Clemson University TigerPrints
All Dissertations Dissertations
May 2020
Synthesis and Characterization of High Sulfur-Content Polymers with Petrochemical and Microbial Comonomers
Timmy Thiounn Clemson University, [email protected]
Follow this and additional works at: https://tigerprints.clemson.edu/all_dissertations
Recommended Citation Thiounn, Timmy, "Synthesis and Characterization of High Sulfur-Content Polymers with Petrochemical and Microbial Comonomers" (2020). All Dissertations. 2607. https://tigerprints.clemson.edu/all_dissertations/2607
This Dissertation is brought to you for free and open access by the Dissertations at TigerPrints. It has been accepted for inclusion in All Dissertations by an authorized administrator of TigerPrints. For more information, please contact [email protected].
SYNTHESIS AND CHARACTERIZATION OF HIGH SULFUR-CONTENT POLYMERS WITH PETROCHEMICAL AND MICROBIAL COMONOMERS
A Dissertation Presented to the Graduate School of Clemson University
In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Chemistry
by Timmy Thiounn May 2020
Accepted by: Dr. Rhett C. Smith, Committee Chair Dr. Stephen Creager Dr. Shiou-Jyh Hwu Dr. Sourav Saha
i ABSTRACT
Plastic consumption has increased at a shocking rate over the past 20 years, consequently the accumulation of plastic waste has followed. The majority of plastic waste tends to get disposed of in landfills or incinerated for energy recovery, however the efficiency of the aforementioned processes is fairly poor. Only 9% of all plastic waste produced is successfully recycled. The miniscule amount of plastic waste that is recycled is either melted and recast into materials or ground up and used for other applications.
However, recycling conventional plastics tend to degrade the plastic material and render it useless after repeated melt-casting cycles. Thus, the current recycling processes are severely limited. One solution to the current recycling process is to synthesize new polymers that can withstand repeated melt processing cycles without loss in mechanical properties or degradation.
In the current contribution, new polymers that have high sulfur-content are studied for durable applications. High sulfur-content polymers are synthesized utilizing a variety of different monomers. The thermal and mechanical properties and recyclability of the materials are considered.
Chapter 2 focuses on the use of a modified commercially available polystyrene derivative as a starting material for reaction with sulfur. It was found that varying the amount of sulfur in the polymer formulation achieved materials with drastically different thermal and mechanical properties. The recycling/thermal healability of higher content sulfur materials were assessed through dynamic mechanical analysis (DMA). The average crosslink length was determined by fractionation studies.
ii Chapter 3 presents work on utilizing an amino acid produced from bacteria as a starting material to react with sulfur. The sulfur content was either 30 or 50 weight percent by mass and the thermal and mechanical properties were analyzed. In both cases the polymers showed to be thermosets and could not be reprocessed through simple melt- processing techniques. The flexural strengths and modulus were shown to be quite high and comparable to that of Portland cement. Additionally, the acid stability of the polymers were also tested and compared to that of Portland cement.
The work presented in Chapter 4 shows a new method to synthesize high sulfur- content materials. A bisphenol A derivative without the presence of any alkene moieties was reacted with elemental sulfur. The following method focuses on using Radical- induced Aryl halide-Sulfur Polymerization (RASP) to synthesize high sulfur-content materials as opposed to inverse vulcanization. The mechanical properties, acid stability, and recyclability/thermal healability of the polymers were analyzed and discussed.
Chapter 5 presents work that combines the inverse vulcanization and RASP process. A modified bisphenol A derivative was synthesized and shown to undergo inverse vulcanization with 80 wt% sulfur. The sample was then subsequently reacted to undergo RASP. The thermal and mechanical properties of the inverse vulcanized and
RASP material were compared to each other.
iii DEDICATION
To my parents Lyn Thiounn and Vannak Thiounn for being the best parents a kid could ever ask for.
iv ACKNOWLEDGMENTS
I would like to extend my sincerest gratitude to my advisor, Dr. Rhett C. Smith, for being such a patient and supportive advisor. I have learned so much from you in my time here at Clemson and could not be more grateful to have had you as my graduate advisor. I would also like to thank the members of my committee: Dr. Stephen Creager,
Dr. Shiou-Jyh Hwu, and Dr. Sourav Saha. Thank you for your time and dedication to my studies here.
I would like to thank the current and former members of the Smith group that have helped me tackle this endeavor and all of the other graduate students that I have met along the way, especially Moira Lauer, Menisha Karunarathna, Kerrick Rees, Beau
Brummel, Tim Lex, Maria Swasy, Ashley Dickey, Jaime Idárraga, and Stephen Vicchio.
I will forever remember all the life memories we have shared inside and outside of the lab.
Finally, I would like to thank my incredible family, my mother, Lyn, who will forever see me as her little boy, my dad, “old man” Vannak, and my sister, Elizabeth, for always being there to annoy me. I love you all.
v TABLE OF CONTENTS
Page
TITLE PAGE ...... i
ABSTRACT ...... ii
DEDICATION ...... iv
ACKNOWLEDGMENTS ...... v
LIST OF ABBREVIATIONS ...... ix
LIST OF TABLES ...... xii
LIST OF FIGURES ...... xiii
LIST OF SCHEMES ...... xviii
CHAPTER
I. ADVANCES AND APPROACHES FOR CHEMICAL RECYCLING OF PLASTIC WASTE...... 1
Abstract ...... 1 Introduction ...... 2 Academic Advances in Plastic Recycling ...... 7 Conclusions ...... 25 Acknowledgements ...... 27 References ...... 28 Attribution of Work ...... 40
II. THERMALLY-HEALABLE NETWORK SOLIDS OF SULFUR-CROSSLINKED POLY(4-ALLYLOXYSTYRENE) ...... 41
Abstract ...... 41 Introduction ...... 42 Experimental Section ...... 45 Results and Discussion ...... 50 Conclusions ...... 64 Conflict of Interest ...... 64 Acknowledgements ...... 65
vi Table of Contents (Continued) Page
Supporting Information ...... 66 References ...... 90 Attribution of Work ...... 99
III. DURABLE, ACID-RESISTANT COPOLYMERS FROM INDUSTRIAL BY-PRODUCT SULFUR AND MICROBIALLY-PRODUCED TYROSINE ...... 100
Abstract ...... 100 Introduction ...... 101 Results and Discussion ...... 104 Conclusions ...... 115 Experimental Section ...... 116 Acknowledgements ...... 118 Supporting Information ...... 119 References ...... 129 Attribution of Work ...... 135
IV. COPOLYMERS OF A BISPHENOL A DERIVATIVE AND ELEMENTAL SULFUR BY THE RASP PROCESS ...... 136
Abstract ...... 136 Introduction ...... 137 Results and Discussion ...... 140 Conclusions ...... 152 Experimental Section ...... 152 Conflicts of Interest ...... 155 Acknowledgements ...... 155 Supporting Information ...... 156 References ...... 164
V. SEQUENTIAL CROSSLINKING FOR MECHANICAL PROPERTY DEVELOPMENT IN HIGH SULFUR-CONTENT COPOLYMERS ..... 173
Abstract ...... 173 Introduction ...... 173 Results and Discussion ...... 177 Conclusions ...... 183 Conflicts of interest ...... 183 Acknowledgements ...... 183 Supporting Information ...... 184
vii Table of Contents (Continued) Page
References ...... 196
viii LIST OF ABBREVIATIONS
α-S orthorhombic sulfur
β-S monoclinic sulfur
ΔHcc cold crystallization enthalpy
ΔHm melting enthalpy
δ delta
χc crystallinity
BG butylene glycol
BHET bis(2-hydroxyethyl)terephthalate
Bmim 1-butyl-3-methylimidazolium hexafluorophosphate
Br4BPA tetrabromobisphenol a
BPA bisphenol a
CaO calcium oxide
CDCl3 chloroform
CS2 carbon disulfide
DEG diethylene glycol
DMA dynamic mechanical analysis
DMSO dimethylsulfoxide
DMT dimethyl terephthalate
DPG dipropylene glycol
DSC differential scanning calorimetry
E' storage modulus
ix List of Abbreviations (Continued)
E'' loss modulus
EDX energy dispersive X-ray analysis
EG ethylene glycol
FT-IR fourier transform infrared spectroscopy
H2S hydrogen sulfide
HDPE high density polyethylene
HHV higher heating values
HSM high sulfur-content material
InV inverse vulcanization
IR infrared spectroscopy
K2CO3 potassium carbonate
LDPE low density polyethylene
LM loss modulus
MPa megapascals
MSW municipal solid waste
NaOH sodium hydroxide
NCC networked covalent composite
PE polyethylene
PET polyethylene terephthalate
PG propylene glycol
PP polypropylene
x List of Abbreviations (Continued)
PS polystyrene
PVC polyvinyl chloride
RASP radical-induced aryl halide-sulfur polymerization
SEM scanning electron microscopy
SM storage modulus
SPI society of plastics industry
STP standard temperature and pressure
Tα-β alpha to beta transition temperature
TA terephthalic acid
Td decomposition temperature
Tg glass transition
TGA thermal gravimetric analysis
Tm melting temperature
xi LIST OF TABLES
Table Page
1.1 The SPI code identification number of polymer resins in plastic products, their structure and common uses ...... 3
2.1 Summary of mass loss in synthesis, TGA, and DSC data ...... 53
2.2 The Tg of PAOS90 and PAOS99 determined from DMA ...... 61
3.1 Summary of properties of materials ...... 108
3.2 Summary of flexural properties of TTSx materials ...... 114
4.1 Fractionation and Sulfur Rank of BASx ...... 146
4.2 Thermal Properties of BASx ...... 148
4.3 Properties and Acid Resistance of BASx ...... 150
5.1 Comparison of properties after each crosslinking reaction ...... 178
S5.1 Tensile data for each trial of I-BPA80 and R-BPA80 ...... 195
xii LIST OF FIGURES
Figure Page
1.1 Common fates for current plastic waste ...... 6
1.2 The differences between HDPE (no branching), LLDPE (short branches), and LDPE (long branches) ...... 17
1.3 The differences between isotactic, syndiotactic, and atactic PP ...... 22
2.1 FT-IR spectra for poly(4-vinylphenol) and PAO over the full scan range (a), poly(4-vinylphenol) and PAO highlighting the range for the monosubstituted alkene C–H bending mode (b), CS2-soluble fractions from PAOSx (x = 40-99, c), and CS2-insoluble fractions from PAOSx (x = 40-99, d)...... 52
2.2 (a) TGA curves of PVP, PAO, elemental sulfur, and PAOSx. (b) The TGA curves for the CS2-extractable (solid line) and CS2-insoluble fraction (dashed line) of PAOS90. (c) The calculated (solid line) TGA curve for independent thermal decomposition of the two fractions overlaid with the experimentally observed TGA curve (dashed line) from the PAOS90 bulk material ...... 57
2.3 Temperature dependence of storage and loss modulus and tan d of PAOS90 (dashed line) and PAOS99 (solid line) in dual cantilever mode after 1.5 h of cooling (graphs a, b, and c) and 96 h of cooling (graphs d, e, and f) ...... 59
2.4 The bulk modulus (measured at –45° C) for PAOS99 after pulverization/remelting cycles remains consistent within error for at least twelve cycles ...... 62
S2.1 Proton NMR spectrum of PVP ...... 66
S2.2 Proton NMR spectrum of PAO ...... 67
S2.3 IR spectrum of PAOS10 ...... 68
S2.4 IR spectrum of PAOS20 ...... 69
S2.5 IR spectrum of PAOS30 ...... 70
S2.6 IR spectrum of PAOS40 ...... 71
S2.7 IR spectrum of PAOS50 ...... 72
xiii List of Figures (Continued) Page
S2.8 IR spectrum of PAOS60 ...... 73
S2.9 IR spectrum of PAOS70 ...... 74
S2.10 IR spectrum of PAOS80 ...... 75
S2.11 IR spectrum of PAOS90 ...... 76
S2.12 IR spectrum of PAOS99 ...... 77
S2.13 DSC curve of PAO ...... 78
S2.14 DSC curve of sulfur ...... 79
S2.15 DSC curve of PAOS10 ...... 80
S2.16 DSC curve of PAOS20 ...... 81
S2.17 DSC curve of PAOS30 ...... 82
S2.18 DSC curve of PAOS40 ...... 83
S2.19 DSC curve of PAOS50 ...... 84
S2.20 DSC curve of PAOS60 ...... 85
S2.21 DSC curve of PAOS70 ...... 86
S2.22 DSC curve of PAOS80 ...... 87
S2.23 DSC curve of PAOS90 ...... 88
S2.24 DSC curve of PAOS99 ...... 89
3.1 Shaped materials can be fabricated from TTSx thermosets by curing the comonomer solution at 150 °C in a mould or on a substrate, as illustrated by the rectangular prismatic brick (left) and patterned disc-shaped item (centre). A backlit sample of TTSx (right) better illustrates the red colour and transparency of the material ...... 106
xiv List of Figures (Continued) Page
3.2 Thermogravimetric analysis (TGA) traces for elemental sulfur (black trace), tetraallyltyrosine (green trace), TTS30 (blue trace) and TTS50 (red trace) ...... 109
3.3 Storage modulus (E’, blue trace), loss modulus (E’’, red trace) and damping factor (tan δ, green trace) for TTS30 (a) and TTS50 (b) ...... 110
3.4 (a) Stress-strain analysis for TTS30 (red trace) and TTS50 (blue trace). The dashed lines are propagations of the linear regions of the respective stress strain curves. (b) comparison of stress-strain analysis for TTS30 (red trace) and TTS50 (blue trace) before (solid traces) and after (coincident black dashed traces) exposure to sulfuric acid for 24 h ...... 113
S3.1 Proton NMR spectrum of tetrallyltyrosine in CDCl3 ...... 120
S3.2 Inset of 1H NMR spectrum between 6–7.5 ppm of tetrallyltyrosine in CDCl3 ...... 121
S3.3 Inset of 1H NMR spectrum between 5.5–6.5 ppm of tetrallyltyrosine in CDCl3...... 122
S3.4 Inset of 1H NMR spectrum between 5–5.5 ppm of tetrallyltyrosine in CDCl3 ...... 123
S3.5 Carbon-13 NMR spectrum of tetrallyltyrosine in CDCl3 ...... 124
S3.6 Infrared spectra of TTS30 (green), TTS50 (orange), and tetraallyltyrosine (black) between 600-1200 cm–1 ...... 125
S3.7 DSC curves for TTS30 (blue line) and TTS50 (red dashed line). Curves represent the third heat/cool cycles ...... 126
S3.8 Trials 1 (top), 2 (middle), and 3 (bottom) of TTS30 stress strain measurements where the red line is the propagation of the linear region of the respective stress strain curve ...... 127
S3.9 Trials 1 (top), 2 (middle), and 3 (bottom) of TTS50 stress strain measurements where the red line is the propagation of the linear region of the respective stress strain curve ...... 128
xv List of Figures (Continued) Page
4.1 Although materials appear very dark under fluorescent room lighting (A), the brown-orange color characteristic of polymeric sulfur is evident in a backlit sample of BAS90 (B). The sample can be broken down (C), then readily remelted and recast (D) ...... 143
4.2 The percent storage modulus retained of BAS90 after 7 pulverization/remelting/recasting cycles ...... 151
S4.1 Differential scanning calorimetry of BAS95 ...... 156
S4.2 Differential scanning calorimetry of BAS90 ...... 157
S4.3 Differential scanning calorimetry of BAS85 ...... 158
S4.4 Differential scanning calorimetry of BAS80 ...... 159
S4.5 Stress strain curve of BAS95 pre (blue line) and post acid (red line) soak for 24 h. The dotted black lines are the extrapolations of the linear region ...... 160
S4.6 Stress strain curve of BAS90 pre (blue line) and post acid (red line) soak for 24 h. The dotted black lines are the extrapolations of the linear region ...... 161
S4.7 Surface analysis of BAS90 by scanning electron microscopy (SEM) revealed a smooth surface consistent with those observed in high sulfur-content materials prepared by inverse vulcanization ...... 162
S4.8 Surface analysis of BAS90 by energy-dispersive X-ray (EDX) analysis revealed even distribution of sulfur (A), carbon (B), oxygen (C), and bromine (D) content on the polymer surface ...... 163
5.1 Photographs of I-BPA80 demonstrate its flexibility (A) colour and transparency (B). Tensile testing demonstrates the elasticity of I-BPA80 as it is elongated to nearly twice its initial length before breaking (C). More highly crosslinked R-BPA80 is a darker colour solid that can be elongated by less than 30% of its initial length prior to breaking (D)...... 176
xvi List of Figures (Continued) Page
5.2 SEM images of I-BPA80 (A) and R-BPA80 (B) and EDX images for of I-BPA80 (C-F) and R-BPA80 (G-J) demonstrating the distribution of sulfur (red), carbon (cyan), oxygen (violet) and bromine (green) in materials...... 179
5.3 DSC analysis of I-BPA80 (A) and R-BPA80 (B) reveal significant changes in morphology after the second crosslinking. The third heating and cooling curves are shown ...... 180
S5.1 Dog bone samples with dimensions used for tensile testing ...... 185
S5.2 Proton NMR of M1 in CDCl3 ...... 188
S5.3 Infrared spectrum of I-BPA80 ...... 189
-1 S5.4 Inset of IR spectrum between 500–1100 cm of I-BPA80 ...... 190
S5.5 Thermogravimetric analysis curve of I-BPA80 ...... 191
S5.6 Infrared spectrum of R-BPA80 ...... 192
-1 S5.7 Inset of IR spectrum between 500–1100 cm of R-BPA80 ...... 193
S5.8 Thermogravimetric analysis curve of R-BPA80 ...... 194
xvii LIST OF SCHEMES
Scheme Page
1.1 The synthesis of PET from terephthalic acid or dimethyl terephthalate ...... 8
1.2 General reactions for the hydrolysis, methanolysis, and aminolysis of PET to synthesize terephthalic acid (TA), dimethyl terephthalate (DMT), and diamines of TA ...... 10
1.3 The glycolysis of PET using ethylene glycol, in the first part of the reaction PET is broken down into oligomers/polyols of PET, which then are further broken down into BHET ...... 13
2.1 Sulfur vulcanization of polyisoprene (a) and by-product sulfur (b) at North Vancouver Sulphur Works, originating from tar sand-processing facilities in Alberta, Canada ...... 43
2.2 The general synthetic scheme of a thiol-ene reaction. (b) The homolytic ring- opening of S8 to form a sulfur-centered radical, which then can react with olefins. (c) Synthetic route to prepare PAO. (d) Synthetic route to PAOSx, where x represents the wt% of S8 ...... 51
3.1 General inverse vulcanization strategy ...... 102
3.2 Preparation of tetrallyltyrosine and its inverse vulcanization to yield TTSx ...... 104
4.1 Inverse vulcanization (A) and RASP (B) routes to HSMs ...... 139
4.2 RASP leads to the direct formation of S–Caryl bonds with loss of S2X2 ...... 140
4.3 Preparation of BASx ...... 141
5.1 Simplified schematics for inverse vulcanization (A) and RASP (B) routes to high sulfur content copolymers ...... 174
5.2 Crosslinking of olefinic carbons by inverse vulcanization produces I-BPA80. At higher temperatures, crosslinking of aryl carbons by RASP can be used to produce R-BPA80 ...... 175
xviii CHAPTER ONE
ADVANCES AND APPROACHES FOR CHEMICAL RECYCLING OF PLASTIC WASTE1
Abstract
The global production and consumption of plastics has increased at an alarming rate over the last few decades. The accumulation of pervasive and persistent waste plastic has concomitantly increased in landfills and the environment. The societal, ecological and economic problems of plastic waste/pollution demand immediate and decisive action.
In 2015, only 9% of plastic waste was successfully recycled in the United States. The major current recycling processes focus on the mechanical recycling of plastic waste, however even this process is limited by the sorting/pretreatment of plastic waste and degradation of plastics during the process. An alternative to mechanical processes is chemical recycling of plastic waste. Efficient chemical recycling would allow for the production of feedstocks for various uses including fuels and chemical feedstocks to replace petrochemicals. This review focuses on the most recent advances for the chemical recycling of three major polymers found in plastic waste: poly(ethylene terephthalate)
(PET), polyethylene (PE), and polypropylene (PP). Processes for conversion of these polymers to monomers for the re-synthesis of the same polymers, as well as efforts to convert the polymers to other feedstocks or fuels will be discussed.
1 This text is reproduced with permission in larger part from: Thiounn, T.; Smith, R. C., Advances and Approaches to Chemical Recycling of Plastic Waste. J. Polym. Sci. 2020, DOI: 10.1002/pol.20190261 Permission documentation is provided at the end of this chapter.
1 Introduction
The term “plastics,” in common verbiage, refers to synthetic polymers that are ubiquitous in modern society, to the extent that each person in the European Union consumes 50 kg per year, and 68 kg per year in the United States.1 Plastics pervade daily life as packaging,2 clothing and sports equipment,3 biomedical devices,4 electronic components,5 and a panoply of other applications. Unfortunately, the majority of high market-share plastics are obtained from the use of non-renewable and ecologically devastating petroleum / natural gas feedstocks and processing techniques. The imperative trial faced by civilization to access new technologies for recycling and repurposing plastics is obviated when one juxtaposes their pernicious origins with their dogged environmental persistence as ocean-bound flotillas and pervasive microplastics,6-8 with newfound landmass invasion of “plasticrusts” about island shores.9 The current review aims to highlight efforts in the past five years (2014 to 2019) by researchers dedicated to developing new methods for recycling some of the most abundantly-produced plastics.
Specifically, efforts to chemically recycle poly(ethylene terephthalate) (PET), polyethylene (PE), and polypropylene (PP), in their various forms, will be discussed.
The Society of Plastics Industry (SPI) has assigned different plastics with codes
1–6 in order to more easily identify the polymer used in the production of the material as well as to expedite the recycling process.10 Table 1.1 summarizes common polymers classified in this way, with some of their most familiar uses highlighted. Organized disposal of vast quantities of plastic waste consists primarily of landfill techniques, while careless and irresponsible practices of dumping into waterways still persist, particularly
2 in less environmentally-regulated areas of the world. Beyond ecological effects of these behaviours, there are grave concerns for economic and geopolitical ramifications of ongoing reliance on petroleum/natural gas products whose scarcity and ever-increasing value have catalyzed conflict since their discovery.
SPI code Polymer Structure Uses 1 Poly(ethylene terephthalate) Soda bottles, water
O O (PET) bottles, medicine jars,
O O n and salad dressing bottles. 2 High density polyethylene Soap bottles, detergent (HDPE) and bleach containers, n and trash bags
3 Polyvinyl chloride Cl Plumbing pipes, (PVC) cables, and fencing n 4 Low density polyethylene Cling wrap, sandwich
(LDPE) n bags, and grocery bags 5 Polypropylene Reusable food (PP) containers, prescription
n bottles, and bottle caps
6 Polystyrene Ph Plastic utensils, (PS) packaging peanuts, and n Styrofoam 7 Other Table 1.1 The SPI code identification number of polymer resins in plastic products, their structure, and common uses.
3 Efforts to recycle plastic waste have grown in meteoric fashion over the past two decades, yet the challenges associated with harnessing post-consumer plastics as feedstock for new products are sufficiently severe that the relative amount of plastics recycled remains embarrassingly low.11 In 2015, approximately 262 Mt of municipal solid waste (MSW) was generated in the United States.12 Of the 262 Mt of MSW, a full
13% (34.5 Mt) was constituted of putatively recyclable plastic waste.12 Of these 34.5 Mt of plastic waste, however, only 9% was recycled. This compares with 16% that was incinerated and 75% that was landfilled.12 Contrast these low recycling numbers with the striking fact that an energy saving to society of approximately 7 barrels of oils is accrued for every ton of mixed plastic waste that is recycled,13 and it becomes self-evident that oil and energy conservation could be astronomical could we implement an effective recycling strategy. Should we reach the apex of accomplishment by recycling all of the plastic waste, consumption of nearly a quarter billion barrels of oil could be saved each year from recycling of U.S. plastic alone.
Certainly, not all plastics can be as easily recycled as others. In 2015, about 18% of PET was recycled, while only 10 % of HDPE, 6% of LDPE/LLDPE and <1% of PP were recycled.14 A plethora of complexities affect recycling rates, from practical considerations of collection, sorting and pretreatment (what contaminants, adhesives, colorants or residues might be present, for example), to more technical considerations such as chemical reactivity.10 To better understand the state-of-the-art in addressing these complexities, it is instructive to introduce the four classifications of recycling: primary recycling, secondary recycling, tertiary recycling, and energy recovery. Primary
4 recycling, also known as closed-loop recycling, is the process of taking uncontaminated discarded plastics and directly turning that material into a the same “new” product, ideally without loss of properties.15 A familiar example of this would be to use clean aluminum pipes and use the metal to make new aluminum pipes. Secondary recycling refers to mechanical recycling, wherein the chemical identity of the polymer is unchanged, but the polymer is in some way physically reprocessed, and thus generally used for a different purpose than its original use.16 An example of this is taking waste tires and using the rubber crumb as an additive in rubber flooring or park benches. In the context of polymers, the most prevalent problems with primary and secondary recycling are related to stability. As the polymers are continually reprocessed, the polymer may degrade to varying extents which will have drastic effects on the mechanical properties of the post-recycled product.16 Additionally, the need for pure/clean plastic waste is a significant barrier when post-consumer, mixed-source plastic is targeted for recycling.
Tertiary recycling, sometimes referred to as chemical recycling, uses chemical processes to break down the polymer into value-added commodities. Typical processes include hydrolysis15 and pyrolysis17 of waste plastics. The product obtained is then used as a feedstock for the production of fuels and polymers.18, 19 The last form of recycling is incineration of the polymer for energy recovery. In this process the polymer is incinerated, and some amount of energy is recovered in the form of heat. This is generally a ‘last resort’ process when no more value-added application is achievable.
Incineration of many plastics also releases hazardous gases and leave behind toxic
5 residues, presenting undesirable hazardous waste remediation and collection costs and downstream ecological consequences.15
The overall efficiency of recycling plastic waste begins with the sorting and pretreatment process of plastic waste. Different plastics have different properties and thus have different recycling methods. From the typical mixed waste model, each type of plastic must first be sorted. Contaminated plastics in the waste stream can lead to unwanted decomposition reactions that will decrease the efficiency of plastic recycling as well as alter the end product. While there are many different types of sorting processes that have been utilized and are currently being studied for optimization,20, 21 this review will focus on chemical recycling methods of plastic waste that occur once sorting is complete.
Fuels/Energy Plastic Products Energy Recovery ! 3° Recycling Landfilling
Chemical Products
1° Recycling
2° Recycling
Figure 1.1 Common fates for current plastic waste.
6
Academic Advances in Plastic Recycling
Recycling of Poly(ethylene terephthalate) (SPI code 1)
Poly(ethylene terephthalate), often abbreviated PET or PETE, is a semi- crystalline, thermoplastic polymer that is known for its high strength. Industrially, PET is synthesized through a polycondensation reaction between ethylene glycol (EG) and terephthalic acid (TA) or through a transesterification reaction between dimethyl terephthalate (DMT) and EG (Scheme 1.1).22 Efficient recycling of PET has reached the most advanced stage of maturity among the common plastics, and a variety of methods have proven utility on a large scale.22-24 There have been instances where PET undergoes primary and secondary recycling, i.e. the recycling of plastic bottles. However, a significant remaining problem with recycling of PET is that the mechanical properties of the non-virgin material are greatly reduced with each reuse. The strain-at-break (the percent of the length that a sample can be stretched before the sample breaks) for virgin
PET, for example, is 42%, whereas after only the fifth cycle of extrusion, the strain-at- break was only 0.7%.25 This downcycling process limits the ability to thermo- mechanically recycle PET. For this reason, tertiary recycling via chemical processes has been the main focus of research in the past few years.22, 26
7 O O
HO OH Terephthalic Acid OH HO
or
O O
H3CO OCH3 Dimethyl terephthalate O O
O O n PET
Scheme 1.1 The synthesis of PET from terephthalic acid or dimethyl terephthalate.
