Entangled polymer solutions and melts exhibit elastic, solid-like resistance to quick deformations and a viscous, fluid-like response to slow deformations. This viscoelastic behaviour reflects the dynamics of individual polymer chains driven by brownian motion: since individual chains can only move in a snake-like fashion through the mesh of surrounding polymer molecules, their diffusive transport, described by reptation, is so slow that the relaxation of suddenly imposed stress is delayed. Entangled polymer solutions and melts therefore elastically resist deforming motions that occur faster than the stress relaxation time. Here we show that the protein myosin II permits active control over the viscoelastic behaviour of actin filament solutions. We find that when each actin filament in a polymerized actin solution interacts with at least one myosin minifilament, the stress relaxation time of the polymer solution is significantly shortened. We attribute this effect to myosin's action as a 'molecular motor', which allows it to interact with randomly oriented actin filaments and push them through the solution, thus enhancing longitudinal filament motion. By superseding reptation with sliding motion, the molecular motors thus overcome a fundamental principle of complex fluids: that only depolymerization makes an entangled, isotropic polymer solution fluid for quick deformations.

The first equilibrium polycondensation polymerization approach to unsaturated poly [carbo-(dimethyl)silanes] is presented. Diallyldimethylsilane (I),4,4,7,9-diene (II), and 1,4-bis(allyldimethylsiIyl)benzene (HI) undergo acyclic diene (metathesis (ADMET) polymerization when catalyzed by highly active tungsten alkylidenes, [(CF3)2CH3CO]2(N-2,6-C6H3-i-Pr2)W=CHC(CH3)2R, where R = CH3 or Ph. These polymerizations, which are performed under bulk conditions, continuously release ethylene to give poly(l,l-dimethyl-l-silapent-3-ene) (VI), poly(1,1,4,4-tetramethyl-1,4-disilaoct-6-ene) (VII), and poly[l-(dimethylsilyl)-4-(l,l-dimethylsilapent-3-en-l-yl)phenylene] (VIII), respectively. Polymers VI and VII produce trace amounts of the cyclic dimer and monomer, respectively, at the end of the polymerizations. Dimethyldivinylsilane (IV) does not homopolymerize; however, it does copolymerize with 1,9-decadiene (V) to give poly(l,l-dimethyl-l-silapropene)-co-octenamer (IX). All vinylsilane linkages in the copolymer are isolated. All polymers were characterized by infrared spectroscopy and , 13C, and "Si NMR spectroscopy.Average molecular weights were determined by gel permeation chromatography and end-group analysis. Synthesis, characterization, and the current scope of this polymerization are discussed.

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The synthesis of unsaturated poly(carbosiloxane)s, a new class of siloxane polymers possessing a perfectly alternating siloxane and alkenylene main chain, is presented. 1,5-Bis(allyl)-1,1,3,3,5,5hexamethyltrisiloxane (4), l,3-bis(4-pentenyl)tetramethyldisiloxane (6), 1,1,3,, and telechelic a,a)-di-4-pentenylpoly(l,l,3,3-tetramethyldisiloxanylpentylene) (10) undergo acyclic diene metathesis (ADMET) polymerization catalyzed by [(CF.ihCHsCOLUV-^.e-CeH.ri-Pr2)Mo=CHC(CH:i)2Ph (1). These polymerizations, which are performed under bulk conditions and at low temperatures, continuously release ethylene to give poly(l,l,3,3,5,5-hexamethyltrisiloxanyl-2-butenylene) (5), poly(1,1,3,3-tetramethyldisiloxanyl-4-octenylene) (7), poly(1,1,3,3-tetramethyldisiloxanyl-p-phenylenel,l,3,3-tetramethyldisiloxanyl-2-butenylene) (9), and poly(l,1,3,3-tetramethyldisiloxanylpentylene-co-l,1,3,3tetramethyldisiloxanyl-4-octenylene) (11), respectively. Bis(vinyl)tetramethyldisiloxane fails to homopolymerize under ADMET conditions, and bis(allyl)tetramethyldisiloxane (2) releases ethylene when catalyzed by 1 in the absence of solvent, to give, exclusively, the ring-closed product, 1,1,3,3-tetramethyldisiloxacyclohept-5-ene (3). Extending the methylene units or the siloxane linkage in the monomer results in facile ADMET polycondensation, in essentially quantitative conversions, affording well-defined, low-Tg, linear polymers with known vinylic end groups. The polymerizations are void of competing reactions except when back-biting reactions are favorable. Polymer 5 undergoes back-biting to generate the nine-membered cyclic silaoxalkene 13, when active polymer is diluted or, to a lesser extent, when in the bulk. All monomers and polymers were characterized by infrared spectroscopy and , 13C, and 29Si NMR spectroscopy. Number average molecular weights were determined by gel permeation chromatography and quantitative 13C NMR end group analysis when possible. Synthesis, characterization, thermal analysis, and the current scope of this polymerization are discussed.

A novel polymer has been developed for use as a thin film dielectric in the interconnect structure of high density integrated circuits. The coating is applied to the substrate as an oligomeric solution, SiLK*, using conventional spin coating equipment and produces highly uniform films after curing at 400 °C to 450 °C. The oligomeric solution, with a viscosity of ca. 30 cPs, is readily handled on standard thin film coating equipment. Polymerization does not require a catalyst. There is no water evolved during the polymerization. The resulting polymer network is an aromatic hydrocarbon with an isotropie structure and contains no fluorine.The properties of the cured films are designed to permit integration with current ILD processes. In particular, the rate of weight-loss during isothermal exposures at 450 °C is ca. 0.7 wt.%/hour. The dielectric constant of cured SiLK has been measured at 2.65. The refractive index in both the in-plane and out-of-plane directions is 1.63. The flow characteristics of SiLK lead to broad topographic planarization and permit the filling of gaps at least as narrow as 0.1 μm. The glass transition temperature for the fully cured film is greater than 490 °C. The coefficient of thermal expansivity is 66 ppm/°C below the glass transition temperature. The stress in fully cured films on Si wafers is ca. 60 MPa at room temperature. The fracture toughness measured on thin films is 0.62 MPa m ½. Thin coatings absorb less than 0.25 wt.% water when exposed to 80% relative humidity at room temperature.