values with an increase in MW (Figure S6, Supporting Information). As DP varied from

11.2 to 50.6, T c (L x ) was enhanced from 32.8 to 48.1 °C, and T c (C x ) increased from 50.6 to 68.8 °C. These results revealed the molecular weight played an important role in ther-moresponsive properties, in which the enhanced T c with extended chain length was primarily resulted from the promoted intra- and interchain hydrophilic interactions. Owing to the presence of repulsive forces between rings and the infl uence of topological constraint on heat-induced dehydration and coil-to-globule collapse, [ 41,42 ] cyclic PDMA exhibited a much higher T c value than its linear precursor, and the Δ T c values were fl uctuated between 17.8 and 20.7 °C. Apart from the turbidity analysis, dynamic light scat-tering (DLS) is another powerful tool to reveal the phase transition behaviors. The DLS curves of C1 and C2 solu-tions at different temperatures are plotted in Figure S7 (Supporting Information). With increasing tempera-ture, the hydrodynamic diameter ( D h ) of polymer solu-tion was slightly changed at a low temperature range, rapidly increased to a maximum value, and then slightly decreased. The temperature range for phase transition pro-cess was roughly comparable to that determined by the turbidity measurement.

Second, L2 and C2 were chosen as typical samples to illustrate the effects of polymer concentration on phase


Figure 4. a) Infl uence of DP, b) polymer concentration, c) salt concentration, and d) pH on transmittances of aqueous solutions ( c = 5.0 mg mL –1 ) comprising linear or cyclic PDMA samples (S).


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transition. As well documented, the T c s tend to increase with reduced concentration due to the dilution effect. [ 32–37 ] As c p varied from 0.5 to 5.0 mg mL –1 , the curves of trans-mittance versus temperature are plotted in Figure 4 b. With lowered concentration, T c (L2) gradually shifted from 39.0 to 70.2 °C, however, the tendency for T c (C2) was a bit different. It fi rst increased from 57.4 ( c p = 5.0 mg mL –1 ) to 75.8 °C ( c p = 1.0 mg mL –1 ) and then stabilized at about 76 °C as c p was further reduced (Figure S8, Supporting Informa-tion), which may be ascribed to the topology effect. At suitable concentrations, the cyclic topology allowed more expanding conformation of pendent groups, and the repulsive forces between rings induced formation of loose packing of polymer chains, leading to similar tempera-ture-dependent phase transition. Another fi nding lies in their close cloud point at lower concentration, in which the Δ T c value noticeably decreased from 18.4 to 6.0 °C as c p dropped from 5.0 to 0.5 mg mL –1 . These results suggest the dilution effect can compete with the topology effect as the polymer concentration is relatively low.

Third, the LCST curves of L2 and C2 aqueous solutions ( c p = 5.0 mg mL –1 ) were measured in the presence of NaCl to reveal the salt effect. By adding inorganic salts into the aqueous solution of thermoresponsive polymers, the cloud points are normally reduced due to the salting-out effect originating from the disruption of hydrogen bonding interactions between polymer chains and water. [ 32–37 ] T c (L2) moderately fell from 39.0 °C (without salt) to 37.2 °C ( c NaCl = 1.0 mol L –1 ) and then further dropped to 30.5 °C ( c NaCl = 5.0 mol L –1 ), while T c (C2) ini-tially stabilized at about 57 °C ( c NaCl = 0–0.5 mol L –1 ) and then drastically reduced to 37.0 °C ( c NaCl = 5.0 mol L –1 ). The T c values of ring PDMA were similar as salt concentration was less than 0.5 mol L –1 , suggesting relatively high con-centration of NaCl was necessary to disrupt the hydrogen bonds between the solvent and polymer chains. The improved stability of cyclic PDMA solution upon addition of salt was in good accordance with the reported results, in which cyclic polymers usually exhibited enhanced thermal and salt stability as compared with their linear counterparts. [ 43 ] For linear PDMA, the increasing dehydra-tion with addition of salt initially led to the intermicellar bridging, and followed by the formation of large agglom-erate as the temperature was further enhanced. However, the chain motions in ring PDMA micelles were topologi-cally prohibited to circumvent bridging, and thus the sta-bility against salt (dehydration) and heating (the thermal motion of chain ends and side groups) was signifi cantly improved. As c NaCl increased from 0.2 to 5.0 mol L –1 , the Δ T c value dropped from 18.7 to 6.5 °C (Figure S9, Sup-porting Information), suggesting that the disruption of hydrogen bonds at high salt concentration can effi ciently reduce the infl uence of cyclic topology on heat-induced phase transition.

