General remarks H NMR (400 MHz), C NMR (100.6 MHz) and Sn NMR (149.2 MHz) spectra were recorded on a Bruker Advance 400 MHz NMR spectrometer with a BBFO 5-mm direct probe. A 1 H pulse width of 30° was used, acquiring a spectral window of 8223 Hz (20 ppm) using a relaxation delay of 1s, acquisition time 3.98s, 32k points (16 scans). The 1 H 90° pulse width was 10.4 µs. A 13 C pulse width of 30° was used, acquiring a spectral window of 24038 Hz (239 ppm) using a relaxation delay of 2s, acquisition time 1.36s, 32k points (4096 scans). The 13 C 90º pulse width was 8.7 µs. A 119 Sn pulse width of 30º was used, 8.75 µs, acquiring a spectral window of 100000 Hz (670 ppm) using a relaxation delay of 1s, acquisition time 0.33s, 32k points (15360 scans) with inverse gated proton decoupling. All results were analysed on MestReNova LITE 5.2.5 software. Chemical shifts were calculated using the chemical deuterated standards as a reference for 1 H and C. The 119 Sn was referenced to SnMe4 as an internal standard. All J coupling values are reported as absolute values. Time-of-flight mass spectrometry analyses were performed at the AIMS Mass Spectrometry Laboratory, University of Toronto using a JMS-T1000LC mass spectrometer (JEOL Inc., Peabody, MA) equipped with a Direct Analysis in Real Time (DART) ionization source (DART-SVP, Ionsense Inc., Saugus, MA). The DART source was operated with He gas and the temperature was adjusted in the range 100-400 C. Isotopic distributions for the observed ionic species were calculated using the Mass Center utility (JEOL) and were in good agreement with the measured mass spectra. Absolute molecular weights of the polymers were determined by GPC using a Viscotek Triple Model 302 Detector system. GPC columns were calibrated versus polystyrene standards (American Polymer Standards). A flow rate of 1.0 mL/min. was used with ACS grade THF as the eluent. GPC samples were prepared using 3-10 mg of each polymer per mL THF, and filtered using a 0.45 μm filter. All samples were run with and without UVA (conc. ≈ 0.001 M) for comparison. The X-ray diffraction data for compounds 12, 14, 18, and 19 were collected on a Bruker Kappa APEX-DUO diffractometer using monochromated Mo-K〈 radiation (Bruker Triumph) and were measured using a combination of  scans and  scans. The data were processed using APEX2 and SAINT programs. Absorption corrections were carried out using SADAB. The structures were solved using SHELXT and refined using SHELXL-2013 for full-matrix least-squares refinement that was based on F 2 . For all structures, H atoms were included in calculated positions and allowed to refine in a ridingmotion approximation with Uiso tied to the carrier atom. All reactions were carried out under a nitrogen atmosphere using Schlenk techniques unless otherwise described. LiAlH4 (1.0 M in Et2O), NaBH4, 1.0 M solution of HCl in Et2O, anhydrous CaCl2, anhydrous MgCl2 and Wilkinson's catalyst were purchased commercially and used without further purification. Solvents were dried by standard procedures prior to use.