PET can undergo pyrolysis to yield its precursor monomers, terephthalic acid and ethylene glycol. Kenny, et al.27 have shown that the pyrolysis of PET at 450 °C yields terephthalic acid and oligomers thereof, which can be further hydrolyzed to obtain the TA monomer. Despite such promising advances, the pyrolysis of PET is seldom used as a method to depolymerize PET into its monomeric units on an industrial scale because pyrolysis generally leads to other liquid and gaseous side-products, reducing process efficiency and necessitating costly separation steps.22
Du et al.28 used PET from carpet waste as a source and studied the thermal and catalytic decomposition of this waste into oils. The catalytic degradation focused on using an aluminosilicate zeolite, ZSM-5, or CaO as the catalyst. They also looked at how steam would affect the final decomposition products. They found that using a catalyst, namely CaO, and steam during the pyrolysis process would yield large percentages of
8 benzene in high purity. These studies foreshadow the promise held by many decades of plastic waste to serve as the next source of what are typically viewed as petrochemicals.
Another chemical process that has shown great promise for the depolymerization of PET into its monomeric units is hydrolysis (reaction with water at elevated temperatures and/or with a catalyst).29 The products yielded from this method are terephthalic acid (or a terephthalate salt) and ethylene glycol. There are three different types of hydrolysis that have been studied in greatest detail: acidic, alkaline, or neutral hydrolysis. While acidic hydrolysis can take place using concentrated acids, such as phosphoric or nitric, the most common acid used is sulfuric acid.30-32 Although the yields obtained from this method are generally high, separation of the ethylene glycol from the highly acidic solution is a major drawback of this method. Additionally, the amount of acid needed to industrialize this process pose economic, process and environmental problems.
Alkaline hydrolysis generally employs aqueous solutions of 4–20 wt% NaOH.31
This process yields the sodium terephthalate and ethylene glycol in relatively good yields, up to 100% PET conversion. However, longer reaction times (3–5 h) and high temperatures (>200 °C) than needed for acidic hydrolysis techniques are notable drawbacks of this method. Polk et al.33 have recently improved on the alkaline hydrolysis process, demonstrating that the addition of a phase transfer catalyst
(trioctylmethylammonium bromide) facilitated the reaction at lower temperatures (70–95
°C) while yielding high purity (99.6%) terephthalic acid in up to 93% yield.
9 O OH TA Hydrolysis
HO O H2O
O OCH O O 3 Methanolysis DMT
CH3OH O O H3CO O n R PET
O NH Aminolysis TA diamines R NH2 HN O
R
Scheme 1.2 General reactions for the hydrolysis, methanolysis, and aminolysis of PET to synthesize terephthalic acid (TA), dimethyl terephthalate (DMT), and diamines of TA.
Neutral hydrolysis employs water or steam in the presence of catalysts.31 This process uses high temperatures (200–300 °C) and elevated pressures (1–4 MPa). Neutral hydrolysis, without the need for stoichiometric acid or base, would be ideal, but these processes generally produces low purity monomers and have relatively slow rate of reaction.31 Campanelli et al.34 found that the use of large ratios of water:PET (5:1) are needed for the complete depolymerization of PET.
Another method to depolymerize PET into its monomers is through methanolysis.
In this process methanol reacts with PET at high temperatures (180–280 °C) and pressures (20–40 atm) in the presence of a catalyst, most commonly zinc acetate.35, 36
This reaction leads to the formation of dimethyl terephthalate and ethylene glycol, which can then be used to resynthesize PET through a transesterification reaction. A major drawback of this method, outside of the high temperatures and pressures, is again the purification process. The crude product contains not only DMT and EG, but also other
10 alcohols and phthalate derivatives.22 A noteworthy advance was developed by Tang et al.,37 who demonstrated that DMT recovered from methanolysis of PET could be exploited as a starting material for the synthesis of gasoline and jet fuel.
Aminolysis of PET is an area that has not been widely exploited, likely because this process requires an amine (which is often toxic or expensive) to depolymerize PET, yielding diamides of TA. The reaction temperatures generally range from 20-100 °C.22 In a study done by Teotia et al.,38 four different amines – methylamine, ethylenediamine, ethanolamine, and butylamine – were reacted with PET. Unprecedented conversion of
PET into lower molecular weight oligomers was achieved at ambient temperatures and pressures, but this required reaction times ranging from 10-85 days. Soni et al.39 also studied aminolysis of PET with a variety of amines and achieved complete degradation of the PET to the diamide after 45 days of reaction. Hoang et al.40 showed that ethelyenediamine was even more effective and was able to depolymerize PET to yield a range of oligomers after only 17 h at 100 °C. Significantly longer reaction times were needed to achieve degradation to small molecules, however. Several catalysts, such as dibutyl tin oxide, sodium acetate, and cetyltrimethylammonium bromide, are under development that show promise for shorter reaction times and improved selectivity, but these have not yet demonstrated large-scale applicability.41, 42
Glycolysis of PET is an area that has been widely studied. This is a very versatile process due to the various potential applications of the products obtained. In this process
PET is depolymerized by glycols to form monomers, oligomers, and/or polyols, which can then be used for different applications. Some of the glycols that have drawn
11 particular recent interest for this application include ethylene glycol, diethylene glycol
(DEG), propylene glycol (PG), butylene glycol (BG), and dipropyleneglycol (DPG).43, 44
If EG is used to depolymerize PET, the major product formed is bis(2-hydroxyethyl) terephthalate (BHET), which can be used to synthesize PET. A challenge in these promising reactions is the immiscibility of PET with the polyols. Liu et al.45 sought to address this problem by conducting a careful study on the role of different solvents in the conversion of PET to BHET. DMSO proved most effective among solvents screened for effective cosolvation of EG and PET, and thus showed an increase in BHET yield to 82%
(when compared to 20% without the addition of DMSO) with remarkably short reaction times of one minute at 190 °C and atmospheric pressure.
Catalyst development has also been an area of active research. For many years, zinc acetate was the primary catalyst used in the glycolysis of PET. Troev et al.46 has recently developed a titanium (IV) phosphate catalyst for the glycolysis of PET using EG,
DEG, or 1,2-propylene glycol. This catalyst achieved shorter reaction times than previous catalysts, with increased yield and selectivity for BHET formation. Titanium is advantageous because it is nontoxic, though the catalyst could not be efficiently recycled.
Wang et al.47 have sought to exploit sustainably-produced organocatalysts such as urea in place of transition metal catalysts. Urea was quite an effective catalyst, facilitating 100% conversion of PET with 74% selectivity of BHET. Additionally, urea could be recycled five times without loss of activity or selectivity for BHET.
12 O O O O Glycolysis
OH HO O O O O n m PET Oligomers/Polyols of PET (2 OH O O O O HO BHET Scheme 1.3 The glycolysis of PET using ethylene glycol, in the first part of the reaction PET is broken down into oligomers/polyols of PET, which then are further broken down into BHET. Ionic liquids can also be used as a catalyst for the glycolysis of PET.48, 49 Despite their high initial cost, ionic liquids have become increasingly attractive tools for green chemistry because of their low volatility and recyclability. Consequently, Yue et al.50 have explored the utility of 1-butyl-3-methylimidazolium hydroxide, ([Bmim]OH) to serve as a catalyst. A 100% conversion of the PET with a selectivity of 72% of BHET was achieved, compared to only 11% conversion of PET without the addition of the ionic liquid as a catalyst. In a follow-up study,51 these researchers found that Lewis acidic ([Bmim]ZnCl3) ionic liquid likewise facilitated a 100% PET conversion, but with improved selectivity of 84% for BHET and minimal catalyst loading (0.16 wt%). Liu et al.52 reported that deep eutectic solvents could also be used to catalyze the glycolysis of PET with EG. They found that the combination of 1,3-dimethylurea (1-3-DMU) with 5 13 wt% Zn(OAc)2 was able to convert 100% of the PET with 82% selectivity for BHET at 190 °C in just 10 min. They associated the high selectivity with relatively mild reaction conditions due to the acid-base synergistic effects between 1-3-DMU and Zn(OAc)2. The catalyst was also recycled up to five times without any loss in conversion efficiency. However the zinc content was shown to decrease by 25% after the 5 cycles which limits further recycling ability of the catalyst. Whereas the foregoing discussion highlights efforts to leverage glycolysis of PET with EG to synthesize BHET as a monomer feedstock, an emerging area of interest is the use of the chemically-recycled PET products as feedstocks for other polymer formulations. Mecit et al.53 used different glycols to convert PET into lower-weight oligomers having hydroxyl end groups that were then reacted with toluene diisocyanate to produce urethane oils. The recycled urethane oils showed similar Koenig hardness values and touch/hard to dry times to commercially available oils. Desai et al.54 synthesized polyol blends by reacting PET waste with plant-derived starch. The oligomers so formed were then esterified with fatty acids – themselves primary constituents of low-value, high-volume waste products from other industries.55-59 The esterified materials were then used to synthesize polyurethanes. These authors also demonstrated the facile tunability of adhesion, flexibility, and chemical resistance properties of the polyurethanes as a function of PET products present in the formulation. Amaro et al.60 used diethylene glycol and PET to synthesize oligomers that proved effective as secondary plasticizers in PVC formulations resulting in improved thermal stability and flexibility of the final PVC product. Furthermore, migration of PET- 14 derived plasticizer migration was greatly decreased compared to traditional PVC plasticizers such as, di(2-ethylhexyl)phthalate (DHEP) which leach out over time. Initial PET oligomerization was achieved in this instance by action of a Ca/Zn stearate catalyst at 250 °C for only 20 min. Recently, Sirohi et al.61 has described alcoholysis of PET to synthesize oligomers that can be used as a plasticizer in nitrile-PVC rubber blends. They used a ZnCl2 catalyst and 1-decanol as the alcohol with reaction temperatures of 190 °C for 4 h. Once resultant depolymerized products were blended with nitrile-PVC rubber blends, the tensile properties and the aging resistance of the materials was significantly buttressed. Polyethylene (SPI codes 2 and 4) Polyethylene is a lightweight and durable thermoplastic that finds use in films, tubing, packaging, plastic bags and bottles, and even automobile parts. Polyethylene is typically made by polymerization of ethylene (C2H4), often facilitated by a Ziegler-Natta or metallocene catalyst.29 The linear alkyl chains comprising the polyethylene backbone lack polar functional groups, and are inert to many chemical reactions, including common acids and bases, under standard conditions. To complicate recycling efforts further, there are multiple types of polyethylene: high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultrahigh molecular weight polyethylene (UHMWPE), and many other crosslinked types of polyethylene.29 Each of these different types of polyethylene are used for different purposes depending primarily on the mechanical property profile required. The most-used polyethylenes 15 include HDPE (SPI code 2) and LDPE/LLDPE (together sharing SPI code 4). HDPE is flexible, translucent, and shows good toughness at low temperatures. There is little to no branching in HDPE, thus it is said to be a linear polymer. LDPE and LLDPE differ from HDPE in that both are branched polymers. LDPE is semi-rigid and has branches that are both short and long throughout the polymer backbone. LLDPE differs from LDPE in that this form of PE features only shorter branches (Figure 1.2). The structural variability and relative chemical inertness of PE have relegated most studies on PE recycling to variations on pyrolysis. A primary difficulty associated with efforts to recycle polyethylene by pyrolysis is that the thermal degradation of PE usually proceeds via random scissions at C–C bonds. This homolytic scission generates two radical chains that can go on to form a complex mixture of olefinic products and highly crosslinked polymeric products.62 Moreover, the melt flow index (a key figure of merit for processability) changes by two orders of magnitude, from 2.25 (reasonable flow) for virgin LDPE to a nearly unprocessable melt flow index of 0.02 g/10 min after 100 extrusion cycles.62 This changing behavior is accompanied by drastic, deleterious drift of mechanical properties of the recycled LDPE, so these approaches do not provide a long term, sustainable path forward for PE recycling. When mechanical processing of PE is no longer feasible, depolymerization of PE through pyrolysis to yield hydrocarbons for fuel/energy applications is often the final stage of use for such materials. 16 HDPE LLDPE LDPE Figure 1.2 The differences between HDPE (no branching), LLDPE (short branches), and LDPE (long branches). There are two major processes for the pyrolysis of PE: thermal or catalytic pyrolysis. Thermal pyrolysis is simply heating PE at high pressure to break down the polymer backbone to form smaller organic molecules. Catalytic pyrolysis utilizes a catalyst in an effort to reduce the temperature and reaction time and thus improve the economic viability and, in some cases, the selectivity. In a study by Ahmad et al.63 the 17 thermal pyrolysis of commercially available HDPE pellets at 350 °C which yielded a liquid oil product in 81% yield. The oil consisted mainly of paraffinic hydrocarbons, most of which contained between six and sixteen carbon atoms. Specificity for a single chemical commodity from PE pyrolysis, however, remains elusive. An intriguing divergent strategy has been devised by Palos et al.,64 who eschewed the selectivity problem and instead sought to take waste HDPE and chemically transform it into a complex mixture akin to “crude” oil. As long as the complex mixture is comprised by molecules similar to those found in petroleum, such a complex mixture could hypothetically be processed by established petroleum refining and cracking techniques. This nascent approach used waste HDPE that had already been sorted, washed, and shredded. The HDPE samples so prepared were then heated in a batch autoclave reactor at 430 °C with short reaction times, up to 38 min, to yield what they refer to as “plastic oil”, with impressive yields of up to 85–90 wt% oil recovered. While the oil obtained is not clean enough to be used directly in place of fuel, this plastic oil can be refined by established methodologies to yield target products. The main drawback of this creative approach is the potentially high energy required to produce the plastic oil on large scales. While many researchers focus on pristine or near-ideal PE sources in their proof- of-principle studies, Das et al.65 examined the important difference between utilizing virgin and waste plastic products. Additionally, a mixed waste feedstock mixture of different polymers was used. The mixture consisted of HDPE, LDPE, and PP. The virgin polymers were obtained commercially, and the waste plastics were obtained in the form primarily of packaging plastics, plastic containers, and bottles. The thermal pyrolysis of 18 this mixed waste was undertaken at 350 or 400 °C and the reaction time was 8 h. Lower temperature reactions yielded lighter hydrocarbons (< C20) while an increase in temperature yielded heavier hydrocarbons (> C20). This offers the potential to tune the oil obtained by varying reaction temperature and holds promise for exploiting mixed waste streams and thereby circumventing some of the challenges associated with waste separation. Miandad et al.66 likewise recognized the importance of developing strategies to deal with mixed waste. In this study, the plastic waste was collected in the form of disposable plates, grocery bags, and cups and comprised not only linear alkyl polymers (PP and PE), but also aromatic-bearing polystyrene and PET. The plastic samples were crushed into fine powders and used on their own as well as mixed together in varying ratios (as little as 20 wt% PE) with reaction conditions of 450 °C for 75 min. Depending on the ratios of the plastic waste liquid oil yields ranged from 24–54%. The initial feedstock used was 1 kg, which shows the potential to scale the system to an industrial level. The oil obtained consisted of large amounts of aromatic compounds and showed higher heating values (HHV) between 41–42 MJ/kg which is comparable to that of commercially available diesel (43 MJ/kg). The area of catalytic pyrolysis of PE has also achieved notable advances in the last few years with regard to increasing yields and lowering reaction time / temperature. 67 Santos et al. looked at three different zeolites, HZSM-5, USY, and NH4ZSM-5. These catalysts differ based on pore size, acidic sites, and surface area. This study garnered important insight that catalysts with larger pore sizes led to a greater yield of liquid 19 products and that the less acidic catalysts would yield lighter fractions of gas and liquid products, thus affording flexibility in the process based on end use. An intriguing study by Chattopadhyay et al.68 examined the effects of catalysts on pyrolysis of an exceedingly complex and disparate waste stream of HDPE, PP, PET and paper biomass (consisting primarily of cellulose). Cobalt complexes were used as catalysts with various chelating agent along with Al2O3 and / or CeO2. The catalytic pyrolysis of the paper waste by itself yielded mainly gaseous and solid products, whereas pyrolysis of the paper/PP/HDPE/PET mixture yielded more liquid products that were rich in aromatics and olefins. The potential for pyrolytic approaches to valorize PE waste is counterbalanced by the high energy cost. A greener avenue that has shown recent promise in the decomposition of plastic waste is the use of supercritical water.69, 70 An added benefit of using supercritical water is that there is no need for a catalyst. Moriya et al.71 undertook one such study on supercritical water as a medium for the thermal cracking of PE for comparison to established pyrolysis techniques. A 5:1 ratio of H2O:HDPE was used, and the mixture was reacted at 425 °C for 2 h, the oil conversion rate was shown to be 90%. Encouragingly, the yield of organic oil produced was higher for supercritical water cracking, while commensurately less coke/char was formed in the process. The oil product mainly was composed of a majority of alkenes with the presence of some alkanes with very little presence of any aromatic moieties. While most of the aforementioned approaches to recycling PE have focused on the pyrolysis to form shorter chain molecules that ultimately tend to be used in fuel 20 applications, the pyrolysis products may also be leveraged as feedstocks to produce other polymers. Recently, Bäckström et al.72 depolymerized HDPE waste to synthesize a mixture of succinic, glutaric, and adipic acid. This was accomplished through a microwave-assisted hydrothermal process with the addition of HNO3. Whereas microwave reactions have in the past been dismissed as impractical for large-scale operations, recent advances and implementation on a commercial scale have begun to dispel these preconceptions.73 The products of the microwave reaction were carried forward as plasticizers in a poly(lactic acid) formulation. Incorporating these PE- derivative plasticizers into the PLA formulation increased the strain at break to 144% as compared to 6% for PLA without the use of any plasticizer. This study illuminates a creative path forward to exploit post-consumer petrochemical waste for improved performance of sustainably-sourced polymers. Polypropylene (SPI code 5) Polypropylene (PP) has a similar backbone to polyethylene, but the presence of an additional methyl group as a sidechain on each repeat unit has a significant impact on the properties. PP is a light weight, tough, crystalline thermoplastic polymer that finds use in reusable food containers, the automotive industry, and even the furniture market.74 PP is synthesized from the propylene (C3H6) using either a Ziegler-Natta or metallocene catalyst. Polypropylene can have three different types of structures, atactic, isotactic, and syndiotactic,29 depending on the relative disposition of the methyl side chains with respect to one another along the backbone (Figure 1.3). 21 isotactic PP syndiotactic PP atactic PP Figure 1.3 The differences between isotactic, syndiotactic, and atactic PP. The configuration of the methyl groups has an effect on the crystallinity of the PP and consequently on bulk properties. The tacticity of PP generally has little effect on recycling techniques, and consequently will not be discussed further here. Very little of the polypropylene that enters the marketplace – less than 1% – ends up being recycled, largely due to the fact that PP is generally found in mixed waste streams14 such as cable coverings, electronic appliances, and rugs. In each of those materials, PP is not the only polymer that is present, thus, having to wash and separate each component are major barriers to recycling efforts.74 In cases where separation is effective and practical, there are still drawbacks to melt re-processing of PP. As with the other polymers discussed herein, recursive heating cycles rapidly degrade the PP backbone. Thermal degradation of PP is more severe than in PE because the tertiary carbon atom present in the PP backbone is susceptible to thermo-oxidative and photo-oxidative degradation.74 While some PP plastics include 22 stabilizer to prevent degradation from occurring, if the PP polymer has been successfully recycled into a new material, this material must then include additional stabilizers to imbue the “newly” made material the same oxidative stability, while the buildup of stabilizer and sacrificial stabilizer degradation products contributes commensurately to the deterioration of PP properties. The elongation-at-break for a virgin, unstabilized sample of PP is 65%; however, after just 10 recycling cycles the elongation-at-break has decreased to 45%.75 When PP can no longer be mechanically recycled through either shredding or melt reprocessing, there are some emerging strategies to convert PP into value-added feedstocks. Interest in tertiary recycling of PP has been growing rapidly. The depolymerization of PP into propylene has even been accomplished by Guddeti et al.76 In this process, PP was depolymerized in an induction-coupled plasma reactor. Under these conditions, PP was converted to gaseous products (up to 78 wt%) and of the gaseous product formed 94% was identified as propylene. Other studies have used plasma as a media to depolymerize PP into other value added commodities.77 One major advantage of using a plasma reactor is that reaction times are very short; however, the cost to setup and operate plasma reactors on an industrial scale detracts from the practical viability of these approaches. Another drawback of using such high temperatures is that any contaminants present, as would be expected in a waste stream, tend to yield many side reactions and have profound impact on product yields. As for PE recycling strategies, supercritical water is a privileged media for contemporary PP recycling as well. Chen et al.78 used supercritical water to convert PP 23 into oil. Optimal reaction conditions were found to be at 425 °C for 2–4 h or 450 °C for 0.5–1 h, in which case up to 91 wt% of the PP was converted into oil. The composition of the oil was found to be olefins, paraffins, cycloalkanes, and aromatics. The analysis of the oil showed that it had similar properties to that of naphtha, which can be further purified to make gasoline. The use of supercritical water to depolymerize PP is a largely sustainable step towards upgrading waste PP into value-added feedstocks. The decomposition of PP can also yield solid carbonaceous products that can be used for various applications. Liu et al.79 described a two-stage reactor. In the first step of the process, PP is catalytically pyrolyzed to yield gaseous and liquid products. These products are then taken to the second stage of the process in which the products are thermally decomposed to make either carbon nanotubes or gaseous products, which mainly consisted of H2 or CH4. This process allows for the collection of gaseous and solid products that can be used in different applications. In a similar vein, Mishra et al.80 successfully synthesized carbon nanotubes and hydrogen gas from waste PP using a chemical vapor deposition method with a nickel-based catalyst. The carbon nanotubes synthesized by this process showed high transmittance (85%) of light at 550 nm which can be exploited as transparent electrodes for optoelectronic devices. The gas evolved was mostly aliphatic molecules, with very little to no presence of aromatics. Given the difficulties associated with obtaining near-pure PP from waste, the incorporation of PP into other waste streams for recycling has drawn interest as well (some of the PE recycling strategies have already illustrated this approach; vide infra). The annual availability of gigaton quantities of biomass (primarily lignocellulosic) make 24 this an attractive waste stream for valorization.81-83 Simple pyrolysis of biomass waste alone generally leads to low-value final products. The addition of PP to biomass waste, however, can enhance the properties of the final products. Zhao et al.,84 for example, mixed bamboo waste with PP and the copyrolysis of the two yielded bio oils that have potential as fuels. To accomplish this transformation, a zeolite catalyst (HZSM-5) was used while the bamboo:PP ratio and catalyst loading were varied. Even under optimal conditions, bamboo alone still gives quite a poor oil yield (30 wt%) and low oil quality. The bamboo-derived oil consisted primarily of aliphatic hydrocarbons. When a 2:1 ratio of bamboo:PP was employed, they found that oil production could be more than doubled to 62 wt% with attendant improvement in oil quality. The bamboo/PP-derived oil consisted of more aromatic and naphthenic compounds. Such compositions are ideal feedstocks for jet fuel production due to the large amounts of heavier hydrocarbons produced. Lee et al.85 further demonstrated the broader scope of such processes by showing that PP addition to high cellulose content agricultural waste from sources other than bamboo can likewise improve the production of oils in high yields and of more desirable composition than can be accomplished by pyrolysis of cellulose alone. Conclusions With many plastic recycling processes focusing on primary and secondary recycling, there is still a parity that exists between the amount of plastic waste versus how much is recycled. We must now turn to other processes that can be used in conjunction 25 with current processes in order to alleviate the problems associated with the mass amounts of plastic waste being produced. As outlined here there has been a lot of work studying how plastic waste can be chemically recycled. The chemical transformation of waste plastics into value added chemicals has been shown to be a convenient avenue to supplement current recycling processes. Depending on temperature, residence times, and initial feedstock, the yields of the desired products (monomers, gases, oils, and solids) can vary. Catalysts have been developed to better increase the overall yields of the reaction, while also focusing on decreasing the reaction temperature and times. While catalysts can increase the overall efficiency of the chemical process, separation, recyclability, and toxicity are factors that are important to the overall usefulness of the catalyst. Continued efforts need to be focused on the catalysts overall efficiency especially when considered to be utilized on an industrial scale. The use of mixed plastic waste streams show how realistic waste streams can be utilized as feedstocks for the recycling process. Additionally, the combination of plastic wastes with other biomass waste to improve the yield and quality of the final product is an avenue that can be further exploited. The exploitation of two different waste streams has the potential to be an efficient solution to the both plastic waste and biomass waste problems. The processing of the chemical products should be limited to simple and non-intensive processes to increase the efficiency of the overall process. The chemical recycling of plastic waste show promise to become one of the main processes in which we can efficiently reduce the amount of waste in landfills. In any process, the ultimate overall 26 efficiency from a chemical and economic standpoint must be assessed in order to fully industrialize any chemical recycling process. Acknowledgements This work was supported by the National Science Foundation (CHE-1708844). 