Last, the pH/CO 2 response was also investigated to depict the role of external stimuli. When L2 and C2 were directly dissolved in neutral water ( c p = 5.0 mg mL –1 ), their aqueous solutions exhibited pH 7.8 (L2) and 7.9 (C2), respectively. As the solution pH was initially adjusted to be less than 7.0 by addition of suitable amount of dilute HCl, no LCST-type phase transition was noted at the tem-perature window of 20−80 °C (fi gure not shown). The LCST curves were further measured under basic conditions by utilization of NaOH aqueous solution, and the transmit-tance–temperature correlations are plotted in Figure 4 d. T c (L2) slightly dropped from 39.0 to 34.9 °C as pH increased from 7.8 to 11.9, while T c (C2) continuously decreased from 57.4 to 49.0 °C as pH rose from 7.9 to 12.1 (Figure S10, Sup-porting Information). The dropped T c with enhanced pH under basic conditions could be mainly ascribed to the deprotonation of the appending protonated dimethyl-amino groups and/or the salting-out effect in the pres-ence of NaOH. As pH jumped from 7.8 to about 12, the maximum reduced T c value was around 4.1 °C (for L2) and 8.4 °C (for C2), while the Δ T c value only slightly decreased from 18.4 to 14.1 °C. These results indicated the topology effect still played a predominant role in phase transition under basic conditions. Similar to other CO 2 -switchable polymers due to protonation of the amine groups, [ 32–34 ] the LCST behaviors of both linear and cyclic PDMAs could be switchable with CO 2 and N 2 (Figures S11 and S12, Sup-porting Information). The solution pH after purge of CO 2 for 5 min was about 6.2 (for L2) and 6.3 (for C2), revealing lack of phase transition under acidic conditions. The pres-ence and absence of thermo-dependent phase transitions could be effi ciently recycled for many times via bubbling with the acidic or inert gas for 2–5 min, and no obvious change in T c s was noted.

3. Conclusions

Well-defi ned cyclic PDMAs were effi ciently synthesized by combination of RAFT polymerization and UV-induced Diels–Alder click reaction. The ring-closure reaction was successfully conducted, evident from 1 H NMR, GPC, and MALDI-TOF MS analyses. As compared with their linear precursors, the cyclic PDMAs were liable to exhibit quite different thermal properties, in which the glass transition temperature was signifi cantly enhanced (Δ T g = 51.8–59.7 °C) due to the differences in confi gu-rational entropy and end groups, while the maximum decomposition temperature corresponding to the loss of pendent 2-(dimethylamino)ethoxy groups was remarkably decreased owing to the cyclic topology. The thermorespon-sive behaviors of PDMAs in water were affected by some factors such as polymerization degree, polymer concen-tration, additive of NaCl, and pH/CO 2 . The cloud points of



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ring PDMAs were usually higher than their linear precur-sors, and the Δ T c values obtained under a fi xed condition could reach up to 20.7 °C due to the combined infl uences of topology effect and other factors. Considering the great potential of RAFT process in construction of complex archi-tectures such as random, block, and alternating copoly-mers, the ring-closure method can be used to generate a wide range of cyclic polymers and their derivatives with rich compositions and versatile functions. This study can not only afford insight into structure–property relation-ships of smart ring homopolymers but also pave the way for exploring the unique properties and applications of multifunctional cyclic polymers and their derivatives.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : This work was fi nancially supported by the National Natural Science Foundation of China (Grant Nos. 21274096, 21374122, and 51203171) and the Project Funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. K.Z. acknowledges the fi nancial support from Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application (Soochow University).

Received: March 10, 2016 ; Revised: April 10, 2016 ; Published online: April 29, 2016 ; DOI: 10.1002/marc.201600152

Keywords: cyclic polymers ; Diels–Alder reactions ; phase transitions ; smart polymers ; thermal properties

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