Z-group Approach Research Articles

RAFT step-growth polymerization via the Z-group approach and deconstruction by RAFT interchange.

Recycling vinyl polymers is essential to mitigate the environmental impact of plastic waste. However, typical polymerization strategies to construct vinyl polymers lack the ability to incorporate degradable linkers throughout the main chain. We report a RAFT step-growth polymerization through the Z-group approach that is directly carried out by using a common class of symmetric trithiocarbonate based RAFT agents and commercially available bismaleimide monomers. Such synthesized RAFT step-growth polymers contain embedded RAFT agents in every structural unit, allowing chain expansion of the step-growth backbone via controlled chain growth to yield linear multiblock (co)polymers. These polymers can undergo deconstruction via the RAFT interchange process with exogeneous RAFT agents, generating smaller uniform species with narrow molecular weight distribution. In addition, the telechelic bifunctional RAFT agent nature after deconstruction allows repolymerization, showing a promising method for recycling common vinyl polymers.

Surface-Initiated PET-RAFT via the Z-Group Approach.

Surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) is a user-friendly and versatile approach for polymer brush engineering. For SI-RAFT, synthetic strategies follow either surface-anchoring of radical initiators (e.g., azo compounds) or anchoring RAFT chain transfer agents (CTAs) onto a substrate. The latter can be performed via the R-group or Z-group of the CTA, with the previous scientific focus in literature skewed heavily toward work on the R-group approach. This contribution investigates the alternative: a Z-group approach toward light-mediated SI photoinduced electron transfer RAFT (SI-PET-RAFT) polymerization. An appropriate RAFT CTA is synthesized, immobilized onto SiO2, and its ability to control the growth (and chain extension) of polymer brushes in both organic and aqueous environments is investigated with different acrylamide and methacrylate monomers. O2 tolerance allows Z-group SI-PET-RAFT to be performed under ambient conditions, and patterning surfaces through photolithography is illustrated. Polymer brushes are characterized via X-ray photoelectron spectroscopy (XPS), ellipsometry, and water contact angle measurements. An examination of polymer brush grafting density showed variation from 0.01 to 0.16 chains nm-2. Notably, in contrast to the R-group SI-RAFT approach, this chemical approach allows the growth of intermittent layers of polymer brushes underneath the top layer without changing the properties of the outermost surface.

Stability of star-shaped RAFT polystyrenes under mechanical and thermal stress

Well-defined three-arm and four-arm star polymers designed via a Z-group approach carrying trithiocarbonate functionalities at the core are prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization featuring molecular weights of Mn,SEC = 156 kDa, Đ = 1.16 (3-arm) and Mn,SEC = 162 kDa, Đ = 1.15 (4-arm) based on multi-angle laser light scattering (MALLS) detection, respectively. The star-shaped polystyrenes are subjected (in bulk) to thermal stress in the temperature range between 140 and 200 °C from 10 minutes up to 96 h. The thermally treated 3-arm and 4-arm star polymers are analyzed via size exclusion chromatography (SEC) to quantify the degradation process at variable temperatures as a function of time under an argon atmosphere. Cleavage rate coefficients of the star polymers are deduced as a function of temperature, resulting in activation parameters for the cleavage process, i.e. Ea = 131 kJ mol−1; A = 3.93 × 1011 s−1 (Mn,SEC = 156 kDa, Đ = 1.16, 3-arm star) and Ea, = 134 kJ mol−1; A = 9.13 × 1011 s−1 (Mn,SEC = 162 kDa, Đ = 1.15, 4-arm star), respectively. Processing of the star-shaped polymers is mimicked via a small scale counter rotating twin screw extrusion to achieve nonlinear shear and elongation flow under pressure. Furthermore, a rheological assessment via the linear shear deformation region (small amplitude oscillatory shear, SAOS) allows for a correlation of the processing conditions with the thermal degradation properties of the star polymers in the melt. Zero shear viscosity (η0) as a criterion of the degradation process is measured in the rheometer and correlated to the weight-average molecular weight, Mw.

RAFT-based Polystyrene and Polyacrylate Melts under Thermal and Mechanical Stress

Although controlled/living radical polymerization processes have significantly facilitated the synthesis of well-defined low polydispersity polymers with specific functionalities, a detailed and systematic knowledge of the thermal stability of the products–highly important for most industrial processes–is not available. Linear polystyrene (PS) carrying a trithiocarbonate mid-chain functionality (thus emulating the structure of the Z-group approach via reversible addition–fragmentation chain transfer (RAFT) based macromolecular architectures) with various chain lengths (20 kDa ≤ Mn,SEC ≤ 150 kDa, 1.27 ≤ Đ = Mw/Mn ≤ 1.72) and chain-end functionality were synthesized via RAFT polymerization. The thermal stability behavior of the polymers was studied at temperatures ranging from 100 to 200 °C for up to 504 h (3 weeks). The thermally treated polymers were analyzed via size exclusion chromatography (SEC) to obtain the dependence of the polymer molecular weight distribution on time at a specific temperature unde...

RAFT mediated polysaccharide copolymers

Graft copolymers were prepared using the RAFT process via a Z-group approach, where xanthate esters were formed directly on a cellulosic substrate. Grafting of vinyl acetate onto the modified cellulosic materials was then carried out via the reversible addition fragmentation chain transfer (RAFT) process. The xanthate RAFT agents on the backbone of the cellulosic materials were identified by Fourier-transform infrared spectroscopy (FT-IR) and ultraviolet–visible spectroscopy (UV–vis). The number average molar masses of the graft copolymers were determined using size exclusion chromatography (SEC) and further characterization was conducted via liquid adsorption chromatography (LAC). The chromatographic results showed that the modified cellulosic materials were successfully grafted with polyvinyl acetate in a controlled manner. Grafted polyvinyl acetate (on the surface) and nongrafted polyvinyl acetate (in the solution) have almost the same molar mass and polydispersity index.