27 References 1. https://www.plasticsinsight.com/world-per-capita-consumption-pe-pp-pvc-resins- 2014/ (accessed 7-4-19). 2. Malik, N.; Kumar, P.; Shrivastava, S.; Ghosh, S. B., An overview on PET waste recycling for application in packaging. Int. J. Plast. Technol. 2017, 21 (1), 1-24. 3. Kaufmann, J., New Materials for Sports Equipment Made of Anisotropic Fiber- reinforced Plastics with Stiffness Related Coupling Effect. Procedia Engineering 2015, 112, 140-145. 4. McKeen, L. W., 3 - Plastics Used in Medical Devices. In Handbook of Polymer Applications in Medicine and Medical Devices, Modjarrad, K.; Ebnesajjad, S., Eds. William Andrew Publishing: Oxford, 2014; pp 21-53. 5. Day, M.; Cooney, J. D.; Touchette-Barrette, C.; Sheehan, S. E., Pyrolysis of mixed plastics used in the electronics industry. Journal of Analytical and Applied Pyrolysis 1999, 52 (2), 199-224. 6. Jeong, J.; Choi, J., Adverse outcome pathways potentially related to hazard identification of microplastics based on toxicity mechanisms. Chemosphere 2019, 231, 249-255. 7. Cole, M.; Lindeque, P.; Halsband, C.; Galloway, T. S., Microplastics as contaminants in the marine environment: A review. Marine Pollution Bulletin 2011, 62 (12), 2588-2597. 28 8. Hidalgo-Ruz, V.; Gutow, L.; Thompson, R. C.; Thiel, M., Microplastics in marine environment review of methods for identification and quantification. Environmental Science and Technology 2012, 46 (6), 3060-3075. 9. Gestoso, I.; Cacabelos, E.; Ramalhosa, P.; Canning-Clode, J., Plasticrusts: A new potential threat in the Anthropocene's rocky shores. Science of the Total Environment 2019, 687, 413-415. 10. Rahimi, A.; García, J. M., Chemical recycling of waste plastics for new materials production. Nature Reviews Chemistry 2017, 1 (6), 0046. 11. Datta, J.; Kopczyńska, P., From polymer waste to potential main industrial products: Actual state of recycling and recovering. Critical Reviews in Environmental Science and Technology 2016, 46 (10), 905-946. 12. Advancing Sustainable Materials: 2015 Fact Sheet; United States Environmental Protection Agency, 2018; pp 1-23. 13. Documentation for Greenhouse Gas Emission and Energy Factors Used in the Waste Reduction Model (WARM); United States Environmental Protection Agency, 2015. 14. Advancing Sustainable Materials Management: 2015 Tables and Figures; United States Environmental Protection Agency, 2018; pp 1-67. 15. Al-Salem, S. M.; Lettieri, P.; Baeyens, J., Recycling and recovery routes of plastic solid waste (PSW): A review. Waste Management 2009, 29 (10), 2625-2643. 16. Ignatyev, I. A.; Thielemans, W.; Vander Beke, B., Recycling of Polymers: A Review. ChemSusChem 2014, 7 (6), 1579-1593. 29 17. Karthikeyan, S.; Sivakumar, N.; Manimekalai, T. K.; Sathiskumar, C., A review on pyrolysis of waste plastics to value added products. Elixir Online J. 2012, (May), 8291-8298. 18. Wong, S. L.; Ngadi, N.; Abdullah, T. A. T.; Inuwa, I. M., Current state and future prospects of plastic waste as source of fuel: A review. Renewable Sustainable Energy Rev. 2015, 50, 1167-1180. 19. Mohanraj, C.; Senthilkumar, T.; Chandrasekar, M., A review on conversion techniques of liquid fuel from waste plastic materials. Int. J. Energy Res. 2017, 41 (11), 1534-1552. 20. Froelich, D.; Maris, E.; Haoues, N.; Chemineau, L.; Renard, H.; Abraham, F.; Lassartesses, R., State of the art of plastic sorting and recycling: Feedback to vehicle design. Minerals Engineering 2007, 20 (9), 902-912. 21. Ahmad, S. R., A New Technology for Automatic Identification and Sorting of Plastics for Recycling. Environmental Technology 2004, 25 (10), 1143-1149. 22. George, N.; Kurian, T., Recent Developments in the Chemical Recycling of Postconsumer Poly(ethylene terephthalate) Waste. Industrial & Engineering Chemistry Research 2014, 53 (37), 14185-14198. 23. Akato, K. M.; Nguyen, N. A.; Bonnesen, P. V.; Harper, D. P.; Naskar, A. K., Recycling Waste Polyester via Modification with a Renewable Fatty Acid for Enhanced Processability. ACS Omega 2018, 3 (9), 10709-10715. 30 24. Geyer, B.; Lorenz, G.; Kandelbauer, A., Recycling of poly(ethylene terephthalate) - A review focusing on chemical methods. eXPRESS Polym. Lett. 2016, 10 (7), 559- 586. 25. López, M. d. M. C.; Ares Pernas, A. I.; Abad López, M. J.; Latorre, A. L.; López Vilariño, J. M.; González Rodríguez, M. V., Assessing changes on poly(ethylene terephthalate) properties after recycling: Mechanical recycling in laboratory versus postconsumer recycled material. Materials Chemistry and Physics 2014, 147 (3), 884-894. 26. Al-Sabagh, A. M.; Yehia, F. Z.; Eshaq, G.; Rabie, A. M.; ElMetwally, A. E., Greener routes for recycling of polyethylene terephthalate. Egyptian Journal of Petroleum 2016, 25 (1), 53-64. 27. Kenny, S. T.; Runic, J. N.; Kaminsky, W.; Woods, T.; Babu, R. P.; Keely, C. M.; Blau, W.; O’Connor, K. E., Up-Cycling of PET (Polyethylene Terephthalate) to the Biodegradable Plastic PHA (Polyhydroxyalkanoate). Environmental Science & Technology 2008, 42 (20), 7696-7701. 28. Du, S.; Valla, J. A.; Parnas, R. S.; Bollas, G. M., Conversion of Polyethylene Terephthalate Based Waste Carpet to Benzene-Rich Oils through Thermal, Catalytic, and Catalytic Steam Pyrolysis. ACS Sustainable Chemistry & Engineering 2016, 4 (5), 2852-2860. 29. Introduction. In Feedstock Recycling of Plastic Wastes, Aguado, J.; Serrano, D. P.; Clark, J. H., Eds. The Royal Society of Chemistry: 1999; pp 1-29. 31 30. Yoshioka, T.; Okayama, N.; Okuwaki, A., Kinetics of Hydrolysis of PET Powder in Nitric Acid by a Modified Shrinking-Core Model. Industrial & Engineering Chemistry Research 1998, 37 (2), 336-340. 31. Paszun, D.; Spychaj, T., Chemical Recycling of Poly(ethylene terephthalate). Industrial & Engineering Chemistry Research 1997, 36 (4), 1373-1383. 32. Yoshioka, T.; Motoki, T.; Okuwaki, A., Kinetics of Hydrolysis of Poly(ethylene terephthalate) Powder in Sulfuric Acid by a Modified Shrinking-Core Model. Industrial & Engineering Chemistry Research 2001, 40 (1), 75-79. 33. Polk, M. B.; Leboeuf, L. L.; Shah, M.; Won, C.-Y.; Hu, X.; Ding, W., Nylon 66, Nylon 46, and Pet Phase-Transfer-Catalyzed Alkaline Depolymerization at Atmospheric Pressure. Polymer-Plastics Technology and Engineering 1999, 38 (3), 459-470. 34. Campanelli, J. R.; Kamal, M. R.; Cooper, D. G., A kinetic study of the hydrolytic degradation of polyethylene terephthalate at high temperatures. Journal of Applied Polymer Science 1993, 48 (3), 443-451. 35. Genta, M.; Iwaya, T.; Sasaki, M.; Goto, M.; Hirose, T., Depolymerization Mechanism of Poly(ethylene terephthalate) in Supercritical Methanol. Industrial & Engineering Chemistry Research 2005, 44 (11), 3894-3900. 36. Goto, M.; Koyamoto, H.; Kodama, A.; Hirose, T.; Nagaoka, S., Depolymerization of polyethylene terephthalate in supercritical methanol. Journal of Physics: Condensed Matter 2002, 14 (44), 11427-11430. 32 37. Tang, H.; Li, N.; Li, G.; Wang, A.; Cong, Y.; Xu, G.; Wang, X.; Zhang, T., Synthesis of gasoline and jet fuel range cycloalkanes and aromatics from poly(ethylene terephthalate) waste. Green Chemistry 2019, 21 (10), 2709-2719. 38. Teotia, M.; Tarannum, N.; Soni, R. K., Depolymerization of PET waste to potentially applicable aromatic amides: Their characterization and DFT study. Journal of Applied Polymer Science 2017, 134 (31), 45153. 39. Soni, R. K.; Singh, S.; Dutt, K., Studies on synthesis and characterization of N-alkyl terephthalamides using different amines from polyethylene terephthalate waste. Journal of Applied Polymer Science 2010, 115 (5), 3074-3080. 40. Hoang, C. N.; Dang, Y. H., Aminolysis of poly(ethylene terephthalate) waste with ethylenediamine and characterization of α,ω-diamine products. Polymer Degradation and Stability 2013, 98 (3), 697-708. 41. Tawfik, M. E.; Eskander, S. B., Chemical recycling of poly(ethylene terephthalate) waste using ethanolamine. Sorting of the end products. Polymer Degradation and Stability 2010, 95 (2), 187-194. 42. Tawfik, M. E.; Ahmed, N. M.; Eskander, S. B., Aminolysis of poly(ethylene terephthalate) wastes based on sunlight and utilization of the end product [bis(2- hydroxyethylene) terephthalamide] as an ingredient in the anticorrosive paints for the protection of steel structures. Journal of Applied Polymer Science 2011, 120 (5), 2842-2855. 33 43. Langer, E.; Waśkiewicz, S.; Lenartowicz-Klik, M.; Bortel, K., Application of waste poly(ethylene terephthalate) in the synthesis of new oligomeric plasticizers. Polymer Degradation and Stability 2015, 119, 105-112. 44. Suh, D. J.; Park, O. O.; Yoon, K. H., The properties of unsaturated polyester based on the glycolyzed poly(ethylene terephthalate) with various glycol compositions. Polymer 2000, 41 (2), 461-466. 45. Liu, B.; Lu, X.; Ju, Z.; Sun, P.; Xin, J.; Yao, X.; Zhou, Q.; Zhang, S., Ultrafast Homogeneous Glycolysis of Waste Polyethylene Terephthalate via a Dissolution- Degradation Strategy. Ind. Eng. Chem. Res. 2018, 57 (48), 16239-16245. 46. Troev, K.; Grancharov, G.; Tsevi, R.; Gitsov, I., A novel catalyst for the glycolysis of poly(ethylene terephthalate). Journal of Applied Polymer Science 2003, 90 (4), 1148-1152. 47. Wang, Q.; Yao, X.; Tang, S.; Lu, X.; Zhang, X.; Zhang, S., Urea as an efficient and reusable catalyst for the glycolysis of poly(ethylene terephthalate) wastes and the role of hydrogen bond in this process. Green Chemistry 2012, 14 (9), 2559-2566. 48. Wang, Q.; Geng, Y.; Lu, X.; Zhang, S., First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET). ACS Sustainable Chemistry & Engineering 2015, 3 (2), 340- 348. 49. Wang, H.; Li, Z.; Liu, Y.; Zhang, X.; Zhang, S., Degradation of poly(ethylene terephthalate) using ionic liquids. Green Chemistry 2009, 11 (10), 1568-1575. 34 50. Yue, Q. F.; Wang, C. X.; Zhang, L. N.; Ni, Y.; Jin, Y. X., Glycolysis of poly(ethylene terephthalate) (PET) using basic ionic liquids as catalysts. Polymer Degradation and Stability 2011, 96 (4), 399-403. 51. Yue, Q. F.; Xiao, L. F.; Zhang, M. L.; Bai, X. F., The glycolysis of poly(ethylene terephthalate) waste: Lewis acidic ionic liquids as high efficient catalysts. Polymers (Basel, Switz.) 2013, 5 (4), 1258-1271, 14 pp. 52. Liu, B.; Fu, W.; Lu, X.; Zhou, Q.; Zhang, S., Lewis Acid–Base Synergistic Catalysis for Polyethylene Terephthalate Degradation by 1,3- Dimethylurea/Zn(OAc)2 Deep Eutectic Solvent. ACS Sustainable Chemistry & Engineering 2019, 7 (3), 3292-3300. 53. Mecit, O.; Akar, A., Synthesis of Urethane Oil Varnishes from Waste Poly(ethylene terephthalate). Macromolecular Materials and Engineering 2001, 286 (9), 513-515. 54. Desai, S. D.; Patel, J. V.; Patel, M. R.; Sinha, V. K., PET waste based polyurethane adhesive for rubber joints. Indian J. Chem. Technol. 2005, 12 (1), 82-87. 55. Leung, D. Y. C.; Wu, X.; Leung, M. K. H., A review on biodiesel production using catalyzed transesterification. Applied Energy 2010, 87 (4), 1083-1095. 56. Ma, F.; Hanna, M. A., Biodiesel production: a review. Bioresource Technology 1999, 70 (1), 1-15. 57. Demirbas, A., Biodiesel fuels from vegetable oils via catalytic and non-catalytic supercritical alcohol transesterifications and other methods: a survey. Energy Conversion and Management 2003, 44 (13), 2093-2109. 35 58. Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C., Biodiesel production from high FFA rubber seed oil. Fuel 2005, 84 (4), 335-340. 59. Canakci, M.; Van Gerpen, J., Biodiesel production from oils and fats with high free fatty acids. Transactions of the ASAE 2001, 44 (6), 1429-1436. 60. Amaro, L. P.; Coiai, S.; Ciardelli, F.; Passaglia, E., Preparation and testing of a solid secondary plasticizer for PVC produced by chemical degradation of post-consumer PET. Waste Management 2015, 46, 68-75. 61. Sirohi, S.; Dobhal, S.; Doshi, M.; Nain, R.; Dutt, K.; Pani, B., Eco-friendly synthesis of PET-based polymeric plasticiser and its application in nitrile-PVC rubber blends. Indian Chemical Engineer 2019, 61 (2), 206-217. 62. Jin, H.; Gonzalez-Gutierrez, J.; Oblak, P.; Zupančič, B.; Emri, I., The effect of extensive mechanical recycling on the properties of low density polyethylene. Polymer Degradation and Stability 2012, 97 (11), 2262-2272. 63. Ahmad, I.; Khan, M. I.; Khan, H.; Ishaq, M.; Tariq, R.; Gul, K.; Ahmad, W., Pyrolysis Study of Polypropylene and Polyethylene Into Premium Oil Products. International Journal of Green Energy 2015, 12 (7), 663-671. 64. Palos, R.; Gutiérrez, A.; Vela, F. J.; Maña, J. A.; Hita, I.; Asueta, A.; Arnaiz, S.; Arandes, J. M.; Bilbao, J., Assessing the potential of the recycled plastic slow pyrolysis for the production of streams attractive for refineries. Journal of Analytical and Applied Pyrolysis 2019, 142, 104668. 65. Das, P.; Tiwari, P., Valorization of packaging plastic waste by slow pyrolysis. Resources, Conservation and Recycling 2018, 128, 69-77. 36 66. Miandad, R.; Barakat, M. A.; Aburiazaiza, A. S.; Rehan, M.; Ismail, I. M. I.; Nizami, A. S., Effect of plastic waste types on pyrolysis liquid oil. International Biodeterioration & Biodegradation 2017, 119, 239-252. 67. Santos, B. P. S.; Almeida, D.; Marques, M. d. F. V.; Henriques, C. A., Petrochemical feedstock from pyrolysis of waste polyethylene and polypropylene using different catalysts. Fuel 2018, 215, 515-521. 68. Chattopadhyay, J.; Pathak, T. S.; Srivastava, R.; Singh, A. C., Catalytic co-pyrolysis of paper biomass and plastic mixtures (HDPE (high density polyethylene), PP (polypropylene) and PET (polyethylene terephthalate)) and product analysis. Energy 2016, 103, 513-521. 69. Su, X.; Zhao, Y.; Zhang, R.; Bi, J., Investigation on degradation of polyethylene to oils in supercritical water. Fuel Processing Technology 2004, 85 (8), 1249-1258. 70. Tan, X.-C.; Zhu, C.-C.; Liu, Q.-K.; Ma, T.-Y.; Yuan, P.-Q.; Cheng, Z.-M.; Yuan, W.-K., Co-pyrolysis of heavy oil and low density polyethylene in the presence of supercritical water: The suppression of coke formation. Fuel Processing Technology 2014, 118, 49-54. 71. Moriya, T.; Enomoto, H., Characteristics of polyethylene cracking in supercritical water compared to thermal cracking. Polymer Degradation and Stability 1999, 65 (3), 373-386. 72. Bäckström, E.; Odelius, K.; Hakkarainen, M., Designed from Recycled: Turning Polyethylene Waste to Covalently Attached Polylactide Plasticizers. ACS Sustainable Chemistry & Engineering 2019, 7 (12), 11004-11013. 37 73. Nagahata, R.; Takeuchi, K., Encouragements for the Use of Microwaves in Industrial Chemistry. The Chemical Record 2019, 19 (1), 51-64. 74. Xanthos, M., Recycling of the #5 Polymer. Science (Washington, DC, U. S.) 2012, 337 (6095), 700-702. 75. Aurrekoetxea, J.; Sarrionandia, M. A.; Urrutibeascoa, I.; Maspoch, M. L., Effects of recycling on the microstructure and the mechanical properties of isotactic polypropylene. J. Mater. Sci. 2001, 36 (11), 2607-2613. 76. Guddeti, R. R.; Knight, R.; Grossmann, E. D., Depolymerization of Polypropylene in an Induction-Coupled Plasma (ICP) Reactor. Industrial & Engineering Chemistry Research 2000, 39 (5), 1171-1176. 77. Tang, L.; Huang, H.; Zhao, Z.; Wu, C. Z.; Chen, Y., Pyrolysis of Polypropylene in a Nitrogen Plasma Reactor. Industrial & Engineering Chemistry Research 2003, 42 (6), 1145-1150. 78. Chen, W.-T.; Jin, K.; Linda Wang, N.-H., Use of Supercritical Water for the Liquefaction of Polypropylene into Oil. ACS Sustainable Chem. Eng. 2019, 7 (4), 3749-3758. 79. Liu, J.; Jiang, Z.; Yu, H.; Tang, T., Catalytic pyrolysis of polypropylene to synthesize carbon nanotubes and hydrogen through a two-stage process. Polymer Degradation and Stability 2011, 96 (10), 1711-1719. 80. Mishra, N.; Das, G.; Ansaldo, A.; Genovese, A.; Malerba, M.; Povia, M.; Ricci, D.; Di Fabrizio, E.; Di Zitti, E.; Sharon, M.; Sharon, M., Pyrolysis of waste 38 polypropylene for the synthesis of carbon nanotubes. Journal of Analytical and Applied Pyrolysis 2012, 94, 91-98. 81. Gallezot, P., Conversion of biomass to selected chemical products. Chemical Society Reviews 2012, 41 (4), 1538-1558. 82. Akato, K. M.; Nguyen, N. A.; Rajan, K.; Harper, D. P.; Naskar, A. K., A tough and sustainable fiber-forming material from lignin and waste poly(ethylene terephthalate). RSC Advances 2019, 9 (54), 31202-31211. 83. Xu, C.; Nasrollahzadeh, M.; Selva, M.; Issaabadi, Z.; Luque, R., Waste-to-wealth: biowaste valorization into valuable bio(nano)materials. Chemical Society Reviews 2019, 48 (18), 4791-4822. 84. Zhao, Y.; Wang, Y.; Duan, D.; Ruan, R.; Fan, L.; Zhou, Y.; Dai, L.; Lv, J.; Liu, Y., Fast microwave-assisted ex-catalytic co-pyrolysis of bamboo and polypropylene for bio-oil production. Bioresource Technology 2018, 249, 69-75. 85. Lee, H. W.; Kim, Y.-M.; Jae, J.; Jeon, J.-K.; Jung, S.-C.; Kim, S. C.; Park, Y.-K., Production of aromatic hydrocarbons via catalytic co-pyrolysis of torrefied cellulose and polypropylene. Energy Conversion and Management 2016, 129, 81-88. 39 Attribution of work My contribution to the work in this chapter has been published in the Journal of Polymer Science . The citation for the above manuscript: J. Polym. Sci., 2020, DOI: 10.1002/pol.20190261 40 CHAPTER TWO THERMALLY-HEALABLE NETWORK SOLIDS OF SULFUR-CROSSLINKED POLY(4-ALLYLOXYSTYRENE)2 Abstract Network polymers of sulfur and poly(4-allyloxystyrene), PAOSx (x = percent by mass sulfur, where x is varied from 10-99), were prepared by reaction between poly(4- allyloxystyrene) with thermal homolytic ring-opened S8 in a thiol-ene-type reaction. The extent to which sulfur content and crosslinking influence thermal/mechanical properties was assessed. Network materials having sulfur content below 50% were found to be thermosets, whereas those having >90% sulfur content are thermally healable and remeltable. DSC analysis revealed that low sulfur-content materials exhibited neither a Tg nor a Tm from –50 to 140 °C, whereas higher sulfur content materials featured Tg or Tm values that scale with the amount of sulfur. DSC data also revealed that sulfur-rich domains of PAOS90 are comprised of sulfur-crosslinked organic polymers and amorphous sulfur, whereas, sulfur-rich domains in PAOS99 are comprised largely of a- sulfur (orthorhombic sulfur). These conclusions are further corroborated by CS2- extraction and analysis of extractable/non-extractable fractions. Calculations based on TGA, FT-IR, H2S trapping experiments, CS2-extractable mass, and elemental combustion microanalysis data were used to assess the relative percentages of free and crosslinked sulfur and average number of S atoms per crosslink. Dynamic mechanical analyses 2 This text is reproduced with permission in larger part from: Thiounn, T.; Lauer, M. K.; Bedford, M. S.; Smith, R. C.; Tennyson, A. G., Thermally-Healable Network Solids of Sulfur-Crosslinked Poly(4-allyloxystyrene). RCS Advances 2018, 8, 39074-39082. Permission documentation is provided at the end of this chapter. 41 indicate high storage moduli for PAOS90 and PAOS99 (on the order of 3 and 6 GPa at = – 37 °C, respectively), with a mechanical Tg between –17 °C and 5 °C. A PAOS99 sample retains its full initial mechanical strength after at least 12 pulverization-thermal healing cycles, making it a candidate for facile repair and recyclability. Introduction Vulcanization is a process developed by Charles Goodyear1 in which the mechanical properties of natural rubber (polyisoprene) are dramatically enhanced by heating it in the presence of up to 5 wt. % elemental sulfur (as S8 at STP). The increase in material strength of a rubber following vulcanization results from covalent crosslinking between polymer chains.2 Such crosslinking is derived from homolytic thermal ring 3 opening of S8 to afford sulfur-centered radicals that subsequently add to C=C bonds, leading to polysulfide crosslinks between polymer chains (Scheme 2.1a). Without the dramatic increase in mechanical strength endowed upon rubber products by polysulfide crosslinks formed during vulcanization, the modern automobile tire would not be accessible, and early automobiles would have been severely limited in the types and duration of tasks they could perform. 42 a) b) Scheme 2.1 Sulfur vulcanization of polyisoprene (a) and by-product sulfur (b) at North Vancouver Sulphur Works, originating from tar sand-processing facilities in Alberta, Canada. Despite the dramatic structural improvement mediated by sulfur as a minor component by mass of vulcanized rubber, elemental sulfur is currently underutilized as a bulk component of materials. The underutilization of sulfur is especially surprising considering the titanic quantities of S8 (Scheme 2.1b) that are produced by petrochemical industries. Petrochemical by-product sulfur is produced by the Claus process,4, 5 wherein H2S is oxidized to S8. Sulfur removal from fuels is necessary to prevent catalyst poisoning in subsequent steps of the petroleum-refining process and to attenuate the environmental impact of acid rain that results from combustion of sulfur-rich fossil 43 6 fuels. Currently, the primary use of the S8 generated from fossil fuels is in the production of sulfuric acid, with lesser quantities being used in fertilizers/pesticides, vulcanization of 7-9 rubber, and as an additive to asphalt. Even after consumption of S8 by these processes, 10 however, 7 million tons of S8 accumulate each year as unused waste. Vast quantities of S8 have been stockpiled at industrial centers and storage sites, so S8 is an inexpensive, abundant, and readily accessible feedstock for valorization.6, 11-13 Furthermore, the formation of S–S bonds is thermally reversible, a property that could be exploited to generate recyclable thermoplastics and thermally self-healing materials.14 Although elemental sulfur itself has very poor mechanical properties, significant advances have been achieved with the advent of inverse vulcanization.15 Inverse vulcanization is a term coined by Pyun for a process in which S8 is the bulk material and organic small molecules form the covalent crosslinks between polysulfide chains.6, 15-27 Pyun’s breakthrough studies on such high sulfur-content materials have inspired a flurry of related studies to better employ byproduct sulfur.12 Materials prepared by inverse vulcanization can retain some of the advantageous properties of S8. For example, S8 exhibits high electrical resistivity (2 × 1023 µΩ-cm) and low thermal conductivity (0.205 –1 –1 W m K ). In addition, S8 actively resists several material degradation pathways, as it is resistant to corrosion by strong acids. It also repels insects and rodents,28 as well as having antibacterial and anti-microbial properties,29-34 and has long been used as a treatment for timbers to prevent decay.35, 36 Moreover, if an organic small molecule used in inverse vulcanization has more than one C=C bond, then that molecule can crosslink 44 multiple polysulfide chains to afford a networked covalent composite (NCC) or polymer material. Many of the inverse vulcanized materials prepared by Pyun exhibit significantly greater durability compared to sulfur itself, even for materials containing > 90% sulfur by weight. Because even small amounts of an alkene-functionalized organic small molecule can afford materials with dramatically enhanced mechanical properties relative to pure S8, we envisioned that using an alkene-functionalized organic polymer would afford an NCC with enhanced networking. We hypothesized that using this strategy to increase networking in NCCs would increase the mechanical strength of the NCCs. Herein we report the synthesis of allyloxy-functionalized polystyrene, its crosslinking with S8 via inverse vulcanization, and the thermomechanical properties of the resulting NCCs, PAOSx (x = percent by mass sulfur in the material), wherein x is varied from 10 to 99. Experimental Section General Considerations All NMR spectra were recorded on a Bruker Avance spectrometer operating at 300 MHz for protons. Thermogravimetric analysis (TGA) was recorded on a TA SDT Q600 instrument over the range 20 to 800 °C, with a heating rate of 5 °C min–1 under a –1 flow of N2 (100 mL min ). Differential Scanning Calorimetry (DSC) was acquired using a Mettler Toledo DSC 3 STARe System over the range of –50 to 140 °C, with a heating –1 –1 rate of 10 °C min under a flow of N2 (200 mL min ). Each DSC measurement was carried out over five heat-cool cycles to confirm consistent results following the first 45 heat-cool cycle. The data reported were taken from the second cycle of the experiment. Dynamic Mechanical Analysis (DMA) was performed using a Mettler Toledo DMA 1 STARe System in dual cantilever mode. DMA samples were cast from silicone resin molds (Smooth-On Oomoo® 30 tin-cure). The sample dimensions were 1.5 × 8 × 50 mm. Clamping force was 1 cN·m and the temperature was varied from –60 to 88 °C with a heating rate of 2 °C min–1. The measurement mode was set to displacement control with a displacement amplitude of 5 µm and a frequency of 1 Hz. Fourier transform infrared spectra were obtained using a Shimadzu IRAffinity-1S instrument operating over the range of 400-4000 cm–1 at ambient temperature using an ATR attachment. Poly(4- vinylphenol) (Mw = 11,000 g/mol, Sigma Aldrich), allyl bromide (Oakwood Chemical), elemental sulfur (99.5+%, Duda Energy, LLC) were used without further purification. Preparation of Poly(4-allyloxystyrene) (PAO) The procedure that follows is based on a reported method.37 To a 500 mL round bottom flask equipped with a Teflon-coated magnetic stir bar was added 22.489 g (185.87 mmol) of allyl bromide, 20.000 g (166.44 mmol) of poly(4-vinylphenol) (PVP) and 200 mL of acetonitrile. The contents of the flask were stirred for 5 min to facilitate complete dissolution of PVP. The flask was then equipped with a water-cooled reflux condenser and placed under an atmosphere of N2. The flask was heated in an oil bath at 80 °C while stirring. While the heated reaction mixture was stirring, 49.999 g (361.77 mmol) of K2CO3 was added portionwise to the reaction mixture. The reaction was then refluxed with stirring for 72 h, after which time the mixture was allowed to cool to room 46 temperature. The flask was opened to the air and deionized water (250 mL) was added to the reaction flask to dissolve the remaining K2CO3. The reaction mixture was then extracted with dichloromethane (450 mL). The organic layer was collected and washed with 1 M HCl(aq) (1 × 250 mL), and then with deionized water (1 × 250 mL). The organic layer was collected, and the solvent was evaporated under reduced pressure. The product was collected as a beige solid. 