Myrcenol-Based Monomer for Carbanionic Polymerization: Functional Copolymers with Myrcene and Bio-Based Graft Copolymers

Abstract

A bio-based hydroxyl group-containing diene monomer, silyl-protected β-myrcenol (MyrOSi), is introduced to the field of carbanionic polymerization. Polymerization in cyclohexane, using sec-butyllithium as an initiator, resulted in homopolymers with well-controlled molecular weights in the range of 7.9−31.3 kg mol −1 and dispersities between 1.10 and 1.27. The silyl protective groups can be removed quantitatively under mild conditions, using tetra-n-butylammonium fluoride (TBAF), resulting in well-defined polymyrcenol. The statistical copolymerization of MyrOSi with β-myrcene was also investigated. The monomer sequence distribution of the copolymers was evaluated by 1 H NMR kinetic studies. Random copolymerization was observed for β-myrcene/MyrOSi copolymerization. Copolymers with varying MyrOH content, in the full range between 0 and 100 mol% MyrOH, were synthesized (Đ ≤ 1.11) and characterized with regard to their glass−transition temperatures and the polydiene microstructure. With an increasing MyrOH content in the polymer backbone, an increase in the number of 3,4-units was observed, resulting in an increase in T g from −67 to −23 °C. The P(Myr-co-MyrOH) copolymers are valuable building blocks for the synthesis of more complex polymer architectures, such as graft copolymers. The P(Myr-co-MyrOH) copolymers were used for the synthesis of graft copolymers consisting of a flexible polymyrcene backbone and poly(L-lactide) side chains using the "grafting from" approach. The graft copolymers are promising candidates for bio-based thermoplastic elastomers.

Anionic Polymerization of Terpene Monomers: New Options for Bio-Based Thermoplastic Elastomers

“…Terpenoids offer a possibility to overcome the lack of functional 1,3-diene monomers for the living anionic polymerization, caused by synthetic difficulties due to the high reactivity of the 1,3-diene entity. By protection of the hydroxyl groups of terpenoids like β-myrcenol with appropriate protective groups for the living anionic polymerization (e.g., silyl groups), a novel type of monomer is obtained. , Furthermore, in recent work myrcenol was copolymerized with β-myrcene or isoprene by using reversible addition–fragmentation chain transfer (RAFT) polymerization or neodymium- or cobalt-catalyst based polymerization. − …”

“…In the case of carbanionic diene polymerization, the polydiene microstructure is mainly influenced by the solvent, polar additives, the counterion, the chain end concentration, and temperature. , Aggregation of the carbanionic chain ends via lithium plays a pivotal role both for reaction kinetics and chain microstructure. The anionic sec -BuLi-inititated polymerization of β-myrcene in cyclohexane results in polymyrcene with 94% of the preferred 1,4-units and 6% 3,4-units, while the polymerization in the more polar solvent THF results in 30% 1,4-units, 57% 3,4-units, and 13% 1,2-units . A high content of 1,4-units of polydienes is required for a low glass transition temperature and improved elasticity, when compared to polydienes with higher 3,4-content. , The glass transition temperature of polydienes generally increases in a linear fashion with increasing vinyl content (1,2- and 3,4-units). , As demonstrated by Henning and Yoo, the slope of the linear correlation of the glass transition strongly correlates with the polydiene architecture and structure of the diene monomer, as shown for different polydienes in Figure .…”

“…Glass transition temperature as a function of vinyl content for polyisoprene, polymyrcene, and polyfarnesene reproduced from Henning et al and expanded by data available for polymyrcene from the literature. ,,, …”

“…In comparison, complete monomer conversion is generally obtained by living anionic polymerization techniques . In a recent work, we were able to introduce the first reported hydroxyl-group-containing 1,3-diene monomer for the living anionic polymerization based on β-myrcenol (2-methyl-6-methylidene­octa-2,7-dienol), which was prepared by allylic oxidation of β-myrcene . The protection of the hydroxyl unit with a silyl protective group (TBDMS-, tert -butyldimethylsilyl group) enables the carbanionic polymerization of silyl-protected β-myrcenol (MyrOSi).…”

Linalool Derivatives for Natural Product-Based 4D Printing Resins

“…Natural products are appealing alternatives to petrochemical sources for polymers, because of their potential to push society toward a more sustainable, circular economy, as well as inherent functionality in many cases. , A popular class of such natural materials is terpenes, of which terpenoids such as linalool are one example. Terpenes have found use in a variety of different polymeric applications, including the development of degradable polyesters and polycarbonates, high T g and performance polymers, and even as initiators or monomers in controlled polymerizations. − The successful utilization of terpenes has generally required modifications to terpenoids or terpenes, such as limonene oxide produced from d -limonene or with epoxidized soybean oils. − Often this yields polymers with degradable backbones, such as with polyesters and polycarbonates synthesized from limonene oxide; the common alternative is the formation of degradable side chains after functionalization with acrylates. ,− More recently, this second strategy has been exploited toward 3D printing of natural products, making use of the available functional groups for modification followed typically by free radical crosslinking to produce nondegradable backbones. − For example, retinol has been conjugated with poly­(vinyl alcohol) for drug delivery applications, cyclodextrane has been functionalized with acrylate groups for vat photopolymerization, linalool and citronellol were grafted to oligomeric chitosan, , and myrcenol has been projected toward producing functional copolymers and brushes . However, there are other strategies toward photopolymerization 3D printing, such as thiol-ene chemistry, which has been widely utilized for postfabrication functionalization but only sparingly used for additive manufacturing. ,− …”