Peer review of: On demand shape memory polymer via light regulated topological defects in a dynamic covalent (2020)

REVIEWER COMMENTS 

Reviewer #1 (Remarks to the Author): 

This study applies the concept of topological isomerization to programing the shape memory 
behavior of polymer networks by introducing internal stresses to enable reversible shape memory 
without applying external stress. Distinct features of this approach include light triggered regulation 
of polymer topology within a solid material without using external reagents, which significantly 
expands the range of practical applications. Within this general concept, the authors address a 
specific problem of harnessing topological defects to control shape, which is has very exciting 
implications for establishing a universal mechanism of topological isomerization. The paper is 
recommended for publication after minor revisions outlined below: 

1) I am skeptical about using the word adaptive in the first and fourth sentences of the introduction, 
unless the authors explain its meaning in the context of shape memory. At low resolution, refs 1-8 
describe different kinds of stimuli-responsive behaviors, including shape memory. The authors may 
want to continue sentence 4 as: 
Indeed, dynamic covalent polymer network exhibits unusual adaptive properties not present in 
typical covalent networks such as….TBC [ref]. 

2) Also, the term network topology should be clearly defined as it is the main focus of this paper. The 
authors may want to continue the following sentence as: 
A counterintuitive opportunity arises when a dynamic covalent network is designed such that bond 
rearrangement leads to different topologies such as TBC… 

3) The authors mentioned that topological defects such as loops affect macroscopic properties. 
What properties are we talking about? How significant is their effect? At small fraction of such 
defects the effect is actually negligible. What fractions of topological defects is produced in these 
systems? 

4) In page 7 and Figure 2d, it should be gel fraction (not content). 

5) PEG melting temperature depends on molecular weight. What is MW range of the PEG sections 
(both tethered and deliberated)? 

6) PEG is a hydrophilic polymer. Therefore, all properties including thermal, mechanical, and shape 
memory depend of water sorption. Please comment. 




Reviewer #2 (Remarks to the Author): 

On Demand Shape Memory Polymer via Light Regulated Topological Defects in a Dynamic Covalent 
Network 
The authors explored covalent adaptable networks (CANs) crosslinked through UV curing of PEG-
diacrylate and N-hydroxyethylacrylamide to design topology isomerizable networks (TINs) with 
varying amount of free hydroxyls. The cured networks were then immersed in a toluene solution 
containing photolabile protected from 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 2 wt%). The dynamic 
nature of the networks proceeded through transesterification catalyzed by TBD, after 
photoliberation, with free hydroxyl groups along the backbone. The authors show that, after 
irradiation, the hydroxyl groups can liberate PEG crosslinks causing an increase in the crystallinity. 
This spaciotemporal control was used for the generation of shape memory materials. The authors 
show an increase in crystallinity of the networks by differential scanning calorimetry (DSC) and 
dynamic mechanical analysis (DMA) after irradiation of UV light, attributed to the liberation of PEG 
crosslinks. This liberation of PEG is also observed by a decrease in the gel content over time by gel 
sol studies and soluble fraction containing PEG. The authors also show that without the presents of 
irradiation or hydroxyl groups the isomerization (loss of PEG) does not happen. The concept and 
results (effecting network crystallinity through light-induced transesterification by TBD) are very 
similar to their recent publication titled “Light-triggered topological programmability in a dynamic 
covalent polymer network” where “TIN” was introduced [Sci. Adv. 2020; 6: DOI eaaz2362]. Although 
this application of defects in covalent adaptable networks for shape memory materials is an 
interesting topic, I do not believe this study to be impactful enough for Nat. Commun. For these 
reasons and the comments below, I believe this manuscript should be published in a more specific 
journal and requires additional experimental work. 


Questions/Concerns: 
(1) Temperature sweeps of networks at various irradiation times/washed samples by DMA should be 
performed to show a change in the rubbery plateau. The rubbery plateau gives information about 
the molecular weight between crosslinks and will give more insight to change in network topology. 
(2) Kinetic studies for the consumption on the two monomers, where PEG-acylate substitutes PEG-
diacylate, should be performed to show the composition of the polymer backbones (statistical, 
gradient, vs homopolymers). 
(3) FTIR at all stages of the network would be nice to show the preservation of the network 
throughout the sample. 
(4) Some of the writing could be reworded for clarity and improved flow: 
e.g., (not comprehensive) 
(1) The word “unusual” is used quite often and it could be replaced as seems misleading to say this 
effect is rare (covalent adaptable networks have become quite common in the literature over past 
two decades). 
(2) Page 2, line 6: “Dynamic covalent bonds, with their unique combination of adaptability and high 
bond strength, have the potential to shift the paradigm.” This sentence seems unfinished. 
(3) Page 2, line 13: “Triggering the dynamic characteristics of the covalent bonds, however, renders 
the network polymers malleable through covalent bond exchange within the network. With 
rheological behaviors similar to inorganic glass, this class of network polymers (vitrimer) can be mold 
reprocessed.” These sentences are disjointed and unrelated. Worse this leads the reader to believe 
that all covalent adaptable networks are vitrimers. This is not the case and should be reworded. 
(4) Page 2, line 19: I don’t believe this sentence is necessarily true, “Although it [solid-state plasticity] 
relies on the same mechanism of network bond rearrangement, the equilibrium for the dynamic 
bond exchange does not need to be pushed to a level required for vitrimers.” Vitrimers are a specific 
class of covalent adaptable networks and solid-state plasticity can be achieved through several types 
of CANs. 
(5) Line 4 in the conclusion states, “This in return…” and should be “This in turn...” 
(5) Moles or mole ratios should be given for experimental procedure. (6) Both NMR figures should be more legible in ESI and integrations for PEG-diacrylate would be 
nice. 



Reviewer #3 (Remarks to the Author): 

In this work, the authors demonstrate a molecular design of a topology isomerizable network. By 
leveraging the BERs between dangling chains and mechanically effective chains, the network 
thermomechanical properties (crystallinity and modulus) can be readily tailored. This unique 
mechanism was further extended to design shape-changing polymers. Overall, I enjoyed reading the 
paper and believe it provides valuable insights to the community to design innovative active 
polymers. I suggest accepting the paper with the following minor comments: 

1. The relationships between heating time and isomerization degree should be characterized. Does 
the crystallinity keep increasing progressively or eventually saturate at some level? 
2. How will it change when more hydroxyl group presents in the network., namely the 
hydroxyl/ester ratio is great than 2? The authors are suggested to elaborate or characterize. 
3. When was the TBD added during the network synthesis? UV light was used to polymerize the 
network, did that release the BER catalyst, and thus affect the network properties? 
4. There are existing studies (e.g., 10.1038/s41467-018-04292-8, 10.1016/j.jmps.2019.02.013) using 
two-stage polymerization to tailor the network crosslinking degree and thermomechanical 
properties, which are similar to the authors’ approach. The authors are suggested to discuss and cite 
the works in the manuscript.