1H NMR (300 MHz, DMSO-d6, �): 1.41 (br, 2H), 1.72 (br, 1H), 4.45 (br, 2H), 5.21 (br, 1H), 5.35 (br, 1H), 5.99 (br, 1H), 6.48-6.69 (br, 4H). Elemental analysis calculated for C11H12O: C, 82.45; H, 7.56%. Found C, 81.88; H, 7.77%. These data are consistent with those previously reported.37 DSC data for this compound are provided in the Supporting Information at the end of this chapter. General synthesis of PAOSx Elemental sulfur was weighed directly into an aluminum reaction vessel. The vessel was heated to 180 °C, over which time the sulfur melted. Once the sulfur turned a viscous dark red-orange color (indicative of thermal ring-opening), the appropriate amount of PAO was slowly added to the molten sulfur. The reaction media was manually stirred with a spatula for 60 min. After 60 min the reaction was stopped, the mixture was allowed to cool, and the solid was removed from the reaction vessel. Reagent masses and results of elemental combustion microanalysis are provided below. 47 Synthesis of PAOS99 CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. The general synthesis above was used to synthesize PAOS99 (99 wt% sulfur) where 9.90 g of elemental sulfur and 0.0994 g of PAO were used in the reaction. Elemental analysis calculated: C, 0.82; S, 99.00; H, 0.07%. Found: C, 1.02; S, 98.21; H, 0.0%. Synthesis of PAOS90 The general synthesis above was used to synthesize PAOS90 (90 wt% sulfur) where 9.00 g of elemental sulfur and 0.998 g of PAO were used in the reaction. Elemental analysis calculated: C, 8.29; S, 90.00; H, 0.69%. Found: C, 8.07; S, 89.37; H, 0.64%. Synthesis of PAOS80 The general synthesis above was used to synthesize PAOS80 (80 wt% sulfur) where 2.40 g of elemental sulfur and 0.600 g of PAO were used in the reaction. Elemental analysis calculated: C, 16.59; S, 80.00; H, 1.40%. Found: C, 16.32; S, 79.70; H, 1.48%. Synthesis of PAOS70 The general synthesis above was used to synthesize PAOS70 (70 wt% sulfur) where 2.10 g of elemental sulfur and 0.900 g of PAO were used in the reaction. 48 Elemental analysis calculated: C, 24.89; S, 70.00; H, 2.08%. Found: C, 25.75; S, 63.18; H, 2.24%. Synthesis of PAOS60 The general synthesis above was used to synthesize PAOS60 (60 wt% sulfur) where 1.80 g of elemental sulfur and 1.20 g of PAO were used in the reaction. Elemental analysis calculated: C, 33.19; S, 60.00; H, 2.79%. Found: C, 34.54; S, 61.91; H, 3.08%. Synthesis of PAOS50 The general synthesis above was used to synthesize PAOS50 (50 wt% sulfur) where 1.50 g of elemental sulfur and 1.50 g of PAO were used in the reaction. Elemental analysis calculated: C, 41.49; S, 50.00; H, 3.49%. Found: C, 41.50; S, 49.90; H, 3.75%. Synthesis of PAOS40 The general synthesis above was used to synthesize PAOS40 (40 wt% sulfur) where 1.20 g of elemental sulfur and 1.80 g of PAO were used in the reaction. Elemental analysis calculated: C, 49.78; S, 40.00; H, 4.18%. Found: C, 51.42; S, 37.65; H, 4.81%. Synthesis of PAOS30 The general synthesis above was used to synthesize PAOS30 (30 wt% sulfur) where 0.900 g of elemental sulfur and 2.10 g of PAO were used in the reaction. 49 Elemental analysis calculated: C, 58.08; S, 30.00; H, 4.89%. Found: C, 60.25; S, 27.08; H, 5.70%. Synthesis of PAOS20 The general synthesis above was used to synthesize PAOS20 (20 wt% sulfur) where 0.600 g of elemental sulfur and 2.40 g of PAO were used in the reaction. Elemental analysis calculated: C, 66.38; S, 20.00; H, 5.58%. Found: C, 66.55; S, 18.22; H, 6.11%. Synthesis of PAOS10 The general synthesis above was used to synthesize PAOS10 (10 wt% sulfur) where 0.300 g of elemental sulfur and 2.70 g of PAO were used in the reaction. Elemental analysis calculated: C, 74.67; S, 10.00; H, 6.28%. Found: C, 73.62; S, 9.40; H, 7.10%. Results and Discussion Synthesis Polystyrene (PS) was selected as the scaffold for the alkene-bearing polymer substrate to access NCCs because a wide variety of functionalized PS derivatives are commercially available with relatively high molecular weights and well-defined polydispersities. The cross-linking mechanism in inverse vulcanization is analogous to the well-known thiol-ene click reaction (Scheme 2.2a).38 Whereas the thiol-ene reaction 50 requires a thiol group and an organic radical initiator to generate a sulfur-centered radical, in inverse vulcanization elemental sulfur (S8) undergoes thermal homolytic ring-opening above 159 °C to generate a S-centered radical that can react with an olefin (Scheme 2.2b). Scheme 2.2 (a) The general synthetic scheme of a thiol-ene reaction. (b) The homolytic ring- opening of S8 to form a sulfur-centered radical, which then can react with olefins. (c) Synthetic route to prepare PAO. (d) Synthetic route to PAOSx, where x represents the wt% of S8. We reasoned that a PS derivative functionalized with hydroxyl groups could be readily modified with allyl bromide, thus poly(4-vinylphenol) (PVP) was employed as the initial starting material. Exhaustive alkylation was achieved upon reaction of PVP with allyl bromide in the presence of K2CO3, affording the desired PS derivative with alkene side chains, poly(4-allyloxystyrene) (PAO), in quantitative yield (Scheme 2.2c).37 51 Analysis of PAO by 1H NMR and FT-IR spectroscopy (Figure 2.1a) revealed that the signals for the OH proton and the O–H stretching vibration (3300 cm–1) had disappeared. Two new peaks appeared in the FT-IR spectrum at 922 and 995 cm–1 that were consistent with monosubstituted alkene (i.e., R–CH=CH2) C–H bending modes (Figure 2.1b). The ratio of integrals for the allyloxy proton signals to the integral of all PS-backbone proton signals revealed that allylation of hydroxyl groups in PVP was quantitative.39, 40 Figure 2.1 FT-IR spectra for poly(4-vinylphenol) and PAO over the full scan range (a), poly(4- vinylphenol) and PAO highlighting the range for the monosubstituted alkene C–H bending mode (b), CS2-soluble fractions from PAOSx (x = 40-99, c), and CS2-insoluble fractions from PAOSx (x = 40-99, d). 52 a b Material Td (°C) Tg (°C) Tm (°C) Tα-β (°C) % Mass loss (±2%) PVP 310 –c –c –c NA PAO 395 43.3 –c –c NA c c S8 210 – 118.7 – NA c c d PAOS99 203 – 117.0 – – c PAOS90 191 –37.0 115.8 – 3 c PAOS80 199 – 116.5 105.3 4 c c PAOS70 192 – 114.3 – 4 c c PAOS60 195 – 114.6 – 3 c PAOS50 238 – 115.2 103.4 2 c PAOS40 241 – 116.6 105.8 2 c c c PAOS30 244 – – – 2 c c c PAOS20 263 – – – 2 c c c PAOS10 290 – – – 7 a b Td was determined by calculating the temperature at which 5% mass was lost. Mass loss was determined by weighing the mass of the product after the reaction. c No visible transition in the range studied. d Was not recorded. Table 2.1 Summary of mass loss in synthesis, TGA, and DSC data. Thermal reaction of PAO with sulfur (Scheme 2.2d) was achieved by heating the polymer to 180 °C in the presence of varying amounts of sulfur to afford the desired NCCs comprising PAO crosslinked with varying relative wt% of sulfur (PAOSx; x = 10, 20, 30, 40, 50, 60, 70, 80, 90, and 99 wt% S). Each PAOSx was a brown-red solid with a glass-like appearance. The general physical appearance was consistent across the series, but the materials became more brittle with decreasing wt% sulfur. Mass losses in the range of 2–7% were observed during the reactions of PAO with S8 (Table 2.1), which were attributed to the release of H2S gas, as confirmed by elemental microanalysis. CAUTION: H2S gas is foul-smelling, toxic, and corrosive and should be trapped as it is produced. Literature examples of H2S-trapping strategies include zinc oxide-titanium dioxide sorbents,41 zinc oxide nanoparticles,42 reaction with + 43 44 Ag salts for precipitation as Ag2S, and other systems. Analysis of PAOSx by FT-IR 53 spectroscopy revealed that not all C=C bonds in PAO had reacted with S8, as peaks were still observed at 922 and 995 cm–1 in all samples (spectra are provided in the supporting information). Unfortunately, 1H NMR spectrometric analysis could not be performed due to the poor solubility of PAOSx in all solvents examined. Combustion analysis was performed for the elements C, H, N, and S for all PAOSx, which demonstrated that S was efficiently incorporated into PAOSx. Given that unreacted C=C linkages remained in each PAOSx NCC, we hypothesized that there might also be uncrosslinked S8. Each NCC was therefore stirred with an excess of carbon disulfide (CS2) to extract and enable quantification of free (non- cross-linked) sulfur. As anticipated, NCCs with higher sulfur contents (x > 50 wt%) comprised substantial amounts of CS2-extractable mass. In contrast, PAOSx materials containing less than 50 wt% sulfur afforded little to no CS2-extractable mass, indicating that essentially all the sulfur is covalently incorporated in these cases. The CS2-soluble and insoluble fractions resulting from extraction of PAOSx, (x = 99, 90, 80, 70, 60, 50, and 40) were isolated and then analyzed by FT-IR spectroscopy (Figure 1d). FT-IR analysis suggested that nearly no organics are present in CS2-soluble fractions for PAOSx (x > 50). Elemental analysis on the CS2-extractable fraction of PAOS90 confirmed their identity as being >99% by mass sulfur. The CS2-insoluble fraction from PAOS90 contained 47% by mass of non-extractable sulfur and the FT-IR spectrum confirms nearly complete consumption of the alkene moiety in the polymer backbone. On the basis of the extent of alkene consumption and the amount of non-extractable (crosslinked) sulfur present, the average crosslink is comprised of five sulfur atoms. 54 Thermal Stability Thermogravimetric analysis (TGA) of PAOSx revealed decomposition temperatures (Td, here defined as the temperature at which 5% mass loss was observed upon heating under an atmosphere of N2) ranging from 191–290 °C (Figure 2.2a). For comparison, the Td values for S8 and PAO are 210 and 395 °C, respectively. For lower sulfur content PAOSx (x = 10–50%), Td values generally decreased as the wt% of S8 increased. However, samples with x > 50% exhibited Td values lower than that of S8 and did not appear to vary significantly in response to changes in S8 content. This depression of Td is attributable to decomposition of an amount of non-crosslinked sulfur corresponding to the amount of CS2-extractable sulfur (vide supra). Higher sulfur content PAOSx (x >70) exhibit very low char yields due to the clean decomposition of sulfur-rich domains, whereas materials containing <70 wt% sulfur – and comprised by correspondingly little non-cross-linked sulfur – have char yields ranging from 20-28% (Figure 2.2a). The first degradation step observed for all PAOSx materials was attributed to the loss of the extractable sulfur portion as H2S gas, which can form via hydrodesulfurization reaction between S and the organic components. To confirm this hypothesis, a bulk sample of PAOS50 was subjected to heating at 250 °C, just above the observed Td for this material. The vessel was equipped with a bubbler containing 1 M AgNO3(aq) to trap evolved H2S gas via reaction with AgNO3 to form insoluble Ag2S. The mass of Ag2S isolated from this experiment accounts for >83% of the mass loss observed in the TGA trace for PAOS50. The shortfall of isolated Ag2S is likely a result of gas escape prior to 55 reaction or incomplete collection of the finely-dispersed solid. PAOSx samples with x < 70 exhibit a second degradation step at roughly 290 °C attributable to degradation of the organic polymer backbone. The assertion that the covalently-linked and extractable sulfur domains undergo thermal decomposition independently even when comingled in PAOS90 is further supported by the observation that the curve resulting from summing mass- normalized TGA traces of CS2-soluble and -insoluble fractions is nearly coincident with that of the TGA trace of the original sample of PAOS90 (Figure 2.2c). 56 Figure 2.2 (a) TGA curves of PVP, PAO, elemental sulfur, and PAOSx. (b) The TGA curves for the CS2-extractable (solid line) and CS2-insoluble fraction (dashed line) of PAOS90. (c) The calculated (solid line) TGA curve for independent thermal decomposition of the two fractions overlaid with the experimentally observed TGA curve (dashed line) from the PAOS90 bulk material. 57 Morphology Differential scanning calorimetry (DSC) data for fully cured samples of PAOSx are summarized in Table 2.1. PAO is shown to have a Tg at 43.3 °C, while pure orthorhombic α-sulfur does not exhibit a glass transition in the range studied. Additionally, sulfur has a melt peak at 118.7 °C. The higher sulfur content PAOSx (x = 60–99) exhibit endothermic features at ~106 °C. The feature at ~106 °C in DSC analysis is attributable to an orthorhombic to monoclinic phase change (Tα-β, for the α-S(s) à β- S(s) process).45-47 Older literature has variably attributed this transition to occurring at 95 °C,48-51 possibly confused by the significant variability in properties of sulfur based on its thermal history and complex phase behavior.36, 49, 52-55 The peak at ~115 °C is attributed to the melt peak of sulfur-rich domains in the materials. The normalized integral of the sulfur melt peak decreases predictably as the wt% of sulfur is decreased (Figure S2.13- S2.24), consistent with progressively lower amounts of non-cross-linked S in these materials. More interestingly, there is no apparent Tg in the measured temperature range for any of the PAOSx materials except for PAOS90. PAOS90 exhibits a weakly- observable Tg at –37.0 °C, a transition which has been suggested in amorphous sulfur as 56 57, 58 well. The lack of a comparable Tg in even more sulfur-rich PAOS99 suggests that a lower ratio of cross-linking of PAO may facilitate continuity of a more purely orthorhombic sulfur state. An alternate explanation is that PAO may act as a plasticizer for amorphous sulfur and stabilize the polymeric and orthorhombic domains in the NCC, consistent with the observed decrease in Tg for PAOS90 relative to amorphous sulfur (–30 °C). DSC data show that the lower wt% sulfur materials (PAOS10-30) have neither a Tg 58 nor a Tm in the temperature range studied. The predominating absence of these thermal transitions or those attributable to either orthorhombic sulfur or polymeric sulfur further supports the formulation of PAOSx as extensively cross-linked materials with oligosulfide chains separating organic polymer chains. Mechanical Properties One of the primary objectives of the current study was to investigate the extent to which low loading of polymers can endow bulk sulfur with enhanced mechanical strength. On this basis, PAOS90 and PAOS99 were selected for study by dynamic mechanical analysis (DMA). Figure 2.3 Temperature dependence of storage and loss modulus and tan δ of PAOS90 (dashed line) and PAOS99 (solid line) in dual cantilever mode after 1.5 h of cooling (graphs a, b, and c) and 96 h of cooling (graphs d, e, and f). 59 The responses of PAOS90 and PAOS99 to flexural stress were studied by DMA in dual cantilever mode with a 1 Hz frequency of oscillation from –60 to 88 °C. The thermal history of sulfur has a dramatic impact on its subsequent thermomechanical properties. To assess whether this would hold true of high sulfur-content NCCs, DMA was undertaken 1.5 h (Figure 2.3a-c) and 96 h (Figure 2.3d-f) of curing at room temperature after initial melt-casting of samples. The TGA and DSC data discussed in the previous sections is for samples that were cured for at least 96 h. One telling metric in assessing the curing process is the Tg. The Tg can be elucidated in many different ways from DMA 59 data. In the present case, the Tg was determined from the onset of storage modulus (Tg,SM) or as a local maximum in either loss modulus (Tg,LM) or tan δ curves (Tg,tan). For samples analysed 1.5 h after preparation, the onset of storage moduli (Figure 2.3a) for PAOS90 and PAOS99 occur at the same temperature (–17 °C), however the magnitude of the storage modulus for PAOS90 is only 50% that of PAOS99. The Tg calculated from loss moduli are –4.7 and –6.3 °C for PAOS90 and PAOS99, respectively. The calculated Tg from the tan δ peaks are 2.3 °C for PAOS90 and 5.0 °C for PAOS99 (Figure 2.3c). The magnitude of the tan δ peak provides evidence for effective damping by the material. The damping factor indicates how well a material can dissipate energy through segmental motions or relaxations. Figure 2.3c shows that over the entire temperature range studied, PAOS99 has a higher tan δ value than PAOS90. This suggests that PAOS99 exhibits far more viscous than elastic properties relative to PAOS90. PAOS99 is thus able to dissipate energy more efficiently than its PAOS90 counterpart. Notably, the tan δ curve for PAOS99 increases from 60 to 88 °C, a result of the large amounts of free sulfur in the 60 material. These insights further support the inference from DSC data that there is a greater predominance of orthorhombic sulfur in PAOS99 than in PAOS90. The 96 h-cured samples of PAOS90 and PAOS99 were subjected to testing by DMA under identical experimental conditions as were the 1.5 h-cured samples. For comparison, a 96 h-cured sample of pure sulfur is too brittle to be clamped in the DMA instrument even at minimum clamping force. The storage and loss moduli and tan δ curves are shown in Figure 2.3d-f. All parameters exhibit modest shifts between 1.5 h and 96 h post-casting. In every case the calculated Tg increased over the 96 h time period. Table 2.2 summarizes the calculated Tg values for PAOS90 and PAOS99 after 1.5 and 96 h post-casting. In both PAOSx materials, the storage modulus is seen to increase over the 96 h period, whereas, the tan δ values for both PAOSx materials are shown to decrease over the entire temperature range. The increase in storage modulus is attributed to the slow reversion of free polymeric sulfur back to the thermodynamically-favored, more 60 crystalline orthorhombic S8. Moreover, the decrease in magnitude of tan δ is an indication that the materials’ elastic behaviour concomitantly increases. Tg,SM (°C) Tg,LM (°C) Tg,tan (°C) Material 1.5 h 96 h 1.5 h 96 h 1.5 h 96 h PAOS90 –17 –10 –4.7 5.7 2.3 9.7 PAOS99 –17 –13 –6.3 –0.5 5.0 7.3 Table 2.2 The Tg of PAOS90 and PAOS99 determined from DMA data. 61 Thermal Healing and Materials Comparison One driving force for the current work was that the thermal reversibility of S–S bond formation offers intriguing possibilities for thermal healing and recycling of high sulfur-content materials. To test the thermal healing/recyclability of PAOS99, a sample of the material was subject to DMA analysis and then pulverized into small pieces prior to reannealing to heal the material, and finally allowing the sample to cool to room temperature over 15 min. After repeating this pulverization-thermal healing process ten times, the mechanical properties of the twelve-times-healed sample were identical to that of the first sample within error of the measurement (Figure 2.4). 120 100 80 60 40 Bulk Modulus (% of Initial) Initial) (%of Modulus Bulk 20 0 0 3 6 9 12 Number of Pulverization/Remelt Cycles Figure 2.4. The bulk modulus (measured at –45° C) for PAOS99 after pulverization/remelting cycles remains consistent within error for at least twelve cycles. 62 Another driving force behind this study was to determine whether crosslinking by small amounts of organic material could improve the mechanical integrity of bulk sulfur to levels that make it viable for practical applications: without additives to stabilize it, polymeric sulfur is unstable, reverting to the S8 form at STP. Comparison of the mechanical features of polymer-cross-linked high-sulfur content PAOS90-99 to other small molecule-cross-linked, high-sulfur content materials is also of interest. Pyun’s group has prepared 1,3,5-triisopropenylbenzene/sulfur crosslinked materials.61 The storage modulus of this small molecularly-cross-linked material comprising 30 wt% 1,3,5- triisopropenylbenzene and 70 wt% sulfur was 1 GPa at 30 °C under a shear stress. Polymer-cross-linked PAOS90 and PAOS99 exhibit a notably higher storage modulus of approximately 2.6 GPa at 30 °C under a flexural stress despite the significantly higher sulfur content in these NCCs. Compared to other reversibly-crosslinked/thermally healable polymers, PAOS99 has a higher flexural storage modulus than for epoxy shape-memory polymers62 and comparable to that of crosslinked and thermally healable (via Diels-Alder reaction) poly(dicyclopentadiene)s.63 The improved mechanical strength of polymer-crosslinked PAOSx over other sulfur-rich materials and the similarity in room-temperature storage modulus to polystyrene (1.8 GPa at 30 °C)64 provide sound proof-of-principle to justify further efforts to prepare value-added materials from affordable/plentiful by-product sulfur by the strategy employed in this study. 63 Conclusions A commercially-available polystyrene derivative was modified to facilitate cross- linking with elemental sulfur. The crosslinked materials, PAOSx were prepared with sulfur content of 10-99% by mass. The thermal and mechanical properties of PAOSx were elucidated. Target high sulfur-content PAOS99 and PAOS90 were comprised of a mixture of sulfur trapped in the amorphous state and short-chain sulfur crosslinks between organic polymer chains. The storage moduli of both PAOS90 and PAOS99 increased over a 96 h setting time as free polymeric sulfur slowly relaxed to the orthorhombic state. The fully-set mechanical strength of even PAOS99, which is 99% by mass sulfur, increased significantly when compared to a pure sulfur sample prepared under the same conditions. PAOS99 maintains its mechanical integrity after physical breakdown and thermally recasting for at least a dozen cycles, demonstrating the facile recyclability of these materials. PAOS90 and PAOS99 materials thus show promise for valorizing elemental sulfur as a component of durable, recyclable material applications for which petrochemicals are currently used. Follow-up studies on improving the mechanical properties of high sulfur-content network solids are currently underway. Conflicts of interest There are no conflicts to declare. 64 Acknowledgements This work was supported by the National Science Foundation (CHE-1708844). Gratitude is kindly extended for assistance from Tucker McFarlane of Stephen F. Foulger’s Group (Clemson University, TGA and DSC). We thank Catherine A. Conrad for help with early work to begin this project. 65 Supporting Information 8.989 8.965 6.552 6.526 6.498 6.483 3.352 2.504 1.729 1.300 1.239 a b n DMSO c d OH e H O 2 e c-d a-b 10 9 8 7 6 5 4 3 2 1 0 −1 ppm 1.00 4.04 3.09 Figure S2.1 Proton NMR spectrum of PVP. 66 a 6.686 6.626 6.476 5.994 5.351 5.211 4.448 3.343 2.510 1.717 1.406 0.853 0.552 b n c d DMSO O e f g H O e 2 c-d f g a-b 9 8 7 6 5 4 3 2 1 0 −1 ppm 3.99 1.03 2.00 1.96 1.07 1.80 Figure S2.2 Proton NMR spectrum of PAO. 67 Figure S2.3 IR spectrum of PAOS10. 68 Figure S2.4 IR spectrum of PAOS20. 69 100 95 90 85 80 %Transmittance 75 70 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Figure S2.5 IR spectrum of PAOS30. 70 Figure S2.6 IR spectrum of PAOS40. 71 Figure S2.7 IR spectrum of PAOS50. 72 Figure S2.8 IR spectrum of PAOS60. 73 Figure S2.9 IR spectrum of PAOS70. 74 Figure S2.10 IR spectrum of PAOS80. 75 Figure S2.11 IR spectrum of PAOS90. 76 Figure S2.12 IR spectrum of PAOS99. 77 Figure S2.13 DSC curve of PAO. 78 Figure S2.14 DSC curve of sulfur. 79 Figure S2.15 DSC curve of PAOS10. 80 Figure S2.16 DSC curve of PAOS20. 81 Figure S2.17 DSC curve of PAOS30. 82 Figure S2.18 DSC curve of PAOS40. 83 Figure S2.19 DSC curve of PAOS50. 84 Figure S2.20 DSC curve of PAOS60. 85 Figure S2.21 DSC curve of PAOS70. 86 Figure S2.22 DSC curve of PAOS80. 87 Figure S2.23 DSC curve of PAOS90. 88 Figure S2.24 DSC curve of PAOS99. 89 References 1. Goodyear, C. IMPROVEMENT IN INDIA-RUBBER FABRICS. US3633A, 1844. 2. Farmer, E. H.; Shipley, F. W., The reaction of sulfur and sulfur compounds with olefinic substances. I. The reaction of sulfur with monöblefins and with Δ1,5- diolefins. Journal of the Chemical Society 1947, 1519-32. 3. Glass, R. S., Sulfur Radicals and Their Application. Topics in Current Chemistry 2018, 376 (3), 1-42. 4. Zhang, X.; Tang, Y.; Qu, S.; Da, J.; Hao, Z., H2S-Selective Catalytic Oxidation: Catalysts and Processes. ACS Catalysis 2015, 5 (2), 1053-1067. 5. Demirbas, A.; Alidrisi, H.; Balubaid, M. A., API Gravity, Sulfur Content, and Desulfurization of Crude Oil. Petroleum Science and Technology 2015, 33 (1), 93- 101. 6. Lim, J.; Pyun, J.; Char, K., Recent Approaches for the Direct Use of Elemental Sulfur in the Synthesis and Processing of Advanced Materials. Angew. Chem., Int. Ed. 2015, 54 (11), 3249-3258. 7. Lee, D.-y., Modification of Asphalt and Asphalt Paving Mixtures by Sulfur Additives. Product R&D 1975, 14 (3), 171-177. 8. Steudel, R.; Editor, Elemental Sulfur and Sulfur-Rich Compounds I. [In: Top. Curr. Chem., 2003; 230]. 