THERMAL CHARACTERIZATION OF POLYMERIC MATERIALS
CONTENTS CHAPTER GENERAL REVIEW ON POLYSILOXANE SYNTHESIS CHAPTER SILICONES IN INDUSTRIAL APPLICATIONS POLYSILOXANES AS TEMPLATES AND BUILDING BLOCKS IN NANOSTRUCTURED MATERIALS PHOTOCHEMISTRY OF POLYSILOXANES POLYSILANES POLYCARBOSILANES POLYSILAZANES POLYFERROCENYLSILANE-BASED POLYMER SYSTEMS DDS = DIMETHYLDICHLOROSILANE LIST OF ABREVIATIONS USED FOR POLYMER IDENTIFICATION: PDMS = POLYDIMETHYLSILOXANE PMHS = POLYMETHYLHYDRIDOSILOXANE, OR POLYMETHYLSILOXANE Å) HENCE, THIS LINKAGE IS MORE COMPLEX THAN A REGULAR ? BOND A LARGE DIFFERENCE IN ELECTRONEGATIVITIES OF THE CONSTITUTIVE SILICON AND OXYGEN ATOMS, WHICH, ACCORDING TO PAULING SCALE, ARE EQUAL TO 1 8 AND 3 5, RESPECTIVELY, RESULTS IN PARTIALLY IONIC CHARACTER, ESTIMATED TO 40%-50% ON THE BASIS OF EMPIRICAL CALCULATIONS [19] O SI O SI O SI O SI O 1 65 A 145 O 110 O 2 5 KJ 1 25 KJ SCHEME 1 ?*(SI1-O) ?*(SI2-O) P1(O) P2(O) SCHEME 2 REACTIVITY OF THE SILOXANE BOND GENERAL REACTIVITY OF THE SILOXANE BOND IS LARGELY A FUNCTION OF ITS ELECTRONIC PROPERTIES THUS, IN VIEW OF THEORETICAL RESULTS, LOW BASICITY OF SILOXANE OXYGEN IS MAINLY DUE TO THE ELECTRON DELOCALIZATION (BY NEGATIVE HYPERCONJUGATION OR P-D BACK DONATION) ALONG THE OSIO (SILAACETAL) LINKAGE ELECTRON DELOCALIZATION AND CONTRIBUTION FROM IONIC STRUCTURES ARE ALSO RESPONSIBLE FOR AN EXTRA STRENGTHENING OF THE SILOXANE BOND [15,21,24] HIGH ENERGY OF THE SILOXANE BOND, (ESTIMATED TO FALL IN THE RANGE OF 450 KJ/MOL [25] TO 570 KJ/MOL [26]), MAKES IT VERY RESISTANT TO HOMOLYTIC CLEAVAGE ON THE OTHER HAND, THE SILOXANE BOND IS STRONGLY POLARIZED MOREOVER, THE SILICON ATOM HAS A RELATIVELY LARGE SIZE AND THE SUBSTITUENTS APPEAR ONLY AT EVERY SECOND ATOM IN THE CHAIN IN CONNECTION WITH THE HIGH FLEXIBILITY OF THE CHAIN, THESE FEATURES ACCOUNT FOR THE RELATIVELY LOW STERIC HINDRANCE AND HIGH ACCESSIBILITY OF SILICON TO A NUCLEOPHILIC ATTACK THIS RESULTS IN RELATIVELY HIGH REACTIVITY OF THE SILOXANE BOND TOWARD HETEROLYTIC CLEAVAGE [3]