This chapter is concerned with the recent progress in cellulose-based thermoplastic plastics and elastomers via homogeneous controlled radical polymerizations (CRPs), including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP). The first section is a brief introduction of cellulose and cellulose graft copolymers. The second section is recent developments in cellulose graft copolymers synthesized by CRPs. The third part is a perspective on design and applications of novel cellulose-based materials. The combination of cellulose and CRPs can provide new opportunities for sustainable materials ranging from thermoplastics to elastomers, and these fascinating materials can find a pyramid of applications in our daily life in the near future.
- graft copolymers
- controlled radical polymerization
As the most abundant biomaterial on earth, cellulose has received enormous attention due to its wide applications in different fields, such as packaging , drug delivery , cosmetics , textiles , membranes , bioengineering , and electronics . Cellulose has some outstanding advantages, including low cost, non-toxicity, good mechanical properties, and excellent biodegradability and biocompatibility . However, cellulose is lack of thermoplasticity and shows poor dimensional stability and crease resistance. Due to the high crystallinity and presence of a large amount of intra- and inter-molecular hydrogen bonding, cellulose is difficult to be dissolved in common solvents . Different solvent systems have been proposed to dissolve cellulose, including
Cellulose graft copolymers, which combine cellulose and grafted side chains in one macromolecule can open new opportunities toward developing novel bio-based materials with tunable properties. Cellulose graft copolymers can be achieved with grafting-to, grafting-through, and grafting-from strategies . Among these methods, the grafting-from strategy is the most effective route for designing cellulose graft copolymers from thermoplastics to thermoplastic elastomers. Atom transfer radical polymerization (ATRP) [26, 27], reversible addition-fragmentation chain transfer (RAFT) [28, 29] polymerization, and nitroxide-mediated polymerization (NMP) [30, 31] are well-established controlled radical polymerizations (CRPs) that can be performed to prepare cellulose-based copolymers with well-defined structures and narrow molecular weight distributions in both heterogeneous and homogeneous conditions. This chapter summarizes recent advances that have been made in cellulose-based thermoplastics and elastomers from native cellulose via homogeneous CRPs.
2. Cellulose graft copolymers via homogeneous CRPs
2.1 Atom transfer radical polymerization (ATRP)
ATRP is the most used grafting method to prepare graft copolymers with cellulose-based macroinitiators and transition-metal catalysts. In general, the macroinitiators can be prepared by substituting the hydroxyl groups on cellulose backbone with chlorine or bromine-contained compounds. As shown in Figure 1, the macroinitiators, cellulose 2-bromopropionate (Cell-Bp), cellulose 2-bromoisobutyrylate (Cell-BiB), and cellulose chloroacetyl (Cell-ClAc) can be synthesized by reacting cellulose with 2-bromopropiomyl bromide, 2-bromoisobutyryl bromide, and chloroacetyl chloride in 1-allyl-3-methylimidazolium chloride (AMIMCl), 1-allyl-3-methylimidazolium chloride (BMIMCl) or DMAc/LiCl solution [32, 33, 34, 35, 36, 37] under homogeneous conditions. The solubility of cellulose-based macroinitiator is strongly related to the DS of acylation, which can be adjusted by the molar ratio of acylating agent/AGU and reaction time [35, 38, 39, 40]. Therefore, it is flexible and convenient to prepare cellulose graft copolymers with controlled grafting density. Different solvents have been utilized as the media to synthesize cellulose graft copolymers via ATRP, including DMF, DMSO, 1,4-dioxane, AMIMCl, and BMIMCl. Moreover, varied catalyst systems, such as copper(I) chloride/2,2′-bipyridine (CuCl/bpy), CuCl/tris(2-(dimethylamino)ethyl)amine) (CuCl/Me6TREN), copper(I) bromide/bpy (CuBr/bpy), CuCl/
ATRP can be performed to synthesize different kinds of thermoplastics and elastomeric polymers due to its high tolerance. As displayed in Figure 2, a variety of vinyl and acrylate monomers, including methyl methacrylate (MMA) [33, 35, 37, 41, 42, 43, 44],
Thermoplastics and elastomers are essential polymeric materials in our daily life, and they are commercially available and can be used in diverse areas, such as household goods, clothing, packaging, auto parts, electronics, sensors, drug delivery, seals, foods, and tissue engineering [63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73]. Cellulose is an interesting natural polymeric material with high strength and durability. The marriage of cellulose and synthetic polymers can promote the development of novel thermoplastics and elastomers with excellent thermal and mechanical properties. Cellulose chains can be used as rigid backbones to design high-performance elastomers. As shown in Figure 3, Cell-
Novel renewable polymers derived from resources have opened the door for sustainable science and engineering. Plant oils, which can be produced from different plants, such as palm, coconut, sunflower, olive, soy, and peanut, are typical natural resources for the chemical industry . By reacting with (meth)acryloyl chloride or methacrylate anhydride, plant oils can be transformed into polymerizable monomers for free radical polymerization. Recently, two soybean oil-based sustainable monomers, SOM1 and SOM2, were designed and used by Wang and coworkers to produce Cell-