2003; p 202 pp. 9. Steudel, R.; Editor, Elemental Sulfur and Sulfur-Rich Compounds II. [In: Top. Curr. Chem.; 2003, 231]. 2003; p 248 pp. 90 10. Kutney, G., Sulfur. History, Technology, Applications & Industry. ChemTec: Toronto, 2007. 11. Survey, U. S. G., Mineral commodity summaries 2017: U.S. Geological Survey. 2017; p 202. 12. Worthington, M. J. H.; Kucera, R. L.; Chalker, J. M., Green chemistry and polymers made from sulfur. Green Chem. 2017, 19 (12), 2748-2761. 13. Crockett, M. P.; Evans, A. M.; Worthington, M. J. H.; Albuquerque, I. S.; Slattery, A. D.; Gibson, C. T.; Campbell, J. A.; Lewis, D. A.; Bernardes, G. J. L.; Chalker, J. M., Sulfur-Limonene Polysulfide: A Material Synthesized Entirely from Industrial By-Products and Its Use in Removing Toxic Metals from Water and Soil. Angew. Chem., Int. Ed. 2016, 55 (5), 1714-1718. 14. Garcia, F.; Smulders, M. M. J., Dynamic covalent polymers. Journal of Polymer Science, Part A: Polymer Chemistry 2016, 54 (22), 3551-3577. 15. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J., The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat Chem 2013, 5 (6), 518-24. 16. Zhang, Y.; Konopka, K. M.; Glass, R. S.; Char, K.; Pyun, J., Chalcogenide hybrid inorganic/organic polymers (CHIPs) via inverse vulcanization and dynamic covalent polymerizations. Polym. Chem. 2017, 8 (34), 5167-5173. 91 17. Zhang, Y.; Griebel, J. J.; Dirlam, P. T.; Nguyen, N. A.; Glass, R. S.; MacKay, M. E.; Char, K.; Pyun, J., Inverse vulcanization of elemental sulfur and styrene for polymeric cathodes in Li-S batteries. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (1), 107-116. 18. Park, J.; Kim, E. T.; Kim, C.; Pyun, J.; Jang, H.-S.; Shin, J.; Choi, J. W.; Char, K.; Sung, Y.-E., The Importance of Confined Sulfur Nanodomains and Adjoining Electron Conductive Pathways in Subreaction Regimes of Li-S Batteries. Adv. Energy Mater. 2017, 7 (19), 1700074 19. Oleshko, V. P.; Herzing, A. A.; Soles, C. L.; Griebel, J. J.; Chung, W. J.; Simmonds, A. G.; Pyun, J., Analytical Multimode Scanning and Transmission Electron Imaging and Tomography of Multiscale Structural Architectures of Sulfur Copolymer-Based Composite Cathodes for Next-Generation High-Energy Density Li-S Batteries. Microsc. Microanal. 2016, 22 (6), 1198-1221. 20. Oleshko, V. P.; Herzing, A. A.; Twedt, K. A.; Griebel, J. J.; McClelland, J. J.; Pyun, J.; Soles, C. L., Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams. Langmuir 2017, 33 (37), 9361-9377. 21. Lim, J.; Jung, U.; Joe, W. T.; Kim, E. T.; Pyun, J.; Char, K., High Sulfur Content Polymer Nanoparticles Obtained from Interfacial Polymerization of Sodium Polysulfide and 1,2,3-Trichloropropane in Water. Macromol. Rapid Commun. 2015, 36 (11), 1103-1107. 92 22. Kim, E. T.; Chung, W. J.; Lim, J.; Johe, P.; Glass, R. S.; Pyun, J.; Char, K., One- pot synthesis of PbS NP/sulfur-oleylamine copolymer nanocomposites via the copolymerization of elemental sulfur with oleylamine. Polym. Chem. 2014, 5 (11), 3617-3623. 23. Kim, E. T.; Park, J.; Kim, C.; Simmonds, A. G.; Sung, Y.-E.; Pyun, J.; Char, K., Conformal Polymeric Multilayer Coatings on Sulfur Cathodes via the Layer-by-Layer Deposition for High Capacity Retention in Li-S Batteries. ACS Macro Lett. 2016, 5 (4), 471-475. 24. Griebel, J. J.; Nguyen, N. A.; Astashkin, A. V.; Glass, R. S.; MacKay, M. E.; Char, K.; Pyun, J., Preparation of Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur. ACS Macro Lett. 2014, 3 (12), 1258-1261. 25. Griebel, J. J.; Nguyen, N. A.; Namnabat, S.; Anderson, L. E.; Glass, R. S.; Norwood, R. A.; MacKay, M. E.; Char, K.; Pyun, J., Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur for Healable Infrared Optical Materials. ACS Macro Lett. 2015, 4 (9), 862-866. 26. Griebel, J. J.; Li, G.; Glass, R. S.; Char, K.; Pyun, J., Kilogram scale inverse vulcanization of elemental sulfur to prepare high capacity polymer electrodes for Li-S batteries. J. Polym. Sci., Part A: Polym. Chem. 2015, 53 (2), 173-177. 27. Dirlam, P. T.; Simmonds, A. G.; Kleine, T. S.; Nguyen, N. A.; Anderson, L. E.; Klever, A. O.; Florian, A.; Costanzo, P. J.; Theato, P.; Mackay, M. E.; Glass, R. S.; Char, K.; Pyun, J., Inverse vulcanization of elemental sulfur with 1,4- 93 diphenylbutadiyne for cathode materials in Li-S batteries. RSC Advances 2015, 5 (31), 24718-24722. 28. Rao, K. J.; Paria, S., Use of sulfur nanoparticles as a green pesticide on Fusarium solani and Venturia inaequalis phytopathogens. RSC Adv. 2013, 3 (26), 10471-10478. 29. E.P.A., U. S., R.E.D. Sulfur Facts. U.S. Environmental Protection Agency, Pesticides and Toxic Substances Branch: Washington, D.C., 1991. 30. Musah, R.; Kim, S.; Kubec, R., Antibacterial and antifungal activity of sulfur- containing compounds from Petiveria Alliacea L. Phosphorus, Sulfur and Silicon and the Related Elements 2005, 180 (5-6), 1455-1456. 31. Kim, S.; Kubec, R.; Musah, R. A., Antibacterial and antifungal activity of sulfur- containing compounds from Petiveria alliacea L. Journal of Ethnopharmacology 2006, 104 (1-2), 188-192. 32. Weld, J. T.; Gunther, A., Antibacterial properties of sulfur. Journal of Experimental Medicine 1947, 85, 531-42. 33. Lawson, G. B., The inhibitory action of sulfur on the growth of tubercle bacilli. American Review of Tuberculosis 1934, 29, 650-1. 34. Libenson, L.; Hadley, F. P.; McIlroy, A. P.; Wetzel, V. M.; Mellon, R. R., Antibacterial effect of elemental sulfur. Journal of Infectious Diseases 1953, 93, 28- 35. 35. Grantham, R.; Warren, J. S., Jr. Method of pressure treating boards. 1997-US19812 9818872, 19971030., 1998. 36. Motai, T. Treating wood. 1976-754266 94 4127686, 19761227., 1978. 37. Wang, C.; Lee, W.-Y.; Nakajima, R.; Mei, J.; Kim, D. H.; Bao, Z., Thiol–ene Cross-Linked Polymer Gate Dielectrics for Low-Voltage Organic Thin-Film Transistors. Chemistry of Materials 2013, 25 (23), 4806-4812. 38. Lowe, A. B., Thiol-ene "click" reactions and recent applications in polymer and materials synthesis: a first update. Polymer Chemistry 2014, 5 (17), 4820-4870. 39. Raghu, A. V.; Gadaginamath, G. S.; Mathew, N.; Halligudi, S. B.; Aminabhavi, T. M., Synthesis, characterization, and acoustic properties of new soluble polyurethanes based on 2,2'-[1,4-phenylenebis(nitrilomethylylidene)diphenol and 2,2'-[4,4'- methylene-di-2-methylphenylene-1,1'-bis(nitrilomethylylidene)]diphenol. Journal of Applied Polymer Science 2007, 106 (1), 299-308. 40. Donawade, D. S.; Raghu, A. V.; Muddapur, U. M.; Gadaginamath, G. S., Chemoselective reaction of benz(g)indole-based bisheterocycle dicarboxylate towards hydrazine hydrate: Synthesis and antimicrobial activity of new triheterocycle-5- pyrrolylaminocarbonyl/mercaptooxadiazolyl/4-allyl-5-mercaptotriazolylmethoxy-1- furfuryl-2-methylbenz(g)indoles. Indian Journal of Chemistry, Section B: Organic Chemistry Including Medicinal Chemistry 2005, 44B (7), 1470-1475. 41. Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M., High-temperature hydrogen sulfide removal from fuel gases by regenerable zinc oxide-titanium dioxide sorbents. Industrial & Engineering Chemistry Research 1989, 28 (5), 535-541. 95 42. Sayyadnejad, M. A.; Ghaffarian, H. R.; Saeidi, M., Removal of hydrogen sulfide by zinc oxide nanoparticles in drilling fluid. International Journal of Environmental Science & Technology 2008, 5 (4), 565-569. 43. Arslan, M.; Kiskan, B.; Yagci, Y., Combining Elemental Sulfur with Polybenzoxazines via Inverse Vulcanization. Macromolecules 2016, 49 (3), 767-773. 44. Potivichayanon, S.; Pokethitiyook, P.; Kruatrachue, M., Hydrogen sulfide removal by a novel fixed-film bioscrubber system. Process Biochemistry 2006, 41 (3), 708- 715. 45. Lian, Q.; Li, Y.; Li, K.; Cheng, J.; Zhang, J., Insights into the Vulcanization Mechanism through a Simple and Facile Approach to the Sulfur Cleavage Behavior. Macromolecules (Washington, DC, United States) 2017, 50 (3), 803-810. 46. Khawaja, S. Z.; Vijay Kumar, S.; Jena, K. K.; Alhassan, S. M., Flexible sulfur film from inverse vulcanization technique. Materials Letters 2017, 203, 58-61. 47. Shankarayya Wadi, V. K.; Jena, K. K.; Khawaja, S. Z.; Yannakopoulou, K.; Fardis, M.; Mitrikas, G.; Karagianni, M.; Papavassiliou, G.; Alhassan, S. M., NMR and EPR Structural Analysis and Stability Study of Inverse Vulcanized Sulfur Copolymers. ACS Omega 2018, 3 (3), 3330-3339. 48. Meyer, B., Solid allotropes of sulfur. Chem. Rev. 1964, 64 (5), 429-51. 49. Meyer, B., Elemental sulfur. Inorg. Sulphur Chem. 1968, 241-58. 50. Meyer, B., The structures of elemental sulfur. Advances in Inorganic Chemistry and Radiochemistry 1976, 18, 287-317. 51. Meyer, B., Elemental sulfur. Chem. Rev. 1976, 76 (3), 367-88. 96 52. Bacon, R. F.; Fanelli, R., The viscosity of sulfur. J. Am. Chem. Soc. 1943, 65, 639-48. 53. Arntson, R. H.; Dickson, F. W.; Tunell, G., Saturation curves of orthorhombic sulfur in the system S-Na2S-H2O at 25 and 50°. Science (Washington, DC, United States) 1958, 128, 716-18. 54. Akahama, Y.; Kobayashi, M.; Kawamura, H., Pressure-induced structural phase transition in sulfur at 83 GPa. Physical Review B: Condensed Matter and Materials Physics 1993, 48 (10), 6862-4. 55. Ferreira, A. G. M.; Lobo, L. Q., The low-pressure phase diagram of sulfur. Journal of Chemical Thermodynamics 2010, 43 (2), 95-104. 56. V., T. A., Polymeric sulfur and related polymers. Journal of Polymer Science Part C: Polymer Symposia 1966, 12 (1), 71-78. 57. Tobolsky, A. V., Polymeric sulfur and related polymers. Journal of Polymer Science, Part C: Polymer Symposia 1966, No. 12, 71-8. 58. Tobolsky, A. V.; MacKnight, W.; Beevers, R. B.; Gupta, V. D., The glass transition temperature of polymeric sulfur. Polymer 1963, 4 (4), 423-7. 59. Menard, K., Dynamic Mechanical Analysis: A Practical Introduction to Techniques and Applications. CRC: 1999; p 250 pp. 60. Batista, N. L.; Olivier, P.; Bernhart, G.; Rezende, M. C.; Botelho, E. C., Correlation between degree of crystallinity, morphology and mechanical properties of PPS/carbon fiber laminates. Materials Research 2016, 19, 195-201. 61. Kleine, T. S.; Nguyen, N. A.; Anderson, L. E.; Namnabat, S.; LaVilla, E. A.; Showghi, S. A.; Dirlam, P. T.; Arrington, C. B.; Manchester, M. S.; Schwiegerling, 97 J.; Glass, R. S.; Char, K.; Norwood, R. A.; Mackay, M. E.; Pyun, J., High Refractive Index Copolymers with Improved Thermomechanical Properties via the Inverse Vulcanization of Sulfur and 1,3,5-Triisopropenylbenzene. ACS Macro Lett. 2016, 5 (10), 1152-1156. 62. Hu, J.; Chen, W.; Fan, P.; Gao, J.; Fang, G.; Cao, Z.; Peng, F., Epoxy shape memory polymer (SMP): Material preparation, uniaxial tensile tests and dynamic mechanical analysis. Polymer Testing 2017, 62, 335–341. 63. Murphy, E. B.; Bolanos, E.; Schaffner-Hamann, C.; Wudl, F.; Nutt, S. R.; Auad, M. L., Synthesis and Characterization of a Single-Component Thermally Remendable Polymer Network: Staudinger and Stille Revisited. Macromolecules 2008, 41, 5203– 5209. 64. Worzakowska, M., Thermal and mechanical properties of polystyrene modified with esters derivatives of 3-phenylprop-2-en-1-ol. Journal of Thermal Analysis and Calorimetry 2015, 121 (1), 235-243. 98 Attribution of work My contribution to the work in this chapter has been published in the journal of RSC Advances. The citation for the above manuscript: RSC Adv., 2018, 8, 39074-39082. 99 CHAPTER THREE DURABLE, ACID-RESISTANT COPOLYMERS FROM INDUSTRIAL BY- PRODUCT SULFUR AND MICROBIALLY-PRODUCED TYROSINE3 Abstract The search for alternative feedstocks to replace petrochemical polymers has centered on plant-derived monomer feedstocks. Alternatives to agricultural feedstock production should also be pursued, especially considering the ecological damage caused by modern agricultural practices. Herein, L-tyrosine produced on an industrial scale by E. coli was derivatized with olefins to give tetraallyltyrosine. Tetraallyltyrosine was subsequently copolymerized via its inverse vulcanization with industrial by-product elemental sulfur in two different comonomer ratios to afford highly-crosslinked network copolymers TTSx (x = wt% sulfur in monomer feed). TTSx copolymers were characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis (DMA). DMA was employed to assess the viscoelastic properties of TTSx through the temperature dependence of the storage modulus, loss modulus and energy damping ability. Stress- strain analysis revealed that the flexural strength of TTSx copolymers (> 6.8 MPa) is more than 3 MPa higher than flexural strengths for previously-tested inverse vulcanized biopolymer derivatives, and more than twice the flexural strength of some Portland cement compositions (which range from 3–5 MPa). Despite the high tyrosine content 3 This text is reproduced with permission in larger part from: Thiounn, T.; Tennyson, A. G.; Smith, R. C., Durable, Acid-resistant Copolymers from Industrial By-product Sulfur and Microbially-produced Tyrosine. RCS Advances 2019, 9, 31460-31465. Permission documentation is provided at the end of this chapter. 100 (50-70 wt%) in TTSx, the materials show no water-induced swelling or water uptake after being submerged for 24 h. More impressively, TTSx copolymers are highly resistant to oxidizing acid, with no deterioration of mechanical properties even after soaking in 0.5 M sulfuric acid for 24 h. The demonstration that these durable, chemically-resistant TTSx copolymers can be prepared from industrial by-product and microbially-produced monomers via a 100% atom-economical inverse vulcanization process portends their potential utility as sustainable surrogates for less ecofriendly materials. Introduction Polymers are ubiquitous in nearly every aspect of modern society to the extent that each person consumes 50 kg of polymers per year in Europe and 68 kg per year in the United States.1 The vast majority of these polymers are made from petrochemicals and are relatively inert to typical biological degradation processes over any reasonable timeframe.2 As a result, anthropogenic polymers accumulate in the environment in landfills, as floating masses and microplastic in the oceans,3-5 and they have even begun to assimilate into landmasses through the formation of “plasticrust” around island shores.6 Plant-derived monomers have been widely pursued as replacements for petrochemicals in the quest to find sustainable and degradable alternatives to polymers of the petrochemical century.7, 8 Agricultural production of crops for chemical feedstocks, however, comes with its own set of ecological problems, as modern agricultural practices wreak havoc on global phosphorus and nitrogen cycles and contaminate water resources through fertilizer and pesticide-laced runoff. Thus, whereas it is certainly imperative that 101 agricultural waste products be leveraged for environmentally and economically advantageous uses, technological advances that rely on increased agricultural activities for the sole purpose of providing a new chemical feedstock should be weighed cautiously. Some attractive alternatives to agricultural production of chemical feedstocks are to employ high production-density aquaculture of algae9, 10 or engineered bacteria11-16 to produce target feedstocks. Such processes can be carried out in a closed system often operating at or slightly above ambient temperatures, thus limiting environmental impact and energy consumption.17 Regardless of how they are produced, chemically-resistant polymers often pose an environmental threat in terms of degradability, yet chemical resistance is required for many applications. New advances in production of chemically-resistant polymers should therefore centre on materials that are recyclable, and/or which degrade to environmentally benign compounds. High sulfur-content materials produced by inverse vulcanization of sustainable olefins have emerged recently as promising candidates in this regard.18, 19 R R Sx S8 >159 ºC Sy R = aryl, alkyl, polymer … Scheme 3.1 General inverse vulcanization strategy. 102 In the inverse vulcanization process,20 sulfur, itself an industrial by-product of fossil fuel refining,21-23 is simply heated with the requisite olefin to form crosslinked network solids with 100% atom economy (Scheme 3.1).24-27 Many high sulfur-content materials can be remelted and recast over multiple cycles without loss of mechanical strength, and the environmental degradation products can be so environmentally benign that inverse vulcanization products have been used as fertilizers to improve the growth of food crops.28, 29 High sulfur-content composites can also exhibit remarkably high mechanical strength. Sulfur composites with lignin30 or cellulose,31 for example, have shown flexural strengths on par with that of Portland cement. For the current work, tyrosine was an attractive initial feedstock because it is produced on an industrial scale by engineered strains of E. coli.32-35 Furthermore, tyrosine is readily functionalized with four allyl groups, providing up to eight sites for the formation of C–S crosslinks upon inverse vulcanization (Scheme 3.2). The mechanical strength of high sulfur-content materials produced by inverse vulcanization increases with increased crosslink density,30, 31, 36 so it was hypothesized that the high crosslink density provided by tetraallyltyrosine would likewise lead to durable copolymers. Inverse vulcanization of tetraallyltyrosine with sulfur was thus undertaken to produce crosslinked network copolymers TTSx (x = wt% sulfur in the monomer feed). Characterization by infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis are discussed for TTSx having two different monomer feed compositions. The resistance of TTSx to water uptake and degradation by sulfuric acid are also assessed. 103 O OH NH2 HO L-tyrosine allylbromide KOH O O N O tetraallyltyrosine >159 ºC Sn S8 S S O Sy O N Sj O Sz Sw Sk Sp Sq TTSx Sx Scheme 3.2 Preparation of tetrallyltyrosine and its inverse vulcanization to yield TTSx. Results and Discussion Synthesis of TTSx A commercial sample of L-tyrosine produced by an engineered strain of E. coli was selected as the initial starting material for crosslinking with industrial by-product sulfur for the current study. Crosslinking of sulfur by inverse vulcanization requires an olefin-derivatized comonomer, so L-tyrosine was first converted to tetraallyltyrosine by 104 the reported method (Scheme 3.2; NMR spectra provided in Figures S3.1-S3.5 of the Supporting Information at the end of this chapter).37 At STP, elemental sulfur exists primarily as S8 rings in an orthorhombic crystal morphology. Upon heating, S8 first melts at 118 °C and then forms polymeric thienyl radicals at temperatures >159 °C (Scheme 3.2).38 These polymeric thienyl radicals are thought to be the active species for reaction with olefins to form crosslinks. In the current case, reaction between thienyl radicals and the four olefin moieties of tetraallyltyrosine affords up to eight S–C crosslinks per molecule. Upon heating a sample of tetraallyltyrosine and sulfur above 159 °C, the solution initially was the dark red colour characteristic of polymeric thienyl radicals and then transformed to a deep brown-red liquid within 5 min. When the sulfur: tetraallyltyrosine mass ratio was >1, the reaction mixture did not homogenize even after extended stirring and heating. Instead, a dark-coloured solid ball of material precipitated from the liquid phase. The separated solid was comprised of sulfur-crosslinked tetraallyltyrosine, whereas the cooled liquid phase was unreacted S8. For reactions in which the sulfur: tetraallyltyrosine mass ratio was ≤1, dark red-brown, homogeneous reaction solutions were observed after 30-45 minutes of heating. Two sulfur: tetraallyltyrosine monomer mixtures, comprised of 50 wt% and 30 wt% sulfur, were thus selected for further investigation to produce TTS50 and TTS30, respectively. 105 Figure 3.1 Shaped materials can be fabricated from TTSx thermosets by curing the comonomer solution at 150 °C in a mould or on a substrate, as illustrated by the rectangular prismatic brick (left) and patterned disc-shaped item (centre). A backlit sample of TTSx (right) better illustrates the red colour and transparency of the material. 30, 31, 36, 39, 40 In contrast to many reported high sulfur-content materials, once TTS50 or TTS30 were set as solids they could not be remelted or reshaped. To prepare materials of desired shapes, it was necessary to pour the homogenized reaction solutions into a mould for subsequent heating to complete the crosslinking process. Moulded samples of TTSx prepared in this manner were hard, dark red-brown, semi-transparent solids with smooth, shiny surfaces (Figure 3.1). The densities of the materials so produced (1.2-1.3 g/cm3) are somewhat higher than that of a typical organic polymer by merit of the high sulfur content, given that the density of pure sulfur is 2 g/cm3. Infrared spectra (Figure S3.6 located in the Supporting Information at the end of this chapter) of TTSx reveals near complete loss of the monosubstituted alkene C–H bending mode (920 and 995 cm–1) from the allyl group in tetraallyltyrosine upon reaction with sulfur, demonstrating the efficiency of the crosslinking process. 106 Some high sulfur-content materials prepared by inverse vulcanization are homogeneous copolymer networks, while others are composites wherein free sulfur occupies voids in a sulfur-crosslinked network polymer.28, 30, 31 Free sulfur is quite soluble in CS2, whereas the crosslinked network is insoluble, so recursive rinsing of TTSx with CS2 was undertaken to extract any free sulfur. A minimal amount of the material was extractable from TTSx by this method (15 and 11% for TTS30 and TTS50, respectively), and the elemental analysis of the soluble fraction matched that of the bulk material, rather than being free sulfur. Differential scanning calorimetry (DSC, vide infra) data likewise did not show thermal transitions associated with free sulfur. These data support the characterization of TTSx as consisting primarily of sulfur-crosslinked networks without appreciable free sulfur present, as they are depicted in Scheme 3.2. Considering near complete consumption of alkene units and the limited amount of free sulfur in each material, the average crosslink was calculated to consist of about 6 or 3 sulfur atoms in TTS30 and TTS50, respectively. The short average crosslink length provides an explanation for the observation that TTSx cannot be remelted. The ability to re-melt high sulfur-content materials generally relies on the thermal reversibility of S–S bond formation. With an average crosslink length of only 3–6 sulfur atoms, a significant fraction of crosslinks will comprise a single sulfur atom, and thus lack the S–S bond needed for reversible bond-formation. The high crosslink density in these materials thus unfortunately precludes TTSx from the re-melting or thermal healing exhibited by materials having longer average oligosulfur crosslink lengths. 107 TTS30 TTS50 a Td (°C) 209 211 Char Yield 26% 23% b Tg (°C) 68 58 ’ c E Tg (°C) 48.4 47.0 ’’ d E Tg (°C) 60.8 58.2 e tan δ Tg (°C) 77.2 70.5 Density (g/cm3) 1.3±0.1 1.2±0.1 f f H2O uptake <0.5% <0.5% a Determined from the onset of the major mass loss peak. b Determined from the onset of transition from the second heating cycle of the DSC curve. c Determined from the onset of the storage modulus. d Determined from the peak of the loss modulus curve. e Determined from the peak of tan δ curve. f No measurable water uptake. Table 3.1 Summary of properties of materials. Thermal Properties of TTSx The thermogravimetric analysis (TGA) data (Table 3.1 and Figure 3.2) reveal that the onset of decomposition for the starting materials, tetraallyltyrosine and sulfur are 182 and 282 °C respectively, and both have char yields of <1%. The decomposition temperatures (Td) for TTS30 (209 °C) and TTS50 (211 °C) lie between those for tetraallyltyrosine and sulfur. The char yields for TTSx materials also show a significant increase from the initial monomers, with an increase to 23-26%. Differential scanning calorimetry was also used to assess the thermal and morphological properties of TTSx (Figure S3.7 located in the Supporting Information at the end of this chapter). The glass transition temperatures (Tg) of TTS30 (68 °C) and TTS50 (58 °C) are both well above room temperature, while the higher Tg of TTS30 reflects fewer degrees of freedom in the more-crosslinked material. Notably, the DSC traces for TTSx do not exhibit a melt peak for orthorhombic sulfur, in agreement with the observation of very little CS2-extractable free sulfur. In contrast, the DSC traces for high sulfur-content composites from which significant quantities of free sulfur are extractable 108 exhibit prominent peaks at 116-120 °C attributable to orthorhombic sulfur melting transitions.30, 31, 36 Peaks attributable to polymeric sulfur domains are also notably absent from the DSC traces of TTSx. Polymeric sulfur domains generally produce endothermic transitions at around –37 °C and sometimes also cold crystallization peaks over a broad temperature range of –20 to +40 °C.30, 31 the absence of such features in the DSC analysis of TTSx further confirm the short average crosslink length. 100 80 60 Wt. % Wt. 40 20 0 0 100 200 300 400 500 600 700 800 Temperature (℃ ) Figure 3.2 Thermogravimetric analysis (TGA) traces for elemental sulfur (black trace), tetraallyltyrosine (green trace), TTS30 (blue trace) and TTS50 (red trace). 109 a) 500 0.7 450 0.6 400 350 0.5 300 Tan Delta 0.4 250 0.3 200 150 0.2 Storage Modulus (MPa) StorageModulus 100 0.1 50 0 0.0 25 35 45 55 65 75 85 95 Temperature (℃) b) 500 1.2 450 1.0 400 350 0.8 300 Tan Delta 250 0.6 200 0.4 150 Storage Modulus (MPa) StorageModulus 100 0.2 50 0 0.0 25 35 45 55 65 75 85 95 Temperature (℃) Figure 3.3 Storage modulus (E’, blue trace), loss modulus (E’’, red trace) and damping factor (tan δ, green trace) for TTS30 (a) and TTS50 (b). 110 Mechanical Properties and Chemical Resistance of TTSx The high crosslink density in TTSx was hypothesized to endow them with high strength and durability. To test this hypothesis, TTS30 and TTS50 were subjected to dynamic mechanical analysis (DMA). The temperature-dependences of storage modulus (E’), loss modulus (E’’) and energy damping factor (tan δ) were initially assessed (Figure 3.3 and Table 3.1). Expectedly, the E’ is higher for TTS30 whereas the tan d is higher for the less-densely-crosslinked TTS50. The storage and loss moduli for both copolymers are significantly higher than those of uncrosslinked elemental sulfur, which is too brittle to even be clamped in the DMA instrument at the minimum clamping force. Stress-strain analysis of TTS30 and TTS50 provide the flexural modulus and minimum flexural strength of TTSx copolymers at room temperature (Figure 3.4a and Table 3.2). The flexural storage modulus of TTS30 is greater than that of TTS50 (860 vs. 580 MPa), indicative of the higher crosslink density of the TTS30 sample. Additionally, the flexural strength for TTS30 is slightly greater than TTS50 sample. Note that because the samples did not break at the maximum force applicable by the instrument (10 N), these already high flexural strengths represent a minimum flexural strength for the materials. The flexural strengths of TTSx exceed those of other sulfur-crosslinked materials for which stress-strain data have been reported, including cellulose/sulfur (up to 3.8 MPa) and lignin/sulfur composites (up to 2.1 MPa). For comparison to a more familiar material, the flexural strength of Portland cement ranges from 3–5 MPa. Other materials utilizing sulfur with dicyclopentadiene, linseed oil, and canola oil led to 41 materials with flexural strengths of up to 6 MPa. The TTSx materials exhibit flexural 111 strengths in excess of 6.8 MPa; this is a lower limit because samples do not break at maximum instrument force. The high strength may be attributable to higher crosslink density afforded by tetraallyltyrosine, which features eight potential sites for crosslinking per molecule. 