Copolymer Compositional Ratio  pattern by a first step of exposure and etching, shifting the  position, processing a second underlay formed below the first  underlay by a second step of exposure through the 1 :3 isolated  left pattern, for forming a half-pitch 1: 1 pattern.  Mw=6,500  Mw/Mn=l.65  This polymer is designated Poly-I.  Synthesis was carried out by the same procedure as in  Synthesis Example 1 aside from using 4.6 g of a monomer of  formula (21) instead of the monomer of formula (20). There  was obtained 12.4 g of a white polymer.  The polymer was analyzed by $^{13}$C-NMR, $^{1}$H-NMR and  GPC, with the analytical results shown below.  Copolymer Compositional Ratio  Mw=7,400  Mw/Mn=l.76  This polymer is designated Poly-2.  Synthesis was carried out by the same procedure as in  Synthesis Example 2 except that 2.4 g of indene was further  added. There was obtained 13.0 g of a white polymer.  The polymer was analyzed by $^{13}$C-NMR, $^{1}$H-NMR and  GPC, with the analytical results shown below.  The polymer was analyzed by $^{13}$C-NMR, $^{1}$H-NMR and  GPC, with the analytical results shown below.  The reaction solution was concentrated to a  ‰  volume and  precipitated in 5.0 L of methanol. The resulting white solids  were filtered and vacuum dried at 40°  C., obtaining 15 .5 g of  a white polymer. The polymer was dissolved in a mixture of  0.4 L methanol and 0.5 L tetrahydrofuran again, to which 140  g of triethylamine and 30 g of water were added. The reactor  was heated to 60°  C., allowing deprotection reaction to occur.  The reaction solution was neutralized with acetic acid, con› centrated, dissolved in 0.5 L ofacetone, and precipitated in 10  L of water. Filtration and drying in the same way as above  gave 12.4 g of a white polymer.  A 100-mL flask was charged with 13. 0 g of acetoxystyrene,  4.1 g ofa monomerof formula (20) below, and 20 g of toluene  as a solvent. The reactor was cooled to - 70°  C. in a nitrogen  atmosphere, whereupon vacuum deaeration and nitrogen  flow were repeated three times. The reactor was warmed up to  room temperature, 1.4 g of AIBN was added as a polymer› ization initiator, and the reactor was further heated to 55°  C.,  at which reaction was effected for 40 hours.  Examples of the invention are given below by way of  illustration and not by way of limitation. The copolymer  composition is expressed in molar ratio. The abbreviation Me  is methyl, AIBN is azobisisobutyronitrile, NMR is nuclear  magnetic resonance. Mw is a weight average molecular  weight, Mn is a number average molecular weight, both as 25  measured by gel permeation chromatography (GPC) versus  polystyrene standards, and Mw/Mn is a molecular weight  dispersity.  A further embodiment of the invention provides a photo- mask blank comprising a chromium compound film and a  coating of the positive resist composition deposited thereon.  In this embodiment, a pattern may be formed by heat treating  the photomask blank, subjecting the resist coating to pattern- wise exposure to high-energy radiation through a photomask  or patternwise exposure to high-energy beam, optionally heat  treating the exposed resist coating, and developing it with a  developer. The chromium compound film may be of CrO,  CrC, CrN, CrON, CrOC, CrONC, CrNC or the like.  EXAMPLES  Synthesis Example 2  Synthesis Example 1  Synthesis Example 3  (20)  OH  Me  ~  0  0  (21)  OH  Me  ~  0  0  196  US 8,968,979 B2  195 