It is convenient to graft polymers with apparently opposite physical properties by grafting from strategy via ATRP and thus to form self-induced nanostructures in bulk or in solutions. As shown in Figure 4, cellulose-
2.2 Reversible addition-fragmentation chain transfer (RAFT) polymerization
RAFT polymerization is an alternative method to synthesize well-defined and narrow distributed polymers with complex topological architectures by choosing a proper chain transfer agent (CTA). RAFT polymerization can be conducted without using any metal catalyst, and thus it is convenient and easy to purify the resultant polymers, which is the biggest advantage over ATRP. To date, only limited attention has been paid to cellulose graft copolymers through homogeneous RAFT polymerization in the literature [78, 79, 80, 81]. The combination of cellulose and RAFT polymerization can provide new opportunities for graft copolymers, especially those that could not be synthesized directly via ATRP strategy. Recently, our group designed a novel cellulose-based macromolecular chain transfer agent by introducing a trithiocarbonate derivative with dodecyl as stabilizing group on cellulose backbone from Cell-BiB for the synthesis of Cell-
2.3 Nitroxide-mediated polymerization (NMP)
NMP is the first and easiest CRP technology controlled by a reversible termination mechanism between the nitroxide moieties and growing propagating macroradicals. A wide range of monomers, including acrylates, styrene derivatives, vinylpyridines, acrylonitrile, acryl acid, acrylamide derivatives, cyclic ketene acetals, and miscellaneous, can be polymerized via NMP to develop well-defined polymers . The primary advantage of NMP is the absence of post-treatment since no catalyst or bimolecular exchange is needed during the polymerization. However, the higher polymerization temperatures and lower polymerization rates limit the wide applications of NMP. To the best of our knowledge, only one study reported the synthesis of cellulose graft copolymers through homogeneous NMP strategy started from microcrystalline cellulose . In this work, pretreated cellulose was dispersed in anhydrous tetrahydrofuran and reacted with 2-bromoisobutyryl bromide and pyridine to prepare Cell-BiB. As shown in Figure 6, Cell-BiB could react with 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) in DMF in the presence of sodium hydride (NaH) to obtain functionalized cellulose (Cell-TEMPOL). At first, PS was grafted from cellulose backbone with Cell-TEMPOL as macroinitiator in the absence of any catalyst to produce Cell-
CRPs are versatile and robust techniques to develop well-defined cellulose graft copolymers as novel thermoplastic and elastomers with desired properties for particular application demands. As the most adopted technique for cellulose graft copolymers, conventional ATRP suffers from the limitation of removing metal catalysts from the resultant products, which may cause undesired toxicity and coloration. Recently, new advances in ARTP have been reported, and metal-free ATRP has been developed to synthesize narrowly distributed polymers with well-defined structures by using photoredox organic catalysts under light irradiation instead of metal catalysts [84, 85, 86]. This new strategy can greatly promote the design and preparation of cellulose graft copolymers for different application fields. Cellulose graft copolymers show improved properties compared to the linear counterparts due to their unique molecular architectures, and the macroscopic performance of cellulose graft copolymers is affected by the degree of polymerization of cellulose backbone, the grafting density (the average number of grafts per anhydroglucose unit), the degree of polymerization, and distribution of grafts. Various block and random copolymers have been grafted from cellulose via CRPs as described above. However, as exhibited in Figure 7a, heterograft and graft-on-graft architectures with cellulose as backbone remain unreported, and these interesting graft copolymers can be synthesized by the combination of RAFT polymerization and ATRP, ring-opening polymerization (ROP) and ATRP or ROP and RAFT polymerization.
Due to the increased attention paid on biomass-derived thermoplastics and elastomers, it becomes necessary and desirable to explore new monomers, which can be synthesized from bioresources, including lignins, terpenes, plant oils, rosin acids, and coumarins. For instance, bioresources, 7-hydroxyl-4-methylcoumarin, vanillin, guaiacol, and oleyl alcohol, can be used to synthesize sustainable monomers by reacting with acryloyl chloride, methacryloyl chloride or methacrylate anhydride as shown in Figure 7b. Among these monomers, 7-acryloyloxy-4-methylcoumarin (AMC), 7-methacryloyloxy-4-methylcoumarin (MMC), acrylated vanillin (AV), methacrylated vanillin (MV), acrylated guaiacol (AG), and methacylated guaiacol (MG) can be utilized as rigid segments, while oleyl acrylate (OA) and oleyl methacrylate (OM) can be utilized as soft segments in the graft copolymers. Though there has been considerable progress, it is still challenging to achieve breakthroughs for the development of cellulose graft copolymers with ultra-strong mechanical properties comparable to commercial petroleum-based products. The marriage of cellulose and novel bio-based monomers via CRPs can provide a variety of opportunities for sustainable materials ranging from thermoplastics to elastomers, and these fascinating materials can find a pyramid of applications in our daily life in the near future.
This work was supported by the National Natural Science Foundation of China (Grants 51603199 and 21776049) and China Postdoctoral Science Foundation (Grant 2017M622629).
Conflict of interest
The authors declare no conflict of interest.