112 a) 8.0 7.0 6.0 5.0 4.0 3.0 Stress (MPa) 2.0 1.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% Strain b) 9.0 8.0 7.0 6.0 5.0 4.0 Stress(MPa) 3.0 2.0 1.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% Strain Figure 3.4 (a) Stress-strain analysis for TTS30 (red trace) and TTS50 (blue trace). The dashed lines are propagations of the linear regions of the respective stress strain curves. (b) comparison of stress-strain analysis for TTS30 (red trace) and TTS50 (blue trace) before (solid traces) and after (coincident black dashed traces) exposure to sulfuric acid for 24 h. 113 With such a high content of tyrosine-derived monomer, water uptake and attendant swelling or loss of mechanical integrity could be a concern for TTSx materials. On the other hand, elemental sulfur is highly hydrophobic, with a critical surface energy of 27 mN/m, which is between the values for Teflon (24 mN/m) and polyethylene (32.6 42 mN/m). Samples of TTSx were thus weighed, submerged in water for 24 h, and reweighed after this soaking period. No water uptake or change in dimensions was measurable for either TTSx sample, confirming exclusion of water by the hydrophobic sulfur content. Water exclusion in polymers comprising polar functional groups is an important property given that uptake of even small quantities of water can significantly impact the mechanical properties of a material. The Tg of nylon-6,6, for example, falls from 100 °C when dry to 43 °C after 3.5 wt.% water uptake – occurring at a relative humidity of just 55% – and the tensile strength at yield falls concomitantly from 80 MPa 43 to 43 MPa. The mechanical strength of TTSx, however, is unchanged even after being submerged in water for 24 h. Material Flexural Storage Flexural Strength (MPa) Modulus (MPa) TTS30 860 ± 110 >7.0 ± 1 b after H2SO4 100% 100% TTS50 580 ± 140 >6.8 ±0.4 b after H2SO4 100% 100% a b Values determined from the average of three runs. percentage of initial metric retained after soaking the sample in 0.5 M H2SO4 (aq) for 24 h. a Table 3.2 Summary of flexural properties of TTSx materials. 114 Whereas the ready decomposition of high sulfur-content materials by soil bacteria can be an asset from a sustainability standpoint, polymers having some targeted chemical resistance may be advantageous or required for a given application. Sulfur-infused masonry and cementitious materials, for example, have proven especially effective in applications requiring resistance to corrosion by acid.44, 45 To assess the extent to which TTSx copolymers could maintain their structural integrity after challenge by acid exposure, a brick of each copolymer was soaked in 0.5 M H2SO4(aq) for 24 h and then the stress-strain curve was re-measured. Sulfuric acid was selected because it is a strong and oxidizing acid that reacts with many common organic functional groups. Impressively, neither TTS30 nor TTS50 showed any change in dimensions or flexural strength / modulus after challenge by sulfuric acid (Figure 3.4b). This is an especially important finding because it demonstrates that at least 50 wt.% tyrosine can be protected by sulfur crosslinking, whereas chemical susceptibility is a common criticism of bioderived polymers comprising polar functional groups as compared to many common petrochemical polymers. Conclusions Durable, chemically-resistant TTSx copolymers have been prepared from industrial by-product sulfur and a derivative of microbially-produced L-tyrosine via the 100% atom-economical inverse vulcanization process. The copolymers are highly- crosslinked network copolymers comprised primarily of sulfur-crosslinked tetraallyltyrosine wherein the average crosslinks are 3–6 sulfur atoms in length. The 115 flexural strengths of TTSx copolymers are significantly higher than that of Portland cement. Despite the high tyrosine content (50-70 wt%), the copolymers exhibit no swelling, water uptake or deterioration of mechanical properties even after soaking in 0.5 M aqueous sulfuric acid for 24 h. Experimental Section General Considerations All NMR spectra were recorded on a Bruker Avance spectrometer operating at 300 MHz for proton nuclei. Fourier transform infrared spectra were obtained using a Shimadzu IR Affinity-1S instrument operating over the range of 400-4000 cm–1 at ambient temperature. Thermogravimetric analysis (TGA) was recorded on a Mettler Toledo TGA 2 STARe System over the range of 25 to 800 °C with a heating rate of 5 °C –1 -1 min under a flow of N2 (20 mL min ). Differential Scanning Calorimetry (DSC) was acquired using a Mettler Toledo DSC 3 STARe System over the range of 0 to 140 °C, –1 –1 with a heating rate of 5 °C min under a flow of N2 (200 mL min ). Each DSC measurement was carried out over 3 heat-cool cycles to get rid of any thermal history. The data reported were taken from the second cycle of the experiment. Dynamic Mechanical Analysis (DMA) was performed using a Mettler Toledo DMA 1 STARe System in single cantilever mode. DMA samples were cast from two stainless steel moulds with sample dimensions of approximately 2 × 11 × 40 mm and 2 × 11 × 19 mm. Temperature scans were taken with the smaller DMA sample while stress strain measurements were taken with the larger samples. The clamping force for both 116 temperature scan and stress strain measurements was set to 1 cN·m. Temperature scans were measured from 25-100 °C and the motor displacement was set to 5 µm and a free length of 5 mm. Static stress-strain measurements were taken at 25 °C in single cantilever mode with a force range of 0-10 N, a ramp rate of 0.1 N/min, and with a free length of 10 mm. Materials and Methods L-Tyrosine (99%, Alfa Aesar), allyl bromide (Oakwood Chemical), elemental sulfur (99.5%, Alfa Aesar) and potassium hydroxide (Fluka Analytical) were used without further purification. Tetraallyltyrosine was prepared as previously reported.37 Water uptake measurements were taken after soaking a sample of TTSx in deionized water for 24 h. A sample of TTSx was taken after the initial stress strain measurement, was re-run, followed by soaking in 0.5 M H2SO4 (aq) solution for 24 h. After 24 h the sample was dried and subjected stress strain measurements. General synthesis of TTSx. CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. Elemental sulfur and tetraallyltyrosine was weighed directly into a 20 mL scintillation vial. The vessel was placed in a thermostat-controlled oil bath set to 150 °C (the internal temperature rises to between 160 and 180 °C). After the sulfur began to melt, two visible layers were formed. The reaction media was vigorously stirred with a Teflon coated stir bar until the reaction mixture became homogenous (30-45 min). Once the reaction mixture became 117 homogenous, the reaction solution was transferred to stainless steel moulds in a 150 °C oven to cure for 4 h. Reagent masses and results of elemental combustion microanalysis are provided below. Synthesis of TTS30. The general synthesis above was used to synthesize TTS30 (30 wt% sulfur) where 3.01 g of elemental sulfur and 7.00 g of tetraallyltyrosine were used in the reaction, with a recovered yield of 95%. Elemental analysis calculated: C, 51.7%; S, 30.0%; H, 5.6%; N, 2.9%. Found: C, 49.6%; S, 33.4%; H, 5.4%; N, 2.9%. Synthesis of TTS50. The general synthesis above was used to synthesize TTS50 (50 wt% sulfur) where 5.02 g of elemental sulfur and 5.02 g of tetraallyltyrosine were used in the reaction, with a recovered yield of 95%. Elemental analysis calculated: C, 36.9%; S, 50.0%; H, 4.0%; N, 2.1%. Found: C, 38.44%; S, 51.39%; H, 3.59%; N, 2.28%. Acknowledgements This work was supported by the National Science Foundation (CHE-1708844) 118 Supporting Information Synthesis of tetraallyltyrosine. To a 500 mL round bottom flask equipped with a Teflon-coated magnetic stir bar was added 9.988 g (0.055 mol) of L-tyrosine and 150 mL of dimethylformamide, the solution was stirred until complete dissolution of tyrosine followed by the addition of 18.216 g (0.325 mol) of KOH. 54.6 g (0.451 mol) of allyl bromide was added drop-wise using an addition funnel, following complete addition of allyl bromide the flask was then equipped with a water-cooled reflux condenser and placed in a 60 °C oil bath and stirred for 24 h. The flask was cooled to room temperature and 100 mL of 0.5 M HCl(aq) was added to the reaction mixture and extracted with dichloromethane (4 × 150 mL). The organic layer was collected and washed with deionized water (4 × 200 mL) and the solvent was evaporated under reduced pressure. The remaining liquid was passed through a 5-cm high silica plug in a 150 mL fritted funnel using dichloromethane as the solvent. The product was collected as a yellow liquid (6.924 g, 37%). 119 7.260 7.094 7.065 6.827 6.798 5.700 5.684 5.433 5.428 5.375 5.370 5.294 5.289 5.267 5.262 5.259 5.254 5.214 5.210 5.205 5.201 5.175 5.170 5.118 5.113 5.106 5.100 5.066 4.556 4.551 4.547 4.542 4.537 4.532 4.528 4.520 4.515 4.510 4.502 4.497 4.492 3.710 3.686 3.683 3.659 3.417 3.400 3.368 3.363 3.352 3.124 3.100 3.076 3.051 2.997 2.970 2.866 2.842 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm 2.00 1.99 1.01 3.01 0.59 0.49 7.07 4.02 0.98 2.02 2.99 1.00 Figure S3.1 Proton NMR spectrum of tetrallyltyrosine in CDCl3. 120 7.260 7.094 7.065 6.827 6.798 7.45 7.40 7.35 7.30 7.25 7.20 7.15 7.10 7.05 7.00 6.95 6.90 6.85 6.80 6.75 6.70 6.65 6.60 ppm 2.00 1.99 1 Figure S3.2 Inset of H NMR spectrum between 6–7.5 ppm of tetrallyltyrosine in CDCl3. 121 6.116 6.098 6.081 6.063 6.041 6.023 6.006 5.988 5.905 5.886 5.870 5.867 5.851 5.848 5.832 5.829 5.813 5.810 5.794 5.775 5.758 5.742 5.734 5.724 5.717 5.708 5.700 5.684 5.676 5.667 5.660 5.651 5.643 5.626 6.45 6.40 6.35 6.30 6.25 6.20 6.15 6.10 6.05 6.00 5.95 5.90 5.85 5.80 5.75 5.70 5.65 5.60 ppm 1.01 3.01 1 Figure S3.3 Inset of H NMR spectrum between 5.5–6.5 ppm of tetrallyltyrosine in CDCl3. 122 5.433 5.428 5.375 5.370 5.294 5.289 5.267 5.262 5.259 5.254 5.214 5.210 5.205 5.201 5.175 5.170 5.118 5.113 5.106 5.100 5.066 5.45 5.40 5.35 5.30 5.25 5.20 5.15 5.10 5.05 ppm 0.59 0.49 7.07 1 Figure S3.4 Inset of H NMR spectrum between 5–5.5 ppm of tetrallyltyrosine in CDCl3. 123 172.23 157.10 136.34 133.43 132.14 130.65 130.29 118.25 117.55 117.17 114.44 77.47 77.05 76.62 68.80 64.79 64.01 53.49 35.00 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 ppm Figure S3.5 Carbon-13 NMR spectrum of tetrallyltyrosine in CDCl3. 124 Transmittance Transmittance 1200 1100 1000 900 800 700 600 Wavenumber (cm–1) Figure S3.6 Infrared spectra of TTS30 (green), TTS50 (orange), and tetraallyltyrosine (black) between 600-1200 cm–1. 125 1.10 0.90 0.70 0.50 0.30 0.10 -0.10 -0.30 Heat Flow (mW) HeatFlow -0.50 -0.70 -0.90 -1.10 0 20 40 60 80 100 120 140 Temperature (℃) Figure S3.7 DSC curves for TTS30 (blue line) and TTS50 (red dashed line). Curves represent the third heat/cool cycles. 126 10.0 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.3% 0.6% 0.9% 1.2% 1.5% Strain 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% Strain 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% Strain Figure S3.8 Trials 1 (top), 2 (middle), and 3 (bottom) of TTS30 stress strain measurements where the red line is the propagation of the linear region of the respective stress strain curve. 127 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% Strain 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% Strain 8.0 6.0 4.0 Stress (MPa) 2.0 0.0 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% 3.5% 4.0% Strain Figure S3.9 Trials 1 (top), 2 (middle), and 3 (bottom) of TTS50 stress strain measurements where the red line is the propagation of the linear region of the respective stress strain curve. 128 References 1. https://www.plasticsinsight.com/world-per-capita-consumption-pe-pp-pvc-resins- 2014/ (accessed 7-4-19). 2. Geyer, R.; Jambeck, J. R.; Law, K. L., Production, use, and fate of all plastics ever made. Sci. Adv. 2017, 3 (7), e1700782/1-e1700782/5. 3. Jeong, J.; Choi, J., Adverse outcome pathways potentially related to hazard identification of microplastics based on toxicity mechanisms. Chemosphere 2019, 231, 249-255. 4. Cole, M.; Lindeque, P.; Halsband, C.; Galloway, T. S., Microplastics as contaminants in the marine environment: A review. Marine Pollution Bulletin 2011, 62 (12), 2588-2597. 5. Hidalgo-Ruz, V.; Gutow, L.; Thompson, R. C.; Thiel, M., Microplastics in marine environment review of methods for identification and quantification. Environmental Science and Technology 2012, 46 (6), 3060-3075. 6. Gestoso, I.; Cacabelos, E.; Ramalhosa, P.; Canning-Clode, J., Plasticrusts: A new potential threat in the Anthropocene's rocky shores. Science of the Total Environment 2019, 687, 413-415. 7. Lanni, L., Infinity Line. LMNO Press: Chapin, SC, USA, 2018; p 290. 8. Kristufek, S. L.; Wacker, K. T.; Timothy Tsao, Y.-Y.; Su, L.; Wooley, K. L., Monomer design strategies to create natural product-based polymer materials. Natural Product Reports 2017, 34 (4), 433-459. 129 9. Wei, N.; Quarterman, J.; Jin, Y.-S., Marine macroalgae: an untapped resource for producing fuels and chemicals. Trends in Biotechnology 2013, 31 (2), 70-77. 10. Durvasula, R. V.; Rao, D. V. S.; Rao, V. S., Microalgal biotechnology: today's (green) gold rush. Biotechnological Applications of Microalgae 2013, 201-227. 11. Zhang, Y.; Kumar, A.; Hardwidge, P. R.; Tanaka, T.; Kondo, A.; Vadlani, P. V., D-Lactic acid production from renewable lignocellulosic biomass via genetically modified Lactobacillus plantarum. Biotechnology Progress 2016, 32 (2), 271-278. 12. Nawabi, P.; Bauer, S.; Kyrpides, N.; Lykidis, A., Engineering Escherichia coli for biodiesel production utilizing a bacterial fatty acid methyltransferase. Applied and Environmental Microbiology 2011, 77 (22), 8052-8061. 13. Magdouli, S.; Yan, S.; Tyagi, R. D.; Surampalli, R. Y., Heterotrophic Microorganisms: A Promising Source for Biodiesel Production. Critical Reviews in Environmental Science and Technology 2014, 44 (4), 416-453. 14. Silva, C. E. d. F.; Bertucco, A., Bioethanol from microalgae and cyanobacteria: A review and technological outlook. Process Biochemistry (Oxford, United Kingdom) 2016, 51 (11), 1833-1842. 15. Liang, M.-H.; Jiang, J.-G., Advancing oleaginous microorganisms to produce lipid via metabolic engineering technology. Progress in Lipid Research 2013, 52 (4), 395- 408. 16. Majidian, P.; Tabatabaei, M.; Zeinolabedini, M.; Naghshbandi, M. P.; Chisti, Y., Metabolic engineering of microorganisms for biofuel production. Renewable & Sustainable Energy Reviews 2018, 82 (Part_3), 3863-3885. 130 17. Erickson, B.; Nelson; Winters, P., Perspective on opportunities in industrial biotechnology in renewable chemicals. Biotechnology Journal 2012, 7 (2), 176-185. 18. Parker, D. J.; Chong, S. T.; Hasell, T., Sustainable inverse-vulcanised sulfur polymers. RSC Adv. 2018, 8 (49), 27892-27899. 19. Worthington, M. J. H.; Kucera, R. L.; Chalker, J. M., Green chemistry and polymers made from sulfur. Green Chem. 2017, 19 (12), 2748-2761. 20. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J., The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat Chem 2013, 5 (6), 518-24. 21. Zhang, X.; Tang, Y.; Qu, S.; Da, J.; Hao, Z., H2S-Selective Catalytic Oxidation: Catalysts and Processes. ACS Catalysis 2015, 5 (2), 1053-1067. 22. Demirbas, A.; Alidrisi, H.; Balubaid, M. A., API Gravity, Sulfur Content, and Desulfurization of Crude Oil. Petroleum Science and Technology 2015, 33 (1), 93- 101. 23. Kutney, G., Sulfur. History, Technology, Applications & Industry. ChemTec: Toronto, 2007. 24. Zhang, Y.; Glass, R. S.; Char, K.; Pyun, J., Recent Advances on the Polymerization of Elemental Sulphur, Inverse Vulcanization and Methods to Functional Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs). Polym. Chem. 2019, Ahead of Print. 131 25. Chalker, J. M.; Worthington, M. J. H.; Lundquist, N. A.; Esdaile, L. J., Synthesis and Applications of Polymers Made by Inverse Vulcanization. Topics in Current Chemistry 2019, 377 (3), 1-27. 26. Salman, M. K.; Karabay, B.; Karabay, L. C.; Cihaner, A., Elemental sulfur-based polymeric materials: Synthesis and characterization. Journal of Applied Polymer Science 2016, 133 (28), n/a. 27. Griebel, J. J.; Glass, R. S.; Char, K.; Pyun, J., Polymerizations with elemental sulfur: A novel route to high sulfur content polymers for sustainability, energy and defense. Progress in Polymer Science 2016, 58, 90-125. 28. Mann, M.; Kruger, J. E.; Andari, F.; McErlean, J.; Gascooke, J. R.; Smith, J. A.; Worthington, M. J. H.; McKinley, C. C. C.; Campbell, J. A.; Lewis, D. A.; Hasell, T.; Perkins, M. V.; Chalker, J. M., Sulfur polymer composites as controlled-release fertilizers. Organic & Biomolecular Chemistry 2019, 17 (7), 1929-1936. 29. Valle, S. F.; Giroto, A. S.; Klaic, R.; Guimaraes, G. G. F.; Ribeiro, C., Sulfur fertilizer based on inverse vulcanization process with soybean oil. Polymer Degradation and Stability 2019, 162, 102-105. 30. Karunarathna, M.; Lauer, M. K.; Thiounn, T.; Smith, R. C.; Tennyson, A. G., Valorisation of Waste to Yield Recyclable Composites of Elemental Sulfur and Lignin. Journal of Materials Chemistry A 2019, 7, 15683-15690. 31. Lauer, M. K.; Estrada-Mendoza, T. A.; McMillen, C. D.; Chumanov, G.; Tennyson, A. G.; Smith, R. C., Durable, Remeltable Materials from Agricultural and Petrochemical Wastes. Advanced Sustainable Systems 2019, accepted for publication. 132 32. Ikeda, M., Towards bacterial strains overproducing L-tryptophan and other aromatics by metabolic engineering. Applied Microbiology and Biotechnology 2006, 69 (6), 615-626. 33. Na, D.; Yoo, S. M.; Chung, H.; Park, H.; Park, J. H.; Lee, S. Y., Metabolic engineering of Escherichia coli using synthetic small regulatory RNAs. Nature Biotechnology 2013, 31 (2), 170-174. 34. Luetke-Eversloh, T.; Santos, C. N. S.; Stephanopoulos, G., Perspectives of biotechnological production of L-tyrosine and its applications. Applied Microbiology and Biotechnology 2007, 77 (4), 751-762. 35. Chavez-Bejar, M. I.; Baez-Viveros, J. L.; Martinez, A.; Bolivar, F.; Gosset, G., Biotechnological production of L-tyrosine and derived compounds. Process Biochemistry (Oxford, United Kingdom) 2012, 47 (7), 1017-1026. 36. Thiounn, T.; Lauer, M. K.; Bedford, M. S.; Smith, R. C.; Tennyson, A. G., Thermally-Healable Network Solids of Sulfur-Crosslinked Poly(4-allyloxystyrene). RCS Advances 2018, 8, 39074-39082. 37. Aoyagi, S.; Shimasaki, T.; Teramoto, N.; Shibata, M., Bio-based polymer networks by thiol-ene photopolymerization of allylated l-glutamic acids and l-tyrosines. European Polymer Journal 2018, 101, 151-158. 38. Meyer, B., Elemental sulfur. Chem. Rev. 1976, 76 (3), 367-88. 39. Griebel, J. J.; Nguyen, N. A.; Namnabat, S.; Anderson, L. E.; Glass, R. S.; Norwood, R. A.; MacKay, M. E.; Char, K.; Pyun, J., Dynamic Covalent Polymers 133 via Inverse Vulcanization of Elemental Sulfur for Healable Infrared Optical Materials. ACS Macro Lett. 2015, 4 (9), 862-866. 40. Smith, A. D.; Thiounn, T.; Lyles, E. W.; Kibler, E. K.; Smith, R. C.; Tennyson, A. G., Combining Agriculture and Energy Industry Waste Products to Yield Recyclable, Thermally Healable Copolymers of Elemental Sulfur and Oleic Acid. Journal of Polymer Science Part A 2019, in Press, DOI:10.1002/pola.29436. 41. Smith, J. A.; Green, S. J.; Petcher, S.; Parker, D. J.; Zhang, B.; Worthington, M. J. H.; Wu, X.; Kelly, C. A.; Baker, T.; Gibson, C. T.; Campbell, J. A.; Lewis, D. A.; Jenkins, M. J.; Willcock, H.; Chalker, J. M.; Hasell, T., Crosslinker Copolymerization for Property Control in Inverse Vulcanization. Chemistry – A European Journal 2019, 25 (44), 10433-10440. 42. Kelebek, S., Critical Surface Tension of Wetting and of Floatability of Molybdenite and Sulfur. Journal of Colloid and Interface Science 1988, 124, 504-514. 43. Pai, C. C.; Jeng, R. J.; Grossman, S. J.; Huang, J. C., Effects of moisture on thermal and mechanical properties of nylon-6,6. Advances in Polymer Technology 1989, 9 (2), 157-63. 44. Weber, H. H., New Applications and Expanding Markets for Sulphur Polymer Cement Concrete. American Concrete Institute 1993, SP-137, 49-72. 45. Weber, H. H.; McBee, W. C. In New market opportunities for sulphur asphalt; Sulphur markets today and tomorrow, Biennial International Symposium , 7th, Washington, D.C. , Washington, D.C. , 2000; p 24. 134 Attribution of work My contribution to the work in this chapter has been published in the journal of RSC Advances. The citation for the above manuscript: RSC Adv., 2019,9, 31460-31465. 135 CHAPTER FOUR COPOLYMERIZATION OF A BISPHENOL A DERIVATIVE AND ELEMENTAL SULFUR BY THE RASP PROCESS4 Abstract The RASP (Radical-induced Aryl halide-Sulfur Polymerization) process has recently emerged as a way to produce high sulfur-content materials (HSMs) through reaction of elemental sulfur with aryl halides. Previously, only one other small molecular aryl halide has been used as a monomer for RASP. In this contribution, a commercial bisphenol A (BPA) derivative, 2,2’,5,5’-tetrabromo(bisphenol A) (Br4BPA), is explored as a potential organic monomer for copolymerization with elemental sulfur by RASP. The resultant copolymers, BASx (x = wt% sulfur in the monomer feed, varied from 80– 95) were characterized by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis. BASx are notable for evaluating the RASP process because BASx are the first HSMs produced via RASP in which benzylic crosslinking is not available to supplement crosslinking due to direct S–Caryl bond-forming reactions. Fractionation of BASx material led to their characterization as a crosslinked network having a sulfur rank of 3–6 and in which polymeric sulfur is stabilized. Several comonomer ratios were explored as a means of assessing the extent to which monomer feed ratio influences thermal and mechanical properties of the resultant materials. Copolymers having lower crosslink density were thermoplastics whereas thermosets were 4 This text is reproduced with permission in larger part from: Thiounn, T.; Lauer, M. K.; Tennyson, A. G.; Smith, R. C., Copolymerization of a Bisphenol A derivative and Elemental Sulfur by the RASP Process. Submitted to Polymer Chemistry, 2020. 136 accomplished with higher crosslink densities. The flexural strengths of the thermally- processable samples were quite high compared to familiar building materials such as portland cement. Furthermore, copolymer BAS90 proved quite resistant to degradation by oxidizing organic acid, maintaining its full flexural strength after soaking in 0.5 M H2SO4 for 24 h . In a demonstration of recyclability and potential as sustainable materials, BAS90 could also be remelted and recast into shapes over many cycles without any loss of mechanical strength. This first study on the effect of monomer ratio on properties of materials prepared by RASP of small molecular aryl halides confirms that highly crosslinked materials with varying physical and mechanical properties can be accessed by this protocol. Introduction High sulfur-content materials (HSMs) are attractive alternatives to petrochemical polymers because they can be synthesized primarily from waste sulfur. Sulfur-sulfur bonds can form in a thermally reversible fashion, so HSMs can also exhibit healable and recyclable properties that are advantageous for sustainability.1-7 Thus far, the inverse vulcanization route, which requires an olefin comonomer for reaction with elemental sulfur (Scheme 4.1A), has been the primary approach to preparing such HSMs.8 Although inverse vulcanization was reported only a few years ago, its potential for facile production of versatile materials was quickly recognized. In a very short time, olefins derived from petroleum,7-16 plant and animal sources,17-22 1, 10, 17, 19, 23-29 30, 31 bacteria32 and algae33 have all proven to be successful monomers for the production of HSMs by 137 inverse vulcanization. These HSMs have subsequently drawn significant attention for their potential as IR transparent lenses for thermal imaging,34, 35 electrode materials,36, 37 absorbents, 10, 12, 38-40 23 fertilizers18, 22 and structural materials.29, 41-46 It would seem that inverse vulcanization’s only limitation is that the organic comonomer must contain olefinic units. We recently developed radical-induced aryl halide-sulfur polymerization (RASP, Scheme 4.1B)47, 48 as a way to produce HSMs. RASP employs an aryl halide as the comonomer with elemental sulfur, thus expanding the scope of potential monomers for HSM preparation beyond the olefins required for inverse vulcanization. To date, chlorolignin48 and 2,4-dimethyl-3,5-dichlorophenol47 are the only organic monomers that have been used in the RASP process, and the extent to which the ratio of organic small molecules to sulfur influences the properties of the resultant HSMs has yet to be elucidated. Herein, RASP of sulfur and a commercial monomer, 2,2’,4,4’- tetrabromo(bisphenol A) (Br4BPA), is undertaken to produce BASx (where x = wt% sulfur in the monomer feed). BASx copolymers were prepared having four different monomer ratios, ranging from 80–95 wt% sulfur in the monomer feed. Characterizing these copolymers provides 1) insight into the influence of monomer ratio on properties in HSMs produced by RASP and 2) the first HSMs produced by RASP wherein there is no possibility for benzylic crosslinking to supplement crosslinking due to direct S–Caryl bond-forming reactions. BASx materials range from thermosets to thermoplastics and all of them form materials wherein polymeric sulfur is stabilized by its incorporation into a 138 crosslinked network. The influence of monomer feed ratio on the processability, composition, thermal properties and mechanical properties was evaluated, and the recyclability of remeltable materials was validated. A) Inverse Vulcanization of Alkenes (Pyun, 2013) B) RASP of Aryl Halides (Karunarathna and Smith, 2019) Scheme 4.1 Inverse vulcanization (A) and RASP (B) routes to HSMs. 139 Results and Discussion Synthesis and Structure The mechanism for RASP is based on the related Macallum reaction.49 In the Macallum mechanism, aryl radicals are thermally generated by loss of halogen radicals. The aryl and halogen radicals subsequently react with elemental sulfur, leading to S–Caryl bond-formation and loss of the halogen as S2X2 (Scheme 4.2). The HSMs initially formed by this reaction contain oligomeric or polymeric sulfur chains that link aryl rings together. Such extended sulfur catenates are unstable and under ordinary conditions will revert back to an S8 ring allotrope, so the end product of the Macallum polymerization is a poly(arylsulfide) in which a single sulfur atom bridges aryl units. If a highly crosslinked network is present, however, the polymeric sulfur domains can be stabilized indefinitely. Stabilizing the polymeric sulfur domains is important for accessing materials with high mechanical strength, so establishing a highly crosslinked structure is a primary objective in selecting the organic monomer for RASP. S8 Ar X−Ar−X n + S2X2 Δ Sx HSM High Sulfur-content Material (HSM) Scheme 4.2 RASP leads to the direct formation of S–Caryl bonds with loss of S2X2. 140 In prior demonstrations of RASP, either a pre-existing polymeric network was 48 supplied in the form of lignin, or a network comprised by a combination of S–Cbenzylic 47 and S–Caryl bonds was prepared. For the current work, an affordable aryl halide monomer that could form a highly-crosslinked network comprised entirely from S–Caryl bond-forming reactions (lacking the possibility for S–Cbenzylic crosslinks) was desired. A commercially available bisphenol A (BPA) derivative, 2,2’,5,5’-tetrabromo(bisphenol A) (Br4BPA), was selected for this purpose. Br4BPA should support a high crosslink density by merit of its ability to form up to four S–Caryl crosslinks per molecule while the organic BPA subunit is a well-established and thermally stable structural element of durable polymers. OH OH OH Br Br Sx Sy Sq Sr S8 Δ S S Br Br Sz S Sp S Sn OH OH OH Scheme 4.3 Preparation of BASx. 141 Another goal of the current research was to study the extent to which the monomer ratio influences thermal and mechanical properties of the resulting HSMs, something that has not been studied for RASP of small molecular aryl halides. Elemental sulfur and Br4BPA were subjected to RASP by heating at 240 °C in a pressure tube. Four different sulfur monomer feeds, ranging from 80–95 wt%, were employed to yield four copolymers BASx (where x = wt% sulfur in the monomer feed, Scheme 4.3). All of the materials have a dark brown-orange colour characteristic of polymeric sulfur (Figure 4.1). Copolymers having ≥90 wt% sulfur in the monomer feed were remeltable whereas BAS85 and BAS80 were thermosets. Elemental analysis for Br is a convenient way to assess the extent of S–Caryl bond formation. The RASP of Br4BPA led to the successful crosslinking of 85–100% of available sites. As indicated in Scheme 4.2, the halogens are initially removed as S2X2, which are toxic and corrosive compound, so caution should be exercised when performing the RASP reaction. After reaction in the pressure tube, the tube should be fully cooled to room temperature before opening it in a fume hood, where S2X2 in the product can be readily hydrolysed by controlled reaction with water or slow reaction with humidity in the air. These RASP reactions can also be performed in a Schlenk flask vented to a water bath into which S2X2 can be distilled as it forms, where it will be hydrolysed to HX. Both the Schlenk flask and pressure tube methods prove effective for preparing the HSMs. In the current context, at least some of the S2Br2 generated during RASP could also react with phenolic sites as well.50 At least some of the phenol 142 functionalities are still present unreacted in the copolymers based on IR spectra that reveal characteristic peaks at 3400 cm-1 (O–H stretch) and 1153 cm-1 (C−O stretch). Figure 4.1 Although materials appear very dark under fluorescent room lighting (A), the brown- orange colour characteristic of polymeric sulfur is evident in a backlit sample of BAS90 (B). The sample can be broken down (C), then readily remelted and recast (D). 143 The prior HSMs prepared by RASP contained some sulfur that was not covalently incorporated into the crosslinked network. Sulfur that is not covalently incorporated is readily extractable into CS2, whereas highly-crosslinked polymers are generally 7, 28, 32 insoluble. This provides a convenient way to fractionate such materials. BASx samples were thus fractionated in CS2, and the relative masses of soluble and insoluble fractions were determined as shown in Table 4.1. Elemental analysis of the CS2-soluble fractions of BASx confirm that they consist of >98.5% sulfur. With the aforementioned data in hand, the sulfur rank (average number of sulfur atoms per crosslinking chain) can be calculated as follows. The number of carbon atoms crosslinked by sulfur (Cxlink) is known by subtracting the remaining bromine (from elemental analysis, Brec) from the initial amount of bromine in the reaction feed (Bri). Fractionation data allowed for the calculation of the number of moles of sulfur atoms covalently incorporated (Scov) into the materials by subtracting the free sulfur removed in the CS2 wash (Sfr) from the total sulfur content (Sadd). From these data the sulfur rank (average number of sulfur atoms in each crosslinking chain) was calculated using equation 1 (the factor of 2 is necessary because each sulfur chain must link to two carbon atoms to form a crosslink). ! ! ! (1) ������ ���� = 2 ∗ !!"# = 2 ∗ !"" !" !!"#$% !!!! !!!" 144 The sulfur ranks in BASx (Table 4.1) are similar to those found in materials prepared by RASP utilizing a dichloroxylenol derivative (sulfur rank = 5)47 but lower than in materials prepared by RASP of a lignin derivative (12-31 sulfur atoms).48 The lower sulfur rank in BASx is expected on the basis of the high number of available crosslink sites. Similar sulfur ranks can also be obtained in HSMs prepared by inverse vulcanization. For example, inverse vulcanization HSMs wherein the olefin was a polystyrene derivative7 or a tyrosine dervative32 both have sulfur ranks of five. The number of available crosslink sites per sulfur in the copolymers appears to dictate the sulfur rank regardless of whether inverse vulcanization or RASP is used to prepare the material. Although the fractionation studies confirm the presence of sulfur that is not covalently bound to the crosslinked material, no phase separation was observed by scanning electron microscopy (SEM) with element mapping by energy dispersive X-ray analysis (EDX), which confirmed uniform distribution of sulfur, carbon and oxygen with almost no Br detectable (Figures S4.7–S4.8 in the Supporting Information supplied at the end of this chapter). 145 Sulfur Rank CS2-Insoluble (%) BAS80 4 28 BAS85 3 20 BAS90 6 17 BAS95 5 8 Table 4.1 Fractionation and Sulfur Rank of BASx. Thermal and Morphological Properties Thermogravimetric analysis of BASx materials revealed that higher sulfur content materials had slightly higher decomposition temperature (Td) values, but the thermal decomposition is driven primarily by sulfur decomposition as would be expected of materials composed of 80-95 wt% sulfur (Table 4.2). As expected, the char yield for BASx materials increased as the sulfur content decreased due to the larger amounts of organic monomer present in the material and the tendency of highly aromatic organics to leave glassy carbon materials after heating to high temperatures. Differential scanning calorimetry (DSC, Table 4.2 and Figure S4.1–S4.4 in the Supporting Information supplied at the end of this chapter) analysis of BASx revealed a number of notable morphological changes over the range of –60 to +140 °C. The typical melt peak for sulfur domains of the materials was observed at 114-116 °C, slightly depressed from the melting point of pure orthorhombic sulfur. The presence of polymeric sulfur for all of the materials was also evident from the observation of the characteristic 146 glass transition (Tg) at between –31 and –36 °C, consistent with the characteristic polymeric sulfur colour of the materials (Figure 4.1). Determination of crystallization and melting enthalpies from the DSC data also allowed the quantification of the percent crystallinity of the copolymers by equation 2. ∆!! !"!! !∆!!! !"#! (2) �! = ∗ 100% ∆!! ! !∆!!! ! Where �c is the percentage crystallinity with respect to sulfur, ∆�! is the melting enthalpy for the indicated material (sulfur or BASx), and ∆�!! is the cold crystallization enthalpy of the indicated material. Crystallinity decreases predictably as more of the organic crosslinking unit is incorporated, to the extent that the crystallinity of BAS95 is only 40% that of sulfur despite being comprised of 95 wt% sulfur. A primarily amorphous polymer is attained with 20 wt% of organic crosslinker in the monomer feed, as reflected in the 8% crystallinity of BAS80. 147 BAS95 BAS90 BAS85 BAS80 a Td (°C) 240 222 233 231 Char Yield 2% 4% 7% 9% b Tg (°C) –36 –34 –35 –31 c Tcc (°C) 14, 41 25, 39 14, 38 39 d Tm (°C) 116 115 115 114 e ΔHm (J/g) 30 28 28 21 f ΔHcc (J/g) 7, 3 18, 1 16, 1 17 g �c(%) 40 20 20 8 a Determined from the onset of the major mass loss peak. b Determined from the onset of transition from the third heating cycle of the DSC curve . c Temperature at which cold crystallization peak maxima are observed. d Temperature of peak minimum for e f g melting of sulfur domains. The melting enthalpy for BASx. The cold crystallization enthalpy for BASx. The percent crystallinity of each sample calculated according to equation 2. Table 4.2 Thermal Properties of BASx. Mechanical Properties The remeltable samples, BAS95 and BAS90, can be fabricated into any number of shapes (Figure 4.1) by simply pouring the molten materials into a silicone mould and allowing the samples to harden upon cooling to room temperature. This allowed the two materials to be conveniently fabricated into rectangular prismatic beams appropriate for flexural strength analysis in a dynamic mechanical analyser (Figures S4.5–4.6 in the Supporting Information provided at the end of this chapter). The flexural strengths (Table 4.3) of both BAS90 (>3.4 MPa) and BAS95 (>4.7 MPa) are comparable to that of Portland cement (3–5 MPa). It is notable that the flexural strength values reported in 148 Table 4.3 represent lower limits to the flexural strengths of the BASx materials because the instrument was unable to apply enough stress to break the samples. In contrast, the Portland cement sample of the same dimensions was broken by the instrument at a force corresponding to a flexural strength of 3.7 MPa. Portland cement is an important target for replacement by more sustainably-sourced and recyclable materials because the production of Portland cement is energy intensive and is responsible for 8–12% of 51, 52 anthropogenic CO2 emissions. In addition to their potential for high strength, sulfur cements, including BASx, exhibit very little water uptake (<3 mass %) compared to Portland cement (28 mass %), making them more resistant to fissure formation resulting from repetitive freeze-thaw 3 cycles. The density of BASx (1.8 g/cm ) is also similar to that of Portland cement (1.5 3 g/cm ). One particularly attractive application space for sulfur cements like BASx is in low-pH environments, because HSMs have proven highly resistant to mechanical deterioration by acid.29 32, 53, 54 Recent studies have shown, for example, that cellulose- sulfur composites are impervious to degradation by sulphuric acid under conditions that completely dissolve a portland cement block of the same dimensions. BASx materials were likewise evaluated for their ability to withstand acid challenge by submerging samples in 0.5 M H2SO4 for 24 h. Both BAS90 and BAS95 maintained their dimensional integrity and flexural strength under these conditions, whereas the Portland cement sample was completely decomposed (Table 4.3). 149 Flexural modulus/strength (MPa) before acid after acida Portland cement 390/3.7 –b c c BAS90 290/>4.7 300/>4.7 c c BAS95 360/>3.4 330/>3.4 a b Acid challenge involved submerging the sample in 0.5 M H2SO4 for 24 h. A Portland cement sample decomposes under the acid challenge conditions. c The flexural strength value represents a lower limit to the strength because the instrument could not apply enough force to break the sample. Table 4.3 Properties and Acid Resistance of BASx. Having shown that BAS90 outperforms portland cement in flexural strength and acid resistance, the next test was to evaluate whether BAS90 material could be recycled. For this test, the flexural strength of a sample of BAS90 was determined after recursive cycles of pulverization, remelting, and curing in a silicone mould. The results of these measurements for seven cycles are shown in Figure 4.2. BAS90 maintains of its mechanical strength, as measured from its storage modulus at 25 °C, within measurement error after this process, although some of the recycling steps produced material with somewhat lower (90-80%) strength. This observation is consistent with some previous reports on the recyclability of HSMs, in which full strength was maintained even after over a dozen cycles.7, 32 The recyclability of HSMs by repetitive melt processing represent a significant improvement over some widely-used petrochemical polymers like polyethylene terephthalate (PETE). The recyclability for PETE is dramatically diminished after only a few thermal processing cycles, to the extent that the strain-at- 150 break falls from 42% for freshly manufactured samples to only 0.7% after only the fifth 55 cycle. Even if BASx can only be recycled four or five times while maintaining its mechanical durability, this would also be a marked improvement over portland cement, the vast majority of which is land filled at its end of life. 100 80 60 40 % of Initial Storage Modulus StorageModulus Initial %of 20 0 1 2 3 4 5 6 7 Remelt Cycle Figure 4.2 The percent storage modulus retained of BAS90 after 7 pulverization/remelting /recasting cycles. 151 Conclusions In conclusion, the first step to evaluating the influence of monomer feed on the properties of copolymers produced by RASP reveals that both thermosets and thermoplastics. RASP also proves effective at stabilizing polymeric sulfur within a network made exclusively by S–Caryl bond-forming reactions, in a manner analogous to the stabilization of polymeric sulfur afforded in HSMs prepared by inverse vulcanization. The resultant copolymers have flexural strengths comparable to familiar commercial building materials like portland cement but have improved capabilities in terms of acid resistance and recyclability. Experimental Section Chemicals and Materials Tetrabromobisphenol A (>98%) was purchased from TCI America. Sulfur powder (99.5%) was purchased from Alfa Aesar. These chemicals were used without further purification. All processes were carried out under ambient conditions unless otherwise specified. General Considerations Thermogravimetric analysis (TGA) data were recorded on a Mettler Toledo TGA 2STARe instrument over the range 25 to 800 °C, with a heating rate of 5 °C min–1 under –1 a flow of N2 (20 mL min ). Differential scanning calorimetry (DSC) data was obtained using Mettler Toledo DSC 3 STARe System, over the range of –60 to 140 °C, with a 152 –1 –1 heating rate of 5 °C min under a flow of N2 (200 mL min ). All the reported data were taken from the third heat/cool cycles. Dynamic Mechanical Analysis (DMA) data was acquired using Mettler Toledo DMA 1 STARe System in single cantilever mode. DMA samples were cast from silicone resin moulds (Smooth-On Oomoo® 30 tin-cure). The sample dimensions were approximately 40×11×2 mm and were clamped with 1 cN·m force. The samples cured for 120 h prior to stress-strain analysis at room temperature. The samples were clamped under single cantilever mode. The force was varied from 0 to 10 N with a ramp rate of 0.1 N·min–1. Fourier transform infrared spectra were obtained using a Shimadzu IR Affinity-1S instrument with ATR attachment, operating over the range of 400-4000 cm–1 at ambient temperature. Synthesis of BAS95 Elemental sulfur (9.5 g) and Br4BPA (0.5 g) was weighed directly into a pressure tube under inert environment. The tube was heated to 240 °C and heating was continued for 24 h with continuous stirring with a magnetic stir bar. The whole procedure was done under N2 gas. Elemental combustion microanalysis calculated: C, 1.67; S, 95.00; H, 0.11%; Br, 2.94%. Found: C, 2.11; S, 96.43; H, 0.00; Br, 0.00%. Synthesis of BAS90 Elemental sulfur (9.0 g) and Br4BPA (1.0 g) was weighed directly into a pressure tube under inert environment. The tube was heated to 240 °C and heating was continued 153 for 24 h with continuous stirring with a magnetic stir bar. The whole procedure was done under N2 gas. Elemental combustion microanalysis calculated: C, 3.31; S, 90.00; H, 0.22; Br, 5.88%. Found: C, 3.76; S, 95.40; H, 0.10; Br, 0.43%. Synthesis of BAS85 Elemental sulfur (8.5 g) and Br4BPA (1.5 g) was weighed directly into a pressure tube under inert environment. The tube was heated to 240 °C and heating was continued for 24 h with continuous stirring with a magnetic stir bar. The whole procedure was done under N2 gas. Elemental combustion microanalysis calculated: C, 4.97; S, 85.00; H, 0.33; Br, 8.82%. Found: C, 6.32; S, 89.88; H, 0.22; Br, 0.28%. Synthesis of BAS80 Elemental sulfur (8.0 g) and Br4BPA (2.0 g) was weighed directly into a pressure tube under inert environment. The tube was heated to 240 °C and heating was continued for 24 h with continuous stirring with a magnetic stir bar. The whole procedure was done under N2 gas. Elemental combustion microanalysis calculated: C, 6.63; S, 80.00; H, 0.44; Br, 11.75%. Found: C, 7.69; S, 87.84; H, 0.21; Br, 0.99%. CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. This reaction may also produce S2Cl2, a toxic and reactive material. 154 CAUTION: Heating material in a sealed tube can generate high pressures. Use caution and consult the manufacturer of the pressure apparatus used for safety guidance Conflicts of interest There are no conflicts to declare. Acknowledgements The authors would like to thank the NSF for funding this work (CHE-1708844). 155 Supporting Information 1.50 0.50 -0.50 -1.50 -2.50 Heat Flow (mW) HeatFlow -3.50 -4.50 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (℃) Figure S4.1 Differential scanning calorimetry of BAS95. 156 3.00 2.00 1.00 0.00 -1.00 -2.00 Heat Flow (mW) HeatFlow -3.00 -4.00 -5.00 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (℃) Figure S4.2 Differential scanning calorimetry of BAS90. 157 1.00 0.50 0.00 -0.50 -1.00 -1.50 -2.00 Heat Flow (mW) HeatFlow -2.50 -3.00 -3.50 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (℃) Figure S4.3 Differential scanning calorimetry of BAS85. 158 1.50 1.00 0.50 0.00 -0.50 -1.00 -1.50 Heat Flow (mW) HeatFlow -2.00 -2.50 -3.00 -3.50 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (℃) Figure S4.4 Differential scanning calorimetry of BAS80. 159 4.0 3.5 3.0 2.5 2.0 Stress(MPa) 1.5 1.0 0.5 0.0 0.0% 0.5% 1.0% 1.5% 2.0% Strain Figure S4.5 Stress strain curve of BAS95 pre (blue line) and post acid (red line) soak for 24 h. The dotted black lines are the extrapolations of the linear region. 160 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Stress(MPa) 1.5 1.0 0.5 0.0 0.0% 1.0% 2.0% 3.0% 4.0% 5.0% 6.0% 7.0% % Strain Figure S4.6 Stress strain curve of BAS90 pre (blue line) and post acid (red line) soak for 24 h. The dotted black lines are the extrapolations of the linear region. 161 Figure S4.7 Surface analysis of BAS90 by scanning electron microscopy (SEM) revealed a smooth surface consistent with those observed in high sulfur-content materials prepared by inverse vulcanization. 162 A) B) C) D) Figure S4.8 Surface analysis of BAS90 by energy-dispersive X-ray (EDX) analysis revealed even distribution of sulfur (A), carbon (B), oxygen (C) and bromine (D) content on the polymer surface. 163 References 1. Hasell, T.; Smith Jessica, A.; Green Sarah, J.; Petcher, S.; Parker Douglas, J.; Zhang, B.; Wu, X.; Kelly Catherine, A.; Baker, T.; Jenkins Mike, J.; Willcock, H.; Worthington Max, J. H.; Gibson Christopher, T.; Campbell Jonathan, A.; Lewis David, A.; Chalker Justin, M., Crosslinker co-polymerisation for property control in inverse vulcanisation. Chemistry (Weinheim an der Bergstrasse, Germany) 2019. 2. Michal, B. T.; Jaye, C. A.; Spencer, E. J.; Rowan, S. J., Inherently Photohealable and Thermal Shape-Memory Polydisulfide Networks. ACS Macro Letters 2013, 2 (8), 694- 699. 3. Griebel, J. J.; Nguyen, N. A.; Namnabat, S.; Anderson, L. E.; Glass, R. S.; Norwood, R. A.; MacKay, M. E.; Char, K.; Pyun, J., Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur for Healable Infrared Optical Materials. ACS Macro Letters 2015, 4 (9), 862-866. 4. Amaral, A. J. R.; Pasparakis, G., Stimuli responsive self-healing polymers: gels, elastomers and membranes. Polymer Chemistry 2017, 8 (42), 6464-6484. 5. Arslan, M.; Kiskan, B.; Yagci, Y., Recycling and Self-Healing of Polybenzoxazines with Dynamic Sulfide Linkages. Sci Rep-Uk 2017, 7 (1), 1-11. 6. Takahashi, A.; Goseki, R.; Ito, K.; Otsuka, H., Thermally Healable and Reprocessable Bis(hindered amino)disulfide-Cross-Linked Polymethacrylate Networks. ACS Macro Letters 2017, 6 (11), 1280-1284. 164 7. Thiounn, T.; Lauer, M. K.; Bedford, M. S.; Smith, R. C.; Tennyson, A. G., Thermally- Healable Network Solids of Sulfur-Crosslinked Poly(4-allyloxystyrene). RCS Advances 2018, 8, 39074-39082. 8. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J., The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat Chem 2013, 5 (6), 518-24. 9. Hasell, T.; Smith, J.; Green, S.; Petcher, S.; Parker, D.; Zhang, B.; Worthington, M. J. H.; Wu, X.; Kelly, C.; Baker, T.; Gibson, C.; Campbell, J.; Lewis, D.; Jenkins, M.; Willcock, H.; Chalker, J. M., Crosslinker Copolymerization for Property Control in Inverse Vulcanization. Chemistry - A European Journal 2019, 25, 10433-10440. 10. Parker, D. J.; Jones, H. A.; Petcher, S.; Cervini, L.; Griffin, J. M.; Akhtar, R.; Hasell, T., Low cost and renewable sulfur-polymers by inverse vulcanization, and their potential for mercury capture. Journal of Materials Chemistry A: Materials for Energy and Sustainability 2017, 5 (23), 11682-11692. 11. Zhang, Y.; Pavlopoulos, N. G.; Kleine, T. S.; Karayilan, M.; Glass, R. S.; Char, K.; Pyun, J., Nucleophilic Activation of Elemental Sulfur for Inverse Vulcanization and Dynamic Covalent Polymerizations. Journal of Polymer Science, Part A: Polymer Chemistry 2019, 57 (1), 7-12. 12. Lin, H.-K.; Lai, Y.-S.; Liu, Y.-L., Cross-Linkable and Self-Foaming Polysulfide Materials for Repairable and Mercury Capture Applications. ACS Sustainable Chemistry & Engineering 2019, 7 (4), 4515-4522. 165 13. Wreczycki, J.; Bielinski, D. M.; Anyszka, R., Sulfur/organic copolymers as curing agents for rubber. Polymers (Basel, Switzerland) 2018, 10 (8), 870/1-870/20. 14. Zhang, Y.; Konopka, K. M.; Glass, R. S.; Char, K.; Pyun, J., Chalcogenide hybrid inorganic/organic polymers via inverse vulcanization and dynamic covalent polymerizations. Polymer Chemistry 2017, 8 (34), 5167-5173. 15. Zhang, Y.; Griebel, J. J.; Dirlam, P. T.; Nguyen, N. A.; Glass, R. S.; MacKay, M. E.; Char, K.; Pyun, J., Inverse vulcanization of elemental sulfur and styrene for polymeric cathodes in Li-S batteries. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (1), 107-116. 16. Diez, S.; Hoefling, A.; Theato, P.; Pauer, W., Mechanical and electrical properties of sulfur-containing polymeric materials prepared via inverse vulcanization. Polymers (Basel, Switzerland) 2017, 9 (2), 59/1-59/16. 17. Herrera, C.; Ysinga Kristen, J.; Jenkins Courtney, L., Polysulfides Synthesized from Renewable Garlic Components and Repurposed Sulfur Form Environmentally Friendly Adhesives. ACS applied materials & interfaces 2019, 11 (38), 35312-35318. 18. Mann, M.; Kruger, J. E.; Andari, F.; McErlean, J.; Gascooke, J. R.; Smith, J. A.; Worthington, M. J. H.; McKinley, C. C. C.; Campbell, J. A.; Lewis, D. A.; Hasell, T.; Perkins, M. V.; Chalker, J. M., Sulfur polymer composites as controlled-release fertilizers. Organic & Biomolecular Chemistry 2019, 17 (7), 1929-1936. 19. Lundquist, N. A.; Worthington, M. J. H.; Adamson, N.; Gibson, C. T.; Johnston, M. R.; Ellis, A. V.; Chalker, J. M., Polysulfides made from re-purposed waste are sustainable materials for removing iron from water. RSC Advances 2018, 8 (3), 1232- 1236. 166 20. Hoefling, A.; Lee, Y. J.; Theato, P., Sulfur-Based Polymer Composites from Vegetable Oils and Elemental Sulfur: A Sustainable Active Material for Li-S Batteries. Macromolecular Chemistry and Physics 2017, 218 (1), n/a. 21. Qin, X.; He, Y.; Khan, S.; Zhang, B.; Chen, F.; Dong, D.; Wang, Z.; Zhang, L., Controllable Synthesis and Characterization of Soybean-Oil-Based Hyperbranched Polymers via One-Pot Method. ACS Sustainable Chemistry & Engineering 2018, 6 (10), 12865-12871. 22. Valle, S. F.; Giroto, A. S.; Klaic, R.; Guimaraes, G. G. F.; Ribeiro, C., Sulfur fertilizer based on inverse vulcanization process with soybean oil. Polymer Degradation and Stability 2019, 162, 102-105. 23. Worthington, M. J. H.; Shearer, C. J.; Esdaile, L. J.; Campbell, J. A.; Gibson, C. T.; Legg, S. K.; Yin, Y.; Lundquist, N. A.; Gascooke, J. R.; Albuquerque, I. S.; Shapter, J. G.; Andersson, G. G.; Lewis, D. A.; Bernardes, G. J. L.; Chalker, J. M., Sustainable Polysulfides for Oil Spill Remediation: Repurposing Industrial Waste for Environmental Benefit. Advanced Sustainable Systems 2018, 2, 1800024. 24. Worthington, M. J. H.; Kucera, R. L.; Albuquerque, I. S.; Gibson, C. T.; Sibley, A.; Slattery, A. D.; Campbell, J. A.; Alboaiji, S. F. K.; Muller, K. A.; Young, J.; Adamson, N.; Gascooke, J. R.; Jampaiah, D.; Sabri, Y. M.; Bhargava, S. K.; Ippolito, S. J.; Lewis, D. A.; Quinton, J. S.; Ellis, A. V.; Johs, A.; Bernardes, G. J. L.; Chalker, J. M., Laying Waste to Mercury: Inexpensive Sorbents Made from Sulfur and Recycled Cooking Oils. Chem. - Eur. J. 2017, 23 (64), 16106. 167 25. Crockett, M. P.; Evans, A. M.; Worthington, M. J. H.; Albuquerque, I. S.; Slattery, A. D.; Gibson, C. T.; Campbell, J. A.; Lewis, D. A.; Bernardes, G. J. L.; Chalker, J. M., Sulfur-Limonene Polysulfide: A Material Synthesized Entirely from Industrial By- Products and Its Use in Removing Toxic Metals from Water and Soil. Angewandte Chemie, International Edition 2016, 55 (5), 1714-1718. 26. Fu, C.; Li, G.; Zhang, J.; Cornejo, B.; Piao, S. S.; Bozhilov, K. N.; Haddon, R. C.; Guo, J., Electrochemical Lithiation of Covalently Bonded Sulfur in Vulcanized Polyisoprene. ACS Energy Letters 2016, 1 (1), 115-120. 27. Hoefling, A.; Nguyen, D. T.; Lee, Y. J.; Song, S.-W.; Theato, P., A sulfur-eugenol allyl ether copolymer: a material synthesized via inverse vulcanization from renewable resources and its application in Li-S batteries. Materials Chemistry Frontiers 2017, 1 (9), 1818-1822. 28. Karunarathna, M. S.; Lauer, M. K.; Thiounn, T.; Smith, R. C.; Tennyson, A. G., Valorization of waste to yield recyclable composites of elemental sulfur and lignin. Journal of Materials Chemistry A: Materials for Energy and Sustainability 2019, 7 (26), 15683-15690. 