Heterotelechelic silicones: Facile synthesis and functionalization using silane-based initiators

The synthetic utility of heterotelechelic polydimethylsiloxane (PDMS) derivatives is limited due to challenges in preparing materials with high chain-end fidelity. In this study, anionic ring-opening polymerization (AROP) of hexamethylcyclotrisiloxane (D3) monomer using a specifically designed silyl hydride (Si–H)-based initiator provides a versatile approach towards a library of heterotelechelic PDMS polymers. A novel initiator, where the Si–H terminal group is connected to a C atom (H–Si–C) and not an O atom (H–Si–O) as in traditional systems, suppresses intermolecular transfer of the Si–H group, leading to heterotelechelic PDMS derivatives with a high degree of control over chain-ends. In-situ termination of the D3 propagating chain end with commercially available chlorosilanes (alkyl chlorides, methacrylates, and norbornenes) yields an array of chain-end functionalized PDMS derivatives. This diversity can be further increased by hydrosilylation with functionalized alkenes (alcohols, esters, and epoxides) to generate a library of heterotelechelic PDMS polymers. Due to the living nature of ring opening polymerization and efficient initiation, narrow-dispersity (Đ < 1.2) polymers spanning a wide range of molar masses (2 – 11 kg mol−1) were synthesized. With facile access to α-Si–H and ω-norbornene functionalized PDMS macromonomers (H–PDMS–Nb), the synthesis of well-defined super-soft (Gʹ = 30 kPa) PDMS bottlebrush networks, which are difficult to prepare using established strategies, was demonstrated.

Methods
Nuclear magnetic resonance spectroscopy

1H-, 13C- and 29Si-NMR spectra were collected on a Bruker Avance NEO 500 MHz using chloroform as a deuterated solvent. IR spectra was measured with a Thermo Nicolet iS10 FTIR spectrometer equipped with a Smart Diamond attenuated total reflectance (ATR) accessory.

Size exclusion chromatography

Size exclusion chromatography (SEC) was measured using Waters e2695 separation module with a Waters 2414 differential refractive index detector equipped with two columns (PLgel, 5 μm MiniMIX-D, 250×4.6 mm columns + guard) with chloroform containing 0.25% TEA at 35 °C as the mobile phase.

Matrix assisted laser desorption ionization-time of flight mass spectrometry

MALDI-TOF MS spectrometry was measured using a Bruker Microflex LRF MALDI TOF mass spectrometer in positive reflection mode; the analyte, matrix (DCTB) were dissolved in chloroform at the concentration of 2.5 and 10 mg/mL respectively, and cationization agent (NaTFA) was dissolved in chloroform at concentrations of 1 mg/mL, then mixed in a volume ratio of 20 : 1 : 1 (DCTB : NaTFA : sample). 0.5 μL of this mixed solution was spotted onto a ground steel target plate and the solvent was allowed to evaporate prior to analysis.

Mechanical analysis

Frequency sweeps and isochronal temperature sweeps were obtained on a TA Instruments ARES G2 in oscillatory shear mode with an 8 mm parallel-plate geometry and a nitrogen-purged forced–convection oven.