Shielding effects in polymer-polymer reactions. II. Reactions between linear and star-branched chains with up to six arms

The shielding effect of surrounding arms and chains on the encounter probability of reactive sites located both at the end of a linear chain and at several positions along the arms of star-branched chains with up to six arms is calculated by means of exact enumeration of samples prepared by Monte Carlo simulation. The changes of parameters that characterize the size and the shape of chain configurations during the approach of reactive centers located at the end of the linear chain and at the center of the star are evaluated. In addition to this specific case, which represents the central reaction step in reversible addition-fragmentation chain transfer star polymerization following the Z-group approach, a general discussion is given on the chain-length dependence of shielding factors associated with distinct segment positions.

Synthesis of poly(vinyl alcohol) combs via MADIX/RAFT polymerization

Poly(vinyl acetate) combs have been prepared via macromolecular design via interchange of xanthate (MADIX)/reversible addition–fragmentation chain-transfer (RAFT) polymerization using xanthate functionalized polymer cores. The comb backbones were prepared using well-defined poly(vinyl alcohol) PVA polymers with a degree of polymerization of 20, 100 and 170, respectively. Functionalization with xanthates via R-group or a Z-group approach resulted in the formation of macromolecular MADIX agents. While Z group designed macromolecular xanthate agents appeared to inhibit the polymerization of vinyl acetate (VAc), R group designed macromolecular xanthate agents achieved to mediate efficiently the bulk polymerization of VAc affording PVAc combs. However, the growth of the combs was accompanied at low conversions by the formation of linear polymer chains as a result of the constant initiation (AIBN) and shoulders, which can be attributed to intermolecular coupling reactions. The proportions of single chains and termination products were observed to increase with the degree of polymerization of the macromolecular MADIX agents broadening the molecular weight distribution. As a result of a stable ester link between the branches and the PVA backbone, the branched PVAc architectures were finally hydrolyzed to afford poly(vinyl alcohol) combs.

Controlled Modification of Cellulosic Surfaces via the Reversible Addition - Fragmentation Chain Transfer (RAFT) Graft Polymerization Process

Background The surface modification of the cellulose fibres through graft polymerization is of great interest to the researcher due to their wide range of potential applications. Living radical polymerization methods have been used to achieve well defined and living polymer grafts from the cellulose surface. [1] RAFT polymerization is a versatile method that allows the synthesis of polymers with complex molecular architectures, low polydispersity, and functionalizable end groups. [2] The controlled surface modification of different solid substrates (such as poly(propylene) lanterns, core-shell microspheres, silica, gold nanoparticles, carbon nanotubes, and the Merrifield resin system) via RAFT has already been reported. [3-7] RAFT graft polymerization from a solid support can be performed using both the R-group approach (the leaving and reinitiating R-group attached to the backbone) and the Z-group approach (the stabilizing group attached to the backbone).The R-group approach follows the 'grafting from' mechanism and higher graft densities can be achieved relative to that achieved using the 'grafting onto' (Z-group) approach due to fewer steric hindrance problems. [7,8] The use of a solid-supported chain transfer agent (the R-group approach) to mediate the controlled graft polymerization from the different cellulosic surfaces such as cotton fabric and filter paper has been the focus of recent studies. [9-11] Current research into RAFT polymerization is covered by two reviews. [12,13] .

Polystyrene comb polymers built on cellulose or poly(styrene-co-2-hydroxyethylmethacrylate) backbones as substrates for the preparation of structured honeycomb films

Comb polymers were prepared using the RAFT process via a Z-group approach. Trithiocarbonate RAFT agents were covalently bound to a polymer backbone via ester linkages. Two different types of comb polymer backbones were prepared, based on either modified cellulose or statistical poly(styrene-co-2-hydroxyethylmethacrylate) backbones. The subsequent polymerization of styrene from these multi-RAFT-functional backbones displayed some unusual kinetics with the rate of polymerization increasing concomitantly with the trithiocarbonate concentration. The molecular weight of each polystyrene branch increased with conversion, however, deviation from the theoretical molecular weight was detected. The polystyrene comb polymers were utilized in a casting process to prepare highly regular honeycomb structured porous films using breath figures. In general, the regularity was found to increase with increasing number of branches on a backbone and with increasing length of the polystyrene branch.

Poly(vinyl ester) Star Polymers via Xanthate-Mediated Living Radical Polymerization: From Poly(vinyl alcohol) to Glycopolymer Stars

Poly(vinyl ester) stars have been synthesized via different macromolecular design via interchange of xanthate (MADIX)/reversible addition−fragmentation chain transfer (RAFT) polymerization methodologies. Two approaches were investigated. The first method involved attaching the xanthate functionality to the core via a nonfragmenting covalent bond (Z-group approach). The second approach involved attaching the xanthate functionality to the core via a fragmenting covalent bond (R-group approach). The R-group approach yielded well-defined poly(vinyl acetate), poly(vinyl pivalate), and poly(vinyl neodecanoate) stars with narrow polydispersities (PDI ≤ 1.4). In contrast, the molecular weight distributions of poly(vinyl acetate) stars prepared using the Z-approach tended to broaden at moderate to high conversions. We attribute this broadening to steric congestion around the xanthate functionality, restricting the access of monomer to the CS bonds. The R-group approach was also found to be superior for preparing p...