29. Lauer, M. K.; Estrada-Mendoza, T. A.; McMillen, C. D.; Chumanov, G.; Tennyson, A. G.; Smith, R. C., Durable Cellulose–Sulfur Composites Derived from Agricultural and Petrochemical Waste. Advanced Sustainable Systems 2019, 3 (10), 1900062. 30. Smith, A. D.; McMillin, C. D.; Smith, R. C.; Tennyson, A. G., Copolymers by Inverse Vulcanization of Sulfur with Pure or Technical Grade Unsaturated Fatty Acids. Journal of Polymer Science 2020, 58, 438-445. 168 31. Smith, A. D.; Thiounn, T.; Lyles, E. W.; Kibler, E. K.; Smith, R. C.; Tennyson, A. G., Combining Agriculture and Energy Industry Waste Products to Yield Recyclable, Thermally Healable Copolymers of Elemental Sulfur and Oleic Acid. Journal of Polymer Science Part A 2019, 57, 1704-1710. 32. Thiounn, T.; Tennyson, A. G.; Smith, R. C., Durable, Acid-Resistant Copolymers from Industrial By-Product Sulfur and Microbially-Produced Tyrosine. RSC Advances 2019, 9, 31460-31465. 33. Oishi, S.; Oi, K.; Kuwabara, J.; Omoda, R.; Aihara, Y.; Fukuda, T.; Takahashi, T.; Choi, J.-C.; Watanabe, M.; Kanbara, T., Synthesis and Characterization of Sulfur-Based Polymers from Elemental Sulfur and Algae Oil. ACS Applied Polymer Materials 2019, 1 (5), 1195-1202. 34. Namnabat, S.; Gabriel, J. J.; Pyun, J.; Norwood, R. A.; Dereniak, E. L.; van der Laan, J., Sulfur copolymers for infrared optical imaging. Proceedings of SPIE 2014, 9070 (Pt. 2, Infrared Technology and Applications XL), 90702H/1-90702H/7. 35. Griebel, J. J.; Namnabat, S.; Kim, E. T.; Himmelhuber, R.; Moronta, D. H.; Chung, W. J.; Simmonds, A. G.; Kim, K.-J.; van der Laan, J.; Nguyen, N. A.; Dereniak, E. L.; MacKay, M. E.; Char, K.; Glass, R. S.; Norwood, R. A.; Pyun, J., New Infrared Transmitting Material via Inverse Vulcanization of Elemental Sulfur to Prepare High Refractive Index Polymers. Advanced Materials (Weinheim, Germany) 2014, 26 (19), 3014-3018. 169 36. Chen, Z.; Droste, J.; Zhai, G.; Zhu, J.; Yang, J.; Hansen, M. R.; Zhuang, X., Sulfur- anchored azulene as a cathode material for Li-S batteries. Chemical Communications (Cambridge, United Kingdom) 2019, 55 (61), 9047-9050. 37. Zhao, F.; Li, Y.; Feng, W., Recent Advances in Applying Vulcanization/Inverse Vulcanization Methods to Achieve High-Performance Sulfur-Containing Polymer Cathode Materials for Li-S Batteries. Small Methods 2018, 2 (11), 1-34. 38. Abraham, A. M.; Kumar, S. V.; Alhassan, S. M., Porous sulphur copolymer for gas- phase mercury removal and thermal insulation. Chemical Engineering Journal (Amsterdam, Netherlands) 2018, 332, 1-7. 39. Akay, S.; Kayan, B.; Kalderis, D.; Arslan, M.; Yagci, Y.; Kiskan, B., Poly(benzoxazine-co-sulfur): An efficient sorbent for mercury removal from aqueous solution. Journal of Applied Polymer Science 2017, 134 (38), n/a. 40. Hasell, T.; Parker, D. J.; Jones, H. A.; McAllister, T.; Howdle, S. M., Porous inverse vulcanized polymers for mercury capture. Chemical Communications (Cambridge, United Kingdom) 2016, 52 (31), 5383-5386. 41. Extension and Replacement of Asphalt and Cement with Sulfur. Transportation, D. o., Ed. Washington, D.C. USA, 1978. 42. Gregor, R.; Hackl, A., A new approach to sulfur concrete. Advances in Chemistry Series 1978, 165 (New Uses Sulfur-2), 54-78. 43. McBee, W. C.; Sullivan, T. A., Sulfur utilization in asphalt paving materials. Advances in Chemistry Series 1978, 165 (New Uses Sulfur-2), 135-60. 170 44. Vroom, A. H., Sulfur polymer concrete and its applications. Polym. Concr., Int. Congr., 7th 1992, 606-21. 45. Yu, S.; Kwon, H.; Noh, H. R.; Park, B.-I.; Park, N. K.; Choi, H.-J.; Choi, S.-C.; Kim, G. D., Preparation and characterization of a new cement-based composite with sulfur polymer. RSC Advances 2015, 5 (45), 36030-36035. 46. Ksiazek, M. M. K., Evaluation of acid corrosion resistance of Portland cement composites impregnated with polymer sulfur composite. Anti-Corrosion Methods and Materials 2017, 64 (3), 1-15. 47. Karunarathna, M. S.; Lauer, M. K.; Tennyson, A. G.; Smith, R. C., Copolymerization of an Aryl Halide and Elemental Sulfur as a Route to High Sulfur Content Materials. Polymer Chemistry 2020, early view. 48. Karunarathna, M. S.; Tennyson, A. G.; Smith, R. C., Facile New Approach to High Sulfur-Content Materials and Preparation of Sulfur-Lignin Copolymers. Journal of Materials Chemistry A 2020, 8, 548-553. 49. Hortling, B., Thermal resistance of crosslinked and substituted poly(phenylene sulfide)s. Die Makromolekulare Chemie 1977, 178 (8), 2185-2196. 50. Goud, D. R.; Pardasani, D.; Purohit, A. K.; Tak, V.; Dubey, D. K., Method for Derivatization and Detection of Chemical Weapons Convention Related Sulfur Chlorides via Electrophilic Addition with 3-Hexyne. Analytical Chemistry (Washington, DC, United States) 2015, 87 (13), 6875-6880. 171 51. Khongprom, P.; Suwanmanee, U., Environmental benefits of the integrated alternative technologies of the portland cement production: case study in Thailand. Engineering Journal 2017, 21 (7), 15-27. 52. Chen, C.; Habert, G.; Bouzidi, Y.; Jullien, A., Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. Journal of Cleaner Production 2010, 18 (5), 478-485. 53. Weber, H. H., New Applications and Expanding Markets for Sulphur Polymer Cement Concrete. American Concrete Institute 1993, SP-137, 49-72. 54. Weber, H. H.; McBee, W. C. In New market opportunities for sulphur asphalt; Sulphur markets today and tomorrow, Biennial International Symposium , 7th, Washington, D.C. , Biennial International Symposium , 7th: Washington, D.C. , 2000; p 24. 55. López, M. d. M. C.; Ares Pernas, A. I.; Abad López, M. J.; Latorre, A. L.; López Vilariño, J. M.; González Rodríguez, M. V., Assessing changes on poly(ethylene terephthalate) properties after recycling: Mechanical recycling in laboratory versus postconsumer recycled material. Materials Chemistry and Physics 2014, 147 (3), 884- 894. 172 CHAPTER FIVE SEQUENTIAL CROSSLINKING FOR MECHANICAL PROPERTY DEVELOPMENT IN HIGH SULFUR-CONTENT COPOLYMERS5 Abstract Herein we demonstrate how sequential crosslinking processes can be applied to develop properties in sulfur-bisphenol A copolymers. Olefinic carbons were first crosslinked by inverse vulcanization at 180 °C and then aryl carbon crosslinking was affected at via radical-induced aryl halide-sulfur polymerization (RASP) at 220 °C. Introduction High sulfur content copolymers can be conveniently prepared by the reaction of elemental sulfur with olefins or aryl halides by inverse vulcanization (InV)1-7 or radical- induced aryl halide-sulfur polymerization (RASP), 8, 9 respectively (Scheme 5.1). The InV mechanism proceeds when olefins are crosslinked by their reaction with thermally- generated sulfur radicals, generally at temperature of 90–180 °C.10-20 RASP involves thermal reaction of aryl halides with elemental sulfur whereby S–Caryl bonds are formed, a process requiring higher temperatures of 220–250 °C. In the current work, it was hypothesized that an olefin-derivatized aryl halide monomer could undergo sequential crosslinking by InV at lower temperatures to facilitate S–Colefin crosslinking, followed by 5 This text is reproduced with permission in larger part from: Thiounn, T.; Slann, L. M.; Lauer, M. K.; Smith, R. C., Sequential Crosslinking for Mechanical Property Development in High Sulfur Content Copolymers. Submitted to Chemical Communications, 2020. 173 RASP at higher temperatures to increase crosslink density through S–Caryl bond formation. Thermal control of sequential crosslinking mechanisms in this way would thus allow stepwise development of thermal and mechanical properties. Given that O-allyl phenols are established substrates for InV,18 while 2,2’,6,6’-tetra-bromobisphenol A 21 (Br4BPA) has proven effective in RASP, O, O’-diallyl-2,2’,5,5’-tetra-bromobisphenol A (M1, Scheme 5.2) was selected as the test monomer for the current study. A copolymer of sulfur and M1 comprising 80 wt% sulfur was targeted for direct comparison to other polymers prepared by RASP, which demonstrated optimal properties for this composition.8, 9 Monomer M1 and elemental sulfur were thus subjected to sequential crosslinking as delineated in Scheme 5.2. A) Inverse Vulcanization of Alkenes (Pyun, 2013) B) RASP of Aryl Halides (Karunarathna and Smith, 2019) Scheme 5.1 Simplified schematics for inverse vulcanization (A) and RASP (B) routes to high sulfur content copolymers. 174 Scheme 5.2 Crosslinking of olefinic carbons by inverse vulcanization produces I-BPA80. At higher temperatures, crosslinking of aryl carbons by RASP can be used to produce R- BPA80. 175 Figure 5.1. Photographs of I-BPA80 demonstrate its flexibility (A) colour and transparency (B). Tensile testing demonstrates the elasticity of I-BPA80 as it is elongated to nearly twice its initial length just before breaking (C). More highly crosslinked R- BPA80 is a darker colour solid that can be elongated by less than 30% of its initial length prior to breaking (D). 176 Results and Discussion Monomer M1 was first subjected to InV by its addition to molten sulfur at 180 °C and mixing for 30 min to yield copolymers I-BPA80. Visually, I-BPA80 is a flexible, remeltable solid with the transparent deep orange-red color characteristic of polymeric sulfur (Figure 5.1). Initially characterization of I-BPA80 was undertaken by elemental microanalysis, infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to assess their composition (Table 5.1 and Figures S5.3–S5.5 provided at the end of this chapter). Elemental analysis for C, H, N, S and Br confirmed that no detectable loss of bromine occurred during the InV process, within the error of the analysis. Having crosslinked olefins by InV, samples of I-BPA80 were subsequently heated to 220 °C to facilitate crosslinking of aryl carbons by RASP. Heating for 1 h produced R- BPA80, in which 40% of available bromides had been successfully displaced in the RASP process, as assessed by elemental analysis. Note that, as shown in Scheme 1B, this reaction produces S2X2, which is toxic and corrosive, so caution should be exercised when performing the RASP reaction. When the samples were heated for longer times (1.5–3 h) in an effort to push the RASP reaction to completion, the molten samples took on a very viscous, tar-like consistency that made it intractable for shaping into samples for tensile measurements. Efforts to heat pre-shaped samples were likewise unsuccessful because bubbling, presumably of S2Br2 produced by the RASP process, led to the samples losing their shape. The sample that had been heated for 1 h was therefore used 177 for subsequent analysis. Although R-BPA80 was still remeltable, it had a darker color and was much less flexible than I-BPA80 (Figure 5.1D). R-BPA80 was characterized by elemental microanalysis, IR spectroscopy, TGA and DSC (see Figures S5.6–S5.8 provided at the end of this chapter). I-BPA80 R-BPA80 a Td (°C) 225 224 Char Yield (%) 8% 9% b Tg (°C) –20 –27 c Tcc (°C) NA 40–80 d Tm (°C) 115 110 Ultimate Tensile Strengthe (MPa) 1.04 ± 0.16 3.0 ± 0.4 Elongation at Breake (%) 89 ± 9 32 ± 4 a Determined from the onset of the major mass loss peak. b Determined from the onset of transition from the third heating cycle of the DSC curve. c Temperature at which cold crystallization peaks occur. d Temperature at peak minimum for melting of the sample. e Value determined from the average of five runs. Table 5.1 Comparison of properties after each crosslinking reaction. 178 Figure 5.2 SEM images of I-BPA80 (A) and R-BPA80 (B) and EDX images for I-BPA80 (C–F) and R-BPA80 (G–J) demonstrating the distribution of sulfur (red), carbon (cyan), oxygen (violet) and bromine (green) in the materials. 179 A) B) Figure 5.3 DSC analysis of I-BPA80 (A) and R-BPA80 (B) reveal significant changes in morphology after the second crosslinking. The third heating and cooling curves are shown. 180 Both I-BPA80 and R-BPA80 were also readily cut or remelted and poured into silicone molds to form dog bone-shaped test items (See Figure S5.1) appropriate for tensile strength measurements (Figures 5.1C–5.1D). The uniform distribution of sulfur, carbon and bromine was also confirmed for both materials by scanning electron microscopy (SEM) with element mapping by energy dispersive X-ray analysis (EDX, Figure 5.2). The decomposition temperatures (Td) for I-BPA80 (225 °C) and R-BPA80 (224 °C) are essentially identical and attributable to decomposition of sulfur domains that comprise the majority of each copolymer. More notable changes in the material following the second crosslinking step are evident in the DSC data (Figure 5.3). Both show the sulfur melt transition with maxima at 110–115 °C, whereas the glass transitions changes from –20 °C for I-BPA80 to –27 °C for R-BPA80. R-BPA80 also exhibits a broad cold crystallization exotherm over a temperature range of 40–80 °C (Tcc) that is absent in the DSC thermogram for I-BPA80. The greater propensity to form crystalline domains within the material may be attributable to the greater pre-ordering of the material induced by the crosslinking or to reorganization of the network to exclude polymeric domains of sulfur, allowing for more orthorhombic sulfur to be trapped in the material. The latter hypothesis is further supported by the change in color of the material as well as the much larger sulfur melting enthalpy in I-BPA80 (3.0 J/g) compared to R-BPA80 (16.8 J/g) in the from DSC data. The difference in mechanical properties between the two materials is perhaps the most obvious. Whereas I-BPA80 is a flexible solid, R-BPA80 is rigid. Inversely 181 vulcanized I-BPA80 had an ultimate tensile strength of 1.04±0.16 MPa (average and standard deviation for five samples) and an 89% elongation at break. Following the increased crosslinking via RASP, R-BPA80 showed a three-fold improvement in ultimate tensile strength (3.0±0.5 MPa, average and standard deviation for five samples) and was much less elastic as evident from the 32% elongation at break. The decrease in elasticity and increased tensile strength are consistent with the increased crosslinking in R-BPA80. The ultimate tensile strengths of I-BPA80 and R-BPA80 are reasonable compared to those reported for other high sulfur content copolymers. For example, copolymers of sulfur (50 wt%) and dicyclopentadiene with either linseed oil, ethylene glycol dimethyl acrylate or limonene exhibit an ultimate tensile strength of 1.5–4.5 MPa.22 Higher tensile strengths of up to 17.5 MPa can be attained when a larger proportion of organic crosslinker (30 wt% sulfur) is utilized to prepare polymers by inverse vulcanization of 1,3- diisopropenylbenzene.23 All of these sulfur copolymers, however, exhibit generally lower tensile strengths than familiar polymers like polyethylene (up to 32 MPa).24 Both I- BPA80 and R-BPA80 do, however, exhibit quite high elongation at break compared to the aforementioned high sulfur content copolymers, which show elongations at break <10%. The elongation at break for common polymers span a wide range, from quite low for polystyrene (<2%) to over 200% for polycarbonate. 182 Conclusions In conclusion, the first demonstration of combining inverse vulcanization and RASP mechanism is reported as a way to sequentially develop the mechanical properties of high sulfur content copolymers. In this case, the inversely vulcanized I-BPA80 is a flexible solid, whereas after further crosslinking by RASP, R-BPA80 is a rigid solid with three times the ultimate tensile strength of I-BPA80. Conflicts of interest There are no conflicts to declare Acknowledgements Funding for this project from the National Science Foundation (CHE-1708844) is gratefully acknowledged. Keywords: Inverse vulcanization • Radical induced polymerization • RASP • crosslinking 183 Supporting Information Experimental Details All NMR spectra were recorded on a Bruker Avance spectrometer operating at 300 MHz for proton nuclei. Fourier transform infrared spectra were obtained using a Shimadzu IR Affinity-1S instrument operating over the range of 400-4000 cm–1 at ambient temperature. Thermogravimetric analysis (TGA) was recorded on a Mettler Toledo TGA 2 STARe System over the range of 25 to 800 °C with a heating rate of 5 °C –1 -1 min under a flow of N2 (20 mL min ). Differential Scanning Calorimetry (DSC) was acquired using a Mettler Toledo DSC 3 STARe System over the range of –60 to 140 °C, –1 –1 with a heating rate of 5 °C min under a flow of N2 (200 mL min ). Each DSC measurement was carried out over 3 heat-cool cycles to get rid of any thermal history. The data reported were taken from the third cycle of the experiment. Fourier transform infrared spectroscopy were taken on a Shimdazu IR Affinity-1S instrument with an ATR attachement, operating between the range of 400-4000 cm-1 at room temperature. Tensile measurements were taken with a Mark-10 ES30 Manual Test Stand equipped with a Mark 10 Force Gauge (Model M3-2 or Model M3-200). The tensile specimens were clamped down with a Mark 10 wedge grip (Model G1061-1). Rigid samples like R- BPA80 had the wedge grips wrapped with a rubber band to prevent the clamp from pre- stressing the sample. Tensile specimens were cut or molded into dog bone shapes with dimensions as depicted in Figure S1. I-BPA80 samples were approximately 1.2 mm in 184 thickness where R-BPA80 samples were approximately 2.5 mm in thickness. The data reported was taken as an average of five runs. Figure S5.1 Dog bone samples with dimensions used for tensile testing. Materials and Methods Tetrabromobisphenol A (>98%, TCI America), elemental sulfur (99.5%, Alfa Aesar), allyl bromide (99% Alfa Aesar), sodium hydroxide (≥97% VWR Chemicals) and acetonitrile (≥99.5% VWR Chemicals) were used without further purification. Synthesis of M1 To a 1000 mL round bottom flask equipped with a Teflon-coated magnetic stir bar was added 75.857 g (0.139 mol) of tetrabromobisphenol a and 500 mL of acetonitrile, 185 the solution was stirred until complete dissolution of tetrabromobisphenol a followed by the addition of 16.611 g (0.415 mol) of NaOH. 42 g (0.347 mol) of allyl bromide was added drop-wise using an addition funnel, following complete addition of allyl bromide the flask was then equipped with a water-cooled reflux condenser and placed in a 60 °C oil bath and stirred for 24 h. The flask was cooled to room temperature and 250 mL of 0.5 M HCl(aq) was added to the reaction mixture and extracted with dichloromethane (4 × 150 mL). The organic layer was collected and washed with deionized water (4 × 200 mL) and the solvent was evaporated under reduced pressure. The product was collected as an off white solid (83.418 g, 96%). Synthesis of I-BPA80 CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. Elemental sulfur (8.0 g, 0.031 mol) was weighed directly into a 20 mL scintillation vial. The vessel was placed in a thermostat-controlled oil bath set to 180 °C. After 5 minutes, the sulfur had visibly melted and became a dark orange color. M1 (2.0 g, 0.003 mol) was added to the vial and the reaction media was vigorously stirred with a Teflon coated stir bar until the reaction mixture became homogenous (3 min), at which point the reaction solution was transferred to an aluminium pan in a 160 °C oven to cure for 30 min. The sample was then taken out of the oven and allowed to cool and cut into tensile samples. Elemental analysis calculated: C, 8.08; S, 80.00; H, 0.65; Br, 10.2%. Found: C, 7.66; S, 81.08; H, 0.46; Br, 10.69%. Yield (99%). 186 Synthesis of R-BPA80 CAUTION: Heating elemental sulfur with organics can result in the formation of H2S gas. H2S is toxic, foul-smelling, and corrosive. After the I-BPA80 tensile samples had been tested, approximately 3.0 g of I-BPA80 was placed into a 20 mL scintillation vial and placed into a 220 °C oven for 1 h after which the sample was taken out and poured into dog bone samples. Elemental analysis calculated: C, 8.08; S, 80.00; H, 0.65; Br, 10.2%. Found: C, 8.77; S, 82.50; H, 0.62; Br, 6.48%. Yield (98%). 187 7.319 7.317 7.281 7.279 6.258 6.241 6.223 6.204 6.184 6.166 6.149 6.130 5.519 5.517 5.514 5.460 5.457 5.345 5.310 4.574 4.556 2.061 2.031 1.619 1.567 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 ppm 5.92 2.09 2.21 2.09 4.31 6.00 Figure S5.2 Proton NMR of M1 in CDCl3. 188 80 75 70 65 60 55 50 % Transmittance %Transmittance 45 40 35 30 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Figure S5.3 Infrared spectrum of I-BPA80. 189 80 75 70 65 60 55 50 % Transmittance %Transmittance 45 40 35 30 1100 1000 900 800 700 600 500 Wavenumber (cm-1) -1 Figure S5.4 Inset of IR spectrum between 500–1100 cm of I-BPA80. 190 100 80 60 wt % wt 40 20 0 0 100 200 300 400 500 600 700 800 Temperature (°C) Figure S5.5 Thermogravimetric analysis curve of I-BPA80. 191 80 75 70 65 60 55 50 % Transmittance %Transmittance 45 40 35 30 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Figure S5.6 Infrared spectrum of R-BPA80. 192 80 75 70 65 60 55 50 % Transmittance %Transmittance 45 40 35 30 1100 1000 900 800 700 600 500 Wavenumber (cm-1) -1 Figure S5.7 Inset of IR spectrum between 500–1100 cm of R-BPA80. 193 100 80 60 wt % wt 40 20 0 0 100 200 300 400 500 600 700 800 Temperature (°C) Figure S5.8 Thermogravimetric analysis curve of R-BPA80. 194 I-BPA80 R-BPA80 Trial Ultimate % Ultimate % Tensile Strength Elongation at Tensile Strength Elongation at (MPa) Break (MPa) Break 1 0.82 73 2.9 32 2 1.01 89 2.4 27 3 1.21 98 2.9 37 4 1.18 92 3.7 32 5 0.99 91 3.0 28 Table S5.1 Tensile data for each trial of I-BPA80 and R-BPA80. 195 References 1. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J., The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat Chem 2013, 5 (6), 518-24. 2. Zhang, Y.; Glass, R. S.; Char, K.; Pyun, J., Recent advances in the polymerization of elemental sulphur, inverse vulcanization and methods to obtain functional Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs). Polymer Chemistry 2019, 10 (30), 4078-4105. 3. Chalker, J. M.; Worthington, M. J. H.; Lundquist, N. A.; Esdaile, L. J., Synthesis and Applications of Polymers Made by Inverse Vulcanization. Topics in Current Chemistry 2019, 377 (3), 1-27. 4. Worthington, M. J. H.; Kucera, R. L.; Chalker, J. M., Green chemistry and polymers made from sulfur. Green Chemistry 2017, 19 (12), 2748-2761. 5. Esdaile Louisa, J.; Chalker Justin, M., The Mercury Problem in Artisanal and Small- Scale Gold Mining. Chemistry (Weinheim an der Bergstrasse, Germany) 2018, 24 (27), 6905-6916. 6. Worthington, M. J. H.; Kucera, R. L.; Albuquerque, I. S.; Gibson, C. T.; Sibley, A.; Slattery, A. D.; Campbell, J. A.; Alboaiji, S. F. K.; Muller, K. A.; Young, J.; Adamson, N.; Gascooke, J. R.; Jampaiah, D.; Sabri, Y. M.; Bhargava, S. K.; Ippolito, S. J.; Lewis, D. A.; Quinton, J. S.; Ellis, A. V.; Johs, A.; Bernardes, G. J. 196 L.; Chalker, J. M., Laying Waste to Mercury: Inexpensive Sorbents Made from Sulfur and Recycled Cooking Oils. Chemistry - A European Journal 2017, 23 (64), 16106. 7. Crockett, M. P.; Evans, A. M.; Worthington, M. J. H.; Albuquerque, I. S.; Slattery, A. D.; Gibson, C. T.; Campbell, J. A.; Lewis, D. A.; Bernardes, G. J. L.; Chalker, J. M., Sulfur-Limonene Polysulfide: A Material Synthesized Entirely from Industrial By-Products and Its Use in Removing Toxic Metals from Water and Soil. Angewandte Chemie, International Edition 2016, 55 (5), 1714-1718. 8. Karunarathna, M. S.; Tennyson, A. G.; Smith, R. C., Facile New Approach to High Sulfur-Content Materials and Preparation of Sulfur-Lignin Copolymers. Journal of Materials Chemistry A 2020, 8, 548-553. 9. Karunarathna, M. S.; Lauer, M. K.; Tennyson, A. G.; Smith, R. C., Copolymerization of an Aryl Halide and Elemental Sulfur as a Route to High Sulfur Content Materials. Polymer Chemistry 2020, Early View. 10. Zhang, B.; Gao, H.; Yan, P.; Petcher, S.; Hasell, T., Inverse vulcanization below the melting point of sulfur. Materials Chemistry Frontiers 2020, 4 (2), 669-675. 11. Zhang, B.; Petcher, S.; Hasell, T., A ternary system for delayed curing inverse vulcanisation. Chemical Communications (Cambridge, United Kingdom) 2019, 55 (72), 10681-10684. 12. Wu, X.; Smith Jessica, A.; Petcher, S.; Zhang, B.; Parker Douglas, J.; Hasell, T.; Griffin John, M., Catalytic inverse vulcanization. Nature communications 2019, 10 (1), 647. 197 13. Smith, A. D.; McMillin, C. D.; Smith, R. C.; Tennyson, A. G., Copolymers by Inverse Vulcanization of Sulfur with Pure or Technical Grade Unsaturated Fatty Acids. Journal of Polymer Science 2020, 58, 438-445. 14. Thiounn, T.; Tennyson, A. G.; Smith, R. C., Durable, Acid-Resistant Copolymers from Industrial By-Product Sulfur and Microbially-Produced Tyrosine. RSC Advances 2019, 9, 31460-31465. 15. Smith, A. D.; Thiounn, T.; Lyles, E. W.; Kibler, E. K.; Smith, R. C.; Tennyson, A. G., Combining Agriculture and Energy Industry Waste Products to Yield Recyclable, Thermally Healable Copolymers of Elemental Sulfur and Oleic Acid. Journal of Polymer Science Part A 2019, 57, 1704-1710. 16. Lauer, M. K.; Estrada-Mendoza, T. A.; McMillen, C. D.; Chumanov, G.; Tennyson, A. G.; Smith, R. C., Durable Cellulose–Sulfur Composites Derived from Agricultural and Petrochemical Waste. Advanced Sustainable Systems 2019, 3 (10), 1900062. 17. Karunarathna, M. S.; Lauer, M. K.; Thiounn, T.; Smith, R. C.; Tennyson, A. G., Valorization of waste to yield recyclable composites of elemental sulfur and lignin. Journal of Materials Chemistry A: Materials for Energy and Sustainability 2019, 7 (26), 15683-15690. 18. Thiounn, T.; Lauer, M. K.; Bedford, M. S.; Smith, R. C.; Tennyson, A. G., Thermally-Healable Network Solids of Sulfur-Crosslinked Poly(4-allyloxystyrene). RCS Advances 2018, 8, 39074-39082. 198 19. Herrera, C.; Ysinga Kristen, J.; Jenkins Courtney, L., Polysulfides Synthesized from Renewable Garlic Components and Repurposed Sulfur Form Environmentally Friendly Adhesives. ACS applied materials & interfaces 2019, 11 (38), 35312-35318. 20. Westerman, C. R.; Jenkins, C. L., Dynamic Sulfur Bonds Initiate Polymerization of Vinyl and Allyl Ethers at Mild Temperatures. Macromolecules (Washington, DC, United States) 2018, 51 (18), 7233-7238. 21. Thiounn, T.; Lauer, M. K.; Tennyson, A. G.; Smith, R. C., Copolymerization of a Bisphenol A Derivative and Elemental Sulfur by the RASP Process. Submitted for publication 2020. 22. Smith, J. A.; Green, S. J.; Petcher, S.; Parker, D. J.; Zhang, B.; Worthington, M. J. H.; Wu, X.; Kelly, C. A.; Baker, T.; Gibson, C. T.; Campbell, J. A.; Lewis, D. A.; Jenkins, M. J.; Willcock, H.; Chalker, J. M.; Hasell, T., Crosslinker Copolymerization for Property Control in Inverse Vulcanization. Chemistry - A European Journal 2019, 25 (44), 10433-10440. 23. Griebel, J. J.; Nguyen, N. A.; Namnabat, S.; Anderson, L. E.; Glass, R. S.; Norwood, R. A.; MacKay, M. E.; Char, K.; Pyun, J., Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur for Healable Infrared Optical Materials. ACS Macro Lett. 2015, 4 (9), 862-866. 24. Crompton, T. R., Physical Testing of Plastics. Smithers Information Limited Shropshire, UK